Wheat Encodes Small, Secreted Proteins That Contribute to Resistance to Septoria Tritici Blotch.

Binbin Zhou,Angela Feechan,Ciarán J Brennan,James I Burke,Ewen Mullins,Sujit J Karki,Fiona M Doohan,Harriet R Benbow,Chanemougasoundharam Arunachalam

doi:10.3389/fgene.2020.00469

Abstract

During plant–pathogen interactions, pathogens secrete many rapidly evolving, small secreted proteins (SSPs) that can modify plant defense and permit pathogens to colonize plant tissue. The fungal pathogen Zymoseptoria tritici is the causal agent of Septoria tritici blotch (STB), one of the most important foliar diseases of wheat, globally. Z. tritici is a strictly apoplastic pathogen that can secrete numerous proteins into the apoplast of wheat leaves to promote infection. We sought to determine if, during STB infection, wheat also secretes small proteins into the apoplast to mediate the recognition of pathogen proteins and/or induce defense responses. To explore this, we developed an SSP-discovery pipeline to identify small, secreted proteins from wheat genomic data. Using this pipeline, we identified 6,998 SSPs, representing 2.3% of all proteins encoded by the wheat genome. We then mined a microarray dataset, detailing a resistant and susceptible host response to STB, and identified 141 Z. tritici- responsive SSPs, representing 4.7% of all proteins encoded by Z. tritici – responsive genes. We demonstrate that a subset of these SSPs have a functional signal peptide and can interact with Z. tritici SSPs. Transiently silencing two of these wheat SSPs using virus-induced gene silencing (VIGS) shows an increase in susceptibility to STB, confirming their role in defense against Z. tritici.

Highlights

One of the most economically important species in the plant kingdom is bread wheat, Triticum aestivum
Using features typical of small secreted proteins, such as a protein length ≤ 250 amino acids and a secretion signal of an N-terminal signal peptide, we investigated the small secretome of wheat, to identify small secreted proteins from that may play a role in the wheat – Z. tritici interaction, and may interact with fungal SSPs that are present in the apoplast during infection
The percentage of SSPs attributed to the 21 wheat chromosomes ranged from 1.9–2.4%, and 3.7% were attributed to chromosome ‘Un’ (Table 1)

Summary

Introduction

One of the most economically important species in the plant kingdom is bread wheat, Triticum aestivum. Wheat dominates the European arable sector, with ∼150 million tons of wheat grown in the European Union annually (FAOSTAT 2019). While yields are generally high across the EU, wheat production is threatened by a range of pests and pathogens. One of the most important of these is Septoria tritici blotch, a foliar disease caused by the pathogenic fungus Zymoseptoria tritici (Z. tritici) (O’Driscoll et al, 2014). Z. tritici is a strictly apoplastic fungus, and is a hostspecific pathogen of wheat. The high selection pressure within intensive agricultural systems [high fungicide usage and dense planting of STB-resistant varieties (Fones and Gurr, 2015)], combined with rapid evolution of the pathogen (Dooley, 2015), has led to the widespread occurrence of

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