The reducing clade IIb polyketide synthase PKS14 acts as a virulence determinant of the entomopathogenic fungus Beauveria bassiana

Abstract

The reducing clade IIb polyketide synthase gene, pks14, is preserved throughout the evolution of entomopathogenic fungi. We examined the functions of pks14 in Beauveria bassiana using targeted gene disruption, and pks14 disruption was verified by Southern blot and PCR analyses. The radial growth, cell dry weight and conidial germination of pks14 were comparable to that of the wild type. Our sequence and gene expression analyses of the pks14 biosynthetic cluster demonstrated: (i) cotranscription and constitutive expression of nearly all the genes of the aforementioned cluster including the C2H2 zinc finger transcription regulator gene, but not pks14 and the cytochrome P450 gene; (ii) expression of the pks14 gene in the insect-containing culture condition only; and (iii) a KAR9-like gene in direct proximity with pks14 is the only gene showing co-regulation. The pks14-infected Spodoptera exigua larvae survived significantly longer than those infected by the wild type, indicating a marked reduction in the virulence of pks14 against the insect. LT50 of pks14 was increased by 1.55 days. Hyphal body formation was decreased in the hemolymph of insects infected by pks14 as compared with those inoculated by the wild type. Our results suggest that PKS14-catalyzed polyketide enhances virulence and pathogenicity of B. bassiana on insects.