Deletion of all three MAP kinase genes results in severe defects in stress responses and pathogenesis in Fusarium graminearum

Jingyi Ren,Zhuyun Bian,Xue Zhang,Huiquan Liu,Yuhua Wang,Yuhan Zhang,Jin-Rong Xu,Chengliang Li,Cong Jiang

doi:10.1007/s44154-021-00025-y

Jingyi Ren, Zhuyun Bian + Show 7 more

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https://doi.org/10.1007/s44154-021-00025-y

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Abstract

Mitogen-activated protein kinase (MAPK) cascades are activated by external stimuli and convert signals to cellular changes. Individual MAPKs have been characterized in a number of plant pathogenic fungi for their roles in pathogenesis and responses to biotic or abiotic stresses. However, mutants deleted of all the MAPK genes have not been reported in filamentous fungi. To determine the MAPK-less effects in a fungal pathogen, in this study we generated and characterized mutants deleted of all three MAPK genes in the wheat scab fungus Fusarium graminearum. The Gpmk1 mgv1 Fghog1 triple mutants had severe growth defects and was non-pathogenic. It was defective in infection cushion formation and DON production. Conidiation was reduced in the triple mutant, which often produced elongated conidia with more septa than the wild-type conidia. The triple mutant was blocked in sexual reproduction due to the loss of female fertility. Lack of any MAPKs resulted in an increased sensitivity to various abiotic stress including cell wall, osmotic, oxidative stresses, and phytoalexins, which are likely related to the defects of the triple mutant in environmental adaptation and plant infection. The triple mutant also had increased sensitivity to the biocontrol bacterium Bacillus velezensis and fungus Clonostachys rosea. In co-incubation assays with B. velezensis, the Gpmk1 mgv1 Fghog1 mutant had more severe growth limitation than the wild type and was defective in conidium germination and germ tube growth. In confrontation assays, the triple mutant was defective in defending against mycoparasitic activities of C. rosea and the latter could grow over the mutant but not wild-type F. graminearum. RNA-seq and metabolomics analyses showed that the MAPK triple mutant was altered in the expression of many ATP-binding cassette (ABC) and major facilitator superfamily (MFS) transporter genes and the accumulation of metabolites related to arachidonic acid, linoleic acid, and alpha-linolenic acid metabolisms. Overall, as the first study on mutants deleted of all three MAPKs in fungal pathogens, our results showed that although MAPKs are not essential for growth and asexual reproduction, the Gpmk1 mgv1 Fghog1 triple mutant was blocked in plant infection and sexual reproductions. It also had severe defects in responses to various abiotic stresses and bacterial- or fungal-fungal interactions.

Highlights

The homothallic ascomycete Fusarium graminearum is a causal agent of Fusarium Head Blight (FHB), a destructive disease of wheat, barley, and other grain cereals worldwide (Harris et al, 2016)
In comparison with the wild type, the triple mutant produced wavy germ tubes and hyphae (Fig. 1b), which was not observed in the Gpmk1 and Gpmk1 mgv1 mutants (Fig. 1b), suggesting an overlapping function of three Mitogen-activated protein kinase (MAPK) in hyphal growth
The roles of individual MAPKs in regulating developmental and infection processes, as well as stress responses have been characterized in a variety of fungal pathogens that differ in host ranges and infection mechanisms (Jiang et al 2018a)

Summary

Introduction

The homothallic ascomycete Fusarium graminearum is a causal agent of Fusarium Head Blight (FHB), a destructive disease of wheat, barley, and other grain cereals worldwide (Harris et al, 2016). Infection of wheat or barley heads by F. graminearum often results in severe yield losses, grain quality reduction, and contamination of mycotoxins such as deoxynivalenol (DON) and zearalenone (Audenaert et al, 2013; Goswami and Kistler 2004). DON is an important virulence factor in this pathogen and TRI genes responsible for DON biosynthesis are expressed in infection cushions (Boenisch and Schafer 2011). For asexual reproduction, it produces macroconidia or conidia, which are infectious and important for spreading infection and colonization of plant tissues. To survive in the nature and infect wheat or barley spikelets, F. graminearum must be able to sense various environmental and plant signals for properly regulating various developmental and infection production as well as DON production (Dilks et al, 2019; Jiang et al, 2019)

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