Journal of Integrative Agriculture Doi ：

Advanced Online Publication 2015 10.1016/S2095-3119(15)61035-8

Breaking wheat yield barriers requires integrated efforts in developing countries1 Saeed Rauf1, Maria Zaharieva2, Marilyn L Warburton3, ZHANG Ping-zhi4, Abdullah M

AL-Sadi5, Farghama

Khalil1, Marcin Kozak6, Sultan A Tariq7 1

Department of Plant Breeding & Genetics, University College of Agriculture, University of Sargodha, Pakistan

2

National Agricultural University La Molina (UNALM), AP 456, Lima, Peru

3

USDA ARS Corn Host Plant Resistance Research Unit, Mississippi State, MS 39762

4

Crop Research Institute, Anhui Academy of Agricultural Sciences, No. 40 South Nongke Road, Hefei 230031,

P.R.China 5

Department of Crop Science, College of Agricultural and Marine Sciences, Sultan Qaboos University, P.O. Box 34,

AlKhoud 123, Oman 6

Department of Botany, Warsaw University of Life Sciences-SSGW, Warsaw, Poland

7

Social Sciences Research Institute, NARC-Islamabad, Pakistan

Abstract: Most yield progress obtained through the so called “green revolution”, particularly in the irrigated areas of Asia, has reached a limit, and major resistance genes are quickly overcome by the appearance of new strains of disease causing organisms. New plant stresses due to a changing environment are difficult to breed for as quickly as the changes occur. There is consequently a continual need for new research programs and breeding strategies aimed at improving yield potential, abiotic stress tolerance and resistance to new, major pests and diseases. Recent advances in plant breeding encompass novel methods of expanding genetic variability and selecting for recombinants, including the development of synthetic hexaploid, hybrid and transgenic wheats. In addition, the use of molecular approaches such as QTL and association mapping may increase the possibility of directly selecting positive chromosomal regions linked with natural variation for grain yield and stress resistance. The present article reviews the potential contribution of these new approaches and tools to the improvement of wheat yield in farmer’s fields, with a special emphasis on the Asian countries, which are major wheat producers, and contain the highest concentration of resource-poor wheat farmers.

Keywords: genetic diversity, heterosis, hybrid wheat, synthetic hexaploid wheat, yield potential 1. Introduction Today, cereals make up around 50% of global food production (FAO 2013). Wheat alone still accounts for one fifth of humanity´s food, providing 21% of the calories and 20% of the protein to more than 4.5 billion people in 94 developing countries (Braun et al. 2010). Bread wheat (Triticum aestivum L. subsp. aestivum) was one of the first domesticated crops and, for 8,000 years, has been the basic staple food of Correspondence Saeed Rauf, PHD, Assistant Professor, Tel: +923321799642, Fax: +92418522313, E-mail: [email protected]

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 the major civilizations of Europe, West Asia and North Africa. It is now grown on 200 million ha with a production of 600 million tons. Global average productivity is around 2.7 t ha-1, with high variability among countries and regions. Global per capita consumption was 67 kg year-1 in 2003. Demand for wheat is expected to increase by 60% in the developing world by 2050 (Rosegrant and Agcaoili 2010). Feeding more people on an increasingly fragile planet is becoming more and more challenging (Fan and Brzeska 2014). Enhancing wheat yield in the present context represents an ambitious target because of the increasing cost of fertilizers, a need to keep their overuse from negatively impacting the environment, and the environmental instability associated with climate change (Ray et al. 2013). Asia is the world’s largest producer of wheat, producing 280 million tons annually (excluding Russia). Southern and Eastern Asia produce 114 million tons each, Western Asia 29 million tons and Central Asia 23 million tons (FAO 2013). The largest wheat producing countries are China with 29 million ha, and India with 26 million ha (FAO 2013). China, India, and Pakistan together represent around 30% of the world wheat harvested area and 36% of the global wheat production (FAO 2013). In China, about 70% of the national production is concentrated in the provinces of Henan, Shandong, Hebei, Anhui and Jiangsu. Shortage of water is a major limiting factor for wheat production in China, particularly in the northern part of Yellow and Huai valleys (He and Bonjean 2013). India contributes around 13% of the global wheat growing area and global production. Main constraints for wheat production in this country are shrinking water and land resources, a changing climate, lack of prebreeding efforts, and screening and phenotyping methodologies (Bansal et al. 2013). In Pakistan, wheat supplies 37% of the daily calorie intake and is cultivated in almost every part of the country. Main production constraints are drought and heat stress, late planting (due to harvesting of kharif crops), non-availability of inputs (eg, seed, fertilizer) and weed infestation (Masood 2013). Among these three neighboring countries, China had the highest wheat yield, followed by India and Pakistan, which had below world average yields in year 2012. Low yield potential of local cultivars and slow gain in yield potential cause stagnant yield in the region (Hobbs and Morris 1996), and increasing prices of fertilizer (especially in Pakistan) and biotic and abiotic stresses are the key yield limiting factors in these countries. In the Punjab, for example, drought, weeds and aphids (together), and lodging are responsible for 46, 28 and 26% of yield losses, respectively (Abbas et al. 2005). Finally, increasing prices of farm inputs have narrowed the profitability of wheat farming, necessitating government prices supports for farmers. In Pakistan for example, the increase in wheat production mainly occurred because of the expansion of the cultivated area due to an increased support to prices by the government (USDA 2014). In consequence, wheat flour prices have doubled during the last eight years in this country. Grain yield (GY) is a complex quantitative trait resulting from interactions between various yieldrelated traits and the environment (Ilker et al. 2013). Breeding work has been largely empirical until recently, with grain yield being the primary trait for selection. The decision to advance or reject a genotype is generally made using a system of multiple cut-offs. Breeders select strongly for traits correlated with yield (including plant type, plant height, growth cycle, and spike fertility) in early

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 segregating generations, and do yield testing at later stages when a certain level of homozygosity has been achieved and large enough seed quantities are available for replicated field trials. Advances in the fields of molecular genetics and cytogenetics have now created new tools for expanding genetic variation, selecting for recombinants and achieving homozygosity or fixation of favorable genotypes in short periods of time. Overdominance can also be exploited to create high-yielding hybrids (Noorka et al. 2013). These tools should be integrated into the breeding process. Despite the importance of the wheat crop, empirical plant breeding techniques continue to play a central role in the development of wheat varieties especially in developing countries such as Pakistan and India. Historically, plant material introduced from CIMMYT was either released as new cultivars after evaluation under multi-location trials, or crossed with local varieties to evolve productive transgressive segregants. Research facilities and modern plant breeding expertise need improvement. Wheat breeding programs have closed or have been reduced in scope over time due to lack of state of the art technologies and unattractive service infrastructure, promotions and packages. No innovation such as the development of single cross hybrids or incorporation of useful translocations within wheat breeding programs has occurred. This situation is true in other developing countries as well. Therefore, this review which examines the integrative role of plant breeding in enhancing wheat yield will provide suggestions for future breeding programs in much of the developing world. After summarizing yield progress, it considers different opportunities and methods to continue increasing wheat productivity in farmers´fields, either by increasing yield potential or by decreasing yield gaps due to biotic or abiotic stresses. A particular emphasis is placed on Asian countries including China, India and Pakistan, where most of the wheat is cultivated, generally by poor farmers. 2. Wheat yield progress World wheat production increased at a rate of 3.3% per year between 1949 and 1978 with peak improvement in the 1960’s (Hawkesford et al. 2013). This is due to a significant increase in global wheat yields that can be largely attributed to significant genetic improvement and a greatly expanded use of irrigation, pesticides and fertilizers (Rauf et al. 2010). Since the introduction of Green Revolution crops in the early 1960s, there has been a linear increase in total cereal production from less than 1 billion tons to 2.6 billion tons in 2011 (FAO 2013). Efforts by wheat breeders at the International Maize and Wheat Improvement Centre (CIMMYT) were particularly fruitful in delivering new high-yielding varieties to various developing countries (Rauf et al. 2012). A good example is the case of the CIMMYT variety Attila, released under different names in India, Pakistan, Afghanistan, Iran, Turkey, Algeria, Tunisia and Morocco. In North-West India, this variety (released as PBW 343) increased yield 10% over the locally bred variety HD2329 and allowed an economic return of 150 million USD (Rajaram and Braun 2008). Yield gains in percentage were quite similar under rainfed and irrigated conditions but in absolute figures, grain yield increased much more in irrigated areas, including the Yaqui Valley in Mexico, the Punjab in India, the Upper Nile Delta in Egypt and irrigated areas of Syria (Rajaram and Braun 2008).

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 Unfortunately, this rate of progress has consistently decreased over recent decades. This stands in contrast to the situation in rice and maize where yield improvements have been maintained (FAO 2013). The rate of increase in wheat production slowed to 1.5% per year between 1981 and 1990, the only exception being China (2.6% per year) and India and Pakistan (3% per year). However, progress in the growth rate of grain yield after 1990 decreased in India and Pakistan as well (Table 1). This is happening precisely when wheat yields must increase at a rate of 3-4% per year to meet future grain requirements of these countries. 3. Improving yield potential Yield potential (YP) can be defined as the yield of a cultivar when grown in environments to which it is adapted, with non-limiting nutrients and water, and with pests, diseases and weeds effectively controlled (Evans and Fisher 1999). Yield potential is determined by genotype characteristics and environmental factors such as solar radiation, temperature, and atmospheric concentration of carbon dioxide. The significant increase in yield potential in wheat has been principally associated with a reduction of plant height, which increased the proportion of carbon partitioned to grain (and harvest index, HI) and simultaneously decreased the risk of yield penalties caused by lodging (Jiang et al. 2003). Harvest index in most modern cultivars now seems to be close to its biological maximum of 60% (Shearman et al. 2005). Further genetic gain in yield potential is not expected to come from single simple traits like plant height. Different ways must be considered for increasing yield potential, and some of these are examined below. 3.1 Increasing spike fertility The extent to which yield components (spikes m-2, number of grains spike-1 and thousand kernel weight) affect final wheat grain production highly depends on the environment (Table 2). Path coefficient analysis showed that in favourable conditions the number of grains spike-1 contributes the most to grain yield, followed by the number of spikes m-2 (Aycicek and Yildirim 2006; Reynolds et al. 2009). As a consequence, yield potential has been found in many countries to be associated to the increase of the number of grains m-2 (Fig. 1). The increase in the number of grains m-2 did not lead to a large reduction of grain weight, probably because photosynthetic capacity during grain filling together with pre-anthesis assimilate reserves exceed the demands of the growing wheat grains during postanthesis (Borras et al. 2004). However, the number of grains spike-1 has a very low heritability (Table 3) and current elite germplasm shows a low genetic variability (18-24%) for this trait. Novel sources of germplasm including synthetic wheats are consequently required to continue increasing this trait. The number of grain spike-1 can be increased by enhancing the number of spikelets spike-1 and the number of grains spikelet-1, which is itself determined by the number of fertile florets (Álvaro et al. 2008). When the number of potential florets per spike remains similar, this last trait results from a higher survival of floret primordia (Slafer and Andrade 1993) which is likely to be due to pleiotropic effects on spike fertility of the two most commercially important gibberellic acid (GA)-insensitive

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 dwarfing genes Rht-B1b and Rht-D1b (Flintham et al. 1997). An association has also been reported between the Xgwm219 alleles, heading time and spike length (Gao et al. 2014). Early heading time was shown to provide favourable conditions and longer duration for spike development. Considering the association reported by Abbate et al. (1995) between spike index (partitioning of dry matter to the spike) and resources available during spike growth, Slafer et al. (2001) proposed that one alternative approach to further raise spike fertility is to increase spike dry matter at anthesis by lengthening the stem elongation phase, provided that this will not change dry matter partitioning and the total period from sowing to anthesis. A recent screening of commercial cultivars of wheat reported by Whitechurch et al. (2007) showed a broad variation in the duration of stem elongation, unrelated to the length of the period from seedling emergence to the onset of stem elongation. It may be speculated that an even wider range of variation may exist in exotic germplasm. 3.2 Altering sinks that compete with spike fertility Genetic coefficient variation in spike index is reported to be in the range of 0.12–0.29 (Siddique et al. 1989; Reynolds et al. 2001; Shearman et al. 2005; Acreche et al. 2008). In theory, this trait can be increased by reducing the competition from alternative sinks during stem elongation. Decreasing wheat root dry weight ratio (root dry weight/total dry weight) to increase spike index is not a viable proposition because optimal yields require optimal access to water and nutrients during all phases of growth. Optimizing rooting patterns for more efficient water and nutrient capture is a better option. Optimal root architecture should be selected for the target environment and may include deeper root distribution (Barraclough et al. 1989; Gregory and Brown 1989). Increased spike index in semi-dwarf wheat has already reduced competition from the growing stem (Siddique et al. 1989). Further reduction of structural stem dry matter would, however, probably reduce grain yield due to lower biomass (Flintham et al. 1997) and reduction of reserves of soluble carbohydrates (mostly fructans) that accumulate in wheat stems and are remobilized to the grain, even under favourable post-anthesis conditions (Foulkes et al. 2007b). In favourable environments, nonproductive tillers are considered to be detrimental to grain yield as there is no net redistribution of assimilate to fertile tillers (Berry et al. 2003). Non-productive tillering can be reduced by selecting favourable alleles of the tin genes, which boost spike fertility but decrease grains per unit area (Duggan et al. 2005). Inconsistent effects of the presence and length of awns on bread wheat yield potential were reported (Foulkes et al. 2007b). The selection of awnless phenotypes appears, therefore, not to be a priority for increasing spike fertility under favourable conditions. 3.3 Increasing photosynthetic capacity Photosynthetic rate is tightly associated with the amount of Rubisco. Rubisco catalyses the assimilation of CO2 by the carboxylation of RuBP in the Calvin Cycle and the oxygenation of RuBP that initiates the process of photorespiration. The relative amounts of carboxylation and oxygenation depend on the concentrations of O2 and CO2 present. The oxygenase activities of Rubisco (and therefore

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 photorespiration) become more significant at higher temperatures, relative to the carboxylation of RuBP (Long et al. 2006). An option to increase photosynthetic rate would be to introduce forms of Rubisco with higher relative specificity for CO2. According to Parry et al. (2007), no substantial increase in the carboxylase relative to oxygenase activity or catalytic rate has yet been achieved by this approach. However, other studies contradict this assumption (Furbank et al. 2013) and further investigation is warranted. Attempts are also made to engineer RuBP regeneration and Rubisco activase and to introduce Rubisco subunits with enhanced catalytic properties (Furbank et al. 2013). Experimental evidence and modeling suggest that RuBP regeneration could be ensured by overexpressing sedoheptulose-1,7bisphosphatase and fructose-1,6-bisphosphate aldolase (Reynolds et al. 2009). A substantial variation for the rate of photosynthesis has been reported among wheat varieties, originated from leaf demand for CO2 and leaf ability to supply this CO2 in the interior tissues of the leaf. It has been noted that wheat yield potential can be increased by 50% or more by exploring natural variation in the catalytic rate of the Rubisco enzyme (Furbank et al. 2013). Another approach to increase photosynthetic capacity is to convert wheat to a C4 crop, installing a system which concentrates CO2 in the compartment where Rubisco is located, eliminating photorespiration and ensuring that Rubisco operates close to its catalytic optimum. The complexity of the anatomical and biochemical traits indicate that it would be necessary to modify or introduce a very large set of genes for this mechanism to operate. As these genes are as yet unknown, this method remains a very distant prospect for yield increase (Furbank et al. 2013). 3.4 Improving biomass and radiation use efficiency There is still considerable potential to boost the biomass of C3 species (Long et al. 2006). Increases in above-ground biomass of wheat have been reported over the last decades (Reynolds et al. 1999; Shearman et al. 2005), in some cases as a result of using exotic germplasm in breeding, including alien introgressions such as the 7DL.7Ag derived from Agropyron elongatum (Host) Nevishi (Reynolds et al. 2001; Monneveux et al. 2003). Another way to increase biomass at anthesis and thus to make more assimilates available to increase spike mass is to enhance pre-anthesis radiation-use efficiency (RUE). Biomass can be expressed as a function of the light intercepted (LI) and RUE. Genetic gains in wheat during the 20 th century have been largely associated with increased harvest index (HI), particularly because of the introgression of Rht genes (Calderini et al. 1995). Traits related to LI, such as stand establishment and delayed senescence, show significant genetic variation in conventional gene pools and are highly amenable to visual selection. RUE can be improved either by increasing cellular and leaf level photosynthesis (Murchie et al. 2009) or by modifying leaf architecture, thus permitting light penetration to canopy levels where photosynthesis responds linearly to light (Fischer 2007). The erect leaves ideotype is preferred over droopy leaves under heat stress to avoid high transpiration and canopy temperature.

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 There is still scope for further optimization of both light and N distribution in the canopy (Mussgnug et al. 2007). Measurement of the relative contribution of spike photosynthesis to overall canopy photosynthesis has rarely been considered as a selection criterion despite the large proportion of light that spikes intercept during grain filling (Tambussi et al. 2007). Sanchez-Bragado et al. (2014) reported an 81.1% contribution of the ear to grain filling under rainfed and high nitrogen level. As sink is still a critical yield limiting factor in wheat (Miralles and Slafer 2007), a promising approach for raising RUE, biomass and yield is by simultaneously improving the balance between source and sink (Shearman et al. 2005) and increasing the number of fertile florets and grains m-2 (see above) and carpel size of florets, thereby boosting potential grain weight (Ugarte et al. 2007). 4. Filling yield gaps 4.1 Constraints causing yield gaps Yields gaps are the differences between yield potential and actual average farm yields for a given area or time (Monneveux et al. 2013). Farm yields are affected by nutrient deficiencies or imbalances, poor soil quality, diseases, insect pests and weed competition, and these factors are influenced by crop management techniques. In most situations in the developing world, the wheat yield gap exceeds 30% of yield potential (Lobell et al. 2005). Assessing the relative contribution of constraints causing yield gaps is essential to define optimal crop management and breeding objectives. These include growth defining factors (solar radiation and temperature), growth limiting factors (water and nutrients) and growth reducing factors (pests, diseases and weeds) (Van Ittersum and Rabbinge 1997). Many plant characteristics that determine the impact of these factors on yield are highly heritable and can be improved via selection. The aim of wheat breeders is consequently not only to enhance the absolute yield of genotypes but also to reduce yield gaps by incorporating resistance against various biotic and abiotic stresses. The proportion of yield increase attributable to plant breeding during the last decades is 47% in wheat in the UK (Silvey 1994). An essential contribution of plant breeding in closing the yield gap over the past decades was by maintaining resistance levels in the face of evolving rusts (Dubin and Brennan 2009). In the case of abiotic constraints, breeding for better tolerance is an effective and sustainable way to improve farmers’ yields (Guttieri et al. 2001; Lobell et al. 2005; Trethowan and Mujeeb-Kazi 2008). By considering the role of breeding in filling the gaps, it is important to note that new varieties often represent a less expensive option for farmers and extension organizations and are generally adopted more readily than new management techniques (Monneveux et al. 2013). 4.2 Genotype by environment interactions Many complex traits show low heritability not only because of the large number of genes controlling the trait, but also because the trait is highly influenced by the environment. A trait may express

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 differently in different environments and breeding for yield often becomes a puzzle to optimize yield in each specific environment, as well as across many different growing conditions. Optimization can be estimated by analyzing the genotype by environment interactions (GEI) that arise due to differential performance of genotypes under a range of environments. Plotting phenotypic variance along with average yield of a cultivar allows the identification of stable and high yielding cultivars. The ability to model yield stability across environments has been well established and various models for estimation of variance stability have been used for the selection of stable varieties (Eberhart and Russell, 1966; Akcura et al. 2005). Using an additive mean multiplicative interaction model Farshadfar et al. (2011) found that the greatest proportion of variation was explained by environmental variation, followed by GEI and genotypes (which was five-fold smaller than GEI). Furthermore, environmental variation due to location was generally higher than variation due to year. Thus, for higher yield stability, selecting over more locations than yield may be the best option (Domitruk et al. 2001; Mustăea et al. 2009). In some cases, the relationship between yield in mild stress and non-stress environments is positive, suggesting that selecting for stress tolerance will not necessarily cause a yield penalty under optimal environments. In many other cases, however, a crossover interaction can be observed, reflecting that in some environments cultivars with high yield potential produce less than other cultivars better adapted to stress. For most cereals grown under water-limited conditions, the crossover occurs at a yield level of around 2–3 t ha-1 (Blum and Pnuel 1990; Ceccarelli and Grando 1991) which represents approximately one-third of the yield potential. Because genetically stable high yielding varieties enjoy greater cultivation area and provide higher return on investment to breeders and seed multipliers, breeding stable wheat varieties without sacrificing yield potential and maximizing return on investment represents the optimal strategy. However, this objective represents a major challenge for wheat breeders (Mustăea et al. 2009). Thus, Dawson et al. (2011) suggested the selection of high yielding genotypes for limited ecological zones and the creation of many, more targeted cultivars for more stressed environments. Obtaining cultivars well adapted to specific environments is dependent on the need to define ideotypes, which consists of a set of morphological and physiological traits contributing for enhanced yield or higher yield than currently prevalent cultivars, for each environment. The original concept of ideotype (Donald 1968) evolved over time with the introduction of genotype by environment covariances (Sedgley 1991), source and sink limitations (Reynolds et al. 2000), crop statistical and dynamic models (Boote et al. 2001) and phenotypic plasticity (Sadras et al. 2009). A different solution to the problem of yield stability may be the use of varietal mixtures (Mundt 2002; Li et al. 2012). Cowger and Weisz (2008) tested 13 mixtures of four wheat varieties chosen for the complementation of disease resistance. Mixtures of varieties showed only a moderate presence of disease over all testing environments and had higher yield and yield stability (determined through stability variance model and principal component analysis) compared to pure stand varieties. Mundt (2002) noted higher lodging resistance, yield and yield stability of varietal mixtures. Mengistu et al.

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 (2010) reported an average yield advantage of 0.4% of varietal mixtures. However, due to differential competition between wheat cultivars, yield performance of varietal mixtures cannot be predicted a priori and must be tested over many locations. 4.3 Tolerance to abiotic stresses Wheat is exposed to a wide range of abiotic stresses including drought, salinity, extreme temperatures and nitrogen deficiencies, leading to a wide range of physiological and biochemical responses. However, analyses of transcripts have shown some similarity in plant responses to multiple stresses at the molecular level. For instance, over expression of the TaZFP15 transcript in roots was also induced under nitrogen deficient, drought and saline conditions (Sun et al. 2012). Transgenic analysis showed that TaZFP15 has the potential to increase plant dry mass under stress condition. It was noted that the transcript was involved in the mediation of the signal transduction under diverse external stresses. The gene YrLT1 for stripe rust resistance is also related to heat resistance in wheat (Ma et al. 2013). Such pleiotropic effects may ultimately make it easier to breed wheat with multiple stress resistance, but negative pleiotropy is also likely in some cases, and may slow breeding progress. The relation between yield with and without abiotic stress varies greatly according to stress intensity and timing (Table 4). To estimate the global impact of climatic factors on wheat yield, Singh and Byerlee (1990) analyzed yield variability in 57 countries over 35 years. The amount and distribution of rainfall was the predominant factor influencing yield variability. Countries in which half of wheat was sown in rainfed conditions experienced twice the yield variability as countries in which wheat was mostly grown under well-watered conditions. At least 60 million ha of wheat is grown in marginal rain-fed environments in developing countries. National average yields in these regions range from 0.8 to 1.5 t ha-1, which represents approximately 10 to 50% of their theoretical potential yield (Morris et al. 1991). Wheat germplasm has been extensively screened for drought resistance and tolerant lines identified on the basis of different drought adaptation mechanisms and drought tolerance related traits (Araus et al. 2008; Monneveux et al. 2012). Azam et al. (2007) selected a drought tolerant wheat strain ‘NRL 2017´that was largely used to develop ICARDA drought tolerant germplasm. Similarly, the wheat variety ‘Raj’ with high yield potential, drought tolerance and good bread quality has been promoted in the drought-prone regions of Pakistan (Khan and Khan 2010). Breeding for salt resistance was achieved through selection for Na+ exclusion, a trait associated to high yield under saline conditions (Munns et al. 2006). Ashraf and O'Leary (1996) showed that the salt tolerant cultivar ‘S24’ had higher yield, number of tillers per plant and thousand kernel weight than its parents ‘Lu-26S’ (elite cultivar) and ‘Kharchia' (salt resistant land race). The salt resistant cultivar ‘S24’ accumulated lower Na+ in leaves and maintained higher K+/Na+ ratio than the elite parent. Munns et al. (2012) introduced the Nx2 gene from Triticum monococcum L. subsp. monococcum into bread wheat to develop the line ‘149’, which exhibited decreased Na+ transportation to the leaves and higher yield under salinity stress.

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 Significant breeding efforts are underway for screening wheat germplasm with enhanced heat tolerance, on the basis of physiological traits such as cell membrane injury (Ortiz et al. 2008). In Egypt, the selection of lines with good tolerance to high temperatures, such as Sakha-64 and Giza-169, allowed for a doubling of yield in heat prone areas (Ferrara et al. 1994). Furthermore, enhancing tolerance to various abiotic stresses may enable the crop to be grown in new environments or seasons of the year (Ashraf and Harris 2005). At CIMMYT, field screening takes place at two sites in the south and center of Mexico, where plants are exposed to two very different environments and day lengths. Selected lines grown in replicated trials are evaluated for secondary traits such as enhanced stand establishment, good tillering, spike fertility, accelerated grain filling and high biomass (Saunders and Hettel 1994). Low soil fertility is another important yield limiting factor in wheat. It is estimated that two thirds of the cultivated area in developing countries is concerned by low nutrient stress (Lal 2000). Most soils in the semi-arid and arid subtropics are characterized by low nitrogen, phosphorus and micronutrients (Fe, Cu, Mn, and Zn) availability (Dhaliwal et al. 2014). Genetic variation with respect to N-use efficiency has been reported in wheat, making breeding for tolerance to N-deficiency feasible and practical (Anderson et al. 1991). Nutrient-use efficiency can be enhanced through the maintenance of photosynthesis under nutrient stress or the improvement of nutrient-uptake capacity, nutrientutilization capacity and translocation efficiency. Nutrient-uptake efficiency of plants can be raised by enhancing the “rhizosphere competence” capacity, i.e., the capacity to establish and sustain efficient plant-growth promoting-rhizosphere bacterial communities and arbuscular mycorrhizas (AM) (Rana et al. 2012). Genetic variability among the wheat cultivars for the abundance of AM colonization has been observed under various fertility levels. It was concluded that wheat genotypes should be selected for the AM compatibility rather than AM dependency (Singh et al. 2012). Hetrick et al. (1993) reported that wheat varieties developed before 1950 were more reliant on mycorrhizal symbiosis than modern ones. Genes controlling the ability to form AM root symbiosis have been identified in wheat (Hetrick et al. 1995; Sisaphaithong et al. 2012) and their introduction into modern varieties could enhance P absorption. The expression analysis of wheat genome under AM inoculated and noninoculated roots has been exploited to tag host plant genes related to AM symbiosis (Sisaphaithong et al. 2012). The genes such as TaPT (10, 11 and myc) showed a marked increase in their expression under inoculated symbiosis. 4.4 Resistance to fungal diseases The major biotrophic fungal diseases of wheat include stem, leaf and stripe rust, powdery mildew, bunts and smuts. Hemibiotrophic fungal diseases include Septoria tritici blotch, Septoria nodorum blotch, spot blotch, tan spot and Fusarium head blight (FHB). Three major rust diseases continue to cause major losses in various parts of the world and hence receive high attention in breeding. These are stem (or black) rust caused by Puccinia graminis Pers. f. sp. tritici Eriks. and E. Henn. (Pgt); leaf (or brown) rust caused by P. triticina Erikss, and stripe (or yellow) rust caused by Puccinia striiformis

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 West. Evolution of new virulence through migration, mutation, recombination of existing virulence genes and their selection is very frequent in rust fungi. Therefore, breeding for resistance to rusts must be a continually ongoing strategic approach to enhance the durability of resistance. Breeders are now looking for alternative approaches to resistance management, such as the slow rusting resistance developed by Van der Plank (1963) and refined for leaf rust (Caldwell 1968), stem rust (Borlaug 1972) and yellow rust (Johnson 1988). High yielding lines that combined four or five additive, minor genes for both leaf and yellow rusts have been developed at CIMMYT by Singh et al. (2000) through three and four way crosses. The wide application of this concept in breeding for leaf rust resistance has dominated CIMMYT’s bread wheat improvement program for almost 40 years with major impacts (Marasas et al. 2003). Simple and three-way crosses are now commonly used in the CIMMYT breeding program with one or more parents carrying different adult plant resistance genes to breed new high yielding, near-immune wheat materials resistant to all three rusts (Singh et al. 2014). To transfer minor gene-based resistance into a susceptible adapted cultivar, a single backcross selected bulk scheme is used. Rust resistant and agronomically desirable plants are selected from large segregating populations grown under artificially created rust epidemics. Selection is practiced from the BC1 generation onward for resistance and other agronomic features under high rust pressure. Because additive genes are partially dominant, BC 1 plants carrying most of the genes show intermediate resistance and can be selected visually. Molecular markers are increasingly used in the selection process to increase resistance to major diseases (Bai et al. 2012; Ma et al. 2013). 4.5 Weed competition As compared to other cereals, wheat is a poor competitor and consequently, weeds represent a major limiting factor for wheat production. Globally, wheat yield loss due to weeds is around 23%, compared to 16% for fungal and bacterial pathogens, 3% for viruses and 9% for animal pests (Oerke and Dehne 2004). Wheat varieties are genetically variable in their ability to compete with weeds (Lemerle et al. 2001). Yield reductions can vary between 17 and 62%, depending on the ability of the cultivar to compete (Balyan et al. 1991). Wheat stand establishment plays an important role in the susceptibility to weeds, particularly in low-rainfall regions (less than 400 mm per year) (Schillinger et al. 1998). Early vigor and early ground cover provide a competitive advantage over early emerging weeds (Bond and Grundy 2001). In wheat, emergence is strongly influenced by coleoptile length, a moderately heritable trait that can be easily incorporated into modern varieties through breeding (Murphy et al. 2008). Allelopathy is another potentially important weed suppression trait. Some wheat accessions have been shown to inhibit the growth of weed species (Wu et al. 2000). Conservation agriculture techniques which leave the previous crop’s residue on the ground may aid wheat in the competition against weeds, or conversely, may increase the weed control problem (Erenstein et al. 2012). 5. Efficient use of genetic diversity

Journal of Integrative Agriculture Advanced Online Publication 2015 10.1016/S2095-3119(15)61035-8

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Genetic variation for traits related to yield potential, resistance to biotic stresses and tolerance to major abiotic stresses is limited in elite germplasm, because bottlenecks caused by two polyploidization events, domestication, and modern breeding led to a loss of potentially beneficial alleles (Rauf et al. 2010; Chen et al. 2012; Bonnin et al. 2014). Genetic diversity provides a buffer against unforeseen constraints, such as those imposed by climate change and evolving pathogens, and expanding the genetic diversity of key traits is expected to ensure greater stability of wheat yield and production (Hughes 2003). Wild relatives are genetically much more diverse than cultivated wheat and constitute useful genetic resources for new trait diversity (Harlan 1992). 5.1 Sources of genetic diversity Harlan and de Wet (1971) proposed the classification of each crop and its related species within three gene pools (primary, secondary, and tertiary), based on evolutionary distance and success rate of hybridization with the cultivated species. The distribution of genetic diversity within wheat gene pools and its exploitation was described by Mujeeb-Kazi and Rajaram (2002) and Mujeeb-Kazi (2003). The primary wheat gene pool includes polyploid Triticum species that have genomes homologues with bread wheat, T. aestivum subsp aestivum (2n=4x=42, BBDDAA) and the diploid donors of the A and D genomes. The desired genes from this gene pool can be transferred to bread wheat via direct hybridization, homologous recombination, backcrossing and selection. Genetic resources of the primary gene pool have been largely used in breeding as no special cytogenetic manipulation, except embryo rescue for some cross combinations, is necessary to produce the F1 hybrids. The Tausch's goatgrass, Aegilops tauschii Coss. (2n = 2x = 14, DD) was successfully used for enhancing genetic diversity in bread wheat (Mujeeb-Kazi and Hettel 1995). High levels of resistance to powdery mildew, Blumeria graminis (DC.) E.O. Speer f. sp. tritici (Bgt) (Schmolke et al. 2012) and leaf rust (Vasu et al. 2001; Zaharieva and Monneveux 2014) were transferred into bread wheat from einkorn wheat, T. monococcum subsp. monococcum (AA). Wild emmer, Triticum turgidum L. subsp. dicoccoides (Korn. ex Asch. & Graebn.) Thell. (2n=4x=28, BBAA), is a valuable source of resistance to powdery mildew, leaf and stripe rust, and tolerance to heat and drought (Nevo et al. 2002). Cultivated emmer, T. turgidum L. subsp. dicoccum (Schrank ex Schubl.) Thell., has been identified as a potential source of drought and heat tolerance and resistance to powdery mildew, leaf and stem rusts (Zaharieva et al. 2010) and has been found to have the lowest natural susceptibility to the Russian wheat aphid (Diuraphis noxia) (Robinson and Skovmand 1992; Lage et al. 2004). The secondary gene pool contains polyploid Triticum and Aegilops species that share one genome (A, B or D) with bread wheat, e.g. T. timopheevii subsp. timopheevii (GGAA). The diploid species of the Sitopsis section carrying the S genome, related to the B genome, are included in this gene pool. Genetic transfers are more difficult as chromosome pairing is reduced, and embryo rescue is required for obtaining F1 hybrids. The utilization of this gene pool for wheat improvement has been limited and concerned mainly Ae. speltoides Tausch genetic resources (Mujeeb-Kazi 2003).

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 Diploid and polyploid Triticeae species with non-homologous genomes to the A, B, and D wheat genomes belong to the tertiary gene pool (e.g. species belonging to Agropyron, Elymus, Hordeum, Psathyrostachys, Thinopyrum and Secale genera). Genetic transfers are complex, chromosome pairing is extremely reduced, and embryo rescue is essential (Qi et al. 2007). However, genes from the tertiary gene pool have been successfully incorporated into bread wheat (Mujeeb-Kazi and Hettel 1995; Friebe et al. 1996; Gill et al. 2008). 5.2 Establishment of core collections Rapid and accurate evaluation of large germplasm collections is feasible only for easily scored traits that do not show high genotype by environment interaction. Most traits of agronomic importance for applied plant breeding research do not fall into this category. The difficulty in evaluation of genetic resources or pre-breeding germplasm for useful agronomic traits is one of the main factors limiting their use in breeding programs. This is particularly true in the case of assessing abiotic stress tolerance, since this generally requires physiological evaluations that are not compatible with large-scale screening. The use of representative core subsets containing reduced numbers of accessions (5-20% of the entire collection), first suggested by Frankel (1984) and Brown (1989), can mitigate this problem. Methods to choose the subsets based on passport, phenotypic and/or genetic information have been well reviewed in the recent past (Odong et al. 2013; Huang et al. 2013; Wang et al. 2014). For example, a worldwide bread wheat core collection was defined by Balfourier et al. (2007) which represents a subsample of a reasonable size with maximized allelic diversity, capturing 98% of the genetic diversity conserved in the whole wheat collection at CIMMYT and ICARDA. This core collection was phenotyped for bread making quality (Bordes et al. 2008) and agronomic traits including nitrogen use efficiency and earliness (Rousset et al. 2011; Le Gouis et al. 2012). It was also genotyped with about one thousand molecular markers (DArT, SNP and SSR) spread over the genome, in order to determine the genetic relationships between the entries of the core collection (Horvath et al. 2009). 5.3 Development of pre-breeding germplasm It is often difficult to compare the performance of germplasm that still maintains some wild or unadapted characteristics such as low harvest index, tall plants or late maturity, with the performance of high yielding elite cultivars. It is thus often more efficient to extensively hybridize this germplasm with elite lines to produce wheat with acceptable agronomic characteristics that can be more accurately compared to elite cultivars. In the case of genetic diversity incorporation from wild wheat relatives, interspecific or intergeneric hybridization is used for transferring desirable traits before characterization can begin (Sharma 1995). Cytogenetic and molecular markers may be used for identification of chromosomes originating from different genomes, and monitoring of alien introgressions into cultivated wheat (Tiwari et al. 2014). A good example of successful introduction of new genetic diversity into bread wheat is through its reconstitution. Since the early 90’s, durum wheat, Triticum turgidum L. subsp. durum (Desf.)

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 (2n=4x=28, BBAA) was used as a female parent in crosses with Ae. tauschii Coss. (2n=2x=14, DD) allowing the production of synthetic hexaploid wheat (SHW). These primary SHWs are amphidiploids (2n=6x=42, BBDDAA), combining the genomes of their parents and can act as a bridge for the introduction of specific characters and genetic diversity from their progenitors into bread wheat, including resistance or tolerance to various biotic and abiotic stresses (Zhang et al. 2005; Trethowan and van Ginkel 2009). SHW were involved in backcrosses with elite bread wheat cultivars to produce synthetic backcross-derived lines (SBL). The SBL showed resistance to major biotic stresses (Li et al. 2014) and out-yielded local varieties under drought conditions in many countries (van Ginkel and Ogbonnaya 2007; Jahanbin et al. 2011; Ogbonnaya et al. 2013; Trethowan 2014). Improvement in performance under drought conditions of SBL was found to be associated with an increased partitioning of root mass to deeper soil profiles (between 60 cm and 120 cm) and increased ability to extract moisture from those depths (Reynolds et al. 2007). An increase in yield due to higher number of grains per spike and spikes per plant (Del Blanco et al. 2001; Aggarwal et al. 2014) as well as thousand kernel weight (Liao et al. 2008) have been reported (Table 5). SBL are increasingly used in breeding programs worldwide leading to a significant increase of cultivated wheat genetic diversity (Warburton et al. 2006). New high-yielding cultivars derived from SLB have been developed and released in China (Yang et al. 2009; Li et al. 2014). Cultivated emmer, T. turgidum L. subsp. dicoccon (Schrank) Thell., has also been used for SHW production to exploit the genetic variation for drought tolerance present in A and B genomes (Zaharieva et al. 2010). Emmer based synthetic wheat and backcross derived lines were found to have a high level of resistance to greenbug (Lage et al. 2003) and Russian wheat aphid (Lage et al. 2004) as well as good grain quality (Lage et al. 2006) and higher yield under drought-prone conditions in Mexico, Pakistan and Eastern India compared to those using durum wheat (Trethowan and MujeebKazi 2008; Trethowan 2014). The diverse SBL are ideal candidates for genetic mapping in order to discern which new allelic diversity is causing the phenotypic differences seen in the SBL (Liao et al. 2008). In addition, genetic fingerprinting of SBLs, their corresponding SHWs and bread wheat parents, and testing for selective advantage of SHW alleles can be used for detecting chromosomal regions causing phenotypes of interest (Zhang et al. 2005). Amphidiploids can be used for the development of a new crop. The man-made crop Triticale (Triticosecale Wittmack) is a cross between wheat (Triticum) and rye (Secale) that was originally developed to combine the yield potential of the wheat with biotic and abiotic resistance of rye (Gupta and Priyadarshan 1982). Triticale has limited use as grain food due to low grain yield, but has been widely exploited as a fodder. However, gain in yield and quality following selection has been reported (Fox et al. 1990). Triticale has been shown to be more tolerant to drought and salinity than most wheat and barley cultivars (Salehi and Arzani 2014). Triticale can be used as a bridge for production of wheat-rye translocations (Lukaszewski and Gustafson 1983).

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 Tritordeum (BBAAHchHch) was developed by crossing wild barley (Hordeum chilense) and Triticum turgidum. Martin et al. (1999) created more than one hundred tritordeum lines. Villegas et al. (2010) found comparable yields in tritordeum and triticale under water stressed conditions. Tritordeum is also tolerant to Septoria leaf blotch and to common bunt diseases (Rubiales 2001). It may be an acceptable substitute crop under water stressed environments and its yield may be improved by selecting for earlier anthesis and longer grain filling period (Pinto et al. 2002; Martinek et al. 2003). Beneficial traits from wild plants have been transferred to the cultivated crop species through chromosome substitution, addition, or translocation lines. The 1BL.1RS (wheat/rye translocation) and the 7DL.7Ag (wheat/Agropyron elongatum translocation) have had a tremendous impact in wheat breeding. However, their potential impact on yield is still restricted by the fact that there is only a limited number of sources (1BL.1RS from one German rye genotype) and not always in the best genetic backgrounds (Villareal et al. 1995; Kim et al. 2004). Translocations have constituted the major recent contribution to yield progress in wheat. The nature of the genetic background determines the expression of a trait associated with a translocation (Ren et al. 2012). The 1BL.1RS wheat translocation has been extensively used in wheat breeding programs across the world to increase wide adaptation (Rabinovich 1998) and yield potential (Rajaram et al. 1990; Villareal et al. 1995; Foulkes et al. 2007a; Kim et al. 2004; Zhou et al. 2004). This translocation has been exploited worldwide for the improvement of yield and more than 300 cultivars carrying this translocation were released in various parts of the world. Of the 179 wheat cultivars and recently bred wheat lines from China, 38% carry 1BL.1RS translocations, and in some major wheat growing areas 1BL.1RS translocations account for up to 59% of the total wheat cultivars (Zhou et al. 2004). In addition to the 1BL.1RS, some other useful wheat-rye translocations have been associated with an increase of yield potential and tolerance to biotic and abiotic stresses. Biomass increase of about 10% has been reported in spring wheat, associated with the introduction of the long arm of chromosome 7D from Agropyron elongatum (Monneveux et al. 2003). Lei et al. (2013) noted that the translocation of 1RafrS and 2RafrS (from Secale africanum) in wheat was associated with the improvement of the agronomic performance of the substituted lines. Song et al. (2014) selected the progeny of wheat-rye translocation for chromosome 5R/5A and 6R/6A in F2 to F5 generations for improved agronomic traits such as “big ears”. The development of wheat/barley introgression lines allowed successful transfer of useful traits from barley into wheat, thus improving earliness, drought and soil salinity tolerance, and various traits of nutritional quality (Molnár-Láng et al. 2014). The first wheat/barley hybrid was produced by Kruse (1973) and the production of the first set of Chinese Spring/Betzes spring wheat/spring barley addition lines was described by Islam et al. (1978). Addition, substitution and translocation lines developed from wheat-barley hybrids were evaluated by Hoffmann et al. (2011) under stress and non-stress conditions, who found great variation for traits that can be used to create new varieties with better adaptation. As there is great genetic variability between barley cultivars for important agronomic traits (two or six-row, winter or spring habit, biotic and abiotic resistance, etc.) it is advisable to develop new

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 introgression lines using different barley genotypes in order to map and transfer favorable agronomical characters from barley. 6. Hybrid wheat 6.1 Heterosis in wheat Development of wheat hybrids for increased grain yield via heterosis was first attempted in the early 1950’s (Johnson and Schmidt 1968). Exploitation of heterosis has historically been limited by the strong inbreeding nature of wheat enforced by its floral architecture and the lack of practical fertility control systems. A lack of effective fertility-restoration genes has inhibited the development of cytoplasmic and genic male sterility systems, and chemical hybridizing agents presented problems of toxicity and selectivity (Whitford et al. 2013). Yield improvement associated with heterosis has been relatively modest, ranging from 3.5 to 15% (Table 6; Zhao et al. 2013; Longin et al. 2013). As a consequence, many hybrid wheat programs were discontinued (Jordaan 1996; Longin et al. 2012) and hybrids currently represent only a minor fraction of the total area sown, and mainly in China, India, France and the United States. Hybrids do have other advantages over inbred cultivars in addition to higher yield, as they also show lower susceptibility to frost (7.2%), leaf rust (8.4%) and Septoria tritici blotch (9.2%) (Longin et al. 2013). Heterosis is expected to increase with the genetic divergence between parents (Melchinger 1999) and the use of lines from different target environments has been suggested as a method to promote genetic diversity among wheat hybrid parents, but this approach is complicated by the different requirements for vernalization, photoperiod, quality, and frost tolerance (Koekemoer et al. 2011). 6.2 Manipulating floral architecture Redesigning the wheat flower may lead to an efficient production of hybrid seed. Ideally, both male and female parental plants for hybrid seed production would possess open flowering spikelets with synchronized flowering time (Murai et al. 2002). In addition, the male ideotype plant would be tall with long extruded anthers producing large quantities of long-lived pollen able to disperse meters away. In comparison, the female ideotype would be a shorter plant with multiple chasmogamous florets to maximize pollen reception. Our recent understanding of the control of floral architecture has greatly improved (Thompson and Hake 2009). Floral development of monocotyledons can be explained by a model whereby organ identities are determined by a specific class or a combination of classes of genes (Theissen 2001). There are a number of known genes, such as various MADS box genes, involved in floral determinacy and differentiation of the glume, lemma, and lodicule (Sreenivasulu and Schnurbusch 2012). It is anticipated that this information will be exploited for the purpose of redesigning the floral architecture of male and female plants. 6.3 Control of male sterility

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 An effective hybrid seed production system requires a reliable system for forcing outcrossing by inducing male sterility or self-incompatibility. The main options include the use of chemical hybridizing agents, cytoplasmic male sterility and genic male sterility (Whitford et al. 2013). Newer possible options include the use of self-incompatibility (SI), a biological mechanism that prevents selfpollination in open-pollinated species, chemically induced male sterility systems, and transgenic construct-driven non-GM systems. Male sterility can be induced in the female inbred parents by spraying a chemical hybridizing agent (CHA). The first CHAs showed strong phytotoxic effects and inadequate male sterility (Hoagland et al. 1953; Porter and Wiese 1961; Rowell and Miller 1971) and only worked in select genotypes and in a narrow application window (Cisar and Cooper 2002) or left toxic residues in F1 seeds of treated plants (Pickett 1993). Sintofen (Croisor®100) from Saaten Union Recherche (EFSA 2010) is the only CHA currently being used in Europe for commercial production of hybrid wheat. The use of cytoplasmic male sterility (CMS), based on rearrangements of mitochondrial DNA leading to the inability to produce fertile pollen (Horn 2014), constitutes another option. Male sterility has been obtained by introducing cytoplasm of Amblyopirum muticum (Boiss.) Eig, Ae. triuncialis L., T. turgidum subsp. dicoccoides (Panayotov 1980) and T. timopheevi subsp. timopheevii (Ahmed et al. 2001) into bread wheat cultivars. Alloplasmic male sterility was developed via Ae. crassa Boiss. (Murai and Tsunewaki 1993). Other cultivars with CMS and fertility restoration genes have been developed (Chen et al. 2011) but suffer from differential ability of the fertility restoration genes (Murai 2002). Fertility restoration systems having multiple genes were found more effective than single genes in terms of seed setting of F1 hybrid. Currently, only the T. timopheevii subsp. timopheevii derived male-sterile cytoplasm have been used for commercial production of wheat hybrids (Longin et al. 2012), although incomplete fertility restoration and shriveled F1 seeds are frequently observed (Adugna et al. 2004). Genic male sterility (GMS) may avoid some of the negative effects on yield noted by using alloplasmic and cytoplasmic male sterility systems. Several spontaneous or induced mutants with GMS have been identified (Pugsley and Oram 1959; Fossati and Ingold 1970; Driscoll and Barlow 1976; Sasakuma et al. 1978; Maan et al. 1987; Deng and Huang 1993; Xing et al. 2003; Zhou et al. 2008). GMS can be either dominant (Ms2, Ms3) or recessive (ms1, ms5) and conditional or non-conditional depending on whether environmental factors revert fertility. Conditional GMS can be temperature and/or photoperiod dependent. The use of a photo-thermo-sensitive GMS (PTGMS) system has been attempted in China, with less than optimal results (Chen et al. 2005). The development of nonconditional GMS mutants face problems in the maintenance, multiplication, and selection of pure malesterile populations, some of which can be overcome through the use of markers linked to male sterility. 6.4 Apomixis to look for heterosis Apomixis, the asexual reproduction of plants via seed, is a widely distributed phenomenon among wild grasses and other species. According to one estimate, there are 400 apomictic species (Bicknell 2000).

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 However, most of the cultivated crop species, including wheat, solely reproduce through sexual means. Apomixis is a genetically controlled trait where the embryo forms without fusion of the male gamete. The resulting embryo is 100% genetically similar to the female parent. One dominant gene has been reported to control apomixis in four grass species (Miles 2007). Apomixis puts restrictions on the free sexual genetic exchanges and origin of variability through genetic recombination. However, its controlled use has many applications in applied crop breeding including the multiplication of unfixed genotypes, avoiding pollen contamination, and the production of cheap commercial hybrid seed. Apomixis has been shown to propagate the heterozygote genotype and is thus one possible way of fixing heterosis in crop species (Miles 2007). It could help to produce cheap wheat hybrid seed on a large scale in developing countries where most of the wheat breeding is carried out by public organizations which do not have enough resources to produce hybrid seed on a commercial scale. The hexaploid wild species Elymus rectisetus (Nees in Lehm.) A. Love and Connor (SSYYWW, 2n=6x=42) has been identified as a potential source of apomixis and various molecular techniques such as florescent in situ hybridization have been exploited to identify introgressions from this species into cultivated wheat (Li et al. 2000). Advanced segregating generation BC2F5-BC2F7 showed three chromosomal substitutions from Elymus rectisetus into the wheat genome. Intergeneric hybrids between Hordeum vulgare cv. Manker × Triticum turgidum was also shown to reproduce apomictically (Mujeeb-Kazi 1981). Despite these potential sources, there are still several challenges in the exploitation of apomixis in wheat, including the polyploid nature of the crop species, the limited sources of apomixis, and the reduced male fertility of Elymus rectisetus (Savidan et al. 1992). Development of apomictic seed technology for its utilization in hybrid seed production is one of the major goals of the genetic engineering. Significant efforts are under way through the combined use of genetic engineering and molecular methods to identify and isolate genes associated with apomixis in other plant species (Barcaccia and Albertini 2013). These efforts may permit the introduction of apomictic genes from wild sources into cultivated species with the lowest linkage drag. 7. Molecular techniques 7.1 QTL mapping for yield components Grain yield and most of the traits associated with it are polygenically controlled and significantly impacted by the environment in which the plants are grown, thus showing very low heritability over and within environments. This makes identification of markers linked to these traits difficult, but at the same time, makes the benefit of using markers associated with yield greater. Despite the difficulty in successfully mapping QTLs for many traits, DNA markers have been identified for various agronomic traits in wheat and can now begin to play an important role in enhancing wheat yield potential, via marker assisted selection (MAS). Molecular markers have been exploited for the selection of yield per se. The markers were helpful in the transfer of positive alleles to the elite cultivars such as Laizhou953 from synthetic hexaploid wheat developed from the cross of Triticum turgidum subsp.

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 carthlicum×Aegilops tauschii (Liu et al. 2006). A review of various studies indicating the effects of major and minor QTLs effecting grain yield is presented in Table 7. Often, multiple traits map on the same QTL, indicating possible pleiotropy or tight linkage (Kumar et al. 2007; Wang et al. 2009). Correlations between traits are common in wheat breeding, and QTL mapping provides possible molecular evidence for the reasons for these correlations. Kumar et al. (2007) identified six QTLs which affected more than one yield component and were consistent over environments. Wang et al. (2009) identified 13 genomic regions showing pleiotropic effects for yield components. Genetic mapping (both linkage based and linkage disequilibrium or genome wide association study based) requires sufficient phenotypic and genetic diversity for successful analysis, and within modern bread wheat, insufficient diversity levels sometimes prevent mapping of some traits. Wild relatives are important sources of unique diversity and contribute positive alleles for yield and other traits, which have been accessed via the synthetic introgression lines. Huang et al. (2003) mapped 11 QTLs for yield and 16 for yield components in a mapping population involving one synthetic wheat, and 60% of the positive alleles originated with the synthetic parent ‘W-7984’, despite its poor agronomic performance. Huang et al. (2004) found that 24 (42.1%) of the QTLs from the synthetic wheat ‘XX86’ had a positive contribution to thousand kernel weight and kernel weight spike -1. Narasimhamoorthy et al. (2006) showed that 3 QTLs were contributed by the synthetic parent ‘TA 4152-4’ for grain hardness, kernel spike-1 and tiller number. 7.2 Functional markers Functional markers are derived from polymorphism within the sequence of the gene(s) which are directly responsible for phenotypic variation. Markers linked to (but not in) genes of interest have been used for marker assisted selection in wheat, but are sometimes limited in usefulness as they work best in the specific populations and parents in which they were mapped. Functional markers (generally single nucleotide polymorphism (SNP) or insertion/deletion (InDel) markers) that are very closely linked to (or even causing) the sequence change responsible for the desired phenotypic change are the most flexible for MAS (Dong et al. 2012). They can be used in any wheat cultivar to select the desired form of the trait because there is no chance for a recombination to break the useful linkage (Bagge et al. 2007). Various approaches such as map based cloning, comparative genomics and targeting induced local lesions in genomes (TILLING) have been recommended for the isolation of genes and development of functional markers. In wheat, a few functional markers for agronomic traits have been developed to date. These include reduction in plant height governed by Rht genes which have four alleles (Rht-BIa, Rht-BIb, Rht-DIa and Rht-DId) for which SNPs have been developed (Ellis et al. 2004). Three loci affecting kernel weight have been isolated and six alleles based on SNPs were developed by Jiang et al. (2011), Su et al. (2011) and Ma et al. (2012). These markers may be helpful to pyramid all positive alleles controlling seed weight into single genotypes of wheat.

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 TILLING combines a standard and efficient technique of mutagenesis using a chemical mutagen such as Ethyl methanesulfonate (EMS) with a sensitive DNA screening-technique to screen germplasm for induced and natural single base mutations in specified genes (Slade and Knauf 2005). TILLING has also been applied to knock out alleles and test our understanding of gene function. The popular method of detecting mutation is the mismatch cleavage assay through endonuclease Cel1 with locus specific primers designed from introns in the gene under study (Dong et al. 2009a). Traditionally, induced mutation breeding programs in wheat were hampered by the inability to identify the desired mutation phenotypically, especially in wheat, where genome redundancy in polyploidy species will mask a mutation in only one of the genomes. The TILLING technique overcomes these drawbacks (Dong et al. 2009b; Uauy et al. 2009). It is estimated that a TILLING population of 5,520 individuals will be sufficient to find a non-sense mutation in every wheat gene (Rawat et al. 2012). Successful identification of genes of interest has been reported in wheat, demonstrating the efficacy of this technique (Chen et al. 2012). The mutations causing the altered phenotypes may, themselves, become targets for MAS as functional markers, as frequently, the desired phenotype is produced by the reduced function of a specific gene. Where this is not the case, however, further sequencing of the gene identified by TILLING in many individuals may reveal other functional markers causing the improved phenotype. 8. Transgenic wheat Although yield is a complex trait, it can be successfully improved by the genetic modification of single genes (Smidansky et al. 2002; Hu et al. 2012a). Despite its global importance, wheat was the last major cereal to be genetically transformed due to its recalcitrance in tissue culture and its genotypedependence on foreign DNA transfer by Agrobacterium (Bhalla 2006). Wheat was transformed by biolistic methods since 1992 and by Agrobacterium methods since 1997. Recently, a highly efficient Agrobacterium mediated transformation protocol has been developed using immature embryo of wheat. The immature embryos were centrifuged and co-cultivated with Agrobacterium to obtain transgenic plants from 40 to 90% of the embryos (Ishida et al. 2015). Transformation of wheat plants with gene constructs affecting photosynthesis, starch biosynthesis, plant architecture, drought tolerance and transcriptional networks controlling plant development have been shown to have a positive impact on grain yield under normal and stressed conditions, and a summary of these many reports is shown in Table 8. Progress with GM wheat in China has been reviewed by Xia et al. (2012), but widespread use in the human diet has found slow acceptance due to cultural resistance. The transgenic Roundup Ready wheat (event MON 71800) developed by Monsanto, using the glyphosate-resistant CP4/maize EPSPS gene, was never marketed (Heller 2006). GM wheat is not currently grown commercially anywhere in the world (Jones and Shewry 2009; GMO Compass 2015). In 2010, Monsanto's partner in India, Maharashtra Hybrid Seeds Co, announced that it planned to seek approval to market GM wheat in India in the next three to five years (Abraham 2010). The ecological risks of cultivating GM wheat

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 have been reviewed by Zaharieva and Monneveux (2006). The main concerns centered on spontaneous hybridization with wild relatives and transfer and gene retention in hybrid progenies, particularly in the occurrence of species sharing the D genome with wheat. The use of transgenic wheat in the developing world also poses unique regulatory and policy problems that must be overcome before this technology is disseminated to growers in those countries. This will be one more tool in the development of stable high yielding wheat cultivars, as soon as its release is allowed. 9. Conclusion There is an urgent need to improve the efficiency of breeding in developing countries, in order to increase productivity and reduce the gap between yield potential and yield in farmer’s fields. Raising yield potential requires increasing photosynthetic capacity and efficiency, accumulating yield potential traits and optimizing partitioning to grain. Significant advances in the understanding of photosynthetic mechanisms are still needed to more efficiently boost canopy photosynthesis, RuBP regeneration, and the thermal stability of Rubisco activase, and eventually to replace wheat Rubisco with that from other species with different kinetic properties. The gap between actual yield and yield potential can be significantly reduced by improving resistance to pests and diseases, competitiveness of the crop with weeds, and tolerance to abiotic stresses (drought, salinity, low fertility, extreme temperatures). The development of multi-rusts resistance and slow rusting resistance has already permitted major impacts. The enhancement of resilience to abiotic stresses will require a better knowledge of the physiology and genetic basis of stress-adaptive traits and their complex interactions. Accurate and cost-effective phenotyping is instrumental in this respect. QTL identification, marker-assisted selection and genome-wide selection all rely on accurate phenotyping. As the cost of genotyping and deep sequencing drops, cost-effective phenotyping will become increasingly limiting for further dissection of stress-adaptive traits. The increasing use of geographic information system tools, soil water balance and plant growth models is expected to better describe the drought scenario faced by the crops in different target environments, compare and cluster phenotyping locations, and finally better understand genotype by environment interactions. The utilization of techniques that allow for a precise control of experimental conditions and a reduction of the experimental noise through precision management of the experiment and the development of more space and time integrative methods (particularly remote sensing methods) are already improving the quality of collected phenotypic data while increasing the cost-effectiveness of phenotyping. A more efficient exploitation of genetic diversity of wild wheat relatives through more targeted introgression of useful alleles is paramount for future breeding advances. This needs a good collaboration with genetic resources managers and a clear strategy in the development of pre-breeding material, two conditions that are sometimes difficult to comply with. Recent progress in genomics and bioinformatics are offering new opportunities for plant breeders who can now use molecular markers to

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 assess and enhance diversity in their germplasm collections, introgress valuable traits from new sources and identify genes that control key traits. Significant progress has also been done in the capacity to introgress beneficial genes through transgenic approaches. The development of in vitro methods and tools is an important component of transgenic approaches, but can also aid in the development of in vitro-selection, the use of somaclonal variants, the facilitation of wide crosses and the acceleration of wheat breeding through haplodiploidisation. Taken together, these technologies will permit the selection of plants with favorable alleles at the underlying genes and accelerate crop improvement timescales. The adoption of molecular breeding and modern phenotyping tools in developing countries is still insufficient and heterogeneous. While emergent countries are progressively using several molecular breeding applications and exploring the latest genomic and phenotyping approaches, developing countries with low or mid-level economies have more difficulties to adopt these methodologies into their breeding programs. This is due mainly to limited human resources and shortage of well trained personnel, poor phenotyping infrastructure, inadequate high-throughput genotyping capacity and lack of information systems or adapted analytic tools. In some of those countries, there is even a need for valorizing the role of agriculture and agricultural research. Implementation of incentives and funding mechanisms is needed to improve field and laboratory infrastructures, information and communication technologies and the social status of scientists. Behind the differences that can be observed from one country to another, a general weakness is the insufficiency of interaction between the different specialties that are contributing in the research and selection process. An integration of methods from a broad set of different disciplinary areas and by consequence, a culture of trans-disciplinarity are urgently required to take full advantage of all these available techniques and tools, improve the efficiency of breeding and reduce the gap between yield potential and yield in farmer’s fields (Fig. 2). This is a condition to overcome the bottlenecks that still impede the integration of genomic tools into breeding programs and to translate the innovations in plant science into concrete benefits for poor farmers. Acknowledgements Authors (Marilyn Warburton, Pingzhi Zhang, Abdullah M Al-Sadi and Saeed Rauf) wish to acknowledge their funding agencies i.e. United States Department of Agriculture, Anhui Academy of Agricultural Sciences, Sultan Qaboos University, Oman and Punjab Agriculture Research Board, Pakistan, for partially supporting the publication charges of the manuscript. References Abbas M, Sheikh A D, Sabir H M, Nighat S. 2005. Factors responsible for low wheat productivity in Central Punjab. Pakistan Journal Agricultural Sciences, 42, 3-4. Abbate P E, Andrade F H, Culot J P. 1995. The effects of radiation and nitrogen on number of grains in wheat. Journal of Agricultural Science, 124, 351–360.

Journal of Integrative Agriculture Advanced Online Publication 2015 10.1016/S2095-3119(15)61035-8

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Journal of Integrative Agriculture Advanced Online Publication 2015 10.1016/S2095-3119(15)61035-8

Fig. 1 Change in grain spike-1 over year in Landmark Pakistani wheat varieties. Source: Wheat Research Institute Faisalabad, Pakistan.

Fig. 2 Integration circle showing the collaborations that are needed to take advantage of the new breeding and genomic tools and methods.

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Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 Table 1 Yield growth rate year-1 of wheat in various decades of key green revolution and non-green revolution countries Country India China Pakistan USA Turkey

1961-1970 4.3 7.5 3.7 2.6 1.9

1971-1980

1981-1990

1.8 5.0 3.4 0.3 4.1

1991-2000

3.0 2.6 3.0 0 1.1

2001-2010

1.8 2.2 2.5 1.9 0.3

0.9 2.7 1.7 1.6 2.2

Yield ha-1 data used for calculation of yield growth rates were obtained from FAO, 2011.

Table 2 Effect of yield components on grain yield under different environments Yield component vs. yield correlations

Intra-yield component correlations

Reference

Number of grains spike-1 has the highest effect on yield, number of spike m-2 has a positive effect

Number of spikes m-2 has negative effect on number of grains spike-1 and thousand kernel weight

Simane et al. (1993)

Thousand kernel weight has the highest effect on yield under non-stress conditions, number of spike m-2 under heat stress conditions and number of grains spike-1 under drought

Number of spike m-2 has negative effects on number of grains spike-1 and thousand kernel weight under heat stress

del Moral et al. (2003)

˗

Thousand kernel weight has a negative effect on yield under drought

Denčić et al. (2000)

Number of grains spike-1 followed by number of spikes m-2 have positive effect on yield under both optimum and late season water stress conditions

Number of grains spike-1 has negative effect on yield via number of spike m-2

Okuyama et al. (2004)

Number of spikes m−2, number of grains spike−1 and thousand kernel weight have similar effects on yield rainfed experiments, variations in grain yield were due mainly to spikes m−2 and to a lesser extent to grains spike−1

Number of spikes m−2 has negative effect on number of grains spike−1

del Moral et al. (2005)

KWS-1 followed by number of grains spike-1 have positive effect on yield

Number of spikelets spike-1 has the highest effect on yield

Mollasadeghi et al. (2011)

Thousand kernel weight has positive effect on yield

Number of spikelets spike-1 has the highest effect on yield via spike length

Iftikhar et al. (2012)

Thousand kernel weight has positive effect on yield under non-stress and drought stress conditions

-

Muhammadi et al. (2012)

-, no data.

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 Table 3 Average values, range, genotypic coefficient of variability (GCV) and heritability (h2) of the number of grains spike-1 under different conditions in Asia Grains per spike

Germplasm

Country

Reference

2

Average

Range

GCV

h

57.6

22-91

87.31

0.41

Mutant (M3)

Iran

Sakin and Yildirim (2004)

40.2

35-44.0

19.46

0.26

Elite cultivar

Turkey

Aycicek and Yildirim (2006)

73.65 vs. 70.27

67.3-81.8

28.22

0.24

Parents vs. F1

Turkey

Analizleri (2008)

35.0

11.4-64.0

84.22

0.27

Synthetic wheat

India

Tayagi et al. (2008)

67.79 vs. 66.5

52.6-76.3

26.22

0.16

Parents vs. F1

Turkey

Sener et al. (2009)

54.8

46.8-62.7

27.34

0.14

P1, P2, F1, F2, BC

Turkey

Erkul et al. (2010)

44.4

41.7-53.5

13.91

0.19

24 Elite cultivars

Turkey

Ilker et al. (2013)

31.1

13-63.5

87.29

0.29

99 synthetic wheats

Iran

Nazem and Arzani (2013)

Table 4 Effect of various abiotic factors on the yield potential of wheat germplasm in various parts of the world Environmental conditions

Yield reduction

Correlation (r value)

Country

Reference

Mild drought

16%

0.80* (non-stress vs. mild stress)

USA

Guttieri et al. (2001)

Severe drought

48%

USA

Guttieri et al. (2001)

Late planting

Pakistan

Mohammad and Ali (2007)

Drought stress

22% to 34% (according to the year) 12%

Egypt

Esmail et al. (2008)

Heavy metals

21%

Egypt

Awaad et al. (2010)

Heat stress

Saudi Arabia

Al-Otayk (2010)

Salinity

21% to 49% (according to the year) 11%

0.43* (non-stress vs. severe stress) 0 to 0.17ns (early planting vs. late planting) 0.61* (non-stress vs. drought stress) 0.91* (non-stress vs. toxicity stress) 0.44* to 0.82* (nonstress vs. heat stress)

Turkey

Bahar and Yildrim (2010)

Heat

67%

Turkey

Bahar and Yildrim (2010)

Drought stress

24% to 65% (according to the year) 27% to 65%

0.41* (non-stress vs. salinity) 0.07ns (non-stress vs. heat stress) 0.21ns to 0.39* (nonstress vs. drought stress) 0.79* to 0.80* (nonstress vs. heat stress)

China

Chen and Wu (2011)

Pakistan

Laghari et al. (2012)

Heat stress

Journal of Integrative Agriculture Doi ：

Advanced Online Publication 2015 10.1016/S2095-3119(15)61035-8 Table 5 Summary of contribution of synthetic wheat to yield and its components Germplasm

Trait

References

282 BC2F2-derived lines from six crosses between different SHWs and four spring wheat cultivars

Eight lines had 11% higher yield than the recurrent parent; number of grains spike-1 and spikes m−2 having highest direct effect on yield

Del Blanco et al. (2001)

‘Prinz’/synthetic wheat ‘W-7984’ (72 BC2F2 lines and BC2F3 families)

24 (60.0%) alleles from the synthetic wheat ‘W7984’ were positively associated with agronomic traits

Huang et al. (2003)

‘Flair’/synthetic wheat ‘XX86’ (111 BC2F2 lines, and BC2F3 families)

57 QTLs derived from ‘XX86’ detected, 24 (42.1%) have positive effect on thousand kernel weight and grain weight per ear

Huang et al. (2004)

‘Am3’ (T. turgidum subsp. carthlicum/ Ae. tauschii)/‘Laizhou953’ (97 BC4F3 lines)

Introgression of favorable alleles for yield and yield components from synthetic wheat

Liu et al. (2006)

‘Karl 92’/synthetic wheat ‘TA 4152-4’ (190 BC2F(2:4) lines)

Synthetic wheat contributed to three QTLs for grain hardness, number of grains spike-1, and tiller number

Narasimhamoorthy et al. (2006)

Cham 6 ///Haurani / Ae. tauschii ICAG400709 //Cham 6 (13 SBLs and 3 SHW genotypes)

A backcross derived line with higher yield under late planting than the recurrent parent Cham 6 suggesting rapid translocation of photosynthetic carbohydrates to the grains after heading

Inagaki et al. (2007)

BCF4 and BCF5 synthetic popultions from 10 SHWs backcrossed to 2 Texas cultivars (TAM 111 and TAM 112)

Synthetic populations produced higher grain yield compared to their recurrent parent and had higher seed size and weight, number of spikes per plant and number of grains spike-1

Cooper et al. (2012)

Table 6 Magnitude of heterosis in grain yield of various wheat hybrids tested in diverse locations Hybrids

Heterosis

Location

References

Commercial hybrid ‘Titan’ 10 hybrids 21 F1 hybrids 83 hybrids 112 hybrids 42 hybrids Beyaziye/Duraking hybrids 28 Tat/SQ hybrids 90 BRS Guamirim/BRS 208 hybrids 20 A899/Wifak hybrids

+11% -10% to +17% +30% -15.48 % to +25.82% −15.33% to +14.13% 17.51% to +35.32% -10.73% to +37.67% -30.93 % to +56.25% -9.2% to +51.51%

Australia Italy Italy India Germany China Turkey Pakistan Brazil

Boland and Walcott (1985) Borghi et al. (1988) Borghi and Perenzin (1994) Singh et al. (2004) Dreisigacker et al. (2005) Bao et al. (2009) Akinci et al. (2009) Hassan and Gul (2006) Beche et al. (2013)

-19.8% to +58.8%

Algeria

Fellahi et al. (2013)

Journal of Integrative Agriculture Doi ：

Advanced Online Publication 2015 10.1016/S2095-3119(15)61035-8 Table 7 Effect of major and minor QTLs affecting grain yield in wheat Marker

Mapping population

Conclusion

References

SSR

BC2F2

Favorable QTL alleles can be transferred from wild relatives for improvement of yield by the advanced backcross QTL strategy and molecular breeding

Huang et al. (2003)

567 RFLP, AFLP, SSR, morphological and biochemical markers

96 doubled haploid lines

17 clusters of yield QTLs identified, QTL effects with strongest effect on yield (due to effect on number of grain spike-1) identified on chromosomes 7AL and 7BL

Quarrie et al. (2005)

RFLP, AFLP, SSR, morphological and biochemical markers

RIL population

SQ1 allele associated with >20% higher ear weight, higher flag leaf chlorophyll content, and wider flag leaves

Quarrie et al. (2006)

SSR

RIL population

Five regions on chromosome 5A contributed effects on yield traits. Increases in grain yield, thousand kernel weight and spikelet number ear-1 determined by complementary QTL alleles from both parents

Kato et al. (2000)

SSR

2 mapping populations

Six QTLs with pleiotropic effects and stable over environments

Kumar et al. (2007)

SSR

RIL population

55 meta QTLs for yield and contributing traits

Zhang et al. (2008)

SSR

260 RAC875/Kukri doubled haploid population

Two QTLs detected on 3B chromosome with large effect on grain yield

Bennett et al. (2012)

163 AFLP and 263 SSR

150 double haploids

12 QTLs for yield and 33 QTLs for plant height detected on all 21 chromosomes, additive effects more important than epistatic effects, major QTLs on 2B, 1B and 4B for yield and its components

Wu et al. (2012)

438 SSR, 253 DArT markers

233 doubled haploid lines (RAC177/‘Monoculm’ //RAC311S)

Several QTLs were detected for agronomic traits on 4B, 2B and 7A, 38 QTLs for metabolites co-localized with QTL for grain yield

Hill et al. (2013)

SSR, DArT, AFLP

95 RIL population

Major QTL identified on chromosome 3A for grain yield (GY), KPS, grain volume weight (GVWT) and spikes per square meter (SPSM) respectively explained 23.2%, 24.2%, 20.5% and 20.2% of the phenotypic variation

Rsutgi et al. (2013)

Journal of Integrative Agriculture Advanced Online Publication 2015 Doi ： 10.1016/S2095-3119(15)61035-8 Table 8 Traits with potential value under abiotic and biotic stresses improved in transgenic wheat Traits

Transgene

Reference

Higher water use efficiency under drought stress (0.66–0.68 g kg−1) as compared to the recipient parent (0.57 and 0.53 g kg−1)

ABA-responsive barley gene HVA1

Sivamani et al. (2000)

Increased ADP-glucose pyrophosphorylase, increased grain weight plant-1 (38%)

Shrunken2 gene (Sh2r6hs) maize

Smidansky et al. (2002)

Lower decrease of fresh weight, dry weight, plant height, and flag leaf length (40%, 8%, 18% and 29%, respectively) compared to the recipient parent (70%, 56%, 40% and 45%, respectively)

mtlD

Abebe et al. (2003)

Higher grain yield and heavier and larger grains under saline conditions

Antiporter gene AtNHX1

Xue et al. (2004)

High drought stress tolerance

Δ1-pyrroline-5-carboxylate synthetase (P5CS)

Vendruscolo et al. 2007

Abiotic stresses

Transgenic plant showed lower degradation of chlorophyll contents under nitrogen stress

ipt

Sýkorováet al. (2008)

Higher photosynthesis rate, light saturation point and carboxylation efficiency (26%, 20% and 22.6%, respectively), increase of seed weight spike-1 and thousand kernel weight (0.23 g and 1.21 g, respectively)

Phosphoenolpyruvate carboxylase gene (pepc)

Hu et al. (2012b)

Better recovery, higher water use efficiency under drought stress and higher yield under well irrigated conditions

DREB1A

Saint Pierre et al. (2012)

Decreased effects of stinking smut (Tilletia tritici)

Antifungal protein KP4

Clausen et al. (2000)

Higher fusarium blight symptoms

NPR1gene (AtNPR1)

Makandar et al. (2006)

Significant reduction of fusarium head blight symptoms

α-1-purothionin, thaumatinlike protein 1 (tlp-1), and β1,3-glucanase

Mackintosh et al. (2007)

Reduced fusarium head blight severity and percentage of visually scabby kernels

class II chitinase

Shin et al. (2008)

Increased leaf rust resistance irrespective of genetic background

Lr34

Risk et al. (2012)

Increased resistance to aphids

(E)-β-farnesene synthase

Yu et al. (2012)

Higher resistance to Gaeumannomyces graminis

TiMYB2R-1

Liu et al. (2013)

Biotic stresses