Iron homeostasis in the absence of ferricrocin and its consequences in fungal development and insect virulence in Beauveria bassiana

The putative ferricrocin synthetase gene ferS in the fungal entomopathogen Beauveria bassiana BCC 2660 was identified and characterized. The 14,445-bp ferS encodes a multimodular nonribosomal siderophore synthetase tightly clustered with Fusarium graminearum ferricrocin synthetase. Functional analysis of this gene was performed by disruption with the bar cassette. ΔferS mutants were verified by Southern and PCR analyses. HPLC and TLC analyses of crude extracts indicated that biosynthesis of ferricrocin was abolished in ΔferS. Insect bioassays surprisingly indicated that ΔferS killed the Spodoptera exigua larvae faster (LT50 59 h) than wild type (66 h). Growth and developmental assays of the mutant and wild type demonstrated that ΔferS had a significant increase in germination under iron depletion and radial growth and a decrease in conidiation. Mitotracker staining showed that the mitochondrial activity was enriched in ΔferS under both iron excess and iron depletion. Comparative transcriptomes between wild type and ΔferS indicated that the mutant was increased in the expression of eight cytochrome P450 genes and those in iron homeostasis, ferroptosis, oxidative stress response, ergosterol biosynthesis, and TCA cycle, compared to wild type. Our data suggested that ΔferS sensed the iron excess and the oxidative stress and, in turn, was up-regulated in the antioxidant-related genes and those in ergosterol biosynthesis and TCA cycle. These increased biological pathways help ΔferS grow and germinate faster than the wild type and caused higher insect mortality than the wild type in the early phase of infection.