Fosp9, a Novel Secreted Protein, Is Essential for the Full Virulence of Fusarium oxysporum f. sp. cubense on Banana (Musa spp.)

The banana vascular wilt pathogen, Fusarium oxysporum f. sp. cubense, delivers a number of different secreted proteins into host plant tissues during infection. Until now, only a few of the secreted proteins from this fungus have been shown to be virulence effectors. Here, the product of fosp9, which is a gene in this pathogen, was found to be a novel virulence effector. The fosp9 gene encodes a hypothetical 185-amino-acid protein which has a functional signal peptide but contains no known motifs or domains. The fosp9 disruptants displayed a significant reduction in producing wilt symptoms on bananas, indicating that fosp9 is essential for the full virulence of this pathogen for banana. These disruptants did not exhibit a change in either saprophytic growth or conidiation on potato dextrose agar medium, but their invasive growth in the rhizomes of banana was markedly compromised, suggesting a pivotal role for fosp9 in the colonization of banana rhizome tissues by this fungus. Live-cell imaging revealed that the Fosp9-GFP fusion protein accumulated in the apoplast of the plant cells. Moreover, transcriptome profiling revealed that a number of virulence-associated genes were differentially expressed in the fosp9 disruptant relative to the wild type. Taken together, these findings suggest that Fosp9 is a genuine effector of F. oxysporum f. sp. cubense. IMPORTANCE Fusarium wilt of bananas (also known as Panama disease), caused by the fungus F. oxysporum f. sp. cubense, is one of the most devastating banana diseases worldwide. The understanding of the molecular mechanism of its pathogenicity is very limited so far. We demonstrated that the secreted protein Fosp9 from this fungus contributes to its virulence against banana hosts and is essential for colonization of banana rhizome tissues by this fungus. In particular, Fosp9 contains no known domains or motifs and has no functionally characterized homologs, implying that it is a novel secreted effector involved in F. oxysporum f. sp. cubense-banana interactions. This work provides insight into molecular mechanisms of F. oxysporum f. sp. cubense pathogenicity, and the characterization of the fosp9 gene will facilitate development of transgenic banana and plantain strains resistant to this disease by silencing this effector gene through host-induced gene silencing or other strategies.