Remarkable recent changes in the genetic diversity of the avirulence gene AvrStb6 in global populations of the wheat pathogen Zymoseptoria tritici.

Christopher Stephens,Andrew Milgate,Florian Hahn,Megan Mcdonald,Fatih Ölmez,Peter Solomon,Anuradha Bansal,Jason Rudd,Bart Fraaije,Kostya Kanyuka,Hannah Blyth,Emine Burcu Turgay,Vladimir Nekrasov,Cyrille Saintenac

doi:10.1111/mpp.13101

Abstract

Septoria tritici blotch (STB), caused by the fungus Zymoseptoria tritici, is one of the most economically important diseases of wheat. Recently, both factors of a gene‐for‐gene interaction between Z. tritici and wheat, the wheat receptor‐like kinase Stb6 and the Z. tritici secreted effector protein AvrStb6, have been identified. Previous analyses revealed a high diversity of AvrStb6 haplotypes present in earlier Z. tritici isolate collections, with up to c.18% of analysed isolates possessing the avirulence isoform of AvrStb6 identical to that originally identified in the reference isolate IPO323. With Stb6 present in many commercial wheat cultivars globally, we aimed to assess potential changes in AvrStb6 genetic diversity and the incidence of haplotypes allowing evasion of Stb6‐mediated resistance in more recent Z. tritici populations. Here we show, using targeted resequencing of AvrStb6, that this gene is universally present in field isolates sampled from major wheat‐growing regions of the world in 2013–2017. However, in contrast to the data from previous AvrStb6 population studies, we report a complete absence of the originally described avirulence isoform of AvrStb6 amongst modern Z. tritici isolates. Moreover, a remarkably small number of haplotypes, each encoding AvrStb6 protein isoforms conditioning virulence on Stb6‐containing wheat, were found to predominate among modern Z. tritici isolates. A single virulence isoform of AvrStb6 was found to be particularly abundant throughout the global population. These findings indicate that, despite the ability of Z. tritici to sexually reproduce on resistant hosts, AvrStb6 avirulence haplotypes tend to be eliminated in subsequent populations.

Highlights

The interactions between plant pathogens and their hosts during infection are highly complex and evolutionarily dynamic
Studies into the changes in Avr gene prevalence in the fungal pathogen Leptosphaeria maculans of Brassica crops over time have focused on single countries such as Canada (Fernando et al, 2018) or Australia (Van de Wouw et al, 2018)
We analysed the diversity of the avirulence factor AvrStb6 in the recent global Z. tritici populations

Summary

| INTRODUCTION

The interactions between plant pathogens and their hosts during infection are highly complex and evolutionarily dynamic. One other isolate, possessing AvrStb isoform I03, which was classified as virulent, induced lower levels of disease on the Stb6-containing wheat compared to their near- isogenic genotypes that lack Stb as reflected by differences in the observed pycnidiospore counts (Figure 3) These differences were significant (at p < 0 .01) only for the Z. tritici isolate T1-A2017.24 possessing the AvrStb isoform I13 and the control avirulent isolate IPO323. As each of the 11 tested isolates has different infection dynamics on this wheat cultivar, the leaf tissues for use in RT-qPCR were sampled at different time points after fungal inoculation, depending on the isolate, when the first symptoms of the disease become visible This infection phase was chosen as many Z. tritici effector genes, including AvrStb, have previously been shown to display maximal expression during transition from the biotrophic to the necrotrophic phase (Rudd et al, 2015; Zhong et al, 2017). These findings argue against the mechanism of Stb resistance breakdown being due to suppression of AvrStb expression

| DISCUSSION

Findings

| EXPERIMENTAL PROCEDURES