Disruptions of the genes involved in lysine biosynthesis, iron acquisition, and secondary metabolisms affect virulence and fitness in Metarhizium robertsii

Based on genomic analysis, polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) pathways account for biosynthesis of the majority of the secondary metabolites produced by the entomopathogenic fungus Metarhizium robertsii. To evaluate the contribution of these pathways to M. robertsii fitness and/or virulence, mutants deleted for mrpptA, the Sfp-type 4' phosphopantetheinyl transferase gene required for their activation were generated. Delta mrpptA strains were deficient in PKS and NRPS activity resulting in colonies that lacked the typical green pigment and failed to produce the nonribosomal peptides (destruxins, serinocylins, and the siderophores ferricrocin and metachelins) as well as the hybrid polyketide-peptides (NG-39x) that are all produced by the wild type (WT) M. robertsii. The Delta mrpptA colonies were also auxotrophic for lysine. Two other mutant strains were generated: Delta mraarA, in which the alpha-aminoadipate reductase gene critical for lysine biosynthesis was disrupted, and Delta mrsidA, in which the L-ornithine N-5-oxygenase gene that is critical for hydroxamate siderophore biosynthesis was disrupted. The phenotypes of these mutants were compared to those of Delta mrpptA to separate effects of the loss of lysine or siderophore production from the overall effect of losing all polyketide and non-ribosomal peptide production. Loss of lysine biosynthesis marginally increased resistance to H2O2 while it had little effect on the sensitivity to the cell wall disruptor sodium dodecyl sulfate (SDS) and no effect on sensitivity to iron deprivation. In contrast, combined loss of metachelin and ferricrocin through the inactivation of mrsidA resulted in mutants that were as hypersensitive or slightly more sensitive to H2O2, iron deprivation, and SDS, and were either identical or marginally higher in Delta mrpptA strains. In contrast to Delta mrpptA, loss of mrsidA did not completely abolish siderophore activity, which suggests the production of one or more non-hydroxamate iron-chelating compounds. Deletion of mrpptA, mrsidA, and mraarA reduced conidium production and conidia of a GFP-tagged Delta mrpptA strain displayed a longer germination delay than WT on insect cuticles, a deficiency that was rescued by lysine supplementation. Compared with WT, Delta mrpptA strains displayed 19-fold reduction in virulence against Drosophila suzukii. In contrast, lysine auxotrophy and loss of siderophores accounted for similar to 2 and similar to 6-fold decreases in virulence, respectively. Deletion of mrpptA had no significant effect on growth inhibition of Bacillus cereus. Our results suggest that PKS and NRPS metabolism plays a significant role in M. robertsii virulence, depresses conidium production, and contributes marginally to resistance to oxidative stress and iron homeostasis, but has no significant antibacterial effect. (C) 2016 Elsevier Inc. All rights reserved.