A polyketide synthase, BbpksP, contributes to conidial cell wall structure and UV tolerance in Beauveria bassiana

Conidial pigments of filamentous fungi play vital roles in fungal biotic/abiotic stress tolerance and are usually synthesized by polyketide synthases or other pigment synthesis proteins. Beauveria bassiana, an important insect pathogenic fungus used worldwide for pest biocontrol, produces white conidia on artificial media, while no conidial pigment has been observed or reported in it. However, real-time PCR and promoter-report analyses reveal a polyketide gene of B. bassiana (named BbpksP), homologous to melanin synthesis genes, is specifically expressed in aerial conidia. We show that deletion of BbpksP does not result in changes in conidial yield, germination rate or colony radial growth; however, the defect impairs conidial cell wall structure. A dense electron layer appears in the outer edge of the cell envelope in wild-type conidia, as observed by TEM, but this dense layer is absent in the ΔBbpksP mutant. The lack of BbpksP gene also reduces the UV-B tolerance of B. bassiana conidia. Bioassay reveals that deletion of BbpksP decreased virulence of B. bassiana against Galleria mellonella larvae via topical infection. These data indicate that the product(s) of BbpksP contributes to the integrity of the B. bassiana conidial cell wall and further affects the tolerance of UV-B stress and insecticidal activity.