FUNGAL RESOURCES FOR BIOLOGICAL CONTROL AND HIGH-VALUE USES
Location: Biological Integrated Pest Management Unit
Title: Genetic basis of destruxin production in the entomopathogen Metarhizium robertsii
Submitted to: Current Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 4, 2012
Publication Date: February 25, 2012
Citation: Donzelli, B., Krasnoff, S., Sun-Moon, Y., Churchill, A., Gibson, D.M. 2012. Genetic basis of destruxin production in the entomopathogen Metarhizium robertsii. Current Genetics. 58:105-116.
Interpretive Summary: Destruxins are among the most well-known small molecular weight metabolites produced by insect pathogenic fungi, but their mode of production was unknown. We identified the gene cluster responsible for their biosynthesis, and knocked out this gene in a series of mutants to investigate the metabolites' role in pathogenicity. The mutants had no significant changes in virulence levels against three different insect species and there were no changes in fungal morphology and development. Gene expression was detectable at low levels during early growth phase and increased with culture time, and gene transcripts were also detectable in later stages of infected insects and in fungal spores. The destruxins appear to be dispensable for virulence against insects, casting doubt on their long-purported role in insect pathogenicity and virulence.
Destruxins are among the most exhaustively researched secondary metabolites of entomopathogenic fungi, yet definitive evidence for their roles in pathogenicity and virulence has yet to be shown. To establish the genetic bases for the biosynthesis of this family of depsipeptides, we identified a 23,742 bp gene in Metarhizium robertsii ARSEF 2575 containing six complete nonribosomal peptide synthetase modules, with an N-methyltransferase domain in each of the last two modules. This domain arrangement is consistent with the positioning of the adjacent amino acids N-methyl-l-valine and N-methyl- l-alanine within the depsipeptide structure of destruxin. DXS expression levels in vitro and in vivo exhibited comparable patterns, beginning at low levels during the early growth phases and increasing with time. Targeted gene knockout using Agrobacterium-mediated transformation produced mutants that failed to synthesize destruxins, in comparison with wild type and ectopic control strains, indicating the involvement of this gene in destruxin biosynthesis. The destruxin synthetase (DXS) disruption mutant was as virulent as the control strain when conidial inoculum was topically applied to larvae of Spodoptera exigua, Galleria mellonella, and Tenebrio molitor indicating that destruxins are dispensable for virulence in these insect hosts. The DXS mutants exhibited no other detectable changes in morphology and development.