The Lipoxygenase Lox1 Is Involved in Light- and Injury-Response, Conidiation, and Volatile Organic Compound Biosynthesis in the Mycoparasitic Fungus Trichoderma atroviride.

Verena Speckbacher,Veronika Ruzsanyi,Stefan Böhmdorfer,Ainhoa Martinez-Medina,Maria Doppler,Ulrike Schreiner,Monika Schmoll,Rainer Schuhmacher,Susanne Zeilinger,Massimo Reverberi,Wolfgang Hinterdobler,Marzia Beccaccioli

doi:10.3389/fmicb.2020.02004

Abstract

The necrotrophic mycoparasite Trichoderma atroviride is a biological pest control agent frequently applied in agriculture for the protection of plants against fungal phytopathogens. One of the main secondary metabolites produced by this fungus is 6-pentyl-α-pyrone (6-PP). 6-PP is an organic compound with antifungal and plant growth-promoting activities, whose biosynthesis was previously proposed to involve a lipoxygenase (Lox). In this study, we investigated the role of the single lipoxygenase-encoding gene lox1 encoded in the T. atroviride genome by targeted gene deletion. We found that light inhibits 6-PP biosynthesis but lox1 is dispensable for 6-PP production as well as for the ability of T. atroviride to parasitize and antagonize host fungi. However, we found Lox1 to be involved in T. atroviride conidiation in darkness, in injury-response, in the production of several metabolites, including oxylipins and volatile organic compounds, as well as in the induction of systemic resistance against the plant-pathogenic fungus Botrytis cinerea in Arabidopsis thaliana plants. Our findings give novel insights into the roles of a fungal Ile-group lipoxygenase and expand the understanding of a light-dependent role of these enzymes.

Highlights

Trichoderma atroviride is a filamentous ascomycete exhibiting a necrotrophic mycoparasitic lifestyle with high antagonistic activity against a broad range of plant pathogenic fungi (Karlsson et al, 2017)
That 6-PP has antifungal activity and that secondary metabolite production contributes to Trichoderma mycoparasitism (Zeilinger et al, 2016), we analyzed lox1 gene expression in T. atroviride upon its mycoparasitic interaction with R. solani
Isotopic labeling experiments in T. atroviride in the early 1990s suggested the oxidation of linoleic acid by a lipoxygenase as a first step in the biosynthesis of 6-PP, one of T. atroviride’s main secondary metabolites with antifungal and plant growth promoting properties (Scarselletti and Faull, 1994; Vinale et al, 2008; GarnicaVergara et al, 2016; Ismaiel and Ali, 2017)

Summary

Introduction

Trichoderma atroviride is a filamentous ascomycete exhibiting a necrotrophic mycoparasitic lifestyle with high antagonistic activity against a broad range of plant pathogenic fungi (Karlsson et al, 2017). T. atroviride in addition is capable of priming the plant’s immune system, thereby inducing systemic resistance as well as enhancing the plant’s resilience against adverse environmental impacts, and promoting plant growth (Harman et al, 2004; Mukherjee et al, 2013b; Lee et al, 2015). Depending on the environmental conditions, T. atroviride produces a plethora of secondary metabolites (e.g., non-ribosomal peptides, polyketides, and terpenoids; Hermosa et al, 2014; Zeilinger et al, 2016) These include volatile organic compounds (VOCs; Stoppacher et al, 2010), which influence the mycoparasitic activity, as well as the interaction of T. atroviride with plants (Druzhinina et al, 2011; Lee et al, 2015; Holzlechner et al, 2016). 6-PP producing Trichoderma species like T. atroviride, Trichoderma gamsii and Trichoderma harzianum encode a lox gene in their genomes, while non-producers such as Trichoderma virens and Trichoderma reesei do not (Atanasova et al, 2013; Zeilinger et al, 2016)

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