Abstract
When infecting a host plant, the fungus Fusarium oxysporum secretes several effector proteins into the xylem tissue to promote virulence. However, in a host plant with an innate immune system involving analogous resistance proteins, the fungus effector proteins may trigger resistance, rather than promoting virulence. Identity of the effector genes of Fusarium oxysporum f. sp. niveum (Fon) races that affect watermelon (Citrullus lanatus) are currently unknown. In this study, the SIX6 (secreted in xylem protein 6) gene was identified in Fon races 0 and 1 but not in the more virulent Fon race 2. Disrupting the FonSIX6 gene in Fon race 1 did not affect the sporulation or growth rate of the fungus but significantly enhanced Fon virulence in watermelon, suggesting that the mutant ΔFon1SIX6 protein allowed evasion of R protein-mediated host resistance. Complementation of the wild-type race 2 (which lacks FonSIX6) with FonSIX6 reduced its virulence. These results provide evidence supporting the hypothesis that FonSIX6 is an avirulence gene. The identification of FonSix6 as an avirulence factor may be a first step in understanding the mechanisms of Fon virulence and resistance in watermelon and further elucidating the role of Six6 in Fusarium-plant interactions.
Highlights
When infecting a host plant, the fungus Fusarium oxysporum secretes several effector proteins into the xylem tissue to promote virulence
To obtain additional information about FonSIX6, the Fol genome was used as a reference for designing specific PCR primers for the flanking sequence
The 1974 bp upstream sequence and the 453 bp downstream of the FonSIX6 open reading frame were cloned via chromosome walking (Supplementary Fig. S2)
Summary
When infecting a host plant, the fungus Fusarium oxysporum secretes several effector proteins into the xylem tissue to promote virulence. Disrupting the FonSIX6 gene in Fon race 1 did not affect the sporulation or growth rate of the fungus but significantly enhanced Fon virulence in watermelon, suggesting that the mutant ΔFon1SIX6 protein allowed evasion of R protein-mediated host resistance. Race 0 is pathogenic only in watermelon cultivars with no resistance genes. Effectors may be defined as pathogen proteins and small molecules that alter host-cell structure and function These alterations either facilitate infection (virulence factors and toxins) or trigger defence responses (avirulence factors and elicitors), or both[9]. Lycopersici (Fol) infecting tomato have been identified through proteomic analysis of xylem sap from tomato plants infected with Fol. The secreted effector proteins of F. oxysporum f. Several functions of the Six proteins www.nature.com/scientificreports/
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