MAPK cascade‐mediated regulation of pathogenicity, conidiation and tolerance to abiotic stresses in the entomopathogenic fungus Metarhizium robertsii

Abstract

Metarhizium robertsii has been used as a model to study fungal pathogenesis in insects, and its pathogenicity has many parallels with plant and mammal pathogenic fungi. MAPK (Mitogen-activated protein kinase) cascades play pivotal roles in cellular regulation in fungi, but their functions have not been characterized in M. robertsii. In this study, we identified the full complement of MAPK cascade components in M. robertsii and dissected their regulatory roles in pathogenesis, conidiation and stress tolerance. The nine components of the Fus3, Hog1 and Slt2-MAPK cascades are all involved in conidiation. The Fus3- and Hog1-MAPK cascades are necessary for tolerance to hyperosmotic stress, and the Slt2- and Fus3-MAPK cascades both mediate cell wall integrity. The Hog1 and Slt2-MAPK cascades contribute to pathogenicity; the Fus3-MAPK cascade is indispensable for fungal pathogenesis. During its life cycle, M. robertsii experiences multiple microenvironments as it transverses the cuticle into the haemocoel. RNA-seq analysis revealed that MAPK cascades collectively play a major role in regulating the adaptation of M. robertsii to the microenvironmental change from the cuticle to the haemolymph. The three MAPKs each regulate their own distinctive subset of genes during penetration of the cuticle and haemocoel colonization, but they function redundantly to regulate adaptation to microenvironmental change.

Evolution of the chitin synthase gene family correlates with fungal morphogenesis and adaption to ecological niches

“…Conidial yields were determined after 18 days of incubation as previously described44. Conidiophores were observed daily beginning at 3 days of incubation as previously described44.…”

Genome‐wide identification of pathogenicity, conidiation and colony sectorization genes in Metarhizium robertsii

“…Compared with the pathogenicity determinants, the mechanisms underlying production of infectious propagules (conidia) in M. robertsii have received scant attention, with only a few conidiation associated genes reported. These include Cag8 that encodes a regulator of G protein signalling (Fang et al ., ), and the Hog1‐MAPK pathway that regulate conidiophore formation and conidial pigmentation (Chen et al ., ). A laccase ( Mlac1 ) and a polyketide synthase ( Mr‐Pks1 ) are involved in conidial pigmentation (Fang et al ., ; Chen et al ., ).…”

“…Significant progress has been made in understanding entomopathogenicity. Characterized pathogenicity determinants include cAMP‐PKA and MAPK signalling pathways that regulate appressorium formation and adaptation to haemolymph (Fang et al ., ; Chen et al ., ), an adhesin (MAD1) responsible for conidial adherence to the cuticle (Wang and St. Leger, ), a series of cuticle degrading enzymes (Bagga et al ., ), a collagen‐like protein MCL1 which is required for evading insect immune responses (Wang and St. Leger, ), and a sterol carrier Mr‐NPC2a that maintains cell membrane integrity while in the host (Zhao et al ., ). The biosynthesis pathway of the secondary metabolite immune modulator Destruxin has also been resolved (Wang et al ., ).…”

Metabolic Conservation and Diversification of Metarhizium Species Correlate with Fungal Host-Specificity

“…Transcriptional and developmental differences have been also observed when Metarhizium species were challenged with host or non-host cuticles. These differences include the up-regulation of divergent G-protein coupled-receptors, differential expression of down-stream singling pathway genes, and finally, production of the infection structure appressoria to determine fungal host recognition and infection (Wang and St Leger, 2005; Gao et al, 2011; Chen et al, 2016). Despite these advances, metabolomic and physiological differences among the Metarhizium species are largely unclear when the fungi are grown under the same or different conditions.…”