A negative melanin regulator, Mamrn, regulates thermo and UV tolerance via distinct mechanisms in Metarhizium

Melanin, a polyphenol pigment found extensively in fungi, has had its biosynthesis pathway clarified in recent years. Nonetheless, the regulatory mechanisms involved in melanin biosynthesis are still not well understood. This study uncovered a new negative regulator of melanin, Mamrn (melanin negative regulator in Metarhizium acridum), which, upon deletion, led to a 75 % increase in melanin production and significantly denser cell walls compared with the wild-type and complemented strains. Mamrn was found to influence melanin biosynthesis in Metarhizium acridum by regulating polyketide synthase (PKS) gene and 1,3,6,8-Tetrahydroxynaphthalene reductase (THR) gene through interaction with their promoter regions. Additionally, the knockdown of Mamrn resulted in delayed conidial germination, a 109.2 % conidial yield increase in nutrient-poor Sucrose yeast extract agar (SYA) medium, and a 72.2 % reduction in conidial yield in nutrient-rich Sabouraud dextrose agar (1/4 SDAY) medium, but with no significant changes in virulence against the 5th instar nymphs of Locusta migratoria compared with the control and wild-type strains. The ΔMamrn strain showed a 76 % reduction in heat shock tolerance and a 178 % increase in UV-B irradiation tolerance. Furthermore, following heat shock, the ΔMamrn strain demonstrated compromised DNA repair capability, a diminished heat shock protein (HSP) response, and a reduced reactive oxygen species (ROS) scavenging capacity of glutathione peroxidase, superoxide dismutase, and catalase. In contrast, after UV-B exposure, the DNA repair system was found to be more active. These findings suggest that Mamrn plays a crucial role in regulating thermotolerance by affecting ROS scavenging capacity and DNA repair via the HSP response, while it negatively impacts UV tolerance by affecting melanin production, cell wall density, and DNA repair mechanisms in Metarhizium acridum. In summary, Mamrn negatively regulates melanin synthesis and influences thermal and UV-B tolerance through different pathways.