Chromosome 2BL Research Articles

Analysis of the expression of transcription factors and other genes associated with aleurone layer development in wheat endosperm

Defective kernel1 (dek1), crinkly4 (cr4) and supernumerary aleurone layer 1 (sal1) have been suggested as key genes involved in aleurone differentiation in cereals. Transcription factors (TFs) may play a role in regulation of aleurone development by controlling the expression of genes involved in this process. In this study, expression patterns of TFs in developing aleurone and starchy endosperm tissues of wheat was studied using RNA-Seq. Transcript profiles were obtained for aleurone and starchy endosperm tissues of the cultivar Banks at 6, 9 and 14 days post anthesis (DPA). The M-type, MIKC, MYB, CSD and NF-Y TF families were highly up-regulated, in the aleurone while in the starchy endosperm, PLATZ and Dof families were significantly up-regulated. Cr4 and sal1 homologs were identified on chromosomes 7AL, 7BL and 7DL. Dek1 was identified on chromosomes 6AL, 6BL and 6DL. Expression of dek1, cr4, sal1 and risbz1 was higher at the earlier stages of seed development. Further, we observed, sub-genome level partitioning of dek1 expression, with B and D sub-genome dek1 alleles abundantly expressed at all three stages of aleurone development. Expression of sal1 was significantly higher than that of dek1 and cr4. Aleurone development is probably regulated by several epistatic genes in wheat.

A Backcross Line of Thatcher Wheat with Adult Plant Leaf Rust Resistance Derived from Duster Wheat has Lr46 and Lr77.

The widely grown hard red winter wheat cultivar Duster released in 2006 has remained highly resistant to leaf rust caused by Puccinia triticina in the southern Great Plains of the United States. In contrast, many of the winter wheat cultivars in this region are susceptible to leaf rust. The goal of this study was to identify the number and chromosome location of leaf rust resistance genes in a line of Thatcher*2/Duster wheat that was selected for adult plant leaf rust resistance. The Thatcher*2/Duster line was crossed with Thatcher (Tc) and a recombinant line inbred line (RIL) population was advanced to the F6 generation by single-seed descent. The parents and RIL population were phenotyped for leaf rust resistance in three field plot tests and in an adult plant greenhouse test. Single-nucleotide polymorphism (SNP) markers derived from the Illumina Infinium iSelect 90K wheat SNP array, kompetitive allele-specific polymerase chain reaction assays on chromosome 3BL, and a sequence tagged site (STS) marker on chromosome 1BL were used to construct a genetic map of the RIL population. The STS marker csLV46G22 that is linked with resistance gene Lr46 on chromosome 1BL, and SNP marker IWB10344 that is linked with Lr77 on chromosome 3BL, were significantly associated with lower leaf rust severity. Duster has at least three adult plant resistance genes for leaf rust resistance because it was previously determined to also have the adult plant resistance gene Lr34. Duster is a valuable source of durable leaf rust resistance for hard red winter wheat improvement in the Great Plains region.

Genetic and transcriptional variations in NRAMP-2 and OPAQUE1 genes are associated with salt stress response in wheat

Key messageSNP alleles on chromosomes 4BL and 6AL are associated with sensitivity to salt tolerance in wheat and upon validation can be exploited in the development of salt-tolerant wheat varieties.The dissection of the genetic and molecular components of salt stress response offers strong opportunities toward understanding and improving salt tolerance in crops. In this study, GWAS was employed to identify a total of 106 SNP loci (R2 = 0.12–63.44%) linked to salt stress response in wheat using leaf chlorophyll fluorescence, grain quality and shoot ionic (Na+ and K+ ions) attributes. Among them, 14 SNP loci individually conferred pleiotropic effects on multiple independent salinity tolerance traits including loci at 99.04 cM (R2 ≥ 14.7%) and 68.45 cM (R2 ≥ 4.10%) on chromosomes 6AL and 4BL, respectively, that influenced shoot Na+-uptake, shoot K+/Na+ ratio, and specific energy fluxes for absorption (ABS/RC) and dissipation (DIo/RC). Analysis of the open reading frame (ORF) containing the SNP markers revealed that they are orthologous to genes involved in photosynthesis and plant stress (salt) response. Further transcript abundance and qRT-PCR analyses indicated that the genes are mostly up-regulated in salt-tolerant and down-regulated in salt-sensitive wheat genotypes including NRAMP-2 and OPAQUE1 genes on 4BL and 6AL, respectively. Both genes showed highest differential expression between contrasting genotypes when expressions of all the genes within their genetic intervals were analyzed. Possible cis-acting regulatory elements and coding sequence variation that may be involved in salt stress response were also identified in both genes. This study identified genetic and molecular components of salt stress response that are associated with Na+-uptake, shoot Na+/K+ ratio, ABS/RC, DIo/RC, and grain quality traits and upon functional validation would facilitate the development of gene-specific markers that could be deployed to improve salinity tolerance in wheat.

Mapping QTL for Resistance to New Virulent Races of Wheat Stripe Rust from Two Argentinean Wheat Cultivars

During the last two decades, new virulent and aggressive races of Puccinia striiformis Westend. f. sp. tritici (Pst) have spread worldwide, causing devastating epidemics and prompting the search for new sources of resistance in wheat (Triticum aestivum L.). Between 2012 and 2017, we mapped four stripe rust resistance quantitative trait loci (QTL) effective against the Pst races present in California, USA, using recombinant inbred lines (RILs) developed from the cross between the Argentinean cultivars ‘Klein Proteo’ and ‘Klein Chajá’. The RIL population showed transgressive segregation in all six growing seasons relative to the parental lines, which showed moderate levels of Pst resistance. Analyses by year detected QTL conferring adult plant resistance on chromosomes 1BL, 2BS, 3D centromeric (from Klein Chajá), and 4DL (from Klein Proteo). QYr.ucw‐1BL, mapped in the Yr29 resistance gene region, was significant in all seasons (P < 0.01) and explained on average 31.0 to 32.8% of the observed variation. QYr.ucw‐2BS showed a stronger effect than QYr.ucw‐1BL in 2013 but was ineffective in 2014 and 2016. This QTL also conferred seedling resistance, suggesting that it is an all‐stage resistance gene. Centromeric QYr.ucw‐3D and QYr.ucw‐4DL showed smaller effects than the previous QTL and were significant only in some of the experiments. No significant interactions were detected among QTL, indicating the absence of digenic epistatic effects. The molecular markers identified in this study can be used to combine these genes and accelerate their deployment in wheat breeding programs.

Genetic analysis and location of a resistance gene for Puccinia striiformis f. sp. tritici in wheat cultivar Zhengmai 7698

Wheat stripe (yellow) rust, caused by Puccinia striiformis West. f. sp. tritici (Pst), is one of the most destructive diseases in many wheat-growing countries, especially in China, the largest stripe rust epidemic area in the world. Growing the resistant cultivars is an effective, economic and environmentally friendly way to control this disease. Wheat cultivar Zhengmai 7698 has shown a high-level resistance to wheat stripe rust. To elucidate its genetic characteristics and location of the resistance gene, Zhengmai 7698 was crossed with susceptible variety Taichung 29 to produce F1, F2 and BC1 progeny generations. The genetic analysis showed that the stripe rust resistance in Zhengmai 7698 to Pst predominant race CYR32 was controlled by a single-dominant gene, named YrZM. Bulked segregant analysis and simple sequence repeat (SSR) markers were used to map the gene. Four SSR markers, Xbarc198, Xwmc179, Xwmc786 and Xwmc398 on chromosome 6BL were polymorphic between the parents and resistance, and susceptible bulks. A linkage genetic map was constructed using 212 F2 plants in the sequential order of Xwmc398, Xwmc179, YrZM, Xbarc198, Xwmc786. As this gene is effective against predominant race CYR32, it is useful in combination with other resistance genes for developing new wheat cultivars with resistance to stripe rust.

Mapping a gene on wheat chromosome 4BL involved in a complementary interaction with adult plant leaf rust resistance gene LrSV2.

A complementary gene to LrSV2 for specific adult plant leaf rust resistance in wheat was mapped on chromosome 4BL, tightly linked to Lr12 / 31. LrSV2 is a race-specific adult plant leaf rust (Puccinia triticina) resistance gene on subdistal chromosome 3BS detected in the cross of the traditional Argentinean wheat (Triticum aestivum) variety Sinvalocho MA and the experimental line Gama6. The analysis of the cross of R46 [recombinant inbred line (RIL) derived from Sinvalocho MA carrying LrSV2 gene and the complementary gene Lrc-SV2 identified in the current paper] and the commercial variety Relmo Siriri (not carrying neither of these two genes) allowed the detection of the unlinked complementary gene Lrc-SV2 because the presence of one dominant allele of both is necessary to express the LrSV2-specific adult plant resistance. Lrc-SV2 was mapped within a 1-cM interval on chromosome 4BL using 100 RILs from the cross Sinvalocho MA × Purple Straw. This genetic system resembles the Lr27+31 seedling resistance reported in the Australian varieties Gatcher and Timgalen where interacting genes map at similar chromosomal positions. However, in high-resolution maps, Lr27 and LrSV2 were already mapped to adjacent intervals on 3BS and Lrc-SV2 map position on 4BL is distal to the reported Lr12/31-flanking microsatellites.

Characterization of TaDREB1 in wheat genotypes with different seed germination under osmotic stress

BackgroundThe cis-acting element DRE/CRT plays an important role in activating gene expression responsive to osmotic stress, low temperature and high-salinity. DREB1/CBF genes encode DRE-binding proteins with the function as transcript activators. TaDREB1 was also found to be induced by osmotic stress.MethodsThe dates of osmotic stress was assessed by seed germination drought resistance index; the full-length cDNA sequences of TaDREB1 gene were downloaded from NCBI datebase; identification of allelic variation and transcript expression were assessed by PCR and semi-quantitive RT-PCR analysis, respectively.ResultsTotal 13 new allele variations of TaDREB1 were identified in the germplasms tested in the paper, including 5 TaDREB1-A on chromosome 3AL, 4 TaDREB1-B on chromosome 3BL and 4 TaDREB1-D on chromosome 3DL. In each variety, there existed two loci of TaDREB1-D genes, named TaDREB1-D1 and TaDREB1-D2, both of which had the similar nucleotide sequence except an 11 bp insertion in the former. In wheat seeds under osmotic stress, we did not detect the transcript expression level of TaDREB1-A and TaDREB1-B, but that of TaDREB1-D.ConclusionsThe capacity of osmotic stress tolerance was closely correlated with the expression level and tendency of TaDREB1-D.

Development of SNP assays for hessian fly response genes, Hfr-1 and Hfr-2, for marker-assisted selection in wheat breeding

BackgroundThe Hessian fly response genes, Hfr-1 and Hfr-2, have been reported to be significantly induced in a Hessian fly attack. Nothing is known about the allelic variants of these two genes in susceptible (S) and resistant (R) wheat cultivars.ResultsBasic local alignment search tool (BLAST) analysis of Hessian fly response genes have identified three alleles of Hessian fly response gene 1 (Hfr-1) on chromosome 4AL and 7DS, and 10 alleles of Hessian fly response gene 2 (Hfr-2) on chromosome 2BS, 2DL, 4BS, 4BL, 5AL and 5BL. Resequencing exons of Hfr-1 and Hfr-2 have identified a single nucleotide polymorphism (SNP) in the lectin domain of each gene that segregates some R sources from S cultivars. Two SNP assays have been developed. The SNP883_Hfr-1 assay characterizes a ‘G/A’ SNP in Hfr-1, which differentiates 14 Hessian fly R cultivars from S ones. The SNP1294_Hfr-2 assay differentiates 12 R cultivars from S ones. Each of the two SNPs identified in Hfr-1 and Hfr-2 is ‘G/A’ and resulted in an amino acid change from isoleucine to valine in the lectin domain of the proteins of the alleles in the R cultivars. In addition to the genotype profiles of Hfr-1 and Hfr-2, generated for a set of 249 wheat cultivars which included a set of 39 R cultivars, this study has genotyped the Hessian fly response gene, HfrDrd, and the H32 gene for the wheat germplasm. Resistant cultivars from different origins with one, two, three or four resistance (R) genes in various combinations/permutations have been identified.ConclusionThis study has identified allelic differences in two Hessian fly response genes, Hfr-1 and Hfr-2, between S and R cultivars and developed one SNP assay for each of the genes. These two SNP assays for Hfr-1 and Hfr-2, together with the published assays for HfrDrd and the H32 gene, can be used for the selection and incorporation of one or more of these 4 R genes identified in the different R sources in wheat breeding programs.

Genome-Wide Linkage Mapping Reveals QTLs for Seed Vigor-Related Traits Under Artificial Aging in Common Wheat (Triticum aestivum).

Long-term storage of seeds leads to lose seed vigor with slow and non-uniform germination. Time, rate, homogeneity, and synchrony are important aspects during the dynamic germination process to assess seed viability after storage. The aim of this study is to identify quantitative trait loci (QTLs) using a high-density genetic linkage map of common wheat (Triticum aestivum) for seed vigor-related traits under artificial aging. Two hundred and forty-six recombinant inbred lines derived from the cross between Zhou 8425B and Chinese Spring were evaluated for seed storability. Ninety-six QTLs were detected on all wheat chromosomes except 2B, 4D, 6D, and 7D, explaining 2.9–19.4% of the phenotypic variance. These QTLs were clustered into 17 QTL-rich regions on chromosomes 1AL, 2DS, 3AS (3), 3BS, 3BL (2), 3DL, 4AS, 4AL (3), 5AS, 5DS, 6BL, and 7AL, exhibiting pleiotropic effects. Moreover, 10 stable QTLs were identified on chromosomes 2D, 3D, 4A, and 6B (QaMGT.cas-2DS.2, QaMGR.cas-2DS.2, QaFCGR.cas-2DS.2, QaGI.cas-3DL, QaGR.cas-3DL, QaFCGR.cas-3DL, QaMGT.cas-4AS, QaMGR.cas-4AS, QaZ.cas-4AS, and QaGR.cas-6BL.2). Our results indicate that one of the stable QTL-rich regions on chromosome 2D flanked by IWB21991 and IWB11197 in the position from 46 to 51 cM, presenting as a pleiotropic locus strongly impacting seed vigor-related traits under artificial aging. These new QTLs and tightly linked SNP markers may provide new valuable information and could serve as targets for fine mapping or markers assisted breeding.

Comparative genome-wide mapping versus extreme pool-genotyping and development of diagnostic SNP markers linked to QTL for adult plant resistance to stripe rust in common wheat

A major stripe rust resistance QTL on chromosome 4BL was localized to a 4.5-Mb interval using comparative QTL mapping methods and validated in 276 wheat genotypes by haplotype analysis. CYMMIT-derived wheat line P10103 was previously identified to have adult plant resistance (APR) to stripe rust in the greenhouse and field. The conventional approach for QTL mapping in common wheat is laborious. Here, we performed QTL detection of APR using a combination of genome-wide scanning and extreme pool-genotyping. SNP-based genetic maps were constructed using the Wheat55K SNP array to genotype a recombinant inbred line (RIL) population derived from the cross Mingxian 169 × P10103. Five stable QTL were detected across multiple environments. A fter comparing SNP profiles from contrasting, extreme DNA pools of RILs six putative QTL were located to approximate chromosome positions. A major QTL on chromosome 4B was identified in F2:4 contrasting pools from cross Zhengmai 9023 × P10103. A consensus QTL (LOD = 26-40, PVE = 42-55%), named QYr.nwafu-4BL, was defined and localized to a 4.5-Mb interval flanked by SNP markers AX-110963704 and AX-110519862 in chromosome arm 4BL. Based on stripe rust response, marker genotypes, pedigree analysis and mapping data, QYr.nwafu-4BL is likely to be a new APR QTL. The applicability of the SNP-based markers flanking QYr.nwafu-4BL was validated on a diversity panel of 276 wheat lines. The additional minor QTL on chromosomes 4A, 5A, 5B and 6A enhanced the level of resistance conferred by QYr.nwafu-4BL. Marker-assisted pyramiding of QYr.nwafu-4BL and other favorable minor QTL in new wheat cultivars should improve the level of APR to stripe rust.

Identification, mapping, and marker development of stem rust resistance genes in durum wheat ‘Lebsock’

Wheat production in many wheat-growing regions is vulnerable to stem rust, caused by Puccinia graminis f. sp. tritici (Pgt). Several previous studies showed that most of the durum cultivars adapted to the upper Great Plains in the USA have good resistance to the major Pgt pathotypes, including the Ug99 race group. To identify the stem rust resistance (Sr) genes in the durum cultivar ‘Lebsock’, a tetraploid doubled haploid (DH) population derived from a cross between Lebsock and Triticum turgidum ssp. carthlicum PI 94749 was screened with the Pgt races TTKSK, TRTTF, and TTTTF. The stem rust data and the genotypic data previously developed were used to identify quantitative trait loci (QTL) associated with resistance. We identified one QTL each on chromosome arms 4AL, 6AS, 6AL, and 2BL. Based on marker and race-specification analysis, we postulated that the QTL on 4AL, 6AS, 6AL, and 2BL correspond to Sr7a, Sr8155B1, Sr13, and likely Sr9e, respectively. The results indicated that most of the US durum germplasm adapted to the upper Great Plains likely harbors the four major Sr genes characterized in this study. Among these genes, Sr8155B1 was recently identified and shown to be unique in that it conferred susceptibility to TTKSK but resistance to variant race TTKST. Two, three, and one thermal asymmetric reverse PCR (STARP) markers were developed for Sr7a, Sr8155-B1, and Sr13, respectively. Knowledge of the Sr genes in durum germplasm and the new STARP markers will be useful to pyramid and deploy multiple Sr genes in future durum and wheat cultivars.

Characterization and mapping of leaf rust resistance in four durum wheat cultivars.

Widening the genetic basis of leaf rust resistance is a primary objective of the global durum wheat breeding effort at the International Wheat and Maize Improvement Center (CIMMYT). Breeding programs in North America are following suit, especially after the emergence of new races of Puccinia triticina such as BBG/BP and BBBQD in Mexico and the United States, respectively. This study was conducted to characterize and map previously undescribed genes for leaf rust resistance in durum wheat and to develop reliable molecular markers for marker-assisted breeding. Four recombinant inbred line (RIL) mapping populations derived from the resistance sources Amria, Byblos, Geromtel_3 and Tunsyr_2, which were crossed to the susceptible line ATRED #2, were evaluated for their reaction to the Mexican race BBG/BP of P. triticina. Genetic analyses of host reactions indicated that leaf rust resistance in these genotypes was based on major seedling resistance genes. Allelism tests among resistant parents supported that Amria and Byblos carried allelic or closely linked genes. The resistance in Geromtel_3 and Tunsyr_2 also appeared to be allelic. Bulked segregant analysis using the Infinium iSelect 90K single nucleotide polymorphism (SNP) array identified two genomic regions for leaf rust resistance; one on chromosome 6BS for Geromtel_3 and Tunsyr_2 and the other on chromosome 7BL for Amria and Byblos. Polymorphic SNPs identified within these regions were converted to kompetitive allele-specific PCR (KASP) assays and used to genotype the RIL populations. KASP markers usw215 and usw218 were the closest to the resistance genes in Geromtel_3 and Tunsyr_2, while usw260 was closely linked to the resistance genes in Amria and Byblos. DNA sequences associated with these SNP markers were anchored to the wild emmer wheat (WEW) reference sequence, which identified several candidate resistance genes. The molecular markers reported herein will be useful to effectively pyramid these resistance genes with other previously marked genes into adapted, elite durum wheat genotypes.

Mapping and characterization of the new adult plant leaf rust resistance gene Lr77 derived from Santa Fe winter wheat.

A new gene for adult plant leaf rust resistance in wheat was mapped to chromosome 3BL. This gene was designated as Lr77. 'Santa Fe' is a hard red winter cultivar that has had long-lasting resistance to the leaf rust fungus, Puccinia triticina. The objective of this study was to determine the chromosome location of the adult plant leaf rust resistance in Santa Fe wheat. A partial backcross line of 'Thatcher' (Tc) wheat with adult plant leaf rust resistance derived from Santa Fe was crossed with Thatcher to develop a Thatcher//Tc*2/Santa Fe F6 recombinant inbred line (RIL) population. The RIL population and parental lines were evaluated for segregation of leaf rust resistance in three field plot tests and in an adult plant greenhouse test. A genetic map of the RIL population was constructed using 90,000 single-nucleotide polymorphism (SNP) markers with the Illumina Infinium iSelect 90K wheat bead array. A significant quantitative trait locus for reduction of leaf rust severity in all four tests was found on chromosome 3BL that segregated as a single adult plant resistance gene. The RILs with the allele from the resistant parent for SNP marker IWB10344 had lower leaf rust severity and a moderately resistant to moderately susceptible response compared to the susceptible RILs and Thatcher. The gene derived from Santa Fe on chromosome 3BL was designated as Lr77. Kompetitive allele-specific polymerase chain reaction assay markers linked to Lr77 on 3BL should be useful for selection of wheat germplasm with this gene.

Global QTL Analysis Identifies Genomic Regions on Chromosomes 4A and 4B Harboring Stable Loci for Yield-Related Traits Across Different Environments in Wheat (Triticum aestivum L.).

Major advances in wheat production are needed to address global food insecurity under future climate conditions, such as high temperatures. The grain yield of bread wheat (Triticum aestivum L.) is a quantitatively inherited complex trait that is strongly influenced by interacting genetic and environmental factors. Here, we conducted global QTL analysis for five yield-related traits, including spike yield, yield components and plant height (PH), in the Nongda3338/Jingdong6 doubled haploid (DH) population using a high-density SNP and SSR-based genetic map. A total of 12 major genomic regions with stable QTL controlling yield-related traits were detected on chromosomes 1B, 2A, 2B, 2D, 3A, 4A, 4B, 4D, 5A, 6A, and 7A across 12 different field trials with timely sown (normal) and late sown (heat stress) conditions. Co-location of yield components revealed significant tradeoffs between thousand grain weight (TGW) and grain number per spike (GNS) on chromosome 4A. Dissection of a “QTL-hotspot” region for grain weight on chromosome 4B was helpful in marker-assisted selection (MAS) breeding. Moreover, this study identified a novel QTL for heat susceptibility index of thousand grain weight (HSITGW) on chromosome 4BL that explains approximately 10% of phenotypic variation. QPh.cau-4B.2, QPh.cau-4D.1 and QPh.cau-2D.3 were coincident with the dwarfing genes Rht1, Rht2, and Rht8, and haplotype analysis revealed their pleiotropic architecture with yield components. Overall, our findings will be useful for elucidating the genetic architecture of yield-related traits and developing new wheat varieties with high and stable yield.

Populations of doubled haploids for genetic mapping in hexaploid winter triticale

To create a framework for genetic dissection of hexaploid triticale, six populations of doubled haploid (DH) lines were developed from pairwise hybrids of high-yielding winter triticale cultivars. The six populations comprise between 97 and 231 genotyped DH lines each, totaling 957 DH lines. A consensus genetic map spans 4593.9 cM is composed of 1576 unique DArT markers. The maps reveal several structural rearrangements in triticale genomes. In preliminary tests of the populations and maps, markers specific to wheat segments of the engineered rye chromosome 1R (RM1B) were identified. Example QTL mapping of days to heading in cv. Krakowiak revealed loci on chromosomes 2BL and 2R responsible for extended vernalization requirement, and candidate genes were identified. The material is available to all parties interested in triticale genetics.

Genome-wide association mapping for resistance to leaf rust, stripe rust and tan spot in wheat reveals potential candidate genes

Key messageGenome-wide association mapping in conjunction with population sequencing map and Ensembl plants was used to identify markers/candidate genes linked to leaf rust, stripe rust and tan spot resistance in wheat.Leaf rust (LR), stripe rust (YR) and tan spot (TS) are some of the important foliar diseases in wheat (Triticum aestivum L.). To identify candidate resistance genes for these diseases in CIMMYT’s (International Maize and Wheat Improvement Center) International bread wheat screening nurseries, we used genome-wide association studies (GWAS) in conjunction with information from the population sequencing map and Ensembl plants. Wheat entries were genotyped using genotyping-by-sequencing and phenotyped in replicated trials. Using a mixed linear model, we observed that seedling resistance to LR was associated with 12 markers on chromosomes 1DS, 2AS, 2BL, 3B, 4AL, 6AS and 6AL, and seedling resistance to TS was associated with 14 markers on chromosomes 1AS, 2AL, 2BL, 3AS, 3AL, 3B, 6AS and 6AL. Seedling and adult plant resistance (APR) to YR were associated with several markers at the distal end of chromosome 2AS. In addition, YR APR was also associated with markers on chromosomes 2DL, 3B and 7DS. The potential candidate genes for these diseases included several resistance genes, receptor-like serine/threonine-protein kinases and defense-related enzymes. However, extensive LD in wheat that decays at about 5 × 107 bps, poses a huge challenge for delineating candidate gene intervals and candidates should be further mapped, functionally characterized and validated. We also explored a segment on chromosome 2AS associated with multiple disease resistance and identified seventeen disease resistance linked genes. We conclude that identifying candidate genes linked to significant markers in GWAS is feasible in wheat, thus creating opportunities for accelerating molecular breeding.

Characterisation and mapping of adult plant stripe rust resistance in wheat accession Aus27284

A new adult plant stripe rust resistance gene, Yr80, was identified in a common wheat landrace Aus27284. Linked markers were developed and validated for their utility in marker-assisted selection. Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is among the most important constraints to global wheat production. The identification and characterisation of new sources of host plant resistance enrich the gene pool and underpin deployment of resistance gene pyramids in new cultivars. Aus27284 exhibited resistance at the adult plant stage against predominant Pst pathotypes and was crossed with a susceptible genotype Avocet S. A recombinant inbred line (RIL) population comprising 121 lines was developed and tested in the field at three locations in 2016 and two in 2017 crop seasons. Monogenic segregation for adult plant stripe rust response was observed among the Aus27284/Avocet S RIL population and the underlying locus was temporarily designated YrAW11. Bulked-segregant analysis using the Infinium iSelect 90K SNP wheat array placed YrAW11 in chromosome 3B. Kompetitive allele specific PCR (KASP) primers were designed for the linked SNPs and YrAW11 was flanked by KASP_65624 and KASP_53292 (3cM) proximally and KASP_53113 (4.9cM) distally. A partial linkage map of the genomic region carrying YrAW11 comprised nine KASP and two SSR markers. The physical position of KASP markers in the pseudomolecule of chromosome 3B placed YrAW11 in the long arm and the location of markers gwm108 and gwm376 in the deletion bin 3BL2-0.22 supported this conclusion. As no other stripe rust resistance locus has been reported in chromosome 3BL, YrAW11 was formally designated Yr80. Marker KASP_ 53113 was polymorphic among 94% of 81 Australianwheat cultivars used for validation.

A new leaf rust resistance gene Lr79 mapped in chromosome 3BL from the durum wheat landrace Aus26582.

A new leaf rust resistance gene Lr79 has been mapped in the long arm of chromosome 3B and a linked marker was identified for marker-assisted selection. Aus26582, a durum wheat landrace from the A. E. Watkins Collection, showed seedling resistance against durum-specific and common wheat-specific Puccinia triticina (Pt) pathotypes. Genetic analysis using a recombinant inbred line (RIL) population developed from a cross between Aus26582 and the susceptible parent Bansi with Australian Pt pathotype showed digenic inheritance and the underlying loci were temporarily named LrAW2 and LrAW3. LrAW2 was located in chromosome 6BS and this study focused on characterisation of LrAW3 using RILs lacking LrAW2. LrAW3 was incorporated into the DArTseq map of Aus26582/Bansi and was located in chromosome 3BL. Markers linked with LrAW3 were developed from the chromosome survey sequence contig 3B_10474240 in which closely-linked DArTseq markers 1128708 and 3948563 were located. Although bulk segregant analysis (BSA) with the 90K Infinium array identified 51 SNPs associated with LrAW3, only one SNP-derived KASP marker mapped close to the locus. Deletion bin mapping of LrAW3-linked markers located LrAW3 between bins 3BL11-0.85-0.90 and 3BL7-0.63. Since no other all stage leaf rust resistance gene is located in chromosome 3BL, LrAW3 represented a new locus and was designated Lr79. Marker sun786 mapped 1.8cM distal to Lr79 and Aus26582 was null for this locus. However, the marker can be reliably scored as it also amplifies a monomorphic fragment that serves as an internal control to differentiate the null status of Aus26582 from reaction failure. This marker was validated among a set of durum and common wheat cultivars and was shown to be useful for marker-assisted selection of Lr79 at both ploidy levels.

Adult Plant Leaf Rust Resistance Derived from Toropi Wheat is Conditioned by Lr78 and Three Minor QTL.

Leaf rust caused by Puccinia triticina is an important disease of wheat in many regions worldwide. Durable or long-lasting leaf rust resistance has been difficult to achieve because populations of P. triticina are highly variable for virulence to race-specific resistance genes, and respond to selection by resistance genes in released wheat cultivars. The wheat cultivar Toropi, developed and grown in Brazil, was noted to have long-lasting leaf rust resistance that was effective only in adult plants. The objectives of this study were to determine the chromosome location of the leaf rust resistance genes derived from Toropi in two populations of recombinant inbred lines in a partial Thatcher wheat background. In the first population, a single gene with major effects on chromosome 5DS that mapped 2.2 centimorgans distal to IWA6289, strongly reduced leaf rust severity in all 3 years of field plot tests. This gene for adult plant leaf rust resistance was designated as Lr78. In the second population, quantitative trait loci (QTL) with small effects on chromosomes 1BL, 3BS, and 4BS were found. These QTL expressed inconsistently over 4 years of field plot tests. The adult plant leaf rust resistance derived from Toropi involved a complex combination of QTL with large and small effects.

Genetic characterization of type II Fusarium head blight resistance derived from transgressive segregation in a cross between Eastern and Western Canadian spring wheat

Fusarium head blight (FHB) caused by Fusarium graminearum is a devastating disease affecting global wheat production, causing significant losses to yield and grain quality. The Eastern Canadian line FL62R1 was developed using a systemic breeding approach and boasts high levels of FHB resistance with good yield, desirable agronomics, and end-use quality traits. The objective of this study was to identify genetic determinants of type II resistance in a cross between FL62R1 and cv. Stettler, a Canada Western Red Spring variety rated moderately susceptible to FHB. Although neither parent displayed strong resistance to FHB spread within spikes following point inoculation (type II resistance) in greenhouses, strong type II resistance was observed in a large number of progeny, including 6% with resistance comparable to the best check line, Sumai 3. Quantitative trait locus (QTL) mapping identified a locus from chromosome 2BL of Stettler which provides Sumai 3 level type II resistance when combined with favorable FHB resistance QTLs with minor effects from FL62R1. This study provides insight into transgressive segregation for FHB, which despite its importance is poorly understood, rare, and difficult to predict. It also begins to dissect the genetic architecture of FHB resistance in wheat derived from the systemic breeding approach.