Chitinases of <Emphasis Type="Italic">Coffea arabica</Emphasis> genotypes resistant to orange rust <Emphasis Type="Italic">Hemileia vastatrix</Emphasis>

Two Coffea arabica — Hemileia vastatrix incompatible interactions (I₁: coffee cv. Caturra — rust race VI and I₂: coffee cv S4 Agaro — rust race II) and a compatible interaction (coffee cv. Caturra — rust race II) were compared in relation to the infection process and chitinase activity. In the two incompatible interactions the fungus ceased growth in the early infection stages, while in the compatible interaction no fungus growth inhibition was observed. A high constitutive level of chitinase activity was detected in the intercellular fluid of healthy leaves. Upon infection, chitinase isoforms were more abundant in incompatible interactions than in the compatible interaction. Immunodetection showed that class I chitinases are particularly relevant in the incompatible interactions and might participate in the defence response of the coffee plants.     

Characterization and mapping of cryptic alien introgression from <Emphasis Type="Italic">Aegilops geniculata</Emphasis> with new leaf rust and stripe rust resistance genes <Emphasis Type="Italic">Lr57</Emphasis> and <Emphasis Type="Italic">Yr40</Emphasis> in wheat

Leaf rust and stripe rust are important foliar diseases of wheat worldwide. Leaf rust and stripe rust resistant introgression lines were developed by induced homoeologous chromosome pairing between wheat chromosome 5D and 5M^g of Aegilops geniculata (U^gM^g). Characterization of rust resistant BC₂F₅ and BC₃F₆ homozygous progenies using genomic in situ hybridization with Aegilops comosa (M) DNA as probe identified three different types of introgressions; two cytologically visible and one invisible (termed cryptic alien introgression). All three types of introgression lines showed similar and complete resistance to the most prevalent pathotypes of leaf rust and stripe rust in Kansas (USA) and Punjab (India). Diagnostic polymorphisms between the alien segment and recipient parent were identified using physically mapped RFLP probes. Molecular mapping revealed that cryptic alien introgression conferring resistance to leaf rust and stripe rust comprised less than 5% of the 5DS arm and was designated T5DL·5DS-5M^gS(0.95). Genetic mapping with an F₂ population of Wichita × T5DL·5DS-5M^gS(0.95) demonstrated the monogenic and dominant inheritance of resistance to both diseases. Two diagnostic RFLP markers, previously mapped on chromosome arm 5DS, co-segregated with the rust resistance in the F₂ population. The unique map location of the resistant introgression on chromosome T5DL·5DS-5M^gS(0.95) suggested that the leaf rust and stripe rust resistance genes were new and were designated Lr57 and Yr40. This is the first documentation of a successful transfer and characterization of cryptic alien introgression from Ae. geniculata conferring resistance to both leaf rust and stripe rust in wheat.     

New slow-rusting leaf rust and stripe rust resistance genes <Emphasis Type="Italic">Lr67</Emphasis> and <Emphasis Type="Italic">Yr46</Emphasis> in wheat are pleiotropic or closely linked

The common wheat genotype ‘RL6077’ was believed to carry the gene Lr34/Yr18 that confers slow-rusting adult plant resistance (APR) to leaf rust and stripe rust but located to a different chromosome through inter-chromosomal reciprocal translocation. However, haplotyping using the cloned Lr34/Yr18 diagnostic marker and the complete sequencing of the gene indicated Lr34/Yr18 is absent in RL6077. We crossed RL6077 with the susceptible parent ‘Avocet’ and developed F₃, F₄ and F₆ populations from photoperiod-insensitive F₃ lines that were segregating for resistance to leaf rust and stripe rust. The populations were characterized for leaf rust resistance at two Mexican sites, Cd. Obregon during the 2008–2009 and 2009–2010 crop seasons, and El Batan during 2009, and for stripe rust resistance at Toluca, a third Mexican site, during 2009. The F₃ population was also evaluated for stripe rust resistance at Cobbitty, Australia, during 2009. Most lines had correlated responses to leaf rust and stripe rust, indicating that either the same gene, or closely linked genes, confers resistance to both diseases. Molecular mapping using microsatellites led to the identification of five markers (Xgwm165, Xgwm192, Xcfd71, Xbarc98 and Xcfd23) on chromosome 4DL that are associated with this gene(s), with the closest markers being located at 0.4 cM. In a parallel study in Canada using a Thatcher × RL6077 F₃ population, the same leaf rust resistance gene was designated as Lr67 and mapped to the same chromosomal region. The pleiotropic, or closely linked, gene derived from RL6077 that conferred stripe rust resistance in this study was designated as Yr46. The slow-rusting gene(s) Lr67/Yr46 can be utilized in combination with other slow-rusting genes to develop high levels of durable APR to leaf rust and stripe rust in wheat.     

Wheat stripe rust resistance genes <Emphasis Type="Italic">Yr5</Emphasis> and <Emphasis Type="Italic">Yr7</Emphasis> are allelic

Stripe rust is one of the most destructive diseases of wheat. Breeding for resistance is the most economical and environmentally acceptable means to control stripe rust. Genetic studies on resistance sources are very important. Previous inheritance studies on Triticum aestivum subsp. spelta cv. album and wheat cultivar Lee showed that each possessed a single dominant gene for stripe rust resistance, i.e., Yr5 and Yr7, respectively. Both were located on the long arm of chromosome 2B, but due to the complexities caused by genetic background effects there was no clear evidence on the allelism or linkage status of these genes. Our study, involving an intercross of Avocet S near-isogenic lines possessing the genes, provided clear evidence for allelism or extremely close linkage of Yr5 and Yr7 based on phenotypic and molecular studies.     

Characterisation of a new stripe rust resistance gene <Emphasis Type="Italic">Yr47</Emphasis> and its genetic association with the leaf rust resistance gene <Emphasis Type="Italic">Lr52</Emphasis>

Two Iranian common wheat landraces AUS28183 and AUS28187 from the Watkins collection showed high levels of seedling resistance against Australian pathotypes of leaf rust and stripe rust pathogens. Chi-squared analyses of rust response segregation among F₃ populations derived from crosses of AUS28183 and AUS28187 with a susceptible genotype AUS27229 revealed monogenic inheritance of leaf rust and stripe rust resistance. As both genotypes produced similar leaf rust and stripe rust infection types, they were assumed to carry the same genes. The genes were temporarily named as LrW1 and YrW1. Molecular mapping placed LrW1 and YrW1 in the short arm of chromosome 5B, about 10 and 15 cM proximal to the SSR marker gwm234, respectively, and the marker cfb309 mapped 8–12 cM proximal to YrW1. LrW1 mapped 3–6 cM distal to YrW1 in two F₃ populations. AUS28183 corresponded to the accession V336 of the Watkins collection which was the original source of Lr52. Based on the genomic location and accession records, LrW1 was concluded to be Lr52. Because no other seedling stripe rust resistance gene has previously been mapped in chromosome 5BS, YrW1 was permanently named as Yr47. A combination of flanking markers gwm234 and cfb309 with phenotypic assays could be used to ascertain the presence of Lr52 and Yr47 in segregating populations. This investigation characterised a valuable source of dual leaf rust and stripe rust resistance for deployment in new wheat cultivars. Transfer of Lr52 and Yr47 into current Australian wheat backgrounds is in progress.     

Genome-wide association mapping for seedling and field resistance to <Emphasis Type="Italic">Puccinia striiformis</Emphasis> f. sp. <Emphasis Type="Italic">tritici</Emphasis> in elite durum wheat

Key message

Genome-wide association analysis in tetraploid wheat revealed novel and diverse loci for seedling and field resistance to stripe rust in elite spring durum wheat accessions from worldwide.

Abstract

Improving resistance to stripe rust, caused by Puccinia striiformis f. sp. tritici, is a major objective for wheat breeding. To identify effective stripe rust resistance loci, a genome-wide association study (GWAS) was conducted using 232 elite durum wheat (Triticum turgidum ssp. durum) lines from worldwide breeding programs. Genotyping with the 90 K iSelect wheat single nucleotide polymorphism (SNP) array resulted in 11,635 markers distributed across the genome. Response to stripe rust infection at the seedling stage revealed resistant and susceptible accessions present in rather balanced frequencies for the six tested races, with a higher frequency of susceptible responses to United States races as compared to Italian races (61.1 vs. 43.1% of susceptible accessions). Resistance at the seedling stage only partially explained adult plant resistance, which was found to be more frequent with 67.7% of accessions resistant across six nurseries in the United States. GWAS identified 82 loci associated with seedling stripe rust resistance, five of which were significant at the false discovery rate adjusted P value <0.1 and 11 loci were detected for the field response at the adult plant stages in at least two environments. Notably, Yrdurum-1BS.1 showed the largest effect for both seedling and field resistance, and is therefore considered as a major locus for resistance in tetraploid wheat. Our GWAS study is the first of its kind for stripe rust resistance in tetraploid wheat and provides an overview of resistance in elite germplasm and reports new loci that can be used in breeding resistant cultivars.

     

Molecular mapping of stripe rust resistance gene <Emphasis Type="Italic">YrHu</Emphasis> derived from <Emphasis Type="Italic">Psathyrostachys huashanica</Emphasis>

Wheat stripe rust is a destructive disease that affects most wheat-growing areas worldwide. Resistance genes from related species and genera add to the genetic diversity available to wheat breeding programs. The stripe rust-resistant introgression line H9020-17-25-6-4 was developed from a cross of resistant Psathyrostachys huashanica with the susceptible wheat cultivar 7182. H9020-17-25-6-4 is resistant to all existing Chinese stripe rust races, including the three most widely virulent races, CYR32, CYR33, and V26. We attempted to characterize this new line by genomic in situ hybridization (GISH) and genetic analysis. GISH using P. huashanica genomic DNA as a probe indicated that the translocated segment was too small to be detected. Genetic analysis involving F₁, F₂, and F_2:3 materials derived from a cross of Mingxian 169 and H9020-17-25-6-4 indicated that a single dominant gene from H9020-17-25-6-4, temporarily designated YrHu, conferred resistance to CYR29 and CYR33. A genetic map consisting of four simple sequence repeat, two sequence-tagged site (STS), and two sequence-related amplified polymorphism markers was constructed. YrHu was located on the short arm of chromosome 3A and was about 0.7 and 1.5 cM proximal to EST-STS markers BG604577 and BE489244, respectively. Both the gene and the closely linked markers could be used in marker-assisted selection.     

A novel Robertsonian translocation event leads to transfer of a stem rust resistance gene (<Emphasis Type="Italic">Sr52</Emphasis>) effective against race Ug99 from <Emphasis Type="Italic">Dasypyrum</Emphasis><Emphasis Type="Italic">villosum</Emphasis> into bread wheat

Stem rust (Puccinia graminis f. sp. tritici Eriks. & E. Henn.) (the causal agent of wheat stem rust) race Ug99 (also designated TTKSK) and its derivatives have defeated several important stem rust resistance genes widely used in wheat (Triticum aestivum L.) production, rendering much of the worldwide wheat acreage susceptible. In order to identify new resistance sources, a large collection of wheat relatives and genetic stocks maintained at the Wheat Genetic and Genomic Resources Center was screened. The results revealed that most accessions of the diploid relative Dasypyrum villosum (L.) Candargy were highly resistant. The screening of a set of wheat–D. villosum chromosome addition lines revealed that the wheat–D. villosum disomic addition line DA6V#3 was moderately resistant to race Ug99. The objective of the present study was to produce and characterize compensating wheat–D. villosum whole arm Robertsonian translocations (RobTs) involving chromosomes 6D of wheat and 6V#3 of D. villosum through the mechanism of centric breakage-fusion. Seven 6V#3-specific EST–STS markers were developed for screening F₂ progeny derived from plants double-monosomic for chromosomes 6D and 6V#3. Surprisingly, although 6D was the target chromosome, all recovered RobTs involved chromosome 6A implying a novel mechanism for the origin of RobTs. Homozygous translocations (T6AS·6V#3L and T6AL·6V#3S) with good plant vigor and full fertility were selected from F₃ families. A stem rust resistance gene was mapped to the long arm 6V#3L in T6AS·6V#3L and was designated as Sr52. Sr52 is temperature-sensitive and is most effective at 16°C, partially effective at 24°C, and ineffective at 28°C. The T6AS·6V#3L stock is a new source of resistance to Ug99, is cytogenetically stable, and may be useful in wheat improvement.     

Microscopy and proteomic analysis of the non-host resistance of <Emphasis Type="Italic">Oryza sativa</Emphasis> to the wheat leaf rust fungus, <Emphasis Type="Italic">Puccinia triticina</Emphasis> f. sp. <Emphasis Type="Italic">tritici</Emphasis>

Rice (Oryza sativa) cv. Nipponbare expresses non-host resistance (NHR) to the wheat leaf rust fungus, Puccinia triticina f. sp. tritici (Ptt). When the leaves of cv. Nipponbare were inoculated with Ptt, approx 93% of the urediniospores germinated on the leaf surface, but only 10% of the germinated spores formed appressoria over the stomata at one day post inoculation (1 dpi). Hydrogen peroxide (H₂O₂) accumulated in host cells around the appressoria at 3 dpi. Approx. 3% of the appressoria produced short hyphae inside the leaf, and fluorescence was observed in tissue invaded by the hyphae by 7 dpi. At 22 dpi, 0.2% of the sites with appressoria formed branching infection hypha in mesophyll cells, but no substomatal vesicles, haustorial mother cells or haustoria were observed. Proteins were extracted from leaves 3 dpi and analyzed by two-dimensional gel electrophoresis (2-DE). A total 33 spots were reproducibly up-regulated and 9 were down-regulated by infection compared to the water inoculated control. Of these, 30 were identified by MALDI-TOF Mass Spectrometry. The identified proteins participate in defense/stress responses, energy/carbohydrate metabolism, oxidation–reduction processes, protein folding/turnover/cleavage/degradation, signal transduction and cell death regulation. The results indicates that NHR of rice to Ptt is consistent with a shift in protein and energy metabolism, increased antimicrobial activities, possibly including phytoalexin accumulation and cell wall reinforcement, increased cell repair, antioxidive and detoxification reactions, and enhanced prevention of plant cell death. Nearly half of the up-regulated identified proteins were associated with chloroplast and mitochondrial physiology suggesting important roles for these organelles during NHR.     

Genome-wide identification of QTL conferring high-temperature adult-plant (HTAP) resistance to stripe rust (<Emphasis Type="Italic">Puccinia striiformis</Emphasis> f. sp<Emphasis Type="Italic">. tritici</Emphasis>) in wheat

High-temperature adult-plant (HTAP) resistance to stripe rust (caused by Puccinia striiformis f. sp. tritici) is a durable type of resistance in wheat (Triticum aestivum L.). This study identified quantitative trait loci (QTL) conferring HTAP resistance to stripe rust in a population consisting of 169 F_8:10 recombinant inbred lines (RILs) derived from a cross between a susceptible cultivar Rio Blanco and a resistant germplasm IDO444. HTAP resistance was evaluated for both disease severity and infection type under natural infection over two years at two locations. The genetic linkage maps had an average density of 6.7 cM per marker across the genome and were constructed using 484 markers including 96 wheat microsatellite (SSR), 632 Diversity Arrays Technology (DArT) polymorphisms, two sequence-tagged-site (STS) from semi-dwarf genes Rht1 and Rht2, and two markers for low molecular-weight glutenin gene subunits. QTL analysis detected a total of eight QTL significantly associated with HTAP resistance to stripe rust with two on chromosome 2B, two on 3B and one on each of 1A, 4A, 4B and 5B. QTL on chromosomes 2B and 4A were the major loci derived from IDO444 and explained up to 47 and 42% of the phenotypic variation for disease severity and infection type, respectively. The remaining five QTL accounted for 7–10% of the trait variation. Of these minor QTL, the resistant alleles at the two QTL QYrrb.ui-3B.1 and QYrrb.ui-4B derived from Rio Blanco and reduced infection type only, while the resistant alleles at the other three QTL, QYrid.ui-1A, QYrid.ui-3B.2 and QYrid.ui-5B, all derived from IDO444 and reduced either infection type or disease severity. Markers linked to 2B and 4A QTL should be useful for selection of HTAP resistance to stripe rust.     

Genome-wide association mapping reveals a rich genetic architecture of stripe rust resistance loci in emmer wheat (<Emphasis Type="Italic">Triticum turgidum</Emphasis> ssp. <Emphasis Type="Italic">dicoccum</Emphasis>)

Key message

SNP-based genome scanning in worldwide domesticated emmer germplasm showed high genetic diversity, rapid linkage disequilibrium decay and 51 loci for stripe rust resistance, a large proportion of which were novel.

Abstract

Cultivated emmer wheat (Triticum turgidum ssp. dicoccum), one of the oldest domesticated crops in the world, is a potentially rich reservoir of variation for improvement of resistance/tolerance to biotic and abiotic stresses in wheat. Resistance to stripe rust (Puccinia striiformis f. sp. tritici) in emmer wheat has been under-investigated. Here, we employed genome-wide association (GWAS) mapping with a mixed linear model to dissect effective stripe rust resistance loci in a worldwide collection of 176 cultivated emmer wheat accessions. Adult plants were tested in six environments and seedlings were evaluated with five races from the United States and one from Italy under greenhouse conditions. Five accessions were resistant across all experiments. The panel was genotyped with the wheat 90,000 Illumina iSelect single nucleotide polymorphism (SNP) array and 5106 polymorphic SNP markers with mapped positions were obtained. A high level of genetic diversity and fast linkage disequilibrium decay were observed. In total, we identified 14 loci associated with field resistance in multiple environments. Thirty-seven loci were significantly associated with all-stage (seedling) resistance and six of them were effective against multiple races. Of the 51 total loci, 29 were mapped distantly from previously reported stripe rust resistance genes or quantitative trait loci and represent newly discovered resistance loci. Our results suggest that GWAS is an effective method for characterizing genes in cultivated emmer wheat and confirm that emmer wheat is a rich source of stripe rust resistance loci that can be used for wheat improvement.

     

Strains of <Emphasis Type="Italic">Bacillus</Emphasis> ssp. regulate wheat resistance to <Emphasis Type="Italic">Septoria nodorum</Emphasis> Berk.

The ability of Bacillus subtilis Cohn and Bacillus thuringiensis Berliner to induce systemic resistance in wheat plants to the casual agent of Septoria nodorum Berk., blotch has been studied. It has been shown that strains of Bacillus ssp. that possess the capacity for endophytic survival have antagonistic activity against this pathogen in vitro. A reduction of the degree of Septoria nodorum blotch development on wheat leaves under the influence of Bacillus spp. was accompanied by the suppression of catalase activity, an increase in peroxidase activity and H₂O₂ content, and expression of defence related genes such us PR-1, PR-6, and PR-9. It has been shown that B. subtilis 26 D induces expression levels of wheat pathogenesis-related (PR) genes which marks a SA-dependent pathway of sustainable development and that B. thuringiensis V-5689 and V-6066 induces a JA/ET-dependent pathway. These results suggest that these strain Bacillus spp. promotes the formation of wheat plant resistance to S. nodorum through systemic activation of the plant defense system. The designed bacterial consortium formed a complex biological response in wheat plants infected phytopathogen.     

Phylogenetic analyses of <Emphasis Type="Italic">Uromyces viciae-fabae</Emphasis> and its varieties on <Emphasis Type="Italic">Vicia</Emphasis>, <Emphasis Type="Italic">Lathyrus</Emphasis>, and <Emphasis Type="Italic">Pisum</Emphasis> in Japan

A pea rust fungus, Uromyces viciae-fabae, has been classified into two varieties, var. viciae-fabae and var. orobi, based on differences in urediniospore wall thickness and putative host specificity in Japan. In principal component analyses, morphological features of urediniospores and teliospores of 94 rust specimens from Vicia, Lathyrus, and Pisum did not show definite host-specific morphological groups. In molecular analyses, 23 Uromyces specimens from Vicia, Lathyrus, and Pisum formed a single genetic clade based on D1/D2 and ITS regions. Four isolates of U. viciae-fabae from V. cracca and V. unijuga could infect and sporulate on P. sativum. These results suggest that U. viciae-fabae populations on different host plants are not biologically differentiated into groups that can be recognized as varieties.Contribution no. 184, Laboratory of Plant Parasitic Mycology, Institute of Agriculture and Forestry, University of Tsukuba, Japan