Mediation of fludioxonil fungicide activity and resistance through Cochliobolus heterostrophus G-protein and MAPK signaling pathways

Abstract

The heterotrimeric G proteins and mitogen-activated protein kinases (MAPKs) conserved signaling pathways are involved in the development, reproduction and pathogenicity in filamentous fungi. The two-component histidine kinase, known also as the HOG MAPK pathway, regulates a similar complex set of responses and is known to mediate the phenylpyrrole fludioxonil fungicide response in fungi. We used Cochliobolus heterostrophus mutant strains deficient in G-protein α (cga1) and/or β (cgb1) subunits or MAPK (chk1, mps1 and hog1) to uncover their role in the mediation of this fungicide’s activity and resistance. The results revealed complex interactions between the G-protein subunits and the MAPK in response to osmotic/ionic and fludioxonil stresses. Under normal conditions, the Hog1 pathway restricts glycerol accumulation since its disruption leads to hyperosmosensitivity and very high cellular glycerol accumulation. The hog1 mutants were also relatively resistance to fludioxonil. Moreover, our results suggest that cgb1, chk1 and mps1 are also weak repressors of this response since mutation in these genes caused relatively high elevation in glycerol levels in the cells. Supporting this is the finding that these three strains exhibit resistance to KCl stress. In contrast, the cga1 strain has only moderate levels of cellular glycerol (higher than those of the wild type, but lower than those of the other mutants) that are little affected by KCl or fludioxonil stress. Indeed, these mutants are highly sensitive to KCl stress. This suggests that Cga1 is a moderate repressor of cellular glycerol under normal conditions and an enhancer of glycerol accumulation under osmotic/ionic stress conditions. Together these findings reveals that the sensitivity to fludioxonil is not only positively controlled by the Hog1 pathway, but also mediated by the Chk1, Mps1, Cga1 and Cgb1 pathways. This study provides insight into the roles of G-protein in mediating the anti-fungal fludioxonil response. A model is proposed for the interactions between the G-protein and MAPK signaling pathways.