The low-affinity calcium channel protein AaFig1 is essential for the growth and development, infection structure differentiation, and pathogenicity of Alternaria alternata

Fig1, as a low-affinity Ca2+ channel, is needed for Ca2+ influx and normal mating found in a few species, but its detailed molecular regulating mechanism in fungi, especially in Alternaria alternata, the causal agent of pear black spot, is still unclear. In this study, Fig1 in A. alternata was cloned, identified and named as AaFig1, and its function was characterized by targeted gene knockout strategy. The results showed that the ΔAaFig1 was severely decreased in vegetative growth, biomass accumulation and sporulation. Fluo-4, AM staining indicated that the Ca2+ content in ΔAaFig1 was decreased. Deletion of AaFig1 reduced the response of A. alternata to exogenous Ca2+ but improved the tolerance to osmotic stresses. Interestingly, the appressorium formation of the ΔAaFig1 strain on hydrophobic and pear wax extract coated surface, and infection hyphae formation on the onion epidermis were significantly reduced, meanwhile, the ΔAaFig1 strain could not penetrate the cellophane. In addition, virulence on wound-inoculated pear fruit and intact pear leaves, melanin production and the activities of cell wall degrading enzymes in AaFig1 strain were reduced. In summary, these results indicate that AaFig1-mediated Ca2+ homeostasis positively regulates the growth and development, infection structure formation and pathogenicity of A. alternata. The findings will provide a basis for developing new strategies to control this pathogen in pear fruit.