A pair of effectors encoded on a conditionally dispensable chromosome of Fusarium oxysporum suppress host-specific immunity

Yu Ayukawa,Yasunori Ichihashi,Martijn Rep,Ayako Tsushima,Tsutomu Arie,Ken Komatsu,Petra M Houterman,Shuta Asai,Ken Shirasu,Arisa Shibata,Pamela Gan

doi:10.1038/s42003-021-02245-4

Abstract

Many plant pathogenic fungi contain conditionally dispensable (CD) chromosomes that are associated with virulence, but not growth in vitro. Virulence-associated CD chromosomes carry genes encoding effectors and/or host-specific toxin biosynthesis enzymes that may contribute to determining host specificity. Fusarium oxysporum causes devastating diseases of more than 100 plant species. Among a large number of host-specific forms, F. oxysporum f. sp. conglutinans (Focn) can infect Brassicaceae plants including Arabidopsis (Arabidopsis thaliana) and cabbage. Here we show that Focn has multiple CD chromosomes. We identified specific CD chromosomes that are required for virulence on Arabidopsis, cabbage, or both, and describe a pair of effectors encoded on one of the CD chromosomes that is required for suppression of Arabidopsis-specific phytoalexin-based immunity. The effector pair is highly conserved in F. oxysporum isolates capable of infecting Arabidopsis, but not of other plants. This study provides insight into how host specificity of F. oxysporum may be determined by a pair of effector genes on a transmissible CD chromosome.

Highlights

Many plant pathogenic fungi contain conditionally dispensable (CD) chromosomes that are associated with virulence, but not growth in vitro
Here we report the identification of a CD chromosome in F. oxysporum that is required for virulence on Arabidopsis
This CD chromosome encodes a pair of effectors (SIX8 and Pair with SIX Eight1 (PSE1)) that are involved in suppressing Arabidopsis-specific immunity, and are conserved in the other F. oxysporum isolates capable of infecting

Summary

Introduction

Many plant pathogenic fungi contain conditionally dispensable (CD) chromosomes that are associated with virulence, but not growth in vitro. Horizontal transfer of the CD chromosomes from Fol4287 or Forc[016] converts non-pathogenic F. oxysporum isolates into pathogens of their respective hosts[6,7,9]. Part of this phytopathogenic conversion is often due to the expression of CD-encoded effectors that modulate host immunity against infection, such as Secreted In. Xylem (SIX) effectors that are, as their name indicates, secreted into xylem elements during infection[10,11]. The CD chromosome of Fol4287 contains all of the SIX genes except SIX4, which is not present in Fol42876,12, but is present in certain other

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