A SIX1 Homolog in Fusarium oxysporum f. sp. conglutinans Is Required for Full Virulence on Cabbage

Fusarium oxysporum is a soil-born fungus that induces wilt and root rot on a variety of plants. F. oxysporum f. sp. conglutinans (Foc) can cause wilt disease on cabbage. This study showed that a homolog of SIX1 protein in the Arabidopsis infecting isolate Fo5176 (Fo5176-SIX1) had four isoforms in the conidia of Foc by proteomic analysis. Thus, we analyzed the roles of protein Foc-SIX1. Gene expression analysis showed that, compared to the expression in mycelia, dramatically altered expression of Foc-SIX1 could be detected after infecting cabbages, and Foc-SIX1 was highly expressed in conidia under axenic culture condition. Furthermore, we knocked out the Foc-SIX1 gene and found that Foc-ΔSIX1 mutants had significantly reduced virulence compared with wild type isolate, and full virulence was restored by complementation of Foc-ΔSIX1 mutants with Foc-SIX1. Thus, we concluded that SIX1 in Foc was required for full virulence on cabbage. We also complemented Foc-ΔSIX1 with SIX1 gene in F. oxysporum f. sp. lycopersici (Fol) and found Foc-ΔSIX1::Fol-SIX1 mutants did not affect the virulence of Foc-ΔSIX1. The results confirmed that Fol-SIX1 was not capable of replacing the role of Foc-SIX1 in Foc on the disease symptom development of cabbage. The roles of Fol-SIX1 on virulence might rely on host specificity.