Mapping genes for resistance to barley stripe rust (Puccinia striiformis f. sp. hordei)

Two genes conferring resistance to the barley stripe rust found in Mexico and South America, previously identified as race 24, were mapped to the M arms of barley chromosomes 7 and 4 in a doubled haploid population using molecular markers and the quantitative trait loci (QTL) mapping approach. The resistance gene on chromosome 7 had a major effect, accounting for 57% of the variation in disease severity. The resistance gene on chromosome 4 had a minor effect, accounting for 10% of the variation in trait expression. Two pairs of restriction fragment length polymorphism markers are being used to introgress the resistance genes to North American spring barley using molecular marker-assisted backcrossing.Ore. Agric Exp Stn J no. 10283     

An overview of stripe rust of wheat (Puccinia striiformis f. sp. tritici) in Pakistan

Stripe rust (yellow rust) caused by Puccinia striiformis f. sp. tritici has been an important disease of wheat in the Indian subcontinent since 1786. Currently, it prevails across all the wheat growing areas from north to south in the country. Due to the favourable weather conditions, the northern uplands have been historically hit by the severe disease epidemics. These epidemics caused significant losses to national wheat production. Acquisition of broader virulence pattern by the pathogen poses a serious threat to national agriculture. Although the deployed national wheat varieties have adequate resistance, these are developed around few major genes and are vulnerable to the new evolving strains of the pathogen. Utilisation of race non-specific durable resistance and seedling resistance via gene pyramiding, based on the current virulence scenario of the pathogen should provide sustainable control. This review focuses on the national milestones that recognise the economic significance of the disease and current status of stripe rust and its management in Pakistan.     

Virulence variation of Puccinia striiformis Westend. f. sp. tritici in Pakistan

Yellow rust populations of Pakistan were characterised for their virulence pathotypes/races and pathogenetic variation using seedling evaluation of differential genotypes under glasshouse conditions in Murree (6000 feet above sea level). Differential genotypes comprised a world set, an European set, near isogenic lines and the universally susceptible bread wheat cultivar “Morocco”. Over the two-year study a total of 18 race groups were identified. Out of these 18 race groups, several (68E0, 64E0, 66E0, 70E0, 6E0, 71E0, 6E0, 2E0, 67E0, and 68E16) were found previously. The new race group 70E32 found probably evolved because of mutation from the previously existing 70E16. Virulence frequencies of yellow rust (Yr) resistance genes were also determined on near isogenic lines. The highest virulence frequencies (%) were found for Yr7 (88%), Yr9 (57%), Yr18 (70%), and Yr24 (69%). Virulence frequencies were low for Yr 1 (4%), Yr5 (7%), Yr10 (10%) and Yr15 (4%). Our studies indicated that virulence existed for almost all yr genes, necessitating regular monitoring of the yellow rust populations and intensifying efforts to identify new sources of resistance to this pathogen.     

Quantitative resistance of some Elite wheat lines to Puccinia striiformis f. sp. tritici

Host resistance is the most economical way to manage wheat stripe rust caused by Puccinia striiformis f. sp. tritici. Slow rusting, a type of quantitative resistance, has been reported to last for a long time. Quantitative resistance, in terms of slow rusting parameters including final rust severity (FRS), apparent infection rate (r), relative area under disease progress curve (rAUDPC) and coefficient of infection (CI), was evaluated in a set of 29 wheat genotypes along with susceptible control during 2008–2009 and 2009–2010 cropping seasons. This study was conducted in field plots at Ardabil Agricultural Research Station (Iran) under natural infection conditions with two times artificial inoculation. Artificial inoculation was carried out by yellow rust inoculum having virulent genes against Yr2, Yr6, Yr7, Yr9, Yr22, Yr23, Yr24, Yr25, Yr26, Yr27, YrA and YrSU. Results of mean comparison for resistance parameters showed that lines C-86-1, C-86-2, C-87-1 and C-87-3 along with susceptible had the highest values of FRS, CI, r and rAUDPC, therefore were selected as susceptible lines. The lines C-86-3, C-86-9, C-87-2, C-87-6, C-87-8, C-87-11 and C-87-18 were susceptible at the seedling stage and had low level infection at adult plant stage. Consequently, these lines with low different parameters most probably have slow rusting resistance. The remaining lines had no infection or were at low level of infection. Thus, they were selected as resistant or moderately resistant lines. In this study, correlation coefficient between different parameters of slow rusting was significantly high (r = 0.92–0.99).     

Genetic evidence of local adaptation of wheat yellow rust (Puccinia striiformis f. sp. tritici) within France

Puccinia striiformis f. sp. tritici (PST), a clonal basidiomycete causing yellow rust disease on wheat, has a long record of 'overcoming' cultivar resistance introduced by breeders. Despite the long dispersal capacity of its spores, the French population of PST presents a strong geographical structure, with the presence of a specific pathotype (array of avirulence genes) at high frequencies in the south of France. The genetic diversity underlying this differentiation was analysed by microsatellite and AFLP markers. A total of 213 French isolates belonging to 10 pathotypes collected over a 15-year period were investigated. For each of the 12 microsatellites used, polymorphism resulted from a unique allelic variant associated to the south-specific pathotype. This pathotype was characterized by 40 specific markers over the total of 63 polymorphins detected using 15 AFLP primer combinations. Phylogeographical analysis indicated a strictly clonal structure of the population, and a strong genomic divergence between the northern population and a south-specific clone. Both virulence and molecular data show that the northern French population belongs to the northwestern European population, whereas the southern clone is most likely related to a Mediterranean population, the two subpopulations resulting from the ancient divergence of two clonal lineages. While the virulence complexity in the northern population may be explained by the successive introduction of corresponding resistance genes in cultivars, the maintenance of a simple virulence type in southern France, despite gene flow between the two populations, may be explained in terms of host cultivars repartition and local adaptation to specific host or climatic conditions.     

Phenotypic and Molecular Characterization of Wheat for Slow Rusting Resistance against Puccinia striiformis Westend. f.sp. tritici

Race‐specific resistance of wheat (Triticum aestivum L.) to yellow rust caused by Puccinia striiformis Westend. f.sp. tritici is often short‐lived. Slow‐rusting resistance has been reported to be a more durable type of resistance. A set of sixteen bread wheat varieties along with a susceptible control Morocco was tested during 2004–05 to 2006–07 in field plots at Peshawar (Pakistan) to identify slow rusting genotypes through epidemiological variables including final rust severity (FRS), apparent infection rate (r), area under disease progress curve (AUDPC), average coefficients of infection (ACI) and leaf tip necrosis (LTN). Epidemiological parameters of resistance were significantly (P < 0.01) different for years/varieties in three seasons, while variety × year interactions remained non‐significant. Sequence tagged site (STS) marker, csLV34 analyses revealed that cultivars Faisalabad‐83, Bahawalpur‐95, Suleman‐96, Punjab‐96, Bakhtawar‐93, Faisalabad‐85, Shahkar‐95 and Kohsar‐95 possessed Yr18 linked allele. Faisalabad‐83, Bahawalpur‐95, Suleman‐96, Punjab‐96, Bakhtawar‐93 and Faisalabad‐85 were relatively more stable over 3‐years where FRS, AUDPC and r values reduced by 80, 84 and 70% respectively compared to control Morocco. These six varieties therefore could be exploited for the deployment of Yr18 in breeding for slow rusting in wheat. Both FRS and ACI are suitable parameters for phenotypic selection.     

TaADF4, an actin‐depolymerizing factor from wheat,is required for resistance to the stripe rust pathogen Puccinia striiformis f. sp. tritici

Actin filament assembly in plants is a dynamic process, requiring the activity of more than 75 actin‐binding proteins. Central to the regulation of filament assembly and stability is the activity of a conserved family of actin‐depolymerizing factors (ADFs), whose primarily function is to regulate the severing and depolymerization of actin filaments. In recent years, the activity of ADF proteins has been linked to a variety of cellular processes, including those associated with response to stress. Herein, a wheat ADF gene, TaADF4, was identified and characterized. TaADF4 encodes a 139‐amino‐acid protein containing five F‐actin‐binding sites and two G‐actin‐binding sites, and interacts with wheat (Triticum aestivum) Actin1 (TaACT1), in planta. Following treatment of wheat, separately, with jasmonic acid, abscisic acid or with the avirulent race, CYR23, of the stripe rust pathogen Puccinia striiformis f. sp. tritici, we observed a rapid induction in accumulation of TaADF4 mRNA. Interestingly, accumulation of TaADF4 mRNA was diminished in response to inoculation with a virulent race, CYR31. Silencing of TaADF4 resulted in enhanced susceptibility to CYR23, demonstrating a role for TaADF4 in defense signaling. Using a pharmacological‐based approach, coupled with an analysis of host response to pathogen infection, we observed that treatment of plants with the actin‐modifying agent latrunculin B enhanced resistance to CYR23, including increased production of reactive oxygen species and enhancement of localized hypersensitive cell death. Taken together, these data support the hypothesis that TaADF4 positively modulates plant immunity in wheat via the modulation of actin cytoskeletal organization.     

Host‐induced gene silencing of an important pathogenicity factor PsCPK1 in Puccinia striiformis f. sp. tritici enhances resistance of wheat to stripe rust

Rust fungi are devastating plant pathogens and cause a large economic impact on wheat production worldwide. To overcome this rapid loss of resistance in varieties, we generated stable transgenic wheat plants expressing short interfering RNAs (siRNAs) targeting potentially vital genes of Puccinia striiformis f. sp. tritici (Pst). Protein kinase A (PKA) has been proved to play important roles in regulating the virulence of phytopathogenic fungi. PsCPK1, a PKA catalytic subunit gene from Pst, is highly induced at the early infection stage of Pst. The instantaneous silencing of PsCPK1 by barley stripe mosaic virus (BSMV)‐mediated host‐induced gene silencing (HIGS) results in a significant reduction in the length of infection hyphae and disease phenotype. These results indicate that PsCPK1 is an important pathogenicity factor by regulating Pst growth and development. Two transgenic lines expressing the RNA interference (RNAi) construct in a normally susceptible wheat cultivar displayed high levels of stable and consistent resistance to Pst throughout the T₃ to T₄ generations. The presence of the interfering RNAs in transgenic wheat plants was confirmed by northern blotting, and these RNAs were found to efficiently down‐regulate PsCPK1 expression in wheat. This study addresses important aspects for the development of fungal‐derived resistance through the expression of silencing constructs in host plants as a powerful strategy to control cereal rust diseases.     

Next generation sequencing provides rapid access to the genome of Puccinia striiformis f. sp. tritici, the causal agent of wheat stripe rust

Background

The wheat stripe rust fungus (Puccinia striiformis f. sp. tritici, PST) is responsible for significant yield losses in wheat production worldwide. In spite of its economic importance, the PST genomic sequence is not currently available. Fortunately Next Generation Sequencing (NGS) has radically improved sequencing speed and efficiency with a great reduction in costs compared to traditional sequencing technologies. We used Illumina sequencing to rapidly access the genomic sequence of the highly virulent PST race 130 (PST-130).

Methodology/Principal Findings

We obtained nearly 80 million high quality paired-end reads (>50x coverage) that were assembled into 29,178 contigs (64.8 Mb), which provide an estimated coverage of at least 88% of the PST genes and are available through GenBank. Extensive micro-synteny with the Puccinia graminis f. sp. tritici (PGTG) genome and high sequence similarity with annotated PGTG genes support the quality of the PST-130 contigs. We characterized the transposable elements present in the PST-130 contigs and using an ab initio gene prediction program we identified and tentatively annotated 22,815 putative coding sequences. We provide examples on the use of comparative approaches to improve gene annotation for both PST and PGTG and to identify candidate effectors. Finally, the assembled contigs provided an inventory of PST repetitive elements, which were annotated and deposited in Repbase.

Conclusions/Significance

The assembly of the PST-130 genome and the predicted proteins provide useful resources to rapidly identify and clone PST genes and their regulatory regions. Although the automatic gene prediction has limitations, we show that a comparative genomics approach using multiple rust species can greatly improve the quality of gene annotation in these species. The PST-130 sequence will also be useful for comparative studies within PST as more races are sequenced. This study illustrates the power of NGS for rapid and efficient access to genomic sequence in non-model organisms.     

A substance inducing teliospore production in wheat leaf rust,Puccinia recondita f. sp.tritici

A substance inducing teliospore production inPuccinia racondita f. sp.tritici was found in water and methanol extracts of wheat leaves with telia of the wheat leaf rust just before harvest time. Methanol (MeOH) and water extracts from uninfected wheat leaves also showed telia-inducing activity. However, the MeOH and water extracts from wheat leaves covered with telia showed much stronger activity than those from uninfected wheat leaves. We obtained a fraction (0.2 mg) showing activity at 2 ng/ml by purification of the water extract.     

小麦条锈菌分子生态学研究进展

小麦条锈病是危害最严重的小麦流行性病害之一,小麦条锈菌的生态学研究对制定合理的防治策略和抗锈育种具有重要意义.近十几年来,DNA分子标记技术被应用于小麦条锈菌的群体遗传学研究,推动了小麦条锈菌分子生态学研究的快速发展,为揭示小麦条锈菌的群体生态特性开辟了一个新的途径.本文系统介绍小麦条锈菌分子生态学研究的主要进展,并就我国当前研究的局限性和发展趋势进行了分析.     

Comparative virulence phenotypes and molecular genotypes of Puccinia striiformis f. sp. tritici, the wheat stripe rust pathogen in China and the United States

Stripe rust (yellow rust) of wheat, caused by Puccinia striiformis f. sp. tritici, is one of the most important diseases in both China and the United States. The Chinese and US populations of the stripe rust fungus were compared for their virulence phenotypes on wheat cultivars used to differentiate races of the pathogen in China and the US and molecular genotypes using simple sequence repeat (SSR) markers. From 86 Chinese isolates, 54 races were identified based on reactions on the 17 Chinese differentials and 52 races were identified based on the 20 US differentials. The selected 51 US isolates, representing 50 races based on the US differentials, were identified as 41 races using the Chinese differentials. A total of 132 virulence phenotypes were identified from the 137 isolates based on reactions on both Chinese and US differentials. None of the isolates from the two countries had identical virulence phenotypes on both sets of differentials. From the 137 isolates, SSR markers identified 102 genotypes, of which 71 from China and 31 from the US. The virulence data clustered the 137 isolates into 20 virulence groups (VGs) and the marker data clustered the isolates into seven molecular groups (MGs). Virulence and SSR data had a low (r = 0.34), but significant (P = 0.01) correlation. Principal component analyses using either the virulence data or the SSR data separated the isolates into three groups: group a consisting of only Chinese isolates, group b consisting of both Chinese and US isolates and group c consisting of mostly US isolates. A neighbour-joining tree generated using the molecular data suggested that the P. striiformis f. sp. tritici populations of China and the US in general evolved independently.     

Reaction of some wheat cultivars and breeding lines to Puccinia striiformis f. sp. tritici hot races in Iran

Many physiological races of Puccinia striiformis f. sp. tritici which cause stripe rust in wheat can be determined in different parts of the world. The emergence of new races with different pathogenicity which happens very quickly breaks cultivars resistant and cause disease. Therefore, breeding cultivar for resistance to different pathogenic races should be continued. In this research, pathogenicity of two isolates collected from two regions of Iran were determined by using wheat yellow rust differential lines, which indicated race 70E50A+ and 6E18A+ The responses of 30 wheat genotypes were separately evaluated in the forms of randomized complete block design with three replicates in the seedling stage under greenhouse condition. The components of resistance including latent period and infection type were recorded. Results indicated genotypes were evaluated in terms of both traits and were significant at 1% level. Also, the results from pathogenicity study indicated of effective gene/s included Yr1, Yr2+, Yr3, Yr4, Yr5, Yr10, Yr15, Yr24, Yr26, YrSP, YrND, YrSD and YrSU. From the genotypes studied in the greenhouse condition, 39% of the genotypes showed complete resistance to both races. Probably, resistance genes, Yr32 and YrCV, or the other unknown genes which are types of seedling resistance are either alone or in combination of one another cause strength in resistant genotypes.