Proper Functions of Peroxisomes Are Vital for Pathogenesis of Citrus Brown Spot Disease Caused by Alternaria alternata

In addition to the production of a host-selective toxin, the tangerine pathotype of Alternaria alternata must conquer toxic reactive oxygen species (ROS) in order to colonize host plants. The roles of a peroxin 6-coding gene (pex6) implicated in protein import into peroxisomes was functionally characterized to gain a better understanding of molecular mechanisms in ROS resistance and fungal pathogenicity. The peroxisome is a vital organelle involved in metabolisms of fatty acids and hydrogen peroxide in eukaryotes. Targeted deletion of pex6 had no impacts on the biogenesis of peroxisomes and cellular resistance to ROS. The pex6 deficient mutant (Δpex6) reduced toxin production by 40% compared to wild type and barely induce necrotic lesions on citrus leaves. Co-inoculation of purified toxin with Δpex6 conidia on citrus leaves, however, failed to fully restore lesion formation, indicating that toxin only partially contributed to the loss of Δpex6 pathogenicity. Δpex6 conidia germinated poorly and formed fewer appressorium-like structures (nonmelanized enlargement of hyphal tips) than wild type. Δpex6 hyphae grew slowly and failed to penetrate beyond the epidermal layers. Moreover, Δpex6 had thinner cell walls and lower viability. All of these defects resulting from deletion of pex6 could also account for the loss of Δpex6 pathogenicity. Overall, our results have demonstrated that proper peroxisome functions are of vital importance to pathogenesis of the tangerine pathotype of A. alternata.