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Published online 1 July 2008
Published in Crop Sci 48:1255-1265 (2008)
© 2008 Crop Science Society of America
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Novel Germplasm Resources for Improving Environmental Stress Tolerance of Hexaploid Wheat

R. M. Trethowana,* and A. Mujeeb-Kazib

a Univ. of Sydney, Plant Breeding Institute, PMB 11, Camden, 2570, NSW, Australia
b National Institute of Biotechnology and Genetic Engineering (NIBGE), Faisalabad, Pakistan

* Corresponding author (rtrethowan{at}camden.usyd.edu.au).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 THE USE OF SYNTHETIC...
 LANDRACES AS A SOURCE...
 ALIEN INTROGRESSION AS A...
 CONCLUSIONS
 REFERENCES
 
Wheat (Triticum spp. L.) breeders have significantly improved wheat adaptation to stress-prone environments around the world. This progress has largely been achieved using empirical selection and genetic variability within the primary wheat gene pool. As most stress tolerance traits are quantitatively inherited, expansion of the available genetic diversity for stress tolerance is necessary if rates of genetic progress are to be maintained. This review explores three sources of novel genetic variability, namely synthetic wheat, landrace cultivars, and alien introgressions and their applicability to applied wheat breeding. Synthetic hexaploid wheat, derived by crossing tetraploid wheat with Aegilops tauschii, provides new genetic variability for adaptation to drought, high temperature, salinity, waterlogging, and soil micronutrient imbalances from the secondary wheat gene pool. Synthetic-derived materials have performed well in many stressed environments globally. There is significant unexploited variation among landraces and modern wheat cultivars to improve the stress adaptation of cultivated wheat. The tertiary gene pool, with a few significant exceptions, has been more difficult to exploit due to complex inheritance, meiotic instability, and linked deleterious effects. Nevertheless, there is sufficient genetic variation in the wheat gene pool to ensure the continued improvement of wheat adaptation to abiotic stress.

Abbreviations: AFLP, amplified fragment length polymorphism • RAPD, random amplification of polymorphic DNA • SAWYT, Semi-Arid Wheat Yield Trial • SSR, simple sequence repeat


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
THE USE OF SYNTHETIC...
 LANDRACES AS A SOURCE...
 ALIEN INTROGRESSION AS A...
 CONCLUSIONS
 REFERENCES
 
ABIOTIC STRESSES such as drought, temperature, salinity, and nutrient imbalances reduce wheat (Triticum spp. L.) yield in many environments. The genetic control of tolerance to these stresses is complex and to a large extent poorly understood (Snape et al., 1997; Worland and Snape, 2001). Nevertheless, wheat breeders have made significant improvements in the adaptation of wheat to stress-prone environments (Byerlee and Moya, 1993; Trethowan et al., 2002; Lantican et al., 2003). This success has largely been achieved through field-based empirical selection for stress tolerance. Selection has been conducted both in the target environment under prevailing stresses (Ceccarelli et al., 1987) and under managed conditions where the intensity of single stresses can be manipulated (Trethowan et al., 2001). Genetic progress has been possible because of additive genetic variance for yield under drought stress found within the existing wheat gene pool (Crossa et al., 2006; Reynolds et al., 2007). However, if genetic progress in the improvement of stress tolerance is to be maintained, it will be essential that new genetic variation be found and combined.

This review examines three potential sources of new genetic variation for abiotic stress tolerance, namely hexaploid synthetic wheat derived from crosses between tetraploid wheat and Aegilops tauschii, the donor of the D genome; landraces or traditional cultivars; and genetic variability in the tertiary wheat gene pool. Emphasis is given to genetic variation that has either been used by wheat breeders, or could practically be used to improve the stress tolerance of hexaploid bread wheat in farmers' fields.


   THE USE OF SYNTHETIC WHEAT IN HEXAPLOID WHEAT IMPROVEMENT
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 THE USE OF SYNTHETIC...
LANDRACES AS A SOURCE...
 ALIEN INTROGRESSION AS A...
 CONCLUSIONS
 REFERENCES
 
Hexaploid Synthetics as a Source of Genetic Variation for Abiotic Stress Tolerance
Hexaploid wheat arose some 8000 to 9,000 years ago, most likely in present day Iran (Feldman, 2001). Feldman considers that cultivated Triticum dicoccom crossed with Aegilops tauschii to produce hexaploid wheat following spontaneous chromosome doubling, as T. dicoccoides, also known as wild emmer, and Ae. tauschii do not share a common geographical distribution. It is likely that very few accessions of both species were involved in the evolution of hexaploid wheat (Feldman, 2001; Pfeiffer et al., 2005). This relatively narrow genetic variation has given rise to a founder effect in hexaploid wheat (Feldman, 2001).

Gene banks around the world have collected more than 1000 accessions of Ae. tauschii (Pfeiffer et al., 2005) and a core collection identifies around 450 accessions (Kawahara et al., 2003). Many of these have been used to produce over 1000 primary synthetic hexaploid wheats (Mujeeb-Kazi, 2003) at the International Maize and Wheat Improvement Center (CIMMYT) and by other organizations world wide (van Ginkel and Ogbonnaya, 2007). Modern tetraploid durum wheat (T. durum L.) has generally been used to produce these new primary synthetic wheats. The primary synthetics are agronomically poor, difficult to thresh, generally tall, low yielding, and frequently have poor quality. However, they do carry useful and often times new variation for a range of economically important characters (Mujeeb-Kazi et al., 2008). Many of these characters are simply inherited disease resistance traits that have been readily used in wheat breeding (Villareal et al., 1992, 1994; Nicholson et al., 1993; Lage et al., 2003a; Smith and Starky, 2003). Potentially new genetic variation among primary synthetics has also been found for tolerance to drought (Villareal et al., 1998), salinity (Gorham, 1990), frost (Maes et al., 2001), high temperatures (Yang et al., 2002), and soil nutrient stress (Cakmak et al., 1999). These primary synthetics have been crossed to adapted wheats and agronomically improved materials have been developed that have superior yield performance compared to check cultivars under drought stress across a range of environments.

Trethowan et al. (2000, 2003) evaluated derived synthetics in Mexico under postanthesis drought stress generated in the field using limited irrigation and compared their yield performance to their adapted recurrent parents and the best local check cultivar. They found the derivatives were 23% higher yielding than their recurrent parents and 33% higher yielding than the check cultivar. However, the value of these materials as a source of yield under drought stress is dependent on the transferability of this performance to other, more variable drought-prone environments. The CIMMYT wheat breeding program has sent synthetic-derived lines to drought-prone environments around the world in yield trials. Lage and Trethowan (2008) analyzed the yield performance of these materials from the deployment of the first synthetic derivatives in the Semi-Arid Wheat Yield Trial (SAWYT) in 1996 (they comprised 8% of total entries) to 2006 where 46% of entries were derived from synthetic wheat. During this period, the average contribution of primary synthetics to parentage was reduced from 75 to 19% based on coefficients of parentage, and microsatellite simple sequence repeat (SSR) analyses indicated that only a small portion of the primary synthetic genome remains in the most recent materials (Zhang et al., 2005). Between 1996 and 2003 the average yield rank of the synthetic-derived materials in the 50 entry SAWYT improved from 30th to 25th. However, synthetic derivatives were identified that were superior to the local check cultivars across a wide range of global environments where mean yields ranged from less than 1 Mg ha–1 to more than 8 Mg ha–1. The better adapted derivatives were generally those with a smaller portion (25% or less) of the primary synthetic in their pedigree; indicating that at least one backcross to an adapted parent is required to obtain synthetic derivatives with high, stable yield performance.

Synthetic derivatives developed in Mexico by CIMMYT under managed terminal drought stress were also tested under Australian conditions in multi-environment trials. The synthetic-derived materials were between 8 and 30% higher yielding than the best local check cultivar, and synthetic derivatives could be found that performed well across all environments (Ogbonnaya et al., 2007). Dreccer et al. (2007) evaluated a different set of 156 derivatives developed by CIMMYT across the Australian wheat belt and found that 56% of the materials were higher yielding than the best local check cultivar. The synthetic derivatives were generally superior in northern environments, but this advantage tended to diminish in southern areas. They noted strong genotype x environment interactions and concluded that these differences in adaptation were likely caused by differences in the length of the respective growing seasons and associated genotype phenology. Genotypes with a mild vernalization requirement tended to perform better in cooler southern areas, an observation consistent with a previous study of non-synthetic based CIMMYT materials (Mathews et al., 2007). Synthetic derivatives developed in Mexico were also evaluated for yield under drought in Iran and were superior to the local check cultivars, producing heavier, longer spikes with better spikelet fertility (Azizinya et al., 2005).

The above findings examine the performance of synthetic derivatives produced in Mexico and tested both in Mexico and in other countries. In contrast, Gororo et al. (2002) developed synthetic hexaploid derivatives from locally produced primary synthetics in Australia and tested these materials in both Australia and Mexico. The synthetic derivatives were up to 32% higher yielding than their recurrent parent and higher yielding in 38 of 42 comparisons across environments in both countries, indicating a significant degree of transferability of drought stress response between northern Mexico and Australia.

The better performance of the synthetic-derived wheats under drought stress is to some extent associated with differences in root distribution. Reynolds et al. (2007) and Reynolds and Trethowan (2007) evaluated synthetic backcross derivatives and their recurrent parents and concluded that superior yield performance (15–33% higher yield) under drought stress was linked to a greater apportionment of root dry weight deeper in the soil profile, rather than a greater total root dry weight. A summary of the influence of synthetics on key traits under moisture stress, adapted from Reynolds and Trethowan (2007), is provided in Table 1 . Synthetic derivatives were 24% higher yielding, produced 57% more biomass, had a 46% lower root:shoot ratio, and were 41% more water-use efficient than their recurrent parents. The ability of synthetic derivatives to maintain seed weight under drought and high-temperature stress is also another important adaptive mechanism (Trethowan et al., 2005). Larger seed size may also have implications for other characters, such as improved milling yield (Marshall et al., 1986).


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  		Table 1. Growth and water use parameters for two synthetic-derived wheat lines and the recurrent parents under moisture stress (average 2 yr) for two genetic backgrounds, northwestern Mexico, 2003–2005 (source, Reynolds and Trethowan, 2007).

 
High-temperature stress limits wheat yield in many environments. Yang et al. (2002) tested synthetic hexaploid wheats under high temperature generated in a growth chamber. They found that chlorophyll content, grain-filling period, grain yield, and kernel weight were negatively correlated with heat susceptibility and concluded that the synthetics are a potentially useful source of genetic variability for heat tolerance, particularly in regions where high-temperature stress is common throughout the crop growth cycle. Low temperature and frost at flowering can severely reduce spike fertility and eventual yield. Maes et al. (2001) tested floret temperatures of primary synthetic wheats that were observed to have better spike fertility following frost at flowering. They found that floret death was delayed by more than 4 min at freezing temperatures in the tolerant synthetics. Frost-tolerant synthetic derivatives with improved agronomic type, developed from the most tolerant primaries, were evaluated for spike fertility across different autumn planting dates in the Toluca highlands of Mexico. Although not as tolerant as the original primary synthetic source, the most tolerant derivatives showed 30% less floret death compared to the frost-intolerant controls (Trethowan, unpublished data, 2005).

Many wheat production areas are constrained by salinity, and saline soils cover 7% of the world's land area (Szabolzs, 1994). The primary mechanism of salt tolerance in the Triticum species is reduced loading of Na+ into the xylem, or salt exclusion (Gorham, 1990). Enhanced K+:Na+ discrimination has been associated with chromosome 4D of Ae. tauschii (Shah et al., 1987) and synthetic hexaploids with better salt tolerance than their durum parents have been identified (Gorham, 1990; Schachtman et al., 1992; Reynolds et al., 2005). The superiority of the synthetics was linked to maintenance of seed weight under saline conditions (Schachtman et al., 1992) and salt-tolerant primary synthetics have subsequently been used in wheat breeding (Reynolds et al., 2005). However, the control of salt tolerance in wheat is complex and likely involves many genes (Colmer et al., 2005). Though synthetics are promising salt-tolerant sources, enhanced K+:Na+ discrimination is not exclusively associated with chromosome 4D. This factor has also been identified on chromosome 2A of durum wheat (Munns et al., 2006). Primary synthetic wheats tolerant to waterlogging have also been identified and the germplasm registered (Villareal et al., 2001). These materials showed significantly less leaf chlorosis under prolonged flooding in the field.

Zinc deficiency is widespread in many grain growing areas of the world (Genc et al., 2006) and genetic variability for Zn efficiency is found in the wheat gene pool (Cakmak et al., 1998). Synthetic wheat also provides useful and potentially new genetic variability for this trait. The level of Zn efficiency among Zn efficient synthetic wheats is reported to be 15% better than the most Zn efficient modern cultivars (Genc and McDonald, 2004). Cakmak et al. (1999) found that Zn efficiency expressed in Ae. tauschii and T. monococcum was expressed in their derived synthetic hexaploids. Synthetic hexaploids with high levels of Zn and Fe concentration in the grain, both essential elements for human nutrition, have also been identified (Genc and McDonald, 2004; Ortiz-Monasterio et al., 2007). Boron toxicity limits wheat yield in some environments (Torun et al., 2001) and B-tolerant primary synthetics have been identified (Dreccer et al., 2003), It is not known, however, if the genes controlling B toxicity and Zn efficiency are different from those identified earlier in the primary wheat gene pool.

Exploiting Tetraploid Variation to Develop Synthetic Hexaploid Wheat
Most of the primary synthetics that have been used extensively in wheat breeding are derived from crosses with modern durum wheat cultivars. Emphasis has been given to combining genetically diverse populations of Ae. tauschii, which has to some extent limited diversity among the tetraploid parents; although this is mitigated by a report that diversity among cultivated durum wheats is significantly greater than hexaploid wheat (Huang et al., 1999). Nevertheless, Nevo (2006) considered wild emmer to be a valuable source of variation for stress tolerance. When tested in well-watered and water-limited environments, wild emmer shows considerable variation in drought response compared to cultivated durum wheat (Peleg et al., 2005). Peleg et al. (2005) identified wild emmer with superior productivity under drought and better water-use efficiency, manifested by lower carbon isotope discrimination. They concluded that wild emmer originating from hot, dry locations exhibited better response to managed drought stress in the field.

Cultivated emmer has also been used to improve the stress tolerance of durum wheat. Al-Hakimi (1998) crossed T. durum with T. dicoccum, T. polonicum, and T. carthlicum to improve the relative water content, carbon isotope discrimination, and ultimately drought tolerance of cultivated durum. These wild tetraploid accessions have the potential to improve the stress adaptation of hexaploid wheat.

Primary synthetic wheat produced by crossing T. dicoccum with Ae. Tauschii offers plant breeders new genetic diversity for stress tolerance (Mujeeb-Kazi et al., 2000). Lage and Trethowan (2008) tested the progeny derived from two backcrosses of adapted hexaploid wheat to a primary T. dicoccum-based synthetic hexaploid, under two levels of moisture stress (Table 2 ). They concluded that synthetic derivatives with significantly higher yield under stress could be identified. Similarly, Valkoun (2001) reported improved drought tolerance from a cross between a primary synthetic wheat, developed by crossing a tetraploid landrace with Ae. tauschii, and an adapted hexaploid wheat in Syria. The genetic variation of tetraploid wheat can also be enhanced by crossing both cultivated and wild tetraploid forms with T. urartu, the A-genome donor (Brandolini et al., 2006), or with T. monococcum to produce tetraploids with new A-genome variation, suitable for introgression into hexaploid wheat. Other diploid genomes can potentially be used for wheat improvement but have still to gain wide acceptance. These are hexaploid amphiploids of the A and B (S) genomes with durum wheat (2n = 6x = 42; AAAABB or AABB BB(SS) (Mujeeb-Kazi 2005). Nevertheless, it must be remembered that most of the genetic variation represented in primary hexaploid synthetics cannot be readily exploited, and considerable effort is required to identify and select for characters that confer improved stress tolerance.


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  		Table 2. Yield of Triticum dicoccon based synthetic derivatives compared to their recurrent parents under two levels of moisture stress (reproduced from Lage and Trethowan, 2008).

 

   LANDRACES AS A SOURCE OF GENETIC VARIATION FOR ABIOTIC STRESS TOLERANCE
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 THE USE OF SYNTHETIC...
 LANDRACES AS A SOURCE...
ALIEN INTROGRESSION AS A...
 CONCLUSIONS
 REFERENCES
 
Landraces are traditional cultivars that have been selected and grown locally by farmers. All modern wheat cultivars are derived from landrace materials. Smale et al. (1996) used pedigree analysis to estimate that the contribution of landraces to the pedigrees of modern wheat cultivars has increased with time as have their geographical origin. Landraces tend to be grown in isolated and more marginal environments—areas where the adoption of modern semi-dwarf cultivars has been considerably slower as farmers tend to be more risk adverse (Byerlee and Moya, 1993). These landraces are frequently perceived to have better drought tolerance but yield poorly under improved growing conditions (Ehdaie and Waines, 1989); although there are also reports that the drought performance of modern cultivars is at least equivalent under drought (Dencic et al., 2000). Nevertheless, landraces offer plant breeders potentially unexploited variation for stress tolerance.

Landraces collected from a wide range of different countries and geographical areas have been shown to be genetically diverse using SSR, amplified fragment length polymorhism (AFLP) and random amplification of polymorphic DNA (RAPD) markers (Strelchenko et al., 2004). Moghaddam et al. (2005) compared the genotypes of drought-tolerant and drought-susceptible Iranian landraces with improved drought-tolerant CIMMYT materials using AFLPs. They concluded that the drought-tolerant landraces and improved materials were genetically distant. Similarly, Reynolds et al. (2007) used DNA fingerprinting to evaluate genetic diversity among drought-tolerant landraces and equally tolerant modern check cultivars; they concluded that the landrace and check cultivars were not only distant from each other, but that the landraces themselves were highly diverse. These are encouraging results for plant breeders as potential additive genetic variance for drought tolerance may be exploited in crosses. Evidence suggests that drought-tolerant landraces extract more water from depth in the soil profile and have a higher concentration of soluble stem carbohydrates (Reynolds et al., 2007, Reynolds and Trethowan, 2007).

Significant variability for heat tolerance also exists among landraces. Heat-tolerant landraces tend to have higher leaf chlorophyll content (Hede et al., 1999) and higher stomatal conductance (Skovmand et al., 2001) compared to modern cultivars.

The Indian landrace ‘Kharchia 65’ was found to be salt tolerant (Mujeeb-Kazi et al., 1993) and has been used by plant breeders to produce salt-tolerant cultivars and breeding lines (Ashraf and O'Leary, 1996; Hollington, 1998; Mahar et al., 2003). Iranian, Nepalese, and Pakistani landraces are also reported to have significantly improved yield under high salinity levels (Martin et al., 1994; Jafari-Shabestari et al., 1995). The Pakistani landrace ‘Shorawaki’ in particular warrants attention as testing has shown it to be superior to Kharchia 65 (Díaz De León et al., 2000)

Triticum spelta, or spelt wheat, is a hexaploid landrace which survives today as a relic wheat in central Europe. The spelts have been reported to have drought- (Cabeza et al., 1993) and waterlogging- (Burgos et al., 2001a; b) tolerance that may be different from that in the primary wheat gene pool. Considerable variation also exists among the spelts of different geographical origins (Elia et al., 2004).


   ALIEN INTROGRESSION AS A SOURCE OF GENETIC VARIATION FOR ABIOTIC STRESS TOLERANCE
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 THE USE OF SYNTHETIC...
 LANDRACES AS A SOURCE...
 ALIEN INTROGRESSION AS A...
CONCLUSIONS
 REFERENCES
 
Synthetic hexaploid wheat represents genetic variability present in the secondary wheat gene pool, primarily represented by Aegilops and Triticum species. All these species have at least one genome in common with hexaploid wheat. However, significant variation for traits of economic importance exists in the more distantly related tertiary gene pool. The genomes of these species are nonhomologous with those of wheat, making hybridization and gene introgression difficult. However, improved tissue culture techniques and the identification of the ph mutants which promote homeologous pairing (Sears, 1976) led to the development of alien addition and substitution lines and the subsequent introgression of segments of alien chromosomes into bread wheat (Sears, 1976; Gupta et al., 2005). Various hybridization techniques are available including fluorescence (FISH), genomic (GISH), and color fluorescence (McFISH) in situ hybridization. Following hybridization, molecular tools can be used to identify quantitative trait loci (QTLs) on these segments (Gupta et al., 2005). Hybrids between hexaploid wheat and distant members of the Triticeae have been made to introduce variability for key economic traits into hexaploid wheat and ultimately, cultivars grown by farmers. Many of the successful wide introgressions have targeted disease resistances, including rust (Puccinia triticina Eriks.) (Tomar and Menon. 2001; Zhang and Ren, 2001; Aghaee-Sarbarzeh et al., 2002; Mago et al., 2005), powdery mildew (Erysiphe graminis DC. f. sp. tritici Em. Marchal) (Alberto et al., 2003), barley yellow dwarf virus (Banks et al., 1995; Hohmann et al., 1996), wheat streak mosaic virus (Friebe et al., 1991; Fedak and Han, 2005), and greenbug [Schizaphis graminum (Rondani)] (Friebe et al., 1991). However, this review will focus only on those introgressions that target abiotic stresses and have a degree of applicability in wheat breeding.

One of the best known and most successful examples of alien introgression has been the 1BL.1RS translocation in wheat (Rajaram et al., 1983, Villareal et al., 1995). This translocation of the long arm of chromosome 1B (1BL) with the short arm of rye (Secale cereale L.) chromosome 1R (1RS) was found in the winter wheat cultivar Kavkaz. It was later transferred to spring bread wheat at CIMMYT, giving rise to the widely grown Veery wheats (Rajaram et al., 1990). The translocation has since been introduced into hexaploid wheat from other rye sources (Schlegel and Korzun, 1997; Ko et al., 2004). Part of the broad adaptation attributed to this rye segment was linked to improved disease resistance (Villareal, 1995). However, the root growth of lines carrying the translocation is also more vigorous (Ehdaie et al., 2003). The 1BL.1RS carrying lines generally have inferior industrial quality, although this is mitigated to some extent by genetic background (Amiour et al., 2002). Wheat rye translocations have also been instrumental in improving wheat adaptation to Zn-deficient (Schlegel et al., 1997) and acidic soils (Ribeiro-Carvalho et al., 1997).

Tolerance to salinity has been transferred from the wild Triticeae into wheat (Zan-Min, 2003). Salt-tolerant wheat germplasm, developed from a translocated segment of Thinopyrum junceum has been recently registered as W4909 and W4910 (Wang et al., 2003). Th. bessarabicum has also been crossed to bread wheat and the salt-tolerance of the resulting amphiploid was found to be significantly improved relative to the recurrent parent (King et al., 1997). The salt-tolerant grasses Lophopyrum elongatum (Host) Á. Löve and Elytrigia pontica (Podp.) Holub also offer potential for improving the salt tolerance of wheat (Dvorak and Knott, 1974; Dvorak et al., 1988).

Wild relatives found in harsh environments such as Ae. geniculata have low carbon isotope discrimination and therefore high water-use efficiency (Zaharieva et al., 2001); these materials represent a potential source of variability for drought and heat tolerance. A translocated chromosome segment from Agropyron elongatum, carrying the leaf rust resistance gene Lr19, was observed to increase yield potential in some hexaploid wheat backgrounds (Reynolds et al., 2001). These authors reported yield increases of 13% over the recurrent parent that were associated with increased biomass partitioning to spike growth at anthesis, increased grain number, and higher radiation-use efficiency. However, the Lr19 segment did not improve drought tolerance or water-use efficiency in moisture stress trials (Monneveux et al., 2003).

Clearly, variability from the secondary and tertiary gene pools offer plant breeders potentially valuable sources of variation for stress related traits. While examples of the successful application of this variability abound for the primary gene pool and to a lesser extent the secondary gene pool, very little variability for abiotic stress tolerance from the tertiary pool has found its way into modern cultivars. Complex inheritance, meiotic instability, and linked deleterious effects have limited their widespread application in applied wheat breeding. Nevertheless, the potential of this resource remains high. Exploitation will require targeting of compensating translocations (Friebe et al., 1994, 1996) and sophisticated genetic manipulation (Mujeeb-Kazi, 2006) facilitated by molecular diagnostics (Ayala et al., 2001).

For translocations to be effectively used by wheat breeders they must carry genes expressing the desired trait(s) with minimal genetic drag from unfavorable linked genes. An example is the stem rust resistance gene Sr26 from Agropyron elongatum located on the translocation chromosome 6AS.6AL-6Ae 1L. This translocation confers effective stem rust resistance but cultivars carrying this translocation are generally lower yielding (The et al., 1988). Homeologous recombination between alien chromosome segments and normal wheat chromosomes can be induced in a ph1bph1b background allowing lines with shortened alien chromatin to be selected using dissociation patterns of molecular markers (Dundas et al., 2007). These techniques have resulted in the identification of Sr26 segments without significant associated yield depression and can be extended to other translocations.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 THE USE OF SYNTHETIC...
 LANDRACES AS A SOURCE...
 ALIEN INTROGRESSION AS A...
 CONCLUSIONS
REFERENCES
 
Novel genetic diversity has had an enormous impact on wheat breeding and wheat cultivation since the inception of the Green Revolution in the 1960s. Most of this impact has been realized through improved disease resistance, particularly for the rusts, derived from variability both within and external to the primary wheat gene pool. However, significant novel genetic variation for tolerance to abiotic stresses has also been identified. This genetic variation when combined with additive genetic variation in the primary wheat gene pool will improve stress adaptation, although this will not be easily done, as most of the new variation is of little economic value and most wheat breeders, particularly those with strict quality-based objectives, are reluctant to introduce excessive diversity into their programs. Therefore, the challenge is to ‘sift’ the economically useful genetic variation from the expanse of available new variation. Targeted backcrossing, large population sizes, and intensive selection can be used to develop new parental materials expressing the desired variation. Genetic gain is realized only when selection is conducted in a repeatable stress environment that is relevant to the wider target area. However, complex inheritance, the confounding effects of genotype x environment interaction, the high cost of managing large populations, and a paucity of molecular markers for abiotic stress tolerance traits has limited progress.

The additive nature of inheritance of most abiotic stress traits suggests that the insertion of a single gene through transgenesis is unlikely to provide broad drought adaptation, notwithstanding preliminary evidence of oxidative signal cascades in other crops (Shou et al., 2004). Candidate genes related to osmotic adjustment and relative water content have been identified (reviewed by Reynolds et al., 2007), although their usefulness in wheat breeding has not been determined. The Dreb1A gene, originally identified in Arabidopsis, when introduced into wheat using particle bombardment showed an encouraging response to moisture stress in the greenhouse (Pellegrineschi et al., 2004) that could not be translated to the field (Matthew Reynolds, personal communication, 2006). The choice of ‘Bobwhite’ as the recipient wheat cultivar may have limited gene expression as Bobwhite carries the 1BL.1RS translocation and already has a degree of drought tolerance.

The synthetic wheats, which provide a bridge between genetic variation in the D-genome progenitor and modern hexaploid wheat, offer wheat breeders the greatest potential for yield advance under stress in the short to medium term. These primary synthetics are directly crossable with adapted wheat and potentially combine genetic variation that has not previously existed in the hexaploid wheat gene pool. The use of synthetic wheats at CIMMYT in applied wheat breeding has not only improved stress adaptation (Lage and Trethowan, 2008), but has significantly increased the genetic diversity of recently developed wheat germplasm (Warburton et al., 2006). If synthetics based on T. dicoccoides and other wild tetraploids are developed, then significantly broader genetic variation can be expected for all three genomes.

The landraces, while offering considerable variation, are less likely to carry novel genes for abiotic stress tolerance as they, like modern cultivated wheat, are derived from the same crosses that gave rise to hexaploid wheat 8000 years ago. Nevertheless, the collection, characterization, and maintenance of these materials are essential, not only as a source of potentially new genetic variation, but because landraces are disappearing from the world's farming systems as farmers seek higher yielding cash crops (Teklu and Hammer, 2006).

The alien species of the Triticeae tribe have been a valuable source of genetic variation for disease resistance and to some extent, abiotic stress tolerance. However, difficulties in transferring this variation into adapted wheat, meiotic instability, and deleterious effects associated with the introduced segments have so far limited their application largely to translocations from cereal rye. Unfortunately, most of the genetic stocks developed by cytogeneticists over the past 40 years have not been evaluated for stress tolerance and are largely maintained as oddities in germplasm collections. If these materials are characterized for stress response, it may be possible to identify alien introgressions that can be used in applied breeding for stress tolerance.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 THE USE OF SYNTHETIC...
 LANDRACES AS A SOURCE...
 ALIEN INTROGRESSION AS A...
 CONCLUSIONS
 REFERENCES
 
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Received for publication August 26, 2007.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
 THE USE OF SYNTHETIC...
 LANDRACES AS A SOURCE...
 ALIEN INTROGRESSION AS A...
 CONCLUSIONS
 REFERENCES
 




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The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome