Responses to Hydric Stress in the Seed-Borne Necrotrophic Fungus Alternaria brassicicola.

Guillaume Quang N’Guyen,Nelly Bataillé-Simoneau,Philippe Simoneau,Sandra Pelletier,Roxane Raulo,Benoit Porcheron,Bruno Hamon,Claire Campion,Chloé Mouchès,Beatrice Iacomi,Remi Lemoine,Thomas Guillemette,Carlos Agustí-Brisach,Anthony Kwasiborski,Jean-Pierre Renou,Muriel Marchi,Franck Bastide

doi:10.3389/fmicb.2019.01969

Abstract

Alternaria brassicicola is a necrotrophic fungus causing black spot disease and is an economically important seed-borne pathogen of cultivated brassicas. Seed transmission is a crucial component of its parasitic cycle as it promotes long-term survival and dispersal. Recent studies, conducted with the Arabidopsis thaliana/A. brassicicola pathosystem, showed that the level of susceptibility of the fungus to water stress strongly influenced its seed transmission ability. In this study, we gained further insights into the mechanisms involved in the seed infection process by analyzing the transcriptomic and metabolomic responses of germinated spores of A. brassicicola exposed to water stress. Then, the repertoire of putative hydrophilins, a group of proteins that are assumed to be involved in cellular dehydration tolerance, was established in A. brassicicola based on the expression data and additional structural and biochemical criteria. Phenotyping of single deletion mutants deficient for fungal hydrophilin-like proteins showed that they were affected in their transmission to A. thaliana seeds, although their aggressiveness on host vegetative tissues remained intact.

Highlights

The fungus Alternaria brassicicola causes black spot disease and is an economically important seed-borne pathogen of Brassicaceae species
Considering that the reference strain is naturally tolerant to water stress (Iacomi-Vasilescu et al, 2008), we analyzed the effect of treatments on three mutant strains derived from the wild-type Abra43 strain, for which we hypothesized a higher sensitivity to water stress
In the same way, when a drastic decrease in relative humidity (RH) was applied to the germlings, no significant difference in mortality between the treated and the silica gel-free conditions was observed for the reference strain

Summary

Introduction

The fungus Alternaria brassicicola causes black spot disease and is an economically important seed-borne pathogen of Brassicaceae species. This necrotrophic fungus strongly depends on seed transmission process for its long-term survival and dispersal (van den Bosch et al, 2010). Fungal and plant factors that impact seed transmission are still poorly described. Such knowledge is crucial to propose strategies for improving the seed health, which remains a major issue for seed companies. Recent studies conducted with the Arabidopsis thaliana/A. brassicicola pathosystem showed that the level of susceptibility of the fungus to water stress strongly influenced its seed transmission ability. Two osmosensitive fungal mutants, defective for the class III Histidine kinase (HK) AbNik (Pochon et al, 2013) and the MAP kinase AbHog (unpublished result), respectively, were highly jeopardized in their ability to colonize

Methods

Results

Conclusion