Identification of LMW glutenin-like genes from Secale sylvestre host

Three low-molecular-weight (LMW) glutenin-like genes (designated as Ssy1, Ssy2 and Ssy3) from Secale sylvestre Host were isolated and characterized. The three genes consist of a predicted highly conservative signal peptide with 20 amino acids, a short N-terminal region with 13 amino acids, a highly variable repetitive domain and a less variable C-terminal domain. The deduced amino acid sequences of the three genes were the LMW-m type due to a methionine residue at the N-terminus. The phylogenic analysis indicated that the prolamin genes could be perfectly clustered into five groups, including HMW-GS, LMW-GS, alpha/beta-, gamma- and omega-prolamin. The LMW glutenin-like genes of S. sylvestre were more orthologous with the LMW-GS genes of wheat and B hordein genes of barley, which also had been confirmed by the homology analysis with the LMW-GS of wheat at Glu-A3, Glu-B3 and Glu-D3 loci. These results indicated that a chromosome locus (designated as Glu-R3) might be located on the R genome of S. sylvestre with the functions similar to the Glu-3 locus in wheat and its related species.     

Low molecular weight glutenin subunit gene composition at <Emphasis Type="Italic">Glu</Emphasis>-<Emphasis Type="Italic">D3</Emphasis> loci of <Emphasis Type="Italic">Aegilops tauschii</Emphasis> and common wheat and a further view of wheat evolution

Key message

A comprehensive comparison of LMW-GS genes between Ae. tauschii and its progeny common wheat.

Abstract

Low molecular weight glutenin subunits (LMW-GSs) are determinant of wheat flour processing quality. However, the LMW-GS gene composition in Aegilops tauschii, the wheat D genome progenitor, has not been comprehensively elucidated and the impact of allohexaploidization on the Glu-D3 locus remains elusive. In this work, using the LMW-GS gene molecular marker system and the full-length gene-cloning method, LMW-GS genes at the Glu-D3 loci of 218 Ae. tauschii and 173 common wheat (Triticum aestivum L.) were characterized. Each Ae. tauschii contained 11 LMW-GS genes, and the whole collection was divided into 25 haplotypes (AeH01–AeH25). The Glu-D3 locus in common wheat lacked the LMW-GS genes D3-417, D3-507 and D3-552, but shared eight genes of identical open reading frame (ORF) sequences when compared to that of Ae. tauschii. Therefore, the allohexaploidization induces deletions, but exerts no influence on LMW-GS gene coding sequences at the Glu-D3 locus. 92.17% Ae. tauschii had 7-9 LMW-GSs, more than the six subunits in common wheat. The haplotypes AeH16, AeH20 and AeH23 of Ae. tauschii ssp. strangulate distributed in southeastern Caspian Iran were the main putative D genome donor of common wheat. These results facilitate the utilization of the Ae. tauschii glutenin gene resources and the understanding of wheat evolution.

     

Comprehensive identification of LMW-GS genes and their protein products in a common wheat variety

Although it is well known that low-molecular-weight glutenin subunits (LMW-GS) from wheat affect bread and noodle processing quality, the function of specific LMW-GS proteins remains unclear. It is important to find the genes that correspond to individual LMW-GS proteins in order to understand the functions of specific proteins. The objective of this study was to link LMW-GS genes and haplotypes characterized using well known Glu-A3, Glu-B3, and Glu-D3 gene-specific primers to their protein products in a single wheat variety. A total of 36 LMW-GS genes and pseudogenes were amplified from the Korean cultivar Keumkang. These include 11 Glu-3 gene haplotypes, two from the Glu-A3 locus, two from the Glu-B3 locus, and seven from the Glu-D3 locus. To establish relationships between gene haplotypes and their protein products, a glutenin protein fraction was separated by two-dimensional gel electrophoresis (2-DGE) and 17 protein spots were analyzed by N-terminal amino acid sequencing and tandem mass spectrometry (MS/MS). LMW-GS proteins were identified that corresponded to all Glu-3 gene haplotypes except the pseudogenes. This is the first report of the comprehensive characterization of LMW-GS genes and their corresponding proteins in a single wheat cultivar. Our approach will be useful to understand the contributions of individual LMW-GS to the end-use quality of flour.     

Development of haplotype-specific molecular markers for the low-molecular-weight glutenin subunits

Low-molecular-weight glutenin subunits (LMW-GSs) are one of the major components of gluten, and their allelic variation has been widely associated with different wheat end-use quality parameters. These proteins are encoded by multigene families located at the orthologous Glu-3 loci (Glu-A3, Glu-B3, and Glu-D3); the genes at each locus are divided by large intergenic and highly recombinogenic regions. Among the methods used for the LMW-GS allele identification, polymerase chain reaction (PCR)-based molecular markers have the advantages of being simple, accurate, and independent from the plant stage of development. However, the available LMW-GS molecular markers are either incapable of capturing the complexity of the LMW-GS gene family or difficult to interpret. In the present study, we report the development of a set of PCR-based molecular markers specific for the LMW-GS haplotypes present at each Glu-3 locus. Based on the LMW-GS gene sequences available in GenBank, single nucleotide polymorphisms (SNPs) specific for each Glu-3 haplotype were identified and the relevant PCR primers were designed. In total, we developed three molecular markers for the Glu-A3 and Glu-B3 loci, respectively, and five molecular markers for the Glu-D3 locus. The markers were tested on 44 bread wheat varieties previously characterized for their LMW-GS genic profile and found to be equally or more efficient than previously developed LMW-GS PCR-based markers. This set of markers allows an easier and less ambiguous identification of specific LMW-GS haplotypes associated with gluten strength and can facilitate marker-assisted breeding for wheat quality.     

Evidence of intralocus recombination at the <Emphasis Type="Italic">Glu-3</Emphasis> loci in bread wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Key message

Recombination at the Glu-3 loci was identified, and strong genetic linkage was observed only between the amplicons representing i-type and s-type genes located, respectively, at the Glu-A3 and Glu-B3 loci.

Abstract

The low-molecular weight glutenin subunits (LMW-GSs) are one of the major components of wheat seed storage proteins and play a critical role in the determination of wheat end-use quality. The genes encoding this class of proteins are located at the orthologous Glu-3 loci (Glu-A3, Glu-B3, and Glu-D3). Due to the complexity of these chromosomal regions and the high sequence similarity between different LMW-GS genes, their organization and recombination characteristics are still incompletely understood. This study examined intralocus recombination at the Glu-3 loci in two recombinant inbred line (RIL) and one doubled haploid (DH) population, all segregating for the Glu-A3, Glu-B3, and Glu-D3 loci. The analysis was conducted using a gene marker system that consists of the amplification of the complete set of the LMW-GS genes and their visualization by capillary electrophoresis. Recombinant marker haplotypes were detected in all three populations with different recombination rates depending on the locus and the population. No recombination was observed between the amplicons representing i-type and s-type LMW-GS genes located, respectively, at the Glu-A3 and Glu-B3 loci, indicating tight linkage between these genes. Results of this study contribute to better understanding the genetic linkage and recombination between different LMW-GS genes, the structure of the Glu-3 loci, and the development of more specific molecular markers that better represent the genetic diversity of these loci. In this way, a more precise analysis of the contribution of various LMW-GSs to end-use quality of wheat may be achieved.

     

Interactions between <Emphasis Type="Italic">Glu-1</Emphasis> and <Emphasis Type="Italic">Glu-3</Emphasis> loci and associations of selected molecular markers with quality traits in winter wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) DH lines

The quality of wheat depends on a large complex of genes and environmental factors. The objective of this study was to identify quantitative trait loci controlling technological quality traits and their stability across environments, and to assess the impact of interaction between alleles at loci Glu-1 and Glu-3 on grain quality. DH lines were evaluated in field experiments over a period of 4 years, and genotyped using simple sequence repeat markers. Lines were analysed for grain yield (GY), thousand grain weight (TGW), protein content (PC), starch content (SC), wet gluten content (WG), Zeleny sedimentation value (ZS), alveograph parameter W (APW), hectolitre weight (HW), and grain hardness (GH). A number of QTLs for these traits were identified in all chromosome groups. The Glu-D1 locus influenced TGW, PC, SC, WG, ZS, APW, GH, while locus Glu-B1 affected only PC, ZS, and WG. Most important marker-trait associations were found on chromosomes 1D and 5D. Significant effects of interaction between Glu-1 and Glu-3 loci on technological properties were recorded, and in all types of this interaction positive effects of Glu-D1 locus on grain quality were observed, whereas effects of Glu-B1 locus depended on alleles at Glu-3 loci. Effects of Glu-A3 and Glu-D3 loci per se were not significant, while their interaction with alleles present at other loci encoding HMW and LMW were important. These results indicate that selection of wheat genotypes with predicted good bread-making properties should be based on the allelic composition both in Glu-1 and Glu-3 loci, and confirm the predominant effect of Glu-D1d allele on technological properties of wheat grains.     

<Emphasis Type="Italic">TaELF3-1DL</Emphasis>, a homolog of <Emphasis Type="Italic">ELF3</Emphasis>, is associated with heading date in bread wheat

Early flowering 3 (ELF3) is a regulator to modulate photoperiod flowering in Arabidopsis. The homologs of ELF3 in rice and barley also have been identified essential for regulation of flowering time. In the current study, TaELF3 genes, homologs of ELF3 in bread wheat (Triticum aestivum L.), were cloned by a comparative genomics approach and located on homologous group 1 chromosomes, designated as TaELF3-1AL, TaELF3-1BL, and TaELF3-1DL, respectively. A sequence-tagged site (STS) marker was developed based on sequence polymorphism at the TaELF3-1DL locus. A quantitative trait locus (QTL) for heading date (HD) co-segregating with TaELF3-1DL explained 7.7–20.6% of the phenotypic variance in a RIL mapping population derived from the Gaocheng 8901/Zhoumai 16 cross genotyped using the wheat 90K iSelect assay. The late HD allele of TaELF3-1DL was prevalently selected in China’s specific wheat-growing regions and other countries. This study produces novel information in better understanding HD and provides a reliable functional marker for molecular marker-assisted selection in wheat breeding.     

Cytogenetic mapping of a major locus for resistance to Fusarium head blight and crown rot of wheat on <Emphasis Type="Italic">Thinopyrum elongatum</Emphasis> 7EL and its pyramiding with valuable genes from a <Emphasis Type="Italic">Th. ponticum</Emphasis> homoeologous arm onto bread wheat 7DL

Key message

A major locus for resistance to different Fusarium diseases was mapped to the most distal end of Th. elongatum 7EL and pyramided with Th. ponticum beneficial genes onto wheat 7DL.

Abstract

Perennial Triticeae species of the Thinopyrum genus are among the richest sources of valuable genes/QTL for wheat improvement. One notable and yet unexploited attribute is the exceptionally effective resistance to a major wheat disease worldwide, Fusarium head blight, associated with the long arm of Thinopyrum elongatum chromosome 7E (7EL). We targeted the transfer of the temporarily designated Fhb-7EL locus into bread wheat, pyramiding it with a Th. ponticum 7el₁L segment stably inserted into the 7DL arm of wheat line T4. Desirable genes/QTL mapped along the T4 7el₁L segment determine resistance to wheat rusts (Lr19, Sr25) and enhancement of yield-related traits. Mapping of the Fhb-7EL QTL, prerequisite for successful pyramiding, was established here on the basis of a bioassay with Fusarium graminearum of different 7EL-7el₁L bread wheat recombinant lines. These were obtained without resorting to any genetic pairing promotion, but relying on the close 7EL-7el₁L homoeology, resulting in 20% pairing frequency between the two arms. Fhb-7EL resided in the telomeric portion and resistant recombinants could be isolated with useful combinations of more proximally located 7el₁L genes/QTL. The transferred Fhb-7EL locus was shown to reduce disease severity and fungal biomass in grains of infected recombinants by over 95%. The same Fhb-7EL was, for the first time, proved to be effective also against F. culmorum and F. pseudograminearum, predominant agents of crown rot. Prebreeding lines possessing a suitable 7EL-7el₁L gene/QTL assembly showed very promising yield performance in preliminary field tests.

     

Adult plant stripe rust resistance gene <Emphasis Type="Italic">Yr71</Emphasis> maps close to <Emphasis Type="Italic">Lr24</Emphasis> in chromosome 3D of common wheat

Australian cultivar Sunco carries three adult plant stripe rust resistance genes. One of these genes corresponded to Yr18 in chromosome 7DS; the second, YrCK, was mapped on chromosome 2D. Here, we describe the characterization of the third adult plant resistance (APR) gene from Sunco. Sunco/2*Avocet S-derived lines SA65 (resistant) and SA67 (susceptible) were crossed and a recombinant inbred line F₆ population was generated. Monogenic segregation among SA65/SA67-derived RIL population was demonstrated and the resistance locus was designated YrSA3. Selective genotyping using an iSelect 90 K Infinium SNP array and SSR markers located YrSA3 on chromosome 3D. Development of KASP markers for SNP loci showing association with YrSA3 allowed construction of a genetic map harboring the resistance gene. Ten KASP markers (KASP_8306, KASP_9142, KASP_10438, KASP_16434, KASP_17207, KASP_20836, KASP_23518, KASP_23615, KASP_57983 and KASP_63653), one SSR marker (gwm114b) and Lr24/Sr24 were mapped 1.8 cM distal to YrSA3. Comparison of marker data indicated that the previously named seedling stripe rust resistance gene Yr45 was located proximal to YrSA3, and therefore the latter was formally designated Yr71. Two recombinants carrying Lr24/Sr24 and Yr71 in combination were identified for use as donor sources in wheat breeding programs. The robustness of gwm114b, KASP_16434, KASP_17207 and KASP_20836 for marker-assisted selection of these genes was demonstrated through tests on 74 Australian wheat cultivars.     

Identification and characterization of <Emphasis Type="Italic">Rht25</Emphasis>, a locus on chromosome arm 6AS affecting wheat plant height,heading time,and spike development

Key message

This study identified Rht25, a new plant height locus on wheat chromosome arm 6AS, and characterized its pleiotropic effects on important agronomic traits.

Abstract

Understanding genes regulating wheat plant height is important to optimize harvest index and maximize grain yield. In modern wheat varieties grown under high-input conditions, the gibberellin-insensitive semi-dwarfing alleles Rht-B1b and Rht-D1b have been used extensively to confer lodging tolerance and improve harvest index. However, negative pleiotropic effects of these alleles (e.g., poor seedling emergence and reduced biomass) can cause yield losses in hot and dry environments. As part of current efforts to diversify the dwarfing alleles used in wheat breeding, we identified a quantitative trait locus (QHt.ucw-6AS) affecting plant height in the proximal region of chromosome arm 6AS (<?0.4 cM from the centromere). Using a large segregating population (~?2800 gametes) and extensive progeny tests (70–93 plants per recombinant family), we mapped QHt.ucw-6AS as a Mendelian locus to a 0.2 cM interval (144.0–148.3 Mb, IWGSC Ref Seq v1.0) and show that it is different from Rht18. QHt.ucw-6AS is officially designated as Rht25, with Rht25a representing the height-increasing allele and Rht25b the dwarfing allele. The average dwarfing effect of Rht25b was found to be approximately half of the effect observed for Rht-B1b and Rht-D1b, and the effect is greater in the presence of the height-increasing Rht-B1a and Rht-D1a alleles than in the presence of the dwarfing alleles. Rht25b is gibberellin-sensitive and shows significant pleiotropic effects on coleoptile length, heading date, spike length, spikelet number, spikelet density, and grain weight. Rht25 represents a new alternative dwarfing locus that should be evaluated for its potential to improve wheat yield in different environments.

     

Discovery and characterization of two new stem rust resistance genes in <Emphasis Type="Italic">Aegilops sharonensis</Emphasis>

Key message

We identified two novel wheat stem rust resistance genes, Sr-1644-1Sh and Sr-1644-5Sh in Aegilops sharonensis that are effective against widely virulent African races of the wheat stem rust pathogen.

Abstract

Stem rust is one of the most important diseases of wheat in the world. When single stem rust resistance (Sr) genes are deployed in wheat, they are often rapidly overcome by the pathogen. To this end, we initiated a search for novel sources of resistance in diverse wheat relatives and identified the wild goatgrass species Aegilops sharonesis (Sharon goatgrass) as a rich reservoir of resistance to wheat stem rust. The objectives of this study were to discover and map novel Sr genes in Ae. sharonensis and to explore the possibility of identifying new Sr genes by genome-wide association study (GWAS). We developed two biparental populations between resistant and susceptible accessions of Ae. sharonensis and performed QTL and linkage analysis. In an F₆ recombinant inbred line and an F₂ population, two genes were identified that mapped to the short arm of chromosome 1S^sh, designated as Sr-1644-1Sh, and the long arm of chromosome 5S^sh, designated as Sr-1644-5Sh. The gene Sr-1644-1Sh confers a high level of resistance to race TTKSK (a member of the Ug99 race group), while the gene Sr-1644-5Sh conditions strong resistance to TRTTF, another widely virulent race found in Yemen. Additionally, GWAS was conducted on 125 diverse Ae. sharonensis accessions for stem rust resistance. The gene Sr-1644-1Sh was detected by GWAS, while Sr-1644-5Sh was not detected, indicating that the effectiveness of GWAS might be affected by marker density, population structure, low allele frequency and other factors.

     

Genetic mapping of a novel recessive allele for non-glaucousness in wild diploid wheat <Emphasis Type="Italic">Aegilops tauschii</Emphasis>: implications for the evolution of common wheat

Cuticular wax on the aerial surface of plants has a protective function against many environmental stresses. The bluish–whitish appearance of wheat leaves and stems is called glaucousness. Most modern cultivars of polyploid wheat species exhibit the glaucous phenotype, while in a wild wheat progenitor, Ae. tauschii, both glaucous and non-glaucous accessions exist. Iw2, a wax inhibitor locus on the short arm of chromosome 2D, is the main contributor to this phenotypic variation in Ae. tauschii, and the glaucous/non-glaucous phenotype of Ae. tauschii is usually inherited by synthetic hexaploid wheat. However, a few synthetic lines show the glaucous phenotype although the parental Ae. tauschii accessions are non-glaucous. Molecular marker genotypes indicate that the exceptional non-glaucous Ae. tauschii accessions share the same genotype in the Iw2 chromosomal region as glaucous accessions, suggesting that these accessions have a different causal locus for their phenotype. This locus was assigned to the long arm of chromosome 3D using an F₂ mapping population and designated W4, a novel glaucous locus in Ae. tauschii. The dominant W4 allele confers glaucousness, consistent with phenotypic observation of Ae. tauschii accessions and the derived synthetic lines. These results implied that glaucous accessions of Ae. tauschii with the W2W2iw2iw2W4W4 genotype could have been the D-genome donor of common wheat.     

Mapping of stripe rust resistance gene in an <Emphasis Type="Italic">Aegilops caudata</Emphasis> introgression line in wheat and its genetic association with leaf rust resistance

A pair of stripe rust and leaf rust resistance genes was introgressed from Aegilops caudata, a nonprogenitor diploid species with the CC genome, to cultivated wheat. Inheritance and genetic mapping of stripe rust resistance gene in backcross-recombinant inbred line (BC-RIL) population derived from the cross of a wheat–Ae. caudata introgression line (IL) T291-2(pau16060) with wheat cv. PBW343 is reported here. Segregation of BC-RILs for stripe rust resistance depicted a single major gene conditioning adult plant resistance (APR) with stripe rust reaction varying from TR-20MS in resistant RILs signifying the presence of some minor genes as well. Genetic association with leaf rust resistance revealed that two genes are located at a recombination distance of 13%. IL T291-2 had earlier been reported to carry introgressions on wheat chromosomes 2D, 3D, 4D, 5D, 6D and 7D. Genetic mapping indicated the introgression of stripe rust resistance gene on wheat chromosome 5DS in the region carrying leaf rust resistance gene LrAc, but as an independent introgression. Simple sequence repeat (SSR) and sequence-tagged site (STS) markers designed from the survey sequence data of 5DS enriched the target region harbouring stripe and leaf rust resistance genes. Stripe rust resistance locus, temporarily designated as YrAc, mapped at the distal most end of 5DS linked with a group of four colocated SSRs and two resistance gene analogue (RGA)-STS markers at a distance of 5.3 cM. LrAc mapped at a distance of 9.0 cM from the YrAc and at 2.8 cM from RGA-STS marker Ta5DS_2737450, YrAc and LrAc appear to be the candidate genes for marker-assisted enrichment of the wheat gene pool for rust resistance.     

<Emphasis Type="Italic">Pm61</Emphasis>: a recessive gene for resistance to powdery mildew in wheat landrace Xuxusanyuehuang identified by comparative genomics analysis

Key message

A single recessive powdery mildew resistance gene Pm61 from wheat landrace Xuxusanyuehuang was mapped within a 0.46-cM genetic interval spanning a 1.3-Mb interval of the genomic region of chromosome arm 4AL.

Abstract

Epidemics of powdery mildew incited by the biotrophic fungus Blumeria graminis f. sp. tritici (Bgt) have caused significant yield reductions in many wheat (Triticum aestivum)-producing regions. Identification of powdery mildew resistance genes is required for sustainable improvement of wheat for disease resistance. Chinese wheat landrace Xuxusanyuehuang was resistant to several Bgt isolates at the seedling stage. Genetic analysis based on the inoculation of Bgt isolate E09 on the F₁, F₂, and F_2:3 populations produced by crossing Xuxusanyuehuang to susceptible cultivar Mingxian 169 revealed that the resistance of Xuxusanyuehuang was controlled by a single recessive gene. Bulked segregant analysis and simple sequence repeat (SSR) mapping placed the gene on chromosome bin 4AL-4-0.80-1.00. Comparative genomics analysis was performed to detect the collinear genomic regions of Brachypodium distachyon, rice, sorghum, Aegilops tauschii, T. urartu, and T. turgidum ssp. dicoccoides. Based on the use of 454 contig sequences and the International Wheat Genome Sequence Consortium survey sequence of Chinese Spring wheat, four EST-SSR and seven SSR markers were linked to the gene. An F₅ recombinant inbred line population derived from Xuxusanyuehuang?×?Mingxian 169 cross was used to develop the genetic linkage map. The gene was localized in a 0.46-cM genetic interval between Xgwm160 and Xicsx79 corresponding to 1.3-Mb interval of the genomic region in wheat genome. This is a new locus for powdery mildew resistance on chromosome arm 4AL and is designated Pm61.

     

Remapping of the stripe rust resistance gene <Emphasis Type="Italic">Yr10</Emphasis> in common wheat

Key message

Yr10 is an important gene to control wheat stripe rust, and the search for Yr10 needs to be continued.

Abstract

Wheat stripe rust or yellow rust is a devastating fungal disease caused by Puccinia striiformis f. sp. tritici (Pst). Host disease resistance offers a primary source for controlling wheat stripe rust. The stripe rust resistance gene Yr10 confers the race-specific resistance to most tested Pst races in China including CYR29. Early studies proposed that Yr10 was a nucleotide-binding site, leucine-rich repeat gene archived as GenBank accession AF149112 (hereafter designated the Yr10 candidate gene or Yr10 _CG ). In this study, we revealed that 15 Chinese wheat cultivars positive for Yr10 _CG are susceptible to CYR29. We then expressed the Yr10 _CG cDNA in the common wheat ‘Bobwhite’. The Yr10 _CG -cDNA positive transgenic plants were also susceptible to CYR29. Thus, it is highly unlikely that Yr10 _CG corresponds to the Yr10 resistance gene. Using the Yr10 donor ‘Moro’ and the Pst-susceptible wheat ‘Huixianhong’, we generated two F₃ populations that displayed a single Mendelian segregation on the Yr10 gene, and used them to remap the Yr10 gene. Six markers were placed in the Yr10 region, with the Yr10 _CG gene now mapping about 1.2-cM proximal to the Yr10 locus and the Xsdauw79 marker is completely linked to the Yr10 locus. Apparently, the Yr10 gene has not yet been identified. Fine mapping and positional cloning of Yr10 is important for gene pyramiding for stripe rust resistance in wheat.

     

Characterization of <Emphasis Type="Italic">Lr75</Emphasis>: a partial,broad-spectrum leaf rust resistance gene in wheat

Key message

Here, we describe a strategy to improve broad-spectrum leaf rust resistance by marker-assisted combination of two partial resistance genes. One of them represents a novel partial adult plant resistance gene, named Lr75.

Abstract

Leaf rust caused by the fungal pathogen Puccinia triticina is a damaging disease of wheat (Triticum aestivum L.). The combination of several, additively-acting partial disease resistance genes has been proposed as a suitable strategy to breed wheat cultivars with high levels of durable field resistance. The Swiss winter wheat cultivar ‘Forno’ continues to show near-immunity to leaf rust since its release in the 1980s. This resistance is conferred by the presence of at least six quantitative trait loci (QTL), one of which is associated with the morphological trait leaf tip necrosis. Here, we used a marker-informed strategy to introgress two ‘Forno’ QTLs into the leaf rust-susceptible Swiss winter wheat cultivar ‘Arina’. The resulting backcross line ‘ArinaLrFor’ showed markedly increased leaf rust resistance in multiple locations over several years. One of the introgressed QTLs, QLr.sfr-1BS, is located on chromosome 1BS. We developed chromosome 1B-specific microsatellite markers by exploiting the Illumina survey sequences of wheat cv. ‘Chinese Spring’ and mapped QLr.sfr-1BS to a 4.3 cM interval flanked by the SSR markers gwm604 and swm271. QLr.sfr-1BS does not share a genetic location with any of the described leaf rust resistance genes present on chromosome 1B. Therefore, QLr.sfr-1BS is novel and was designated as Lr75. We conclude that marker-assisted combination of partial resistance genes is a feasible strategy to increase broad-spectrum leaf rust resistance. The identification of Lr75 adds a novel and highly useful gene to the small set of known partial, adult plant leaf rust resistance genes.

     

Evolutionary divergence of the rye <Emphasis Type="Italic">Pm17</Emphasis> and <Emphasis Type="Italic">Pm8</Emphasis> resistance genes reveals ancient diversity

Key message

We have isolated a novel powdery mildew resistance gene in wheat that was originally introgressed from rye. Further analysis revealed evolutionary divergent history of wheat and rye orthologous resistance genes.

Abstract

Wheat production is under constant threat from a number of fungal pathogens, among them is wheat powdery mildew (Blumeria graminis f. sp. tritici). Deployment of resistance genes is the most economical and sustainable method for mildew control. However, domestication and selective breeding have narrowed genetic diversity of modern wheat germplasm, and breeders have relied on wheat relatives for enriching its gene pool through introgression. Translocations where the 1RS chromosome arm was introgressed from rye to wheat have improved yield and resistance against various pathogens. Here, we isolated the Pm17 mildew resistance gene located on the 1RS introgression in wheat cultivar ‘Amigo’ and found that it is an allele or a close paralog of the Pm8 gene isolated earlier from ‘Petkus’ rye. Functional validation using transient and stable transformation confirmed the identity of Pm17. Analysis of Pm17 and Pm8 coding regions revealed an overall identity of 82.9% at the protein level, with the LRR domains being most divergent. Our analysis also showed that the two rye genes are much more diverse compared to the variants encoded by the Pm3 gene in wheat, which is orthologous to Pm17/Pm8 as concluded from highly conserved upstream sequences in all these genes. Thus, the evolutionary history of these orthologous loci differs in the cereal species rye and wheat and demonstrates that orthologous resistance genes can take different routes towards functionally active genes. These findings suggest that the isolation of Pm3/Pm8/Pm17 orthologs from other grass species, additional alleles from the rye germplasm as well as possibly synthetic variants will result in novel resistance genes useful in wheat breeding.

     

A new leaf rust resistance gene <Emphasis Type="Italic">Lr79</Emphasis> mapped in chromosome 3BL from the durum wheat landrace Aus26582

Key message

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.

Abstract

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 90 K 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 1.8 cM 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.

     

<Emphasis Type="Italic">Sr33</Emphasis> and <Emphasis Type="Italic">Sr35</Emphasis> gene homolog identification in genomes of cereals related to <Emphasis Type="Italic">Aegilops tauschii</Emphasis> and <Emphasis Type="Italic">Triticum monococcum</Emphasis>

Using bioinformatics analysis, the homologs of genes Sr33 and Sr35 were identified in the genomes of Triticum aestivum, Hordeum vulgare, and Triticum urartu. It is known that these genes confer resistance to highly virulent wheat stem rust races (Ug99). To identify amino acid sites important for this resistance, the found homologs were compared with the Sr33 and Sr35 protein sequences. It was found that sequences S5DMA6 and E9P785 are the closest homologs of protein RGAle, a Sr33 gene product, and sequences M7YFA9 (CNL-C) and F2E9R2 are homologs of protein CNL9, a Sr35 gene product. It is assumed that the homologs of genes Sr33 and Sr35, which were obtained from the wild relatives of wheat and barley, can confer resistance to various forms of stem rust and can be used in the future breeding programs aimed at improvement of national wheat varieties.