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International audience; The deployment of plant varieties carrying resistance genes (R) exerts strong selection pressure on pathogen populations. Rapidly evolving avirulence genes (Avr) allow pathogens to escape R-mediated plant immunity through a variety of mechanisms, leading to virulence. The poplar rust fungus Melampsora larici-populina is a damaging pathogen of poplars in Europe. It underwent a major adaptive event in 1994, with the breakdown of the poplar RMlp7 resistance gene. Population genomics studies identified a locus in the genome of M. larici-populina that probably corresponds to the candidate avirulence gene AvrMlp7. Here, to further characterize this effector, we used a population genetics approach on a comprehensive set of 281 individuals recovered throughout a 28-year period encompassing the resistance breakdown event. Using two dedicated molecular tools, genotyping at the candidate locus highlighted two different alterations of a predominant allele found mainly before the resistance breakdown: a nonsynonymous mutation and a complete deletion of this locus. This results in six diploid genotypes: three genotypes related to the avirulent phenotype and three related to the virulent phenotype. The temporal survey of the candidate locus revealed that both alterations were found in association during the resistance breakdown event. They pre-existed before the breakdown in a heterozygous state with the predominant allele cited above. Altogether, these results suggest that the association of both alterations at the candidate locus AvrMlp7 drove the poplar rust adaptation to RMlp7-mediated immunity. This study demonstrates for the first time a case of adaptation from standing genetic variation in rust fungi during a qualitative resistance breakdown.

The recent availability of genome-wide sequencing techniques has allowed systematic screening for molecular signatures of adaptation, including in nonmodel organisms. Host–pathogen interactions constitute good models due to the strong selective pressures that they entail. We focused on an adaptive event which affected the poplar rust fungus Melampsora larici-populina when it overcame a resistance gene borne by its host, cultivated poplar. Based on 76 virulent and avirulent isolates framing narrowly the estimated date of the adaptive event, we examined the molecular signatures of selection. Using an array of genome scan methods based on different features of nucleotide diversity, we detected a single locus exhibiting a consistent pattern suggestive of a selective sweep in virulent individuals (excess of differentiation between virulent and avirulent samples, linkage disequilibrium, genotype–phenotype statistical association, and long-range haplotypes). Our study pinpoints a single gene and further a single amino acid replacement which may have allowed the adaptive event. Although our samples are nearly contemporary to the selective sweep, it does not seem to have affected genome diversity further than the immediate vicinity of the causal locus, which can be explained by a soft selective sweep (where selection acts on standing variation) and by the impact of recombination in mitigating the impact of selection. Therefore, it seems that properties of the life cycle of M. larici-populina, which entails both high genetic diversity and outbreeding, has facilitated its adaptation.

Wheat stem rust, a devastating disease of wheat and barley caused by the fungal pathogen Puccinia graminis f. sp. tritici, was largely eradicated in Western Europe during the mid-to-late twentieth century. However, isolated outbreaks have occurred in recent years. Here we investigate whether a lack of resistance in modern European varieties, increased presence of its alternate host barberry and changes in climatic conditions could be facilitating its resurgence. We report the first wheat stem rust occurrence in the United Kingdom in nearly 60 years, with only 20% of UK wheat varieties resistant to this strain. Climate changes over the past 25 years also suggest increasingly conducive conditions for infection. Furthermore, we document the first occurrence in decades of P. graminis on barberry in the UK . Our data illustrate that wheat stem rust does occur in the UK and, when climatic conditions are conducive, could severely harm wheat and barley production. Clare Lewis et al. report the first identification in nearly 60 years of a cultivated wheat plant infected with the fungal pathogen Puccinia graminis f.sp. tritici (wheat stem rust) in the United Kingdom. They find that only 20% of UK wheat varieties are resistant to this strain and urge growers to resume resistance breeding programs.

Made available in DSpace on 2018-12-11T17:06:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-10-03 Background: In February 2016, a new fungal disease was spotted in wheat fields across eight districts in Bangladesh. The epidemic spread to an estimated 15,000 hectares, about 16% of the cultivated wheat area in Bangladesh, with yield losses reaching up to 100%. Within weeks of the onset of the epidemic, we performed transcriptome sequencing of symptomatic leaf samples collected directly from Bangladeshi fields. Results: Reinoculation of seedlings with strains isolated from infected wheat grains showed wheat blast symptoms on leaves of wheat but not rice. Our phylogenomic and population genomic analyses revealed that the wheat blast outbreak in Bangladesh was most likely caused by a wheat-infecting South American lineage of the blast fungus Magnaporthe oryzae. Conclusion: Our findings suggest that genomic surveillance can be rapidly applied to monitor plant disease outbreaks and provide valuable information regarding the identity and origin of the infectious agent. Bangabandhu Sheikh Mujibur Rahman Agricultural University Department of Biotechnology Institute of Integrative Biology ETH Zurich Plant Pathology INRA UMR 385 Biologie et génétique des interactions plantes-pathogènes BGPI University of Exeter College of Life and Environmental Sciences Norwich Research Park Earlham Institute Bangladesh Agricultural Research Institute Argo-Environmental Remote Sensing and Modeling Lab, Joydebpur 1701 Directorate of Grains Industry Department of Agriculture and Food Western Australia (DAFWA), 3 Baron-Hay Court Brazilian Agricultural Research Enterprise - EMBRAPA Wheat/Trigo University of São Paulo State - UNESP Department of Crop Protection Rural Engineering and Soil Science, IlhaSolteira Campus CIRAD UMR 385 Biologie et génétique des interactions plantes-pathogènes BGPI Norwich Research Park The Sainsbury Laboratory Max Planck Institute for Plant Breeding Research, Carl-von-Linné-Weg 10 INRA UMR 1290 Biologie et Gestion des Risques en agriculture BIOGER Norwich Research Park John Innes Centre University of São Paulo State - UNESP Department of Crop Protection Rural Engineering and Soil Science, IlhaSolteira Campus

Melampsora larici-populina is a fungal pathogen responsible for foliar rust disease on poplar trees, which causes damage to forest plantations worldwide, particularly in Northern Europe. The reference genome of the isolate 98AG31 was previously sequenced using a whole genome shotgun strategy, revealing a large genome of 101 megabases containing 16,399 predicted genes, which included secreted protein genes representing poplar rust candidate effectors. In the present study, the genomes of 15 isolates collected over the past 20 years throughout the French territory, representing distinct virulence profiles, were characterized by massively parallel sequencing to assess genetic variation in the poplar rust fungus. Comparison to the reference genome revealed striking structural variations. Analysis of coverage and sequencing depth identified large missing regions between isolates related to the mating type loci. More than 611,824 single-nucleotide polymorphism (SNP) positions were uncovered overall, indicating a remarkable level of polymorphism. Based on the accumulation of non-synonymous substitutions in coding sequences and the relative frequencies of synonymous and non-synonymous polymorphisms (i.e., PN/PS ), we identify candidate genes that may be involved in fungal pathogenesis. Correlation between non-synonymous SNPs in genes encoding secreted proteins (SPs) and pathotypes of the studied isolates revealed candidate genes potentially related to virulences 1, 6, and 8 of the poplar rust fungus.

Wheat stripe rust, caused by the fungal pathogen Puccinia striiformis f. sp. tritici (Pst), occurs in all major wheat-growing regions worldwide and poses a constant threat to production. In South Africa, Pst first emerged in 1996 in the Western Cape and has since caused frequent epidemics with three further distinct races (pathotypes) recorded to date. Herein, we undertook detailed genomic-based analyses of four Pst isolates that represent the four dominant Pst races in South Africa recorded between 1996 and 2005. This analysis identified a number of polymorphic genes with features of known effector proteins and provided additional support of the likely stepwise changes in virulence profile of these South African Pst isolates. Next, we carried out comparative genomic-based analyses with 54 additional Pst isolates collected across wheat-growing regions within South Africa between 1996 and 2017 and 58 Pst isolates from East Africa, Pakistan, the UK, and France. This revealed a close genetic relationship between Pst isolates in South Africa and a number from East Africa. Furthermore, we found the South African Pst isolates also grouped closely with isolates identified in the UK in 2013 that were specifically found on triticale, illustrating long-distance transmission of Pst isolates either between these regions or from a common independent source area. This highlights the critical need for close monitoring of Pst. With wheat being the most planted winter cereal crop in South Africa, investment in continuous surveillance is essential to rapidly identify any future introductions that could quickly lead to rust epidemics.