Insight into the transcriptional regulation of Msn2 required for conidiation, multi-stress responses and virulence of two entomopathogenic fungi

Abstract

Msn2/4 transcription factors in some fungi have null effects on virulence and cellular stress responses. Here we found that the transcriptional regulation of Msn2 orthologs is vital for the conidiation, virulence and multi-stress responses of Beauveria bassiana (Bb) and Metarhizium robertsii (Mr), which lack Msn4 orthologs. Compared to wild-type and complemented strains of each fungus with all similar phenotypes, ΔBbmsn2 and ΔMrmsn2 showed remarkable defects in conidial yield (∼40% decrease) and virulence (∼25% decrease). Both delta mutants lost 20–65% of their tolerances to hyperosmolarity, oxidation, carbendazim, cell wall perturbing and high temperature at 34°C during colony growth. Their conidia were also significantly (18–41%) less tolerant to oxidation, hyperosmolarity, wet-heat stress at 45°C and UV-B irradiation. Accompanied with the defective phenotypes, several conidiation- and virulence-associated genes were greatly repressed in ΔBbmsn2 and ΔMrmsn2. Moreover, differentially expressed genes in the transcriptomes of ΔBbmsn2 versus wild type were ∼3% more under oxidative stress, but ∼12% fewer under heat shock, than those in the ΔMrmsn2 counterparts. Many stress-responsive effector genes and cellular signaling factors were remarkably downregulated. Taken together, the two entomopathogens could have evolved somewhat distinct stress-responsive mechanisms finely tuned by Msn2, highlighting the biological significance of Msn2 orthologs for filamentous fungi.