The siderophore-iron transporter BbMirB is required for the fungal pathogen Beauveria bassiana to repress insect immunity and promote proliferation during colonization of hemocoel.

Abstract

Nutritional immunity plays a vital role in limiting the proliferation of microbial pathogens in vertebrates and invertebrates. During coevolution with their hosts, pathogens have developed tactics to counteract the host nutritional immunity by acquiring iron via siderophore-iron transporter (SIT)-mediated uptake of siderophore-iron chelates. However, little is known about the role of SITs of insect fungal pathogens in combating host immune defense. Here, we demonstrate that Beauveria bassiana, an important insect pathogenic fungus, modulates dominant extracellular siderophores and co-expresses the SITs BbMirA and BbMirB for adaptation to the host in vivo. BbMirB, but not BbMirA, is crucial for B. bassiana full virulence. Disruption of BbMirB significantly reduces the virulence against Galleria mellonella larvae, delays immune evasion and in vivo hyphal body proliferation, and alters the host immune responses, including host nutritional immunity and immune-related gene expression. Loss of BbMirA or BbMirB leads to increased extracellular coprogen-type siderophores, including coprogen B, dimerumic acid, and their derivatives, suggesting these metabolites as their potential substrates. BbMirA and BbMirB co-localized to the plasma membrane, early/late endosomes, and vacuoles, implicating their roles in vesicular trafficking and siderophore-iron uptake. These findings highlight the importance of siderophore-iron transporters, particularly BbMirB, in fungal vesicular trafficking and host-pathogen interactions.IMPORTANCESiderophores are essential for iron uptake under iron-limiting conditions and are involved in fungal niche competition and pathogenicity via siderophore-iron transporters (SITs)-mediated uptake of specific substrates. However, the details of many SIT-mediated substrates in fungal colonization of hosts remain limited. Here, we identify two SITs, BbMirA and BbMirB, in the entomopathogenic fungus Beauveria bassiana, which are highly expressed during colonization of insect hemocoel. The two SITs have complementary functions in siderophore-iron uptake in vitro, but only BbMirB dominantly mediates the uptake of a derivative of dimerumic acid in vivo and plays a critical role in the pathogenic process via disturbing insect immune defense responses. These findings provide insights into the mechanisms of SITs mediating the interaction of fungal pathogens with their hosts.