Responses of the Necrotrophic Fungus Alternaria brassisicola to the Indolic Phytoalexin Brassinin.

Guillaume Quang N’Guyen,Sandra Pelletier,Bruno Hamon,Thomas Guillemette,Pietrick Hudhomme,Antoine Porquier,Jean-Pierre Renou,Anthony Kwasiborski,Nelly Bataillé-Simoneau,Roxane Raulo,Philippe Simoneau,Beatrice Iacomi,Justine Colou,David Macherel,Claire Campion,Abdelilah Benamar

doi:10.3389/fpls.2020.611643

Abstract

Alternaria brassicicola causes black spot disease in Brassicaceae. During host infection, this necrotrophic fungus is exposed to various antimicrobial compounds, such as the phytoalexin brassinin which is produced by many cultivated Brassica species. To investigate the cellular mechanisms by which this compound causes toxicity and the corresponding fungal adaptive strategies, we first analyzed fungal transcriptional responses to short-term exposure to brassinin and then used additional functional approaches. This study supports the hypothesis that indolic phytoalexin primarily targets mitochondrial functions in fungal cells. Indeed, we notably observed that phytoalexin treatment of A. brassicicola disrupted the mitochondrial membrane potential and resulted in a significant and rapid decrease in the oxygen consumption rates. Secondary effects, such as Reactive oxygen species production, changes in lipid and endoplasmic reticulum homeostasis were then found to be induced. Consequently, the fungus has to adapt its metabolism to protect itself against the toxic effects of these molecules, especially via the activation of high osmolarity glycerol and cell wall integrity signaling pathways and by induction of the unfolded protein response.

Highlights

Plant antimicrobial secondary metabolites are one of the key elements of host immune systems
Phytoalexin sensitivity assays were first performed in 96well plates by monitoring initial growth stages using laser nephelometry in liquid medium amended with different brassinin concentrations (Figure 1A)
This brassinin concentration corresponds to concentrations likely to exist in localized leaf areas surrounding necrotic lesions (Kliebenstein et al, 2005; Stefanato et al, 2009)

Summary

Introduction

Plant antimicrobial secondary metabolites are one of the key elements of host immune systems. Two classes of plant antibiotics may be distinguished based on the type of synthesis in host cells (VanEtten et al, 1994). Phytoanticipins constitutively accumulate in plants or are produced during infection from preexisting constituents (Osbourn, 1996). Phytoalexins are produced by plants in response to biotic or abiotic stresses, and accumulate to the infected site. The two types of molecules include a wide range of chemical families, but closely related plants generally synthesize phytoalexins of similar chemical structures. Phytoalexins have been demonstrated to have inhibitory activity in vitro against various bacteria and fungi, and they confered disease protection in several parasitic plant-host interactions (González-Lamothe et al, 2009; Ahuja et al, 2012; Großkinsky et al, 2012)

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Results

Conclusion