Deletion of PdMit1, a homolog of yeast Csg1, affects growth and Ca²⁺ sensitivity of the fungus Penicillium digitatum, but does not alter virulence

GDP-mannose:inositol-phosphorylceramide (MIPC) and its derivatives are important for Ca²⁺ sensitization of Saccharomyces cerevisiae and for the virulence of Candida albicans, but its role in the virulence of plant fungal pathogens remains unclear. In this study, we report the identification and functional characterization of PdMit1, the gene encoding MIPC synthase in Penicillium digitatum, one of the most important pathogens of postharvest citrus fruits. To understand the function of PdMit1, a PdMit1 deletion mutant was generated. Compared to its wild-type control, the PdMit1 deletion mutant exhibited slow radial growth, decreased conidia production and delayed conidial germination, suggesting that PdMit1 is important for the growth of mycelium, sporulation and conidial germination. The PdMit1 deletion mutant also showed hypersensitivity to Ca²⁺. Treatment with 250mmol/l Ca²⁺ induced vacuole fusion in the wild-type strain, but not in the PdMit1 deletion mutant. Treatment with 250mmol/lCaCl₂ upregulated three Ca²⁺-ATPase genes in the wild-type strain, and this was significantly inhibited in the PdMit1 deletion mutant. These results suggest that PdMit1 may have a role in regulating vacuole fusion and expression of Ca²⁺-ATPase genes by controlling biosynthesis of MIPC, and thereby imparts P. digitatum Ca²⁺ tolerance. However, we found that PdMit1 is dispensable for virulence of P. digitatum.