Abstract
Fusarium oxysoporum f. sp. radicis‐cucumerinum (Forc) is able to cause disease in cucumber, melon, and watermelon, while F. oxysporum f. sp. melonis (Fom) can only infect melon plants. Earlier research showed that mobile chromosomes in Forc and Fom determine the difference in host range between Forc and Fom. By closely comparing these pathogenicity chromosomes combined with RNA‐sequencing data, we selected 11 candidate genes that we tested for involvement in the difference in host range between Forc and Fom. One of these candidates is a putative effector gene on the Fom pathogenicity chromosome that has nonidentical homologs on the Forc pathogenicity chromosome. Four independent Forc transformants with this gene from Fom showed strongly reduced or no pathogenicity towards cucumber, while retaining pathogenicity towards melon and watermelon. This suggests that the protein encoded by this gene is recognized by an immune receptor in cucumber plants. This is the first time that a single gene has been demonstrated to determine a difference in host specificity between formae speciales of F. oxysporum.
Highlights
The Fusarium oxysporum (Fo) species complex contains both pathogenic and nonpathogenic strains, and a sexual stage has not been observed so far (Michielse and Rep, 2009; Pietro et al, 2003)
Cucumber, melon, and watermelon-infecting strains are classified as Fo f. sp. radicis-cucumerinum (Forc), while melon-infecting strains are classified as Fo f. sp. melonis (Fom) (Edel-Hermann and Lecomte, 2019)
Based on characteristics of effector genes identified previously in plant-pathogenic fungi, the following criteria were used for selecting candidate genes: (a) highly expressed in the relevant interaction, because we expect that genes involved in host–pathogen interaction are highly expressed; (b) absent or with less than 100% DNA sequence identity in either f. sp., because we expect that proteins that are not completely identical may have escaped recognition by immune receptors or have altered their function; and (c) presence of a signal peptide for secretion in the predicted translation product, because we assume that effector proteins need to be secreted to function as host-specific virulence or avirulence factors
Summary
The Fusarium oxysporum (Fo) species complex contains both pathogenic and nonpathogenic strains, and a sexual stage has not been observed so far (Michielse and Rep, 2009; Pietro et al, 2003). The third gene absent in Fom, g410, with unknown function, is highly expressed in Forc and has a mimp upstream of its coding sequence These three genes were selected as candidate cucumber/watermelon virulence genes. To assess whether the unique candidate avirulence gene in Fom, g14035, can turn Forc nonpathogenic to cucumber and/or watermelon, this gene with its native promoter and terminator was cloned and transformed randomly into the genome of Forc, and four transformants with correct sequences (confirmed by PCR) were selected for further analysis. For transformants of Fom with putative Forc-specific virulence genes, except Fom005_g287_g288_g410#4 and Fom005_ g287_g288_g410#9, all Fom transformants caused severe disease on melon plants, comparable to the parental strain (Figure 2a). Candidate Forc-specific virulence genes were not able to turn Fom pathogenic to cucumber or watermelon plants, while a single gene was able to turn Forc nonpathogenic to cucumber plants
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