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RESEARCH PAPER |
Identification of variation in adaptively important traits and genome-wide analysis of trait–marker associations in Triticum monococcum
1Centre for Sustainable Pest and Disease Management, Department of Plant Pathology and Microbiology, Rothamsted Research, Harpenden, Herts AL5 2JQ, UK
2N. I. Vavilov Research Institute of Plant Industry, St Petersburg, 190000, Russian Federation
3Department of Crop Genetics, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK
4Ukrainian Institute for Plant Variety Examination, Centre for Certification Trials, 15 Henerala Rodimtseva Street, Kyiv 03041, Ukraine
* To whom correspondence should be addressed. E-mail: kim.hammond-kosack{at}bbsrc.ac.uk
Received 8 April 2007; Revised 1 August 2007 Accepted 28 August 2007
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Abstract |
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Einkorn wheat Triticum monococcum (2n=2x=14, AmAm) is one of the earliest domesticated crops. However, it was abandoned for cultivation before the Bronze Age and has infrequently been used in wheat breeding. Little is known about the genetic variation in adaptively important biological traits in T. monococcum. A collection of 30 accessions of diverse geographic origins were characterized for phenotypic variation in various agro-morphological traits including grain storage proteins and endosperm texture, nucleotide-binding site (NBS) domain profiles of resistance (R) genes and resistance gene analogues (RGAs), and germination under salt and drought stresses. Forty-six SSR (single sequence repeat) markers from bread wheat (T. aestivum, 2n=6x=42, AABBDD) A genome were used to establish trait–marker associations using linear mixed models. Multiple significant associations were identified, some of which were on chromosomal regions containing previously known genetic loci. It is concluded that T. monococcum possesses large genetic diversity in multiple traits. The findings also indicate that the efficiency of association mapping is much higher in T. monococcum than in other plant species. The use of T. monococcum as a reference species for wheat functional genomics is discussed.
Key words: Association mapping, biological and agronomic traits, disease resistance, genetic variation, grain storage proteins, grain texture, salt and drought tolerance, T. monococcum
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionSupplementary materialReferences |
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The diploid species Triticum monococcum (2n=2x=14, AmAm), commonly known as einkorn wheat coined from the German expression of ‘one grain’, was widely cultivated during the pioneering human farming activities in the Fertile Crescent. It was domesticated from its wild progenitor T. boeoticum near the Karacadag mountains in southeast Turkey (Heun et al., 1997). Domesticated einkorn wheat differs from the wild T. boeoticum in three major traits: larger and plumper seeds, a tough rachis which prevents spikelets falling apart at maturity, and relatively easy threshing (Salamini et al., 2002). Although dominating Neolithic agriculture, einkorn wheat was less favoured after the Bronze Age when the cultivation of high-yielding polyploid wheat species began. It has since been literally left untouched growing in its natural habitats for thousands of years and has not been exposed to intensive human selection (Zohary and Hopf, 1993). Thus, T. monococcum may retain its ancient level of genetic diversity and provide an ideal cereal model to study diversity of important traits and genetic diversity after domestication.
The Am genome of T. monococcum and the Au genome of T. urartu are closely related and diverged 0.5–1 million years ago (Huang et al., 2002; Dvorak et al., 2004). Triticum urartu has been the dominant A genome donor of the most important polyploid wheat species including the durum or macaroni wheat T. turgidum (AABB), T. timopheevii (AAGG), and common wheat T. aestivum (AABBDD). In contrast, T. monococcum has only been used for the generation of T. zhukovskyi (AmAmAAGG) (Dvorak et al., 1993; Dubcovsky et al., 1995; Baum and Bailey, 2004). Thus, the Am genome is under-represented in hexaploid wheat, and the exploitation of genetic diversity in T. monococcum and discovery of novel variant alleles may provide opportunities for further wheat genetic improvement. Indeed, T. monococcum has been used for improving various traits of polyploid wheat species (Valkoun, 2001). For example, bread-making quality was improved by introgression of genes encoding the high molecular weight (HMW) glutenin subunits from T. boeoticum and T. monococcum (Rogers et al., 1997; Tranquilli et al., 2002a). The incorporation of additional copies of Pina and Pinb genes from T. monococcum into the cultivar Chinese Spring resulted in significantly softer grains than those of the progenitor cultivar, improving the biscuit- and cake-making quality (Tranquilli et al., 2002b; See et al., 2004). Various einkorn wheat genetic loci were successfully introgressed into the hexaploid triticale and bread wheat to provide resistance to leaf rust and powdery mildew (Shi et al., 1998; Vasu et al., 2001; Sodkiewicz and Strzembicka, 2004), prevent pre-harvest sprouting (Sodkiewicz, 2002), and increase zinc uptake efficiency (Cakmak et al., 1999). A durum wheat cultivar with Nax1 and Nax2 genes introgressed from T. monococcum exhibited greatly enhanced salt exclusion ability (James et al., 2006). These studies, albeit focused on only a few traits, suggest that T. monococcum is useful for wheat genetic improvement. Fregeau-Reid and Abdel-Aal (2005) recently noted variation in numerous traits in the diploid einkorn wheat worthy of exploitation, such as dietary fibre, milling characteristics, and lutein content.
The large genome size and the co-existence of three homoeologous genomes in hexaploid wheat present a huge challenge for the genetic dissection of phenotype–genotype relationships. It is more feasible to use a relatively smaller diploid genome such as that of T. monococcum for genetic studies. Also wheat has biological questions which cannot be studied using unrelated model plant species. One particular example is the interaction between wheat and a fungal pathogen Mycosphaerella graminicola, which is an exclusive pathogen of some Triticum species causing Septoria tritici leaf blotch disease (Keon et al., 2007). It is unlikely that the bona fide resistance mechanisms can be defined by studying resistance in non-host model species such as rice, Arabidopsis, barley, or Brachypodium distachyon. Triticum monococcum is a host of this pathogen and can be used as an alternative route to study the genetics of resistance (H-C Jing and K Hammond-Kosack, unpublished data). It has been convincingly demonstrated that T. monococcum is a good model for assisting with the cloning of genes from hexaploid wheat and for gene function studies (Stein et al., 2000; Feuillet et al., 2003; Yan et al., 2003; Yahiaoui et al., 2004; Uauy et al., 2006).
Our research aims to develop T. monococcum into a reference species for wheat genetics and genomics. Here, the variation in several important morphological and agronomic traits was characterized in T. monococcum, including plant growth- and yield-related components, various grain features, the profiles of resistance (R) gene and resistance gene analogues (RAGs), as well as germination under salt and drought stresses. Genetic segregation and association analyses were performed to define simple sequence repeat (SSR) markers (microsatellites) linked with multiple important biological traits. This is the first report on genome-wide trait–marker associations in T. monococcum. The potential to explore novel variation in T. monococcum for modern wheat improvement and to use T. monococcum as a model for wheat genetics and genomes is discussed.
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Materials and methods |
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The T. monococcum accessions
In total 30 T. monococcum accessions were used in this study (Table 1). These included 26 accessions from the NI Vavilov Research Institute of Plant Industry (VIR), St Petersburg, Russia, three accessions from the John Innes Centre (JIC, Norwich, UK), and one accession (DV92, referred to as MDR308 in our collection) from Professor Jorge Dubcovsky, University of California at Davis. The 26 VIR accessions were selected based on their resistance/susceptibility to important Russian wheat pathogens such as powdery mildew, leaf rust, and aphids (Lebedeva and Peusha, 2006). MDR050 (V97031) is from Victor Vallega, Italy which has a large grain size and had been used for studying starch biosynthesis in cereal endosperm (Kay Denyer, JIC, personal communication). This genotype has been selected from the progeny of a cross between T. monococcum and T. sinskajae (Korzun et al., 1998b) and is characterized by short and compact ears which lack awns and easy threshing. The accession DV92 had been used for generation of a genetic restriction fragment length polymorphism (RFLP) map, construction of a bacterial artificial chromosome (BAC) library, and isolation of a number of genes (Dubcovsky et al., 1996; Lijavetzky et al., 1999; Yan et al., 2003; Yahiaoui et al., 2004; Uauy et al., 2006).
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Evaluation of morphological and agronomic traits
Seeds were germinated and seedlings were vernalized at 4–6 °C for 8 weeks and then grown to maturity in temperature-controlled glasshouse compartments equipped with supplementary lighting. Five plants from each T. monococcum accession were arranged in a randomized block design and, at harvest, traits were measured. Produced seeds were dried to
10% water content and stored for 6 months at 6 °C and 20% relative humidity prior to grain hardness measurements and germination tests.
Analysis of grain
Electrophoretic profiles of seed storage proteins were generated as described (Shewry et al., 2006). Each profile was verified by comparison of three independent extractions. The gel images were analysed for polymorphism using Totallab image analysis software (Nonlinear Dynamics, Newcastle, UK).
Endosperm hardness was assessed using a single kernel characterization system (Perten SKCS 4100, Perten Instruments AB, Huddinge, Sweden). The weight, length, diameter, and moisture content of 250 individual grains per accession were measured. Grain texture was visualized by scanning electron microscopy (SEM). For this purpose, mature seeds were quench-frozen in liquid nitrogen, transferred to a Cryo SEM preparation chamber (Gatan Alto 2100), fractured, etched by sublimation at 85 °C for 2 min, sputter coated with gold, and finally examined at 5–15 kV in a JEOL JSM-6360 LV scanning electron microscope.
NBS (nucleotide-binding site) profiling
NBS profiling was carried out essentially as described (van der Linden et al., 2004) with some modification. Briefly, 200 ng of genomic DNA were subjected to MseI restriction and ligation in 30 µl of buffer containing 10 mM TRIS acetate, pH 7.5, 10 mM magnesium acetate, 50 mM potassium acatate, 5 mM dithiothreitol, 1.5 µg of bovine serum albumin, 1 mM ATP, 5 U of MseI, 0.5 U of T4 DNA ligase, and 0.025 nmol adaptor primers. The amplification of NBS-specific fragments involved a two-step procedure with the second using [
-33P]ATP-end-labelled NBS-specific primers. PCR products were separated on 6% polyacrylamide gels and imaged with a Typhoon 8600 Variable Mode Imager (Amersham). The images were processed using the Totallab image analysis software (Nonlinear Dynamics, Newcastle, UK).
Seed germination test
Seeds (three lots of 25 seeds per accession) were surface-sterilized with bleach containing 1% sodium hypochloride for 5 min, rinsed vigorously five times with deionized water for 5 min, and imbibed on two layers of Whatman filter paper soaked with 8 ml of deionized water, 150 mM NaCl solution, or 135 g kg–1 PEG-6000 solution (–0.8 MPa), respectively. Petri dishes were sealed with Parafilm and maintained at 25 °C with a 16 h/8 h day/night cycle. The germination percentage was scored daily for 7 d.
Generation of mapping populations and genetic segregation analysis
Accessions MDR002, MDR308, and MDR043 were used to generate two mapping populations. Anthers from female plants were emasculated using a fine pair of tweezers 15 d after ear emergence, and pollination was carried out 2–3 d after emasculation. To increase the rate of success, a single anther was used to pollinate a single floret. The pollinated ears were then covered with cheesecloth pockets to prevent cross-pollination and allowed to set seed. The electrophoretic profiles of the endosperm tips from the resulting F1 seeds were compared with parental lines to confirm their authenticity. F2 progeny were grown in greenhouses for assessing segregation of various growth and morphological traits.
Microsatellite genotyping
Genomic DNA was isolated from the second leaf of 2-week-old seedlings using a QIAgene DNA mini-kit. To test intra-accession genetic variation, DNA was extracted from five seedlings per accession.
Primer sequences for microsatellites mapped to the bread wheat A genome were obtained from the Graingenes database (http://wheat.pw.usda.gov/GG2/index.shtml) and used to amplify genomic DNA templates from T. monococcum accessions. Each 10 µl reaction contained 50 ng of template DNA, 1.5 mM Mg2+, 1.5 mM of dNTPs, 1.5 µM of each primer, 1 µl of 10x PCR buffer, and 1.25 U of Taq DNA polymerase (Promega). The PCR conditions were 2 min at 95 °C, followed by 30 cycles of 94 °C/30 s, Tm/30 s, and 72 °C/60 s, ending with an extension of 72 °C/5 min. The Tm varied between 50 °C and 63 °C depending on the SSR markers (Table 2). The PCR products of 46 microsatellites were analysed using either an ABI 3730 DNA analyser or a 3% agarose gel consisting of one-third of Nusieve® 3:1 agarose and two-thirds of MetaPhor® agarose (Cambrex Bio Science, Rockland, ME, USA). The microsatellite profiles were scored for clustering and association analyses.
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Linkage and association statistical analysis
GenStatTM (release 9.2 2007, Lawes Agricultural Trust, Rothamsted Research) was used to perform statistical analyses. Genetic diversity of T. monococcum accessions was assessed by clustering analysis, in which a Jaccard similarity matrix was generated using the microsatellite banding data, and the UPGMA (unweighted pair group mean average) method was used for generating clustering dendrograms.
Linkages between SSR markers and two morphological traits, awn colour and leaf pubescence, were analysed using the marker regression function of the software Map Manager QTX (http://www.mapmanager.org/mmQTX.html). For association mapping, linear mixed models using residual maximum likelihood (REML) were employed to identify associations between SSR markers and genetic loci controlling traits. For each trait, the following linear model was fitted using each SSR marker with the directive REML:
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Results |
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Genetic purity and diversity of T. monococcum accessions as assessed by SSR analysis
Wheat A genome-specific SSR markers were used to assess the genetic diversity within the T. monococcum accessions. Out of 101 primer pairs tested, 73 amplified products effectively from T. monococcum templates following some minor adjustment to the PCR conditions, giving a transferability rate of >70%. Forty-six SSR markers were selected to assess the genetic purity of the accessions based on their genome coverage (Table 2).
The VIR accessions were collections of various landraces, and therefore genetic variation within these accessions was expected. Amongst the 26 accessions examined, seven showed genetic heterogeneity (data not shown), suggesting that VIR landraces are fairly homogenous genetically. For these seven accessions, the dominant genotype was selected and multiplied to produce a pure line.
The same set of 46 SSR markers was also used to assess genetic diversity of the 26 T. monococcum accessions from the Vavilov Institute, and three accessions requested from the JIC and DV92 (Table 1). In total, 293 polymorphic bands were identified, and this gave an average of six polymorphic bands per marker. A Jaccard similarity matrix was generated using these polymorphic bands, which was then used to construct a phylogenetic tree deciphering the genetic relationships of the 30 accessions (Fig. 1). The minimal similarity was <0.3, suggesting an overall high genetic variation in these accessions. MDR050 clustered well with other T. monococcum accessions, confirming the notion that T. sinskajae was generated from a spontaneous mutation in T. monococcum (Korzun et al., 1998b). The clustering analysis also clearly indicates that the genetic variation only partially correlates with the geographic origin. For instance, accessions MDR025 and MDR026 from the Ukraine and accessions MDR034, MDR037, and MDR038 from Armenia were in the same clads, while accession MDR001 from Algeria was distantly related to all the other accessions. However, the two accessions from Turkey, MDR031 and MDR044, were split into different clads. Accession MDR308 (DV92) from Italy was clustered together with MDR043 from Greece, but was distantly related to another Italian accession MDR032. The SSR clustering also only partially correlated with subspecies classification. These results were not anticipated and were in contrast to those obtained for barley, where a good correlation between geographic origin and SSR marker clustering was discovered (Malysheva-Otto et al., 2006). To explore the genetic diversity and geographic location association in greater detail, an additional 66 T. monococcum and 13 T. boeoticum accessions were genotyped using the same set of SSR markers (see Supplementary Table S1 at JXB online). These results again indicate that only a partial correlation exists between SSR marker clustering and the geographic origin, although T. monococcum was reasonably well separated from T. boeoticum (see Supplementary Fig. S1 at JXB online).
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Variation in morphological and agronomic traits
Table 3 shows a large variation in 11 scored morphological traits of agronomic relevance. Several significant correlations were evident (Table 4). As could be expected, the numbers of tillers were negatively correlated with plant height, peduncle length, and spikelet numbers. The grain weight was positively correlated with many traits including seed volume, ear length, peduncle length, and plant height, but was negatively correlated with spikelet numbers.
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Variation in grain features
Figure 2 shows the banding patterns for both gliadin and glutenin subunits. In the experimental system used, both HMW and low molecular weight (LMW) glutenin and the
and fractions of gliadin were detected. Highly polymorphic bands were observed for the gliadin fractions and the LMW glutenin subunits, whereas the HMW glutenin subunits were rather monomorphic. The 30 accessions showed discrete electrophoretic profiles.
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Grain texture measurement indicated that all the T. monococcum accessions examined had a hardness index <35, a minimal threshold level of hard endosperm suggesting that overall T. monococcum has a soft grain texture (Fig. 3A). Interestingly, the 30 T. monococcum accessions fell into two groups. While the majority of the accessions showed a minus value of hardness index, four accessions, MDR001, MDR002, MDR047, and MDR308, exhibited a hardness index over +10, suggesting that these four accessions may be the ‘hard grains’ in T. monococcum. SEM examination (Fig. 3B) showed that the MDR308 grain cryofracture images were similar to the representative hard-grain wheat Mercia, while those of MDR040 resembled the representative soft-grain wheat Riband. Thus, these T. monococcum accessions can be used to explore the genetic controls of grain texture.
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Polymorphism in disease R genes and RGAs in T. monococcum
Figure 4 shows representative NBS profiles obtained for the T. monococcum accessions in comparison with those for hexaploid wheat. Multiple novel polymorphic bands were observed in einkorn wheat NBS profiles which were absent in hexaploid wheat using primers specifically targeting NBS2 and NBS5 domains. However, the NBS3 profiles in T. monococcum were monomorphic (data not shown). Thus, there exists a high level of polymorphism in R genes and/or resistance gene analogue (RGA) genes in T. monococcum. Further exploitation may allow the identification of novel variant alleles conferring high disease resistance to important wheat pathogens.
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Responses of T. monococcum to salt and drought stresses
The T. monococcum accessions were explored for tolerance to salt and drought stresses in a germination assay (Fig. 5). Seeds of the tested accessions could reach a total germination of >90% and there were no significant differences in germination rates and total germinations when seeds were imbibed in water (data not shown). However, variation in germination rate and total germination was observed when seeds were imbibed in the presence of 150 mM NaCl or –0.8 MPa. Seeds of MDR0001, MDR033, MDR034, MDR037, MDR038, MDR047, and MDR308 exhibited slow germination rates and could not reach full germination under salt stress, while seeds of MDR001, MDR037, MDR038, MDR047, and MDR049 showed poor germination under drought stress. Thus, T. monococcum accessions possess different levels of tolerance to salt and drought stresses.
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Identification of SSR markers associated with genetic loci controlling awn colour and leaf pubescence
Awn colour and leaf pubescence are two prominent traits in wheat. In the T. monococcum accessions, MDR002 has strong leaf pubescence and black awns, whereas most of the accessions have glabrous leaves and yellow awns (Fig. 6). MDR002 was crossed with MDR043 and MDR308 to study the inheritance of these two traits. The leaf pubescence and black awn phenotypes were evident in F1 hybrids of both crosses (22 and 18 F1 seeds were examined for crosses MDR002xMDR043 and MDR308xMDR002, respectively). In MDR308xMDR002 F2 populations, these two traits segregated in a 3:1 ratio (79 black awn:25 yellow awn,
2=0.0513; 73 pubescence:23 glabrousness, 2=0.0556). These results confirm that both traits are controlled by single dominant genes. Genetic loci controlling glume colour and leaf pubescence have been previously mapped in hexaploid wheat (T. aestivum) to 1AS and 4B and 7B, respectively (Borner et al., 2002; Taketa et al., 2002). Therefore, 94 F2 individuals from the MDR308 and MDR002 cross were genotyped using SSR markers in the vicinity of these loci and it was found that awn colour was associated with the SSR marker Xwmc336 locus on 1Am (P <0.00001) and leaf pubescence with the Xcfd39 locus on 5Am (P=0.00002) (Table 5).
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Associations between SSR markers and other traits in T. monococcum
To analyse further the genetic basis of the variation in the 14 other traits examined, their associations with 46 SSR markers were tested (Table 5). Strong linkages with SSR markers were found for quantitative traits such as grain hardness, seed germination under salt and drought stresses, and various yield components, as well as the traits which had qualitative scores such as seed storage protein profiles and NBS profiles. Interestingly, a few SSR markers were linked to multiple traits. For example, SSR markers BARC52 and GWM179 were associated with several yield-related components, grain hardness as well as germination under salt tolerance. On the other hand, variation in one trait could be linked to SSR markers from various chromosomal regions as exemplified by grain moisture content, ear length, grain hardness, and germination under salt stresses. Some of the associations were on chromosomal regions containing previously known genetic loci in hexaploid wheat. This is particularly true for the associations between SSR markers and NBS profiles. Comparing these with map locations of RGA expressed sequence tags (ESTs) in bread wheat (McFadden et al., 2006), it was found that Xgwm164 was estimated to be only 0.7 cM away from RGA10 on chromosome 1A, while Xgwm293 and Xgwm129 were tightly linked to RGA71 on chromosome 5A. Furthermore, a quantitative trait locus (QTL) controlling germination under salt and drought stresses was linked to Xwgm179, which is within a 10 cM distance to the Nax2 gene (Byrt et al., 2007). The present data also indicate that a relatively high map resolution can be achieved by association mapping in T. monococcum. For instance, ear length is linked to Xgwm636 and Xwmc177 which are only
4 cM apart on 2A; hardness to Xwmc177 and Xbarc5 loci which are also 7 cM apart on 2A; and an NBS2 band to Xgwm293 and Xgwm129 loci which are <0.5 cM apart on 5A. These data imply that association mapping is a powerful tool to identify trait–marker links in T. monococcum. |
Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionSupplementary materialReferences |
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Triticum monococcum is one of the most ancient small grain cereals which turned Stone Age nomads into farmers. Various agriculturally important traits were analysed and their associations with SSR markers in T. monococcum were examined. Several important trait–marker associations were identified efficiently using only a small core collection. The results demonstrate that T. monococcum possesses genetic variation in 16 useful traits and is a good model for wheat genetic study.
Genetic diversity and trait variation in T. monococcum
Using microsatellites spanning the genome, a genetic similarity as low as 0.3 was found in the T. monococcum collection examined. A similar estimation was reported in an earlier study on 26 T. monococcum accessions using 20 microsatellites (Korzun et al., 1998b). Studies using RFLP and rapid amplification of polymorphic DNA (RAPD) indicated that the genetic diversity in T. monococcum was lower than that of T. boeoticum and T. urartu, and there is high genetic diversity in the three A genome diploid wheat species (Vierling and Nguyen, 1992; Castagna et al., 1994). Interestingly, the microsatellite clustering of T. monococcum accessions correlated only to a limited degree with the geographical origins. This implies that T. monococcum has been widely spread after domestication, but has not undergone significant genetic changes during the past 10 000 years (Zohary and Hopf, 1993). Furthermore, the cultivation and spread of T. monococcum had already declined before the Bronze Age 7000 years ago, far earlier than the start of human crop breeding activities, which would narrow the genetic diversity of local landraces but increase the genetic distance for accessions with different geographical origins. In line with this argument, the correlation between geographical origin and microsatellite clustering is high in barley which has been intensively selected for over centuries (Malysheva-Otto et al., 2006).
The genetic diversity observed in T. monococcum is well reflected by the variation in multiple biological traits. For instance, several-fold differences were found in yield-related components. Triticum monococcum is generally considered as a soft grain owing to the control of Pina and Pinb genes (Luo et al., 2005). However, grain hardness indexes between –20 and +20 were found in the accessions tested. In a Danish T. monococcum collection, grain hardness indexes between –7.3 and +27.2 were reported (Loje et al., 2003). Triticum monococcum therefore can be exploited for genetic variation in grain hardness. This information may then be used to alter the grain hardness for various purposes in polyploid wheat species, which lost the Pin genes during polyploidization (Shewry and Halford, 2002). Over a dozen different electrophoretic profiles of grain storage proteins and a high level of diversity in the gliadin and LMW glutenin exist in the 30 T. monococcum accessions examined, which may provide new sources for bread wheat improvement (An et al., 2006).
Global climate changes are predicted to bring new biotic and abiotic stresses to the wheat crop and impose further impacts on water and other natural resources (Reynolds and Borlaug, 2006). These changes may render the current elite wheat cultivars and/or cropping systems inappropriate. Plant R genes and RGAs are prominent components in induced defence responses conferring resistance in either a race-specific or race-non-specific manner (Hammond-Kosack and Parker, 2003; Chisholm et al., 2006). The NBS domain of R proteins and RGAs contains the following characteristic motifs: P loop (phosphate-binding domain), kinase-2 motif, and GLPL-motif (Meyers et al., 1999). The NBS profiling in T. monococcum indicates that homologues of the potential R genes containing NBS2 and NBS5 are highly polymorphic. Triticum monococcum possesses a high level of resistance to a range of diseases and pests including leaf rust (Hussien et al., 1998), stem rust (Bai et al., 1998), powdery mildew (Shi et al., 1998), cereal aphid (Migui and Lamb, 2004), Russian wheat aphid (Deol et al., 1995), and Hessian fly (Sharma et al., 1997). The present T. monococcum collection contains accessions resistant to soil-borne cereal mosaic viruses and partially resistant to the virus vector Polymyxa graminis (Kanyuka et al., 2004; Ward et al., 2005). All the 30 T. monococcum accessions tested exhibited high resistance to Septoria tritici blotch under UK wheat production conditions, and in planta fungal sporulation was not observed throughout the growing season for four consecutive years (Jing et al., 2005; H-C Jing and K Hammond-Kosack, unpublished data).
Tolerance to abiotic stresses is pivotal for the success of crop production (Reynolds and Borlaug, 2006). Durum wheat containing the Nax1 (Na+ exclusion) and Nax2 genes introgressed from T. monococcum exhibited greatly enhanced ability for salt exclusion and hence tolerance (James et al., 2006). This suggests that T. monococcum also has novel genes which can be used to enhance tolerance to abiotic stresses. In the 30 T. monococcum accessions, differences in tolerance to salt and drought stress were found using a germination assay. In a salt exclusion assay using young seedlings, a >10 times difference in leaf sodium content has been identified in these accessions (Y Shavrukov and H-C Jing, unpublished data). These results provide a strong basis to explore the genetic control of salt tolerance in T. monococcum.
Genetic basis of variation in multiple traits
Two types of genetic analyses were carried out to establish trait–marker associations in T. monococcum. First, segregation and linkage analyses were performed to identify genetic loci controlling awn colour and leaf pubescence. In bread wheat, the awn colour and glume colour are suggested to be controlled by the same genetic loci, which are associated with RFLP loci QRaw.ipk-1A on 1AS and QRaw.ipk-1D on 1D, respectively (Borner et al., 2002). It is not clear whether these two traits are linked in T. monococcum. The accession MDR002 has black awns but yellow glumes, suggesting that these two traits may be controlled by independent loci. However, the tightly linked SSR locus Xwmc336 is located at 21.52 cM on chromosome 1A, which is in the vicinity of the bread wheat 1AS genetic locus controlling the black awn and glume trait. Furthermore, the T. monococcum black glume trait was previously mapped to a similar region on 1AmS using two different mapping populations, and it was suggested that there is allelic variation in the black glume locus in T. monococcum (Dubcovsky et al., 1996). Hence, it is most likely that in T. monococcum black awn and black glume are controlled by one single dominant locus. It was found that in T. monococcum leaf pubescence is dominant over leaf glabrousness. A tight linkage of the hairy leaf locus with Xcfd39 at 83.19 cM on chromosome 5AL was found. In hexaploid wheat the hairy leaf loci Hl1 and Hl2 have been mapped to 4B and 7B, respectively (Taketa et al., 2002). After polyploidization there are serial events of chromosomal translocations amongst 5AL, 4AL, 4AS, and 7BS (Devos et al., 1995). It is likely that the mapped hairy leaf locus in T. monococcum is allelic to that on 7BS in hexaploid wheat. Interestingly, the hairy leaf loci Hbs and Hp1 have been found in homologous chromosomal regions in barley and rye (Korzun et al., 1998a, 1999).
Association genetics analyses the variation of particular phenotypes amongst plants to detect and measure the degree of association between molecular markers and traits of interest (Gupta et al., 2005). This approach has been successfully used to identify a range of marker–trait associations in hexaploid wheat (Roy et al., 2006). The present study points to some interesting marker–trait associations in T. monococcum, even though only a limited numbers of 30 accessions were used (Table 5). Remarkably, some of the associations identify chromosomal regions containing previously known genetic loci. For instance, several SSR markers associated with NBS profiles are in the vicinity of mapped RGAs (McFadden et al., 2006). The association mapping also identified that germination under salt and drought stresses is probably linked to Nax2. Both Nax1 and Nax2 genes were identified in a seedling salt exclusion assay (James et al., 2006). However, it appears that only Nax2 is involved in salt tolerance during both seed germination and seedling growth. These may correlate with the divergence of the two genes in terms of function. Nax1 works to reduce sodium content in leaf blades (Huang et al., 2006), whereas Nax2 removes sodium from xylem in the roots (Byrt et al., 2007). In addition to confirming previous marker–trait associations, the association mapping has identified many new associations which merit further study.
In hexaploid wheat, grain texture is mainly controlled by the Hardness (Ha) locus on 5D consisting of the Pina-D1, Pinb-D1, and Gsp-D1 genes (Gautier et al., 1994; Sourdille et al., 1996). These Ha-related genes were shown to be arranged in a highly conserved manner on 5Am (Tranquilli et al., 1999). Interestingly, it was not possible to identify associations with the predominant Ha locus; none of the identified SSR markers is located on the 5AS region containing PinA, PinB, and Gsp genes. Also the previously known Ha locus SSR markers are not linked to variation in hardness in T. monococcum (Table 5; Bonafede et al., 2007). This may imply that the observed variation in grain hardness in T. monococcum is controlled by genetic loci other than the Hardness locus. Indeed, there is a report indicating that additional QTLs exist in hexaploid wheat controlling grain texture (Turner et al., 2004). Furthermore, our preliminary results showed that the sequences of Pina and Pinb genes are highly conserved in these T. monococcum accessions (M Wilkinson and P Shewry, unpublished data).
A number of other important associations between microsatellites and traits were found in this study, including plant height, peduncle length, grain moisture content, and ear length. Thus, via screening only a small pool of genotypes, possible loci conferring a specific trait are detected in T. monococcum, which often requires a large pool of germplasm to be screened in other species (Gupta et al., 2005). The findings in this report indicate that the efficiency of association mapping is much higher in T. monococcum than in other plant species. Mapping populations are currently being generated and more SSR markers are being applied to construct a high density genetic map with the bulk segregating populations from a cross between MDR308 and MDR002 and to narrow down the linkage intervals.
The genetic diversity of modern hexaploid wheat has been achieved through the introgression of novel genetic materials (Reif et al., 2005). The unique characteristics and evolution of T. monococcum make it ideal as a reference species for wheat genetics and genomics. Examples exist demonstrating the success of gene cloning using a subgenome approach in T. monococcum (Keller et al., 2005). Furthermore, useful traits and genic variants can be introgressed into elite wheat varieties using conventional breeding approaches assisted by molecular makers (Korzun, 2002). Over the years, many approaches have been developed to facilitate the introgression of novel traits through conventional breeding (Potgieter et al., 1991). These include the utilization of a unique T. monococcum accession (PI355520), containing two dominant genetic loci which could help achieve high rates of viable and functional hybrids when crossed to hexaploid wheat (Cox et al., 1991). In addition, a range of bridge species have been developed, including a multiploid mutant or amphiploid of durum wheat (Multani et al., 1988; Klindworth and Williams, 2003), a synthetic allotetraploid T. monococcum/Secale cereale (AmAmRR) (Kison and Neumann, 1993), or tetraploid and hexaploid triticale (Sodkiewicz and Apolinarska, 2000; Sodkiewicz and Strzembicka, 2004).
In the current post-genomic era, many molecular genetic resources and technology breakthroughs are ready or under development for crop sciences. TILLING (Targeting Induced Local Lesions IN Genomes) and VIGS (virus-induced gene silencing) have been efficiently used for functional genomics in cereals (Hein et al., 2005; Scofield et al., 2005; Slade et al., 2005). All these will be helpful for exploiting T. monococcum as a reference species to establish tight trait–marker associations, and eventually leading to gene function studies using both forward and reverse genetic approaches.
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TopAbstractIntroductionMaterials and methodsResultsDiscussionSupplementary materialReferences |
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The supplementary material available at JXB online includes the following: (1) Supplementary Table S1 showing the additional 66 T. monococcum and 13 T. boeoticum accessions used for assessing genetic diversity; (2) Supplementary Fig. S1 showing cluster analysis of the 96 T. monococcum and 13 T. boeoticum accessions.
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Acknowledgements |
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We thank Mike A Field (Advanta Seeds UK) for help on grain protein profiling, and Alan Todd and Salvador A Gezan for statistical analysis. The Rothamsted Bioimaging Centre and Jean Devonshire are acknowledged for the SEM analyses. We appreciated constructive discussions with Peter Shewry and Paola Tosi. The UK Small Grains Cereal Workshop Network is acknowledged for providing a travel grant for HCJ to visit the John Innes Centre to carry out the NBS profiling work at the laboratory of Dr Robert Koebner. This research is part of the core project of the Wheat Genetic Improvement Network which is supported by a grant from the Department for Environment, Food and Rural Affairs (Defra, AR0709). DK and AZ were supported by Rothamsted International Fellowships. Both Rothamsted Research and the John Innes Centre receive strategic grants from the Biotechnology and Biological Sciences Research Council (BBSRC).
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References |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionSupplementary materialReferences |
|---|
An X, Zhang Q, Yan Y, Li Q, Zhang Y, Wang A, Pei Y, Tian J, Wang H, Hsam SLK, Zeller FJ. Cloning and molecular characterization of three novel LMW-i glutenin subunit genes from cultivated einkorn (Triticum monococcum L.). Theoretical and Applied Genetics (2006) 113:383–395.[CrossRef][Web of Science][Medline]
Bai D, Knott DR, Zale JM. The inheritance of leaf and stem rust resistance in Triticum monococcum L. Canadian Journal of Plant Science (1998) 78:223–226.[Web of Science]
Baum BR, Bailey LG. The origin of the A genome donor of wheats (Triticum: Poaceae)—a perspective based on the sequence variation of the 5S DNA gene units. Genetic Resources and Crop Evolution (2004) 51:183–196.[CrossRef][Web of Science]
Bonafede M, Kong L, Tranquilli G, Ohm H, Dubcovsky J. Reduction of a Triticum monococcum chromosome segment carrying the softness genes Pina and Pinb translocated to bread wheat. Crop Science (2007) 47:821–828.
Borner A, Schumann E, Furste A, Coster H, Leithold B, Roder MS, Weber WE. Mapping of quantitative trait loci determining agronomic important characters in hexaploid wheat (Triticum aestivum L.). Theoretical and Applied Genetics (2002) 105:921–936.[CrossRef][Web of Science][Medline]
Byrt CS, Platten DJ, Spielmeyer W, James RA, Lagudah ES, Dennis ES, Tester M, Munns R. HKT1;5-like cation transporters linked to Na+ exclusion loci in wheat, Nax2 and Kna1. Plant Physiology (2007) 143:1918–1928.
Cakmak I, Cakmak O, Eker S, Ozdemir A, Watanabe N, Braun HJ. Expression of high zinc efficiency of Aegilops tauschii and Triticum monococcum in synthetic hexaploid wheats. Plant and Soil (1999) 215:203–209.[CrossRef][Web of Science]
Castagna R, Maga G, Perenzin M, Heun M, Salamini F. RFLP-based genetic relationships of Einkorn wheats. Theoretical and Applied Genetics (1994) 88:818–823.[CrossRef][Web of Science]
Chisholm ST, Coaker G, Day B, Staskawicz BJ. Host–microbe interactions: shaping the evolution of the plant immune response. Cell (2006) 124:803–814.[CrossRef][Web of Science][Medline]
Cox TS, Harrell LG, Chen P, Gill BS. Reproductive behavior of hexaploid diploid wheat hybrids. Plant Breeding (1991) 107:105–118.[CrossRef][Web of Science]
Deol GS, Wilde GE, Gill BS. Host plant resistance in some wild wheats to the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae). Plant Breeding (1995) 114:545–546.[CrossRef][Web of Science]
Devos KM, Dubcovsky J, Dvorak J, Chinoy CN, Gale MD. Structural evolution of wheat chromosomes 4A, 5A, and 7B and its impact on recombination. Theoretical and Applied Genetics (1995) 91:282–288.[CrossRef][Web of Science]
Dubcovsky J, Luo M, Dvorak J. Differentiation between homoeologous chromosomes 1A of wheat and 1Am of Triticum monococcum and its recognition by the wheat Ph1 locus. Proceedings of the National Academy of Sciences, USA (1995) 92:6645–6649.
Dubcovsky J, Luo MC, Zhong GY, Bransteitter R, Desai A, Kilian A, Kleinhofs A, Dvorak J. Genetic map of diploid wheat, Triticum monococcum L. and its comparison with maps of Hordeum vulgare L. Genetics (1996) 143:983–999.[Abstract]
Dvorak J, Diterlizzi P, Zhang HB, Resta P. The evolution of polyploid wheats—identification of the A-genome donor species. Genome (1993) 36:21–31.[Medline]
Dvorak J, Yang ZL, You FM, Luo MC. Deletion polymorphism in wheat chromosome regions with contrasting recombination rates. Genetics (2004) 168:1665–1675.
Feuillet C, Travella S, Stein N, Albar L, Nublat A, Keller B. Map-based isolation of the leaf rust disease resistance gene Lr10 from the hexaploid wheat (Triticum aestivum L.) genome. Proceedings of the National Academy of Sciences, USA (2003) 100:15253–15258.
Fregeau-Reid J, Abdel-Aal E-SM. Einkorn: a potential functional wheat and genetic resource. In: Specialty grains for food and feed—Abdel-Aal E, Wood P, eds. (2004) St Paul, MN: American Association of Cereal Chemists. 37–61.
Gautier MF, Alelman ME, Guirao A, Marion D, Joudrier P. Triticum aestivum puroindolines, two basic cysteine-rich seed proteins: cDNA sequence analysis and developmental gene expression. Plant Molecular Biology (1994) 25:43–57.[CrossRef][Web of Science][Medline]
Gupta PK, Rustgi S, Kulwal PL. Linkage disequilibrium and association studies in higher plants: present status and future prospects. Plant Molecular Biology (2005) 57:461–485.[CrossRef][Web of Science][Medline]
Hammond-Kosack KE, Parker JE. Deciphering plant–pathogen communication: fresh perspectives for molecular resistance breeding. Current Opinion in Biotechnology (2003) 14:177–193.[CrossRef][Web of Science][Medline]
Hein I, Barciszewska-Pacak M, Hrubikova K, Williamson S, Dinesen M, Soenderby IE, Sundar S, Jarmolowski A, Shirasu K, Lacomme C. Virus-induced gene silencing-based functional characterization of genes associated with powdery mildew resistance in barley. Plant Physiology (2005) 138:2155–2164.
Heun M, SchaferPregl R, Klawan D, Castagna R, Accerbi M, Borghi B, Salamini F. Site of einkorn wheat domestication identified by DNA fingerprinting. Science (1997) 278:1312–1314.[CrossRef][Web of Science]
Huang S, Sirikhachornkit A, Su X, Faris J, Gill B, Haselkorn R, Gornicki P. Genes encoding plastid acetyl-CoA carboxylase and 3-phosphoglycerate kinase of the Triticum/Aegilops complex and the evolutionary history of polyploid wheat. Proceedings of the National Academy of Sciences, USA (2002) 99:8133–8138.
Hussien T, Bowden RL, Gill BS, Cox TS. Chromosomal locations in common wheat of three new leaf rust resistance genes from Triticum monococcum. Euphytica (1998) 101:127–131.[CrossRef][Web of Science]
James RA, Davenport RJ, Munns R. Physiological characterisation of two genes for Na+ exclusion in durum wheat: Nax1 and Nax2. Plant Physiology (2006) 142:1537–1547.
Jing HC, Lovell D, Kornyukhin D, Kanyuka K, Tearall K, Phillips A, Orford S, Koebner R, Mitrofanova OP, Hammond-Kosack KE. New approaches for durable disease resistance in wheat. BCPC International Congress & Exhibition—Crop Science and Technology (2005) 963–970.
Kanyuka K, Lovell DJ, Mitrofanova OP, Hammond-Kosack K, Adams MJ. A controlled environment test for resistance to soil-borne cereal mosaic virus (SBCMV) and its use to determine the mode of inheritance of resistance in wheat cv. Cadenza and for screening Triticum monococcum genotypes for sources of SBCMV resistance. Plant Pathology (2004) 53:154–160.[CrossRef][Web of Science]
Keller B, Feuillet C, Yahiaoui N. Map-based isolation of disease resistance genes from bread wheat: cloning in a supersize genome. Genetical Research (2005) 85:93–100.[CrossRef][Web of Science][Medline]
Keon J, Antoniw J, Carzaniga R, Deller S, Ward JL, Baker JM, Beale MH, Hammond-Kosack K, Rudd JJ. Transcriptional adaptation of Mycosphaerella graminicola to programmed cell death (PCD) of its susceptible wheat host. Molecular Plant–Microbe Interaction (2007) 20:178–193.[CrossRef]
Kison HU, Neumann M. The introgression of genetic information from Triticum monococcum L into hexaploid triticale by hybridization with a T monococcumxSecale cereale amphiploid. 2. Stabilizing basic material after passing an intermediate octoploid stage. Plant Breeding (1993) 110:283–289.[CrossRef][Web of Science]
Klindworth DL, Williams ND. Interspecific hybridization of a multiploid mutant of durum wheat with rye and Triticum monococcum L. results in pentaploid hybrids. Plant Breeding (2003) 122:213–216.[CrossRef][Web of Science]
Korzun V. Use of molecular markers in cereal breeding. Cellular and Molecular Biology Letters (2002) 7:811–820.[Medline]
Korzun V, Malyshev S, Kartel N, Westermann T, Weber WE, Börner A. A genetic linkage map of rye (Secale cereale L.). Theoretical and Applied Genetics (1998a) 96:203–208.[CrossRef][Web of Science]
Korzun V, Malyshev S, Pickering RA, Börner A. RFLP mapping of a gene for hairy leaf sheath using a recombinant line from Hordeum vulgare L.xHordeum bulbosum L. cross. Genome (1999) 42:960–963.
Korzun V, Röder M, Ganal M, Hammer K, Filatenko A. Genetic diversity and evolution of the diploid wheats T. urartu, T. boeoticum and T. monococcum revealed by microsatellite markers. Schriften zu Genetischen Ressourcen (1998b) 8:244–247.
Lebedeva TV, Peusha HO. Genetic control of the wheat Triticum monococcum L. resistance to powdery mildew. Russian Journal of Genetics (2006) 42:60–66.[CrossRef][Web of Science]
Lijavetzky D, Muzzi G, Wicker T, Keller B, Wing R, Dubcovsky J. Construction and characterization of a bacterial artificial chromosome (BAC) library for the A genome of wheat. Genome (1999) 42:1176–1182.[Medline]
Loje H, Moller B, Laustsen AM, Hansen A. Chemical composition, functional properties and sensory profiling of einkorn (Triticum monococcum L.). Journal of Cereal Science (2003) 37:231–240.[CrossRef][Web of Science]
Luo M-C, Deal KR, Yang Z-L, Dvorak J. Comparative genetic maps reveal extreme crossover localization in the Aegilops speltoides chromosomes. Theoretical and Applied Genetics (2005) 111:1098–1106.[CrossRef][Web of Science][Medline]
Malysheva-Otto LV, Ganal MW, Roder MS. Analysis of molecular diversity, population structure and linkage disequilibrium in a worldwide survey of cultivated barley germplasm (Hordeum vulgare L.). BMC Genetics (2006) 7:6.[CrossRef][Medline]
McFadden HC, Lehmensiek A, Lagudah ES. Resistance gene analogues of wheat: molecular genetic analysis of ESTs. Theoretical and Applied Genetics (2006) 113:987–1002.[CrossRef][Web of Science][Medline]
Meyers BC, Dickerman AW, Michelmore RW, Sivaramakrishnan S, Sobral BW, Young ND. Plant disease resistance genes encode members of an ancient and diverse protein family within the nucleotide-binding superfamily. The Plant Journal (1999) 20:317–332.[Web of Science][Medline]
Migui SM, Lamb RJ. Seedling and adult plant resistance to Sitobion avenae (Hemiptera: Aphididae) in Triticum monococcum (Poaceae), an ancestor of wheat. Bulletin of Entomological Research (2004) 94:35–46.[CrossRef][Web of Science][Medline]
Multani DS, Dhaliwal HS, Singh P, Gill KS. Synthetic amphiploids of wheat as a source of resistance to Karnal Bunt (Neovossia indica). Plant Breeding (1988) 101:122–125.[CrossRef][Web of Science]
Potgieter GF, Marais GF, Dutoit F. The transfer of resistance to the Russian wheat aphid from Triticum monococcum L. to common wheat. Plant Breeding (1991) 106:284–292.[CrossRef][Web of Science]
Reif JC, Zhang P, Dreisigacker S, Warburton ML, van Ginkel M, Hoisington D, Bohn M, Melchinger AE. Wheat genetic diversity trends during domestication and breeding. Theoretical and Applied Genetics (2005) 110:859–864.[CrossRef][Web of Science][Medline]
Reynolds MP, Borlaug NE. Impacts of breeding on international collaborative wheat improvement. Journal of Agricultural Science (2006) 144:3–17.[CrossRef]
Rogers WJ, Miller TE, Payne PI, Seekings JA, Sayers EJ, Holt LM, Law CN. Introduction to bread wheat (Triticum aestivum L.) and assessment for bread-making quality of alleles from T-boeoticum Boiss ssp thaoudar at Glu-A1 encoding two high-molecular-weight subunits of glutenin. Euphytica (1997) 93:19–29.[CrossRef][Web of Science]
Roy JK, Bandopadhyay R, Rustgi S, Balyan HS, Gupta PK. Association analysis of agronomically important traits using SSR, SAMPL and AFLP markers in bread wheat. Current Science (2006) 90:683–389.[Web of Science]
Salamini F, Ozkan H, Brandolini A, Schafer-Pregl R, Martin W. Genetics and geography of wild cereal domestication in the near east. Nature Reviews Genetics (2002) 3:429–441.[Web of Science][Medline]
Scofield SR, Huang L, Brandt AS, Gill BS. Development of a virus-induced gene-silencing system for hexaploid wheat and its use in functional analysis of the Lr21-mediated leaf rust resistance pathway. Plant Physiology (2005) 138:2165–2173.
See DR, Giroux M, Gill BS. Effect of multiple copies of puroiudoline genes on grain softness. Crop Science (2004) 44:1248–1253.
Sharma HC, Ohm HW, Patterson FL, Benlhabib Q, Cambron S. Genetics of resistance to Hessian fly (Mayetiola destructor) (Diptera: Cecidomyiidae) biotype L in diploid wheats. Phytoprotection (1997) 78:61–65.[Web of Science]
Shewry PR, Halford NG. Cereal seed storage proteins: structures, properties and role in grain utilization. Journal of Experimental Botany (2002) 53:947–958.
Shewry PR, Powers S, Field JM, et al. Comparative field performance over 3 years and two sites of transgenic wheat lines expressing HMW subunit transgenes. Theoretical and Applied Genetics (2006) 113:128–136.[CrossRef][Web of Science][Medline]
Shi AN, Leath S, Murphy JP. A major gene for powdery mildew resistance transferred to common wheat from wild einkorn wheat. Phytopathology (1998) 88:144–147.[Medline]
Slade AJ, Fuerstenberg SI, Loeffler D, Steine MN, Facciotti D. A reverse genetic, nontransgenic approach to wheat crop improvement by TILLING. Nature Biotechnology (2005) 23:75–81.[CrossRef][Web of Science][Medline]
Sodkiewicz W, Apolinarska B. Development of secondary tetraploid triticale with a complete A-genome through crossing primary amphiploids with secondary hexaploid triticale. Cereal Research Communications (2000) 28:49–56.[Web of Science]
Sodkiewicz W. Diploid wheat: Triticum monococcum as a source of resistance genes to preharvest sprouting of triticale. Cereal Research Communications (2002) 30:323–328.[Web of Science]
Sodkiewicz W, Strzembicka A. Application of Triticum monococcum for the improvement of triticale resistance to leaf rust (Puccinia triticina). Plant Breeding (2004) 123:39–42.[CrossRef][Web of Science]
Sourdille P, Perretant MR, Charmet G, Leroy P, Gautier MF, Joudrier P, Nelson JC, Sorrells ME, Bernard M. Linkage between RSLP markers and genes affecting kernel hardness in wheat. Theoretical and Applied Genetics (1996) 93:580–586.[CrossRef][Web of Science]
Stein N, Feuillet C, Wicker T, Schlagenhauf E, Keller B. Subgenome chromosome walking in wheat: a 450-kb physical contig in Triticum monococcum L. spans the Lr10 resistance locus in hexaploid wheat (Triticum aestivum L. Proceedings of the National Academy of Sciences, USA (2000) 97:13436–13441.
Taketa S, Chang CL, Ishii M, Takeda K. Chromosome arm location of the gene controlling leaf pubescence of a Chinese local wheat cultivar ‘Hong-mang-mai’. Euphytica (2002) 125:141–147.[CrossRef][Web of Science]
Tranquilli G, Cuniberti M, Gianibelli MC, Bullrich L, Larroque OR, MacRitchie F, Dubcovsky J. Effect of Triticum monococcum glutenin loci on cookie making quality and on predictive tests for bread making quality. Journal of Cereal Science (2002a) 36:9–18.[CrossRef][Web of Science]
Tranquilli G, Heaton J, Chicaiza O, Dubcovsky J. Substitutions and deletions of genes related to grain hardness in wheat and their effect on grain texture. Crop Science (2002b) 42:1812–1817.
Tranquilli G, Lijavetzky D, Muzzi G, Dubcovsky J. Genetic and physical characterization of grain texture-related loci in diploid wheat. Molecular and General Genetics (1999) 262:846–850.[CrossRef][Web of Science]
Turner AS, Bradburne RP, Fish L, Snape JW. New quantitative trait loci influencing grain texture and protein content in bread wheat. Journal of Cereal Science (2004) 40:51–60.[CrossRef][Web of Science]
Uauy C, Distelfeld A, Fahima T, Blechl A, Dubcovsky J. A NAC gene regulating senescence improves grain protein, zinc, and iron content in wheat. Science (2006) 314:1298–1301.
Valkoun JJ. Wheat pre-breeding using wild progenitors. Euphytica (2001) 119:17–23.[CrossRef][Web of Science]
van der Linden CG, Wouters DC, Mihalka V, Kochieva EZ, Smulders MJ, Vosman B. Efficient targeting of plant disease resistance loci using NBS profiling. Theoretical and Applied Genetics (2004) 109:384–393.[Web of Science][Medline]
Vasu K, Singh H, Singh S, Chhuneja P, Dhaliwal HS. Microsatellite marker linked to a leaf rust resistance gene from Triticum monococcum L transferred to bread wheat. Journal of Plant Biochemistry and Biotechnology (2001) 10:127–132.[Web of Science]
Ward E, Kanyuka K, Motteram J, Kornyukhin D, Adams MJ. The use of conventional and quantitative real-time PCR assays for Polymyxa graminis to examine host plant resistance, inoculum levels and intraspecific variation. New Phytologist (2005) 165:875–885.[CrossRef][Web of Science][Medline]
Yahiaoui N, Srichumpa P, Dudler R, Keller B. Genome analysis at different ploidy levels allows cloning of the powdery mildew resistance gene Pm3b from hexaploid wheat. The Plant Journal (2004) 37:528–538.[CrossRef][Web of Science][Medline]
Yan L, Loukoianov A, Tranquilli G, Helguera M, Fahima T, Dubcovsky J. Positional cloning of the wheat vernalization gene VRN1. Proceedings of the National Academy of Sciences, USA (2003) 100:6263–6268.
Zohary D, Hopf M. Domestication of plants in the Old World: the origin and spread of cultivated plants in West Asia, Europe, and the Nile Valley (1993) 2nd edn. Oxford: Oxford University Press.
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