Lr70, a new gene for leaf rust resistance mapped in common wheat accession KU3198

Key message

KU3198 is a common wheat accession that carries one novel leaf rust resistance (Lr) gene, Lr70 , and another Lr gene which is either novel, Lr52 or an allele of Lr52.

Abstract

Leaf rust, caused by Puccinia triticina Eriks. (Pt), is a broadly distributed and economically important disease of wheat. Deploying cultivars carrying effective leaf rust resistance (Lr) genes is a desirable method of disease control. KU3198 is a common wheat (Triticum aestivum L.) accession from the Kyoto collection that was highly resistant to Pt in Canada. An F₂ population from the cross HY644/KU3198 showed segregation for two dominant Lr genes when tested with Pt race MBDS which was virulent on HY644. Multiple bulk segregant analysis (MBSA) was employed to find putative chromosome locations of these Lr genes using SSR markers that provided coverage of the genome. MBSA predicted that the Lr genes were located on chromosomes 5B and 5D. A doubled haploid population was generated from the cross of JBT05-714 (HY644*3/KU3198), a line carrying one of the Lr genes from KU3198, to Thatcher. This population segregated for a single Lr gene conferring resistance to Pt race MBDS, which was mapped to the terminal region of the short arm of chromosome 5B with SSR markers and given the temporary designation LrK1. One F₃ family derived from the HY644/KU3198 F₂ population that segregated only for the second Lr gene from KU3198 was identified. This family was treated as an F₂-equivalent population and used for mapping the Lr gene, which was located to the terminal region of chromosome 5DS. As no other Lr gene has been mapped to 5DS, this gene is novel and has been designated as Lr70.     

Inheritance of resistance to Ug99 stem rust in wheat cultivar Norin 40 and genetic mapping of Sr42

Stem rust, caused by Puccinia graminis f. sp. tritici, is a devastating disease of wheat. The emergence of race TTKSK (Ug99) and new variants in Africa threatens wheat production worldwide. The best method of controlling stem rust is to deploy effective resistance genes in wheat cultivars. Few stem rust resistance (Sr) genes derived from the primary gene pool of wheat confer resistance to TTKSK. Norin 40, which carries Sr42, is resistant to TTKSK and variants TTKST and TTTSK. The goal of this study was to elucidate the inheritance of resistance to Ug99 in Norin 40 and map the Sr gene(s). A doubled haploid (DH) population of LMPG-6/Norin 40 was evaluated for resistance to the race TTKST. Segregation of 248 DH lines fitted a 1:1 ratio (χ (2) 1:1= 0.58, p = 0.45), indicating a single gene in Norin 40 conditioned resistance to Ug99. This was confirmed by an independent F(2:3) population also derived from the cross LMPG-6/Norin 40 where a 1:2:1 ratio (χ (2)1:2:1 = 0.69, p = 0.71) was observed following the inoculation with race TTKSK. Mapping with DNA markers located this gene to chromosome 6DS, the known location of Sr42. PCR marker FSD_RSA co-segregated with Sr42, and simple sequence repeat (SSR) marker BARC183 was closely linked (0.5 cM) to Sr42. A previous study found close linkage between FSD_RSA and SrCad, a temporarily designated gene that also confers resistance to Ug99, thus Sr42 may be the same gene or allelic. Marker FSD_RSA is suitable for marker-assisted selection (MAS) in wheat breeding programs to improve stem rust resistance, including Ug99.     

Genetic mapping of a major gene in triticale conferring resistance to bacterial leaf streak

Key message

A major gene conferring resistance to bacterial leaf streak was mapped to chromosome 5R in triticale.

Abstract

Bacterial leaf streak (BLS), caused by Xanthomonas translucens pv. undulosa (Xtu), is an important disease of wheat and triticale around the world. Although resistance to BLS is limited in wheat, several triticale accessions have high levels of resistance. To characterize the genetic basis of this resistance, we developed triticale mapping populations using a resistant accession (Siskiyou) and two susceptible accessions (UC38 and Villax St. Jose). Bulked segregant analysis in an F₂ population derived from the cross of Siskiyou × UC38 led to the identification of a simple sequence repeat (SSR) marker (XSCM138) on chromosome 5R that co-segregated with the resistance gene. The cross of Siskiyou × Villax St. Jose was advanced into an F_2:5 recombinant inbred line population and evaluated for BLS reaction. Genetic linkage maps on this population were assembled with markers generated using genotyping-by-sequencing as well as several SSR markers previously identified on 5R. Quantitative trait locus (QTL) mapping revealed a single major QTL on chromosome 5R, underlined by the same SSR marker as in the Siskiyou × UC38 population. The F₁ hybrids of the two crosses were highly resistant to BLS, indicating that resistance is largely dominant. This work will facilitate introgression of this rye-derived BLS resistance gene into the wheat genome by molecular marker-mediated chromosome engineering.

     

Genetics and mapping of stem rust resistance to Ug99 in the wheat cultivar Webster

New races of wheat stem rust, namely TTKSK (Ug99) and its variants, pose a threat to wheat production in the regions where they are found. The accession of the wheat cultivar Webster (RL6201) maintained at the Cereal Research Centre in Winnipeg, Canada, shows resistance to TTKSK and other races of stem rust. The purpose of this study was to study the inheritance of seedling resistance to stem rust in RL6201 and genetically map the resistance genes using microsatellite (SSR) markers. A population was produced by crossing the stem rust susceptible line RL6071 with Webster. The F₂ and F₃ were tested with TPMK, a stem rust race native to North America. The F₃ was also tested with TTKSK. Two independently assorting genes were identified in RL6201. Resistance to TPMK was conferred by Sr30, which was mapped with microsatellites on chromosome 5DL. The second gene, temporarily designated SrWeb, conferred resistance to TTKSK. SrWeb was mapped to chromosome 2BL using SSR markers. Comparison with previous genetic maps showed that SrWeb occupies a locus near Sr9. Further analysis will be required to determine if SrWeb is a new gene or an allele of a previously identified gene.     

Simultaneous transfer, introgression, and genomic localization of genes for resistance to stem rust race TTKSK (Ug99) from Aegilops tauschii to wheat

Wheat production is currently threatened by widely virulent races of the wheat stem rust fungus, Puccinia graminis f. sp. tritici, that are part of the TTKSK (also known as ‘Ug99’) race group. The diploid D genome donor species Aegilops tauschii (2n = 2x = 14, DD) is a readily accessible source of resistance to TTKSK and its derivatives that can be transferred to hexaploid wheat, Triticum aestivum (2n = 6x = 42, AABBDD). To expedite transfer of TTKSK resistance from Ae. tauschii, a direct hybridization approach was undertaken that integrates gene transfer, mapping, and introgression into one process. Direct crossing of Ae. tauschii accessions with an elite wheat breeding line combines the steps of gene transfer and introgression while development of mapping populations during gene transfer enables the identification of closely linked markers. Direct crosses were made using TTKSK-resistant Ae. tauschii accessions TA1662 and PI 603225 as males and a stem rust-susceptible T. aestivum breeding line, KS05HW14, as a female. Embryo rescue enabled recovery of F₁ (ABDD) plants that were backcrossed as females to the hexaploid recurrent parent. Stem rust-resistant BC₁F₁ plants from each Ae. tauschii donor source were used as males to generate BC₂F₁ mapping populations. Bulked segregant analysis of BC₂F₁ genotypes was performed using 70 SSR loci distributed across the D genome. Using this approach, stem rust resistance genes from both accessions were located on chromosome arm 1DS and mapped using SSR and EST-STS markers. An allelism test indicated the stem rust resistance gene transferred from PI 603225 is Sr33. Race specificity suggests the stem rust resistance gene transferred from TA1662 is unique and this gene has been temporarily designated SrTA1662. Stem rust resistance genes derived from TA1662 and PI 603225 have been made available with selectable molecular markers in genetic backgrounds suitable for stem rust resistance breeding.     

Development and characterization of wheat lines carrying stem rust resistance gene Sr43 derived from Thinopyrum ponticum

Key message

Wheat lines carrying Ug99-effective stem rust resistance gene Sr43 on shortened alien chromosome segments were produced using chromosome engineering, and molecular markers linked to Sr43 were identified for marker-assisted selection.

Abstract

Stem rust resistance gene Sr43, transferred into common wheat (Triticum aestivum) from Thinopyrum ponticum, is an effective gene against stem rust Ug99 races. However, this gene has not been used in wheat breeding because it is located on a large Th. ponticum 7el₂ chromosome segment, which also harbors genes for undesirable traits. The objective of this study was to eliminate excessive Th. ponticum chromatin surrounding Sr43 to make it usable in wheat breeding. The two original translocation lines KS10-2 and KS24-1 carrying Sr43 were first analyzed using simple sequence repeat (SSR) markers and florescent genomic in situ hybridization. Six SSR markers located on wheat chromosome arm 7DL were identified to be associated with the Th. ponticum chromatin in KS10-2 and KS24-1. The results confirmed that KS24-1 is a 7DS·7el₂L Robertsonian translocation as previously reported. However, KS10-2, which was previously designated as a 7el₂S·7el₂L-7DL translocation, was identified as a 7DS-7el₂S·7el₂L translocation. To reduce the Th. ponticum chromatin carrying Sr43, a BC₂F₁ population (Chinese Spring//Chinese Spring ph1bph1b*2/KS10-2) containing ph1b-induced homoeologous recombinants was developed, tested with stem rust, and genotyped with the six SSR markers identified above. Two new wheat lines (RWG33 and RWG34) carrying Sr43 on shortened alien chromosome segments (about 17.5 and 13.7 % of the translocation chromosomes, respectively) were obtained, and two molecular markers linked to Sr43 in these lines were identified. The new wheat lines with Sr43 and the closely linked markers provide new resources for improving resistance to Ug99 and other races of stem rust in wheat.     

Molecular mapping of Yr53, a new gene for stripe rust resistance in durum wheat accession PI 480148 and its transfer to common wheat

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most damaging diseases of wheat worldwide. It is essential to identify new genes for effective resistance against the disease. Durum wheat PI 480148, originally from Ethiopia, was resistant in all seedling tests with several predominant Pst races in the US under controlled greenhouse conditions and at multiple locations subject to natural infection for several years. To map the resistance gene(s) and to transfer it to common wheat, a cross was made between PI 480148 and susceptible common wheat genotype Avocet S (AvS). Resistant F₃ plants with 42 chromosomes were selected cytologically and by testing with Pst race PST-100. A total of 157 F₄ plants from a single F₃ plant with 2n = 42 tested with PST-100 segregated in a 3 resistant: 1 susceptible ratio, indicating that a single dominant gene from PI 480148 conferred resistance. Using the F_3:4 population and the resistance gene-analog polymorphism (RGAP) and simple sequence repeat (SSR) markers, the gene was mapped to the long arm of chromosome 2B. SSR marker Xwmc441 and RGAP marker XLRRrev/NLRRrev ₃₅₀ flanked the resistance gene by 5.6 and 2.7 cM, respectively. The effective resistance of the gene to an Australian Pst isolate virulent to Yr5, which is also located on 2BL and confers resistance to all US Pst races, together with an allelism test of the two genes, indicated that the gene from PI 480148 is different from Yr5 and should be a new and useful gene for resistance to stripe rust. Resistant common wheat lines with plant types similar to AvS were selected for use in breeding programs.     

Fine genetic mapping of a locus controlling short internode length in melon (Cucumis melo L.)

Compact and dwarfing vining habits in melon (Cucumis melo L.; 2n = 2x = 24) may have commercial importance since they can contribute to the promotion of concentrated fruit set and can be planted in higher plant densities than standard vining types. A study was designed to determine the genetics of dwarfism associated with a diminutive (short internodes) melon mutant line PNU-D1 (C. melo ssp. cantalupensis). PNU-D1 was crossed with inbred wild-type melon line PNU-WT1 (C. melo ssp. agrestis), and resultant F₁ progeny were then self-pollinated to produce an F₂ population that segregated as dwarf and vining plant types. Primary stem length of F₂ progeny assessed under greenhouse conditions indicated that a single recessive gene, designated mdw1, controlled dwarfism in this population. To identify the chromosomal location associated with mdw1, an simple sequence repeat (SSR)-based genetic linkage map was constructed using 94 F₂ progeny. Using 76 SSR markers positioned on 15 linkage groups spanning 462.84 cM, the location of mdw1 was localized to Chromosome 7. Using the putative dwarfing-associated genes, fine genetic mapping of the mdw1 genomic region was facilitated with 1,194 F₂ progeny that defined the genetic distance between mdw1 and cytokinin oxidase gene, a candidate gene for compact growth habit (cp) in cucumber, to be 1.7 cM. The candidate gene ERECTA (serin/threonine kinase) and UBI (ubiquitin) were also mapped to genomic regions flanking mdw1 at distances of 0.6 and 1.2 cM, respectively.     

Identification and mapping of resistance to stem rust in the European winter wheat cultivars Spark and Rialto

A mapping population of 126 doubled haploid (DH) lines derived from a cross between the English winter wheat cultivars Spark and Rialto was evaluated for response to Puccinia graminis f. sp. tritici in the greenhouse and in artificially inoculated field plots at two locations over 3 years (2011, 2012 and 2013). Genetic analysis indicated the involvement of two seedling genes (Sr5 and Sr31, contributed by Rialto) and three adult plant resistance genes. QTL analyses of field data showed the involvement of three consistent effects QTL on chromosome arms 1BS (contributed by Rialto), and 3BS and chromosome 5A (contributed by Spark) in the observed resistance to stem rust. These QTLs explained average phenotypic variation of 78.5, 9.0 and 5.9 %, respectively. With the presence of virulence for Sr5 and absence of Sr31 virulence in the field, the QTL detected on 1BS (QSr.sun-1BS) was attributed to the major seedling resistance gene Sr31. The QTL located on chromosome arm 3BS (QSr.sun-3BS) was closely associated with SSR marker gwm1034, and the QTL detected on 5A (QSr.sun-5A) was closely linked with SSR marker gwm443. DH lines carrying the combination of QSr.sun-3BS and QSr.sun-5A exhibited lower stem rust responses indicating the additive effects of the two APR genes in reducing disease severity. The markers identified in this study can be useful in pyramiding these QTLs with other major or minor genes and marker assisted selection for stem rust resistance in wheat.     

Molecular mapping of an adult plant stem rust resistance gene Sr56 in winter wheat cultivar Arina

Key message

This article covers detailed characterization and naming of QSr.sun - 5BL as Sr56 . Molecular markers linked with adult plant stem rust resistance gene Sr56 were identified and validated for marker-assisted selection.

Abstract

The identification of new sources of adult plant resistance (APR) and effective combinations of major and minor genes is well appreciated in breeding for durable rust resistance in wheat. A QTL, QSr.sun-5BL, contributed by winter wheat cultivar Arina providing 12–15 % reduction in stem rust severity, was reported in an Arina/Forno recombinant inbred line (RIL) population. Following the demonstration of monogenic segregation for APR in the Arina/Yitpi RIL population, the resistance locus was formally named Sr56. Saturation mapping of the Sr56 region using STS (from EST and DArT clones), SNP (9 K) and SSR markers from wheat chromosome survey sequences that were ordered based on synteny with Brachypodium distachyon genes in chromosome 1 resulted in the flanking of Sr56 by sun209 (SSR) and sun320 (STS) at 2.6 and 1.2 cM on the proximal and distal ends, respectively. Investigation of conservation of gene order between the Sr56 region in wheat and B. distachyon showed that the syntenic region defined by SSR marker interval sun209-sun215 corresponded to approximately 192 kb in B. distachyon, which contains five predicted genes. Conservation of gene order for the Sr56 region between wheat and Brachypodium, except for two inversions, provides a starting point for future map-based cloning of Sr56. The Arina/Forno RILs carrying both Sr56 and Sr57 exhibited low disease severity compared to those RILs carrying these genes singly. Markers linked with Sr56 would be useful for marker-assisted pyramiding of this gene with other major and APR genes for which closely linked markers are available.     

Molecular cytogenetic identification of B genome chromosomes linked to blackleg disease resistance in Brassica napus × B. carinata interspecific hybrids

Key message

Provide evidence that the Brassica B genome chromosome B3 carries blackleg resistance gene, and also the B genome chromosomes were inherited several generations along with B. napus chromosomes.

Abstract

Blackleg disease caused by fungus Leptosphaeria maculans causes significant yield losses in Brassica napus. Brassica carinata possesses excellent resistance to this disease. To introgress blackleg resistance, crosses between B. napus cv. Westar and B. carinata were done. The interspecific-hybrids were backcrossed twice to Westar and self-pollinated three times to produce BC₂S₃ families. Doubled haploid lines (DH1) were produced from one blackleg resistant family. SSR markers were used to study the association between B genome chromosome(s) and blackleg resistance. The entire B3 chromosome of B. carinata was associated with blackleg resistance in DH1. A second DH population (DH2) was produced from F₁s of resistant DH1 lines crossed to blackleg susceptible B. napus cv. Polo where resistance was found to be associated with SSR markers from the middle to bottom of the B3 and top of the B8 chromosomes. The results demonstrated that the B3 chromosome carried gene(s) for blackleg resistance. Genomic in situ hybridization (GISH) and GISH-like analysis of the DH2 lines revealed that susceptible lines, in addition to B. napus chromosomes, possessed one pair of B genome chromosomes (2n = 40), while resistant lines had either one (2n = 40) or two pairs (2n = 42) of B chromosomes. The molecular and GISH data suggested that the B chromosome in the susceptible lines was B7, while it was difficult to confirm the identity of the B chromosomes in the resistant lines. Also, B chromosomes were found to be inherited over several generations along with B. napus chromosomes.     

Mapping gibberellin-sensitive dwarfing locus <Emphasis Type="Italic">Rht18</Emphasis> in durum wheat and development of SSR and SNP markers for selection in breeding

Gibberellin-sensitive dwarfing gene Rht18 was mapped in two durum wheat recombinant inbred lines (RIL) populations developed from crosses, Bijaga Yellow/Icaro and HI 8498/Icaro. Rht18 was mapped within genetic interval of 1.8 cM on chromosome 6A. Simple sequence repeat (SSR) markers S470865SSR4, barc37 and TdGA2ox-A9 specific marker showed co-segregation with Rht18 in Bijaga Yellow/Icaro population consisting 256 RILs. Effect of Rht18 on plant height was validated in HI 8498/Icaro RIL population which segregated for Rht18 and Rht-B1b. Rht-B1b from HI 8498 showed pleiotropic effect on plant height and coleoptile length, on the other hand, Rht18 did not show effect on coleoptile length. The SSR and SNP markers linked to Rht18 were also validated by assessing their allelic frequency in 89 diverse durum and bread wheat accessions. It was observed that 204 bp allele of S470865SSR4 could differentiate Icaro from rest of the wheat accessions except HI 8498, suggesting its utility for selection of Rht18 in wheat improvement programs. Rht18 associated alleles of TdGA2ox-A9, IAW4371 and IAW7940 were absent in most of the tall Indian local durum wheat and bread wheat, hence could be used to transfer Rht18 to bread wheat and local durum wheat. SSR marker barc3 showed high recombination frequency with Rht18, though it showed allele unique to Icaro. Since semidwarf wheat with GA-sensitive dwarfing genes are useful in dry environments owing to their longer coleoptile, better emergence and seedling vigor, Rht18 may provide a useful alternative to widely used GA-insensitive dwarfing genes under dry environments.     

Population genetic analyses of the endangered alpine <Emphasis Type="Italic">Sinadoxa corydalifolia</Emphasis> (Adoxaceae) provide insights into future conservation

With increasing temperature and anthropogenic activity, endangered alpine species in the high altitudes of the Qinghai-Tibet Plateau face high risk of extinction; however, they have received little attention in the past. In this study, we used 12 nuclear and nine chloroplast microsatellites (simple sequence repeats, SSR) to assess genetic diversity within and among the only two populations of the highly endangered alpine species Sinadoxa corydalifolia (Adoxaceae). We identified only one individual exhibiting clonal reproduction across all 160 extant plants. The levels of genetic variability were estimated to be very low, with the allele number N_a = 3.2 and the expected heterozygosity H_e = 0.368. The genetic differentiation is extremely high between the two regional populations (F_ST = 0.214), with a limited rate of gene flow in the recent past. In addition, numerous endemic alleles were found for each subpopulation within each population. Our analyses suggest that it is critical not only to conserve all surviving individuals of the two populations in situ but also to mediate gene flow artificially between subpopulations within each population in this endangered species.     

Chromosomal location of genomic SSR markers associated with yellow rust resistance in Turkish bread wheat (Triticum aestivum L.)

We have previously reported Xgwm382 as a diagnostic marker for disease resistance against yellow rust in Izgi2001 × ES14 F₂ population. Among the same earlier tested 230 primers, one SSR marker (Xgwm311) also amplified a fragment which is present in the resistant parent and in the resistant bulks, but absent in the susceptible parent and in the susceptible bulks. To understand the chromosome group location of these diagnostic markers, Xgwm382 and Xgwm311, in the same population, we selected 16 SSR markers mapped only in one genome of chromosome group 2 around 1–21 cM distance to these diagnostic markers based on the SSR consensus map of wheat. Out of 16 SSRs, Xwmc658 identified resistant F₂ individuals as a diagnostic marker for yellow rust disease and provided the location of Xgwm382 and Xgwm311 on chromosome 2AL in our plant material.     

Genetic and comparative genomics mapping reveals that a powdery mildew resistance gene Ml3D232 originating from wild emmer co-segregates with an NBS-LRR analog in common wheat (Triticum aestivum L.)

Powdery mildew caused by Blumeria graminis f. sp. tritici is one of the most important wheat diseases worldwide and breeding for resistance using diversified disease resistance genes is the most promising approach to prevent outbreaks of powdery mildew. A powdery mildew resistance gene, originating from wild emmer wheat (Triticum turgidum var. dicoccoides) accessions collected from Israel, has been transferred into the hexaploid wheat line 3D232 through crossing and backcrossing. Inoculation results with 21 B. graminis f. sp. tritici races indicated that 3D232 is resistant to all of the powdery mildew isolates tested. Genetic analyses of 3D232 using an F₂ segregating population and F₃ families indicated that a single dominant gene, Ml3D232, confers resistance in the host seedling stage. By applying molecular markers and bulked segregant analysis (BSA), we have identified polymorphic simple sequence repeats (SSR), expressed sequence tags (EST) and derived sequence tagged site (STS) markers to determine that the Ml3D232 is located on chromosome 5BL bin 0.59–0.76. Comparative genetic analyses using mapped EST markers and genome sequences of rice and Brachypodium established co-linearity of the Ml3D232 genomic region with a 1.4 Mb genomic region on Brachypodium distachyon chromosome 4, and a 1.2 Mb contig located on the Oryza sativa chromosome 9. Our comparative approach enabled us to develop new EST–STS markers and to delimit the genomic region carrying Ml3D232 to a 0.8 cM segment that is collinear with a 558 kb region on B. distachyon. Eight EST markers, including an NBS-LRR analog, co-segregated with Ml3D232 to provide a target site for fine genetic mapping, chromosome landing and map-based cloning of the powdery mildew resistance gene. This newly developed common wheat germplasm provides broad-spectrum resistance to powdery mildew and a valuable resource for wheat breeding programs.     

Genome-wide association analysis identified SNPs closely linked to a gene resistant to Soil-borne wheat mosaic virus

Key message

Using association and linkage mapping, two SNP markers closely linked to the SBWMV resistance gene on chromosome 5D were identified and can be used to select the gene in breeding.

Abstract

Soil-borne wheat mosaic virus (SBWMV) disease is a serious viral disease of winter wheat growing areas worldwide. SBWMV infection can significantly reduce grain yield up to 80 %. Developing resistant wheat cultivars is the only feasible strategy to reduce the losses. In this study, wheat Infinium iSelect Beadchips with 9 K wheat SNPs were used to genotype an association mapping population of 205 wheat accessions. Six new SNPs from two genes were identified to be significantly associated with the gene for SBWMV resistance on chromosome 5D. The SNPs and Xgwm469, an SSR marker that has been reported to be associated with the gene, were mapped close to the gene using F₆-derived recombinant inbred lines from the cross between a resistant parent ‘Heyne’ and a susceptible parent ‘Trego’. Two representative SNPs, wsnp_CAP11_c209_198467 and wsnp_JD_c4438_5568170, from the two linked genes in wheat were converted into KBioscience Competitive Allele-Specific Polymerase assays and can be easily used in marker-assisted selection to improve wheat resistance to SBWMV in breeding.     

Earliness per se QTLs and their interaction with the photoperiod insensitive allele Ppd-D1a in the Cutler × AC Barrie spring wheat population

Earliness per se regulates flowering time independent of environmental signals and helps to fine tune the time of flowering and maturity. In this study, we aimed to map earliness per se quantitative trait loci (QTLs) affecting days to flowering and maturity in a population developed by crossing two spring wheat cultivars, Cutler and AC Barrie. The population of 177 recombinant inbred lines (RILs) was genotyped for a total of 488 SSR and DArT polymorphic markers on all 21 chromosomes. Three QTLs of earliness per se affecting days to flowering and maturity were mapped on chromosomes 1B (QEps.dms-1B1 and QEps.dms-1B2) and 5B (QEps.dms-5B1), in individual environments and when all the environments were combined. A QTL affecting flowering time (QFlt.dms-4A1) was identified on chromosome 4A. Two grain yield QTLs were mapped on chromosome 5B, while one QTL was mapped on chromosome 1D. The population segregated for the photoperiod insensitive gene, Ppd-D1a, and it induced earlier flowering by 0.69 days and maturity by 1.28 days. The photoperiod insensitive allele Ppd-D1a interacted in an additive fashion with QTLs for flowering and maturity times. The earliness per se QTL QFlt.dms-5B.1 inducing earlier flowering could help to elongate grain filling duration for higher grain yield. Hence, chromosome 5B possesses promising genomic regions that may be introgressed for higher grain yield with earlier maturity through marker-assisted selection in bread wheat.     

An intercalary translocation from <Emphasis Type="Italic">Agropyron</Emphasis><Emphasis Type="Italic">cristatum</Emphasis> 6P chromosome into common wheat confers enhanced kernel number per spike

Main conclusion

This study explored 6P chromosomal translocations in wheat, and determined the effects of 6P intercalary chromosome segments on kernel number per wheat spike. Exploiting and utilising gene(s) from wild relative species has become an essential strategy for wheat crop improvement. In the translocation line Pubing2978, the intercalary 6P chromosome segment from Agropyron cristatum (L.) Gaertn. (2n = 4x = 28, PPPP) carried valuable multi-kernel gene(s) and was selected from the offspring of the common wheat plant Fukuho and the irradiated wheat-A. cristatum 6P disomic substitution line 4844-8. Genomic in situ hybridisation (GISH), dual-colour fluorescence in situ hybridisation (FISH), and molecular markers were used to detect the small segmental 6P chromosome in the wheat background and its translocation breakpoint. Cytological studies demonstrated that Pubing2978 was a T1AS-6PL-1AS·1AL intercalary translocation with 42 chromosomes. The breakpoint was located near the centromeric region on the wheat chromosome 1AS and was flanked by the markers SSR12 and SSR283 based on an F₂ linkage map. The genotypic data, combined with the phenotypic information, implied that A. cristatum 6P chromosomal segment plays an important role in regulating the kernel number per spike (KPS). By comparison, the mean value of KPS in plants with translocations was approximately 10 higher than that in plants without translocations in three segregated populations. Moreover, the improvement in KPS was likely achieved by increasing both the spikelet number per spike (SNS) and the kernel number per spikelet. These excellent agronomic traits laid the foundation for further investigation of valuable genes and make the Pubing2978 line a promising germplasm for wheat breeding.

     

Development of an efficient method for assessing resistance to the net type of net blotch (Pyrenophora teres f. teres) in winter barley and mapping of quantitative trait loci for resistance

Breeding for resistance against Pyrenophora teres f. teres in barley is difficult due to the high virulence diversity of the pathogen and the fact that in field trials a simultaneous infection with Rhynchosporium commune, Puccinia hordei or Blumeria graminis f. sp. hordei often takes place. To avoid this, a so-called “summer hill trial” was developed in which winter barley is sown at the beginning of August at optimum conditions for P. teres infection. These trials allowed an unequivocal scoring of P. teres resistance. Using this approach, strong correlations of the results obtained in 3 years at two locations were observed and heritability was estimated at h ² = 0.80 for the doubled haploid (DH) population Uschi × HHOR3073 and h ² = 0.62 for (Post × Viresa) × HHOR9484. In parallel, genetic maps based on DArT, SSR and SNP markers were constructed, comprising 705.7 cM for the DH population Uschi × HHOR3073 and 1,035.8 cM for (Post × Viresa) × HHOR9484. In the population Uschi × HHOR3073, one quantitative trait locus (QTL) was detected on each of chromosomes 2H and 3H and two on chromosome 5H, explaining between 9.4 and 19.0 % of the phenotypic variance. In the population (Post × Viresa) × HHOR9484, three QTL were detected on chromosome 5H and one on chromosome 7H, explaining between 12.6 and 34.7 % of the phenotypic variance. These results show that the new summer hill trial design is best suited to obtain reliable phenotypic data for P. teres resistance under field conditions, as on the one hand already known QTL were confirmed and on the other hand new QTL were detected.     

Mapping and validation of simple sequence repeat markers linked to a major gene controlling seed cadmium accumulation in soybean [Glycine max (L.) Merr]

Daily consumption of cadmium (Cd) contaminated foods poses a risk to human health. Cultivar selection is an important method to limit Cd uptake and accumulation, however, analyzing grain Cd concentration is costly and time-consuming. Developing markers for low Cd accumulation will facilitate marker assisted selection (MAS). Inheritance studies using a threshold value of 0.2 mg kg^?1 for low and high and an F_2:3 population showed that low Cd accumulation in soybean seed is under the control of a major gene (Cda1, proposed name) with the allele for low accumulation being dominant. A recombinant inbred line (RIL) population (F_6:8) derived from the cross AC Hime (high Cd accumulation) and Westag-97 (low Cd accumulation) was used to identify the DNA markers linked to Cda gene(s) or quantitative trait loci (QTLs) controlling low Cd accumulation. We screened 171 simple sequence repeat (SSR) primers that showed polymorphism between parents on the 166 RILs. Of these, 40 primers were newly developed from the soybean genomic DNA sequence. Seven SSR markers, SatK138, SatK139, SatK140 (0.5 cM), SatK147, SacK149, SaatK150 and SattK152 (0.3 cM), were linked to Cda1 in soybean seed. All the linked markers were mapped to the same linkage group (LG) K. The closest flanking SSR markers linked to Cda1 were validated using a parallel population (RILs) involving Leo × Westag-97. Linked markers were also validated with diverse soybean genotypes differing in their seed Cd concentration and showed that SSR markers SatK147, SacK149, and SattK152 clearly differentiated the high and low Cd accumulating genotypes tested. To treat Cd uptake as a quantitative trait, QTL analysis using a linkage map constructed with 161 markers identified a major QTL associated with low Cd concentration in the seeds. The QTL was also mapped to the same location as Cda1 on LG-K. This QTL accounted for 57.3% of the phenotypic variation. Potential candidate genes (genes with known or predicted function that could influence the seed Cd concentration) like protein kinase, putative Adagio-like protein, and plasma membrane H⁺-ATPase were found to be located in the locus of interest. Of the four SSR markers located in the region, SattK152 was localized in the plasma membrane H⁺-ATPase gene. SSR markers closely linked to Cda1 in seeds of soybean were identified and have potential to be used for MAS to develop low Cd accumulating cultivars in a breeding program.