Biocontrol Activity of Nonpathogenic Strains of Fusarium oxysporum: Colonization on the Root Surface to Overcome Nutritional Competition.

Yuichiro Iida,Aya Ogata,Hiroki Kanda,Takashi Tsuge,Oumi Nishi,Hirotoshi Sushida,Yumiko Higashi

doi:10.3389/fmicb.2022.826677

Abstract

Fusarium oxysporum is a soil-borne fungal pathogen that causes vascular wilts in a wide variety of crops. Certain nonpathogenic strains of F. oxysporum are known to protect crops against F. oxysporum pathogens. We assessed the biocontrol activities of nonpathogenic mutants of F. oxysporum ff. spp. melonis and lycopersici generated by disruption of the FOW2 gene, which encodes a Zn(II)2Cys6-type transcriptional regulator essential for their pathogenicity. Pre-inoculation of melon or tomato roots with strain ΔFOW2 conidia markedly reduced disease incidence caused by the parental wild-type strain in a concentration-dependent manner of conidial suspensions of ΔFOW2 strains. The biocontrol effect caused by the ΔFOW2 pre-inoculation lasted for at least 7 days. Pre-inoculation of melon roots with the wild-type or ΔFOW2 strain of F. oxysporum f. sp. lycopersici and nonpathogenic F. oxysporum strain also led to biocontrol activity against F. oxysporum f. sp. melonis, indicating that the biocontrol activity of ΔFOW2 strains is due to its nonpathogenic nature, not to the FOW2 disfunction. Conidial germination and hyphal elongation of only the wild-type strain were inhibited on melon root surface pre-inoculated with conidia of strains nonpathogenic to melon plants. Expression of defense-related genes was not significantly induced in roots and aboveground parts of melon seedlings preinoculated with ΔFOW2 conidia. Carbon source competition assay showed that nonpathogenic strains competed with the wild-type strain for a carbon source in soil. Strain ΔFOW2 also competed with the oomycete pathogen Pythium aphanidermatum for carbon source and protected melon plants from P. aphanidermatum. Our results suggest that the biocontrol activity of the nonpathogenic F. oxysporum strains used in this study mainly depends on their extensive colonization of the root surface and outcompeting pathogens for nutrients.

Highlights

The soil-borne pathogen Fusarium oxysporum is a facultative fungus that causes economically important losses in a wide range of crops (Michielse and Rep, 2009; Edel-Hermann and Lecomte, 2019)
Imazaki et al (2007) isolated FOW2 strains from strain Mel02010 of F. oxysporum f. sp. melonis and found that the mutants were not pathogenic to host plants
In tests of three strains with a mutation in one of three pathogenicity genes that are conserved in F. oxysporum for activity against a pathogenic strain of the fungus, pre-inoculation with the FOW2 strain yielded the best biocontrol against pathogenic strains of F. oxysporum ff. spp. melonis and lycopersici, and its activity was concentration dependent, like that of the biocontrol by nonpathogenic F. oxysporum strain MFG6

Summary

Introduction

The soil-borne pathogen Fusarium oxysporum is a facultative fungus that causes economically important losses in a wide range of crops (Michielse and Rep, 2009; Edel-Hermann and Lecomte, 2019). Interactions between pathogenic and nonpathogenic F. oxysporum strains in suppressive soils directly or indirectly contribute to disease control; nonpathogenic strains have been developed as biocontrol agents (Ogawa and Komada, 1984; Postma and Rattink, 1992; Alabouvette et al, 1993) Their main modes of action include competition for nutrients and trace elements in the rhizosphere and for infection sites on the root surface and induction of plant resistance (Ogawa and Komada, 1984; Postma and Luttikholt, 1996; Fuchs et al, 1997; Larkin and Fravel, 1998; Fravel et al, 2003; Alabouvette et al, 2009).

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