MaPmt4, a protein O-mannosyltransferase, contributes to cell wall integrity, stress tolerance and virulence in Metarhizium acridum

In eukaryotic cells, protein O-glycosylation is an essential protein modification. Analysis of the Metarhizium acridum genome database revealed a total of three O-glycoside mannosyltransferase homologs (Pmt1, Pmt2 and Pmt4), closely related to Saccharomyces cerevisiae Pmt1, Pmt2, and Pmt4. In this study, the functions of MaPmt4, encoding a protein O-mannosyltransferase in M. acridum, were characterized using disruption and complementation strategies. Disruption of MaPmt4 delayed the conidial germination and reduced the fungal tolerances to heat shock and UV-B irradiation, but did not affect conidial yield. Inactivation of MaPmt4 displayed increased sensitivity to cell wall-perturbing agents, formed thinner cell walls, and changed composition of fungal cell wall, demonstrating that MaPmt4 was also important to maintain fungal cell wall integrity. Bioassays by topical inoculation and intrahemocoel injection showed that the MaPmt4 deletion mutant exhibited greatly reduced virulence. The subsequent examination revealed that the inactivation of MaPmt4 impaired appressorium formation, decreased fungal growth in locust hemolymph in vitro, and boosted insect immune responses, the latter in part potentially owing to the changes in conidial surface structures, and thus attenuated the virulence of MaPmt4 deletion mutant. Furthermore, the results of comparative proteomics showed that MaPmt4 played important roles in fungal cell wall integrity, stress tolerances, and virulence via broad genetic pathways.