Plant phenolics and related compounds provide a signal and a nutrient source to pathogens, but may also be a toxic stress. The transcription factor ChAP1 of the Dothideomycete Cochliobolus heterostrophus responds to oxidative stress by retention in the nucleus, up-regulating the expression of genes for proteins mediating antioxidant responses. Phenolics also promote nuclear localization of ChAP1, without triggering a strong antioxidant response. By microarray hybridization and candidate gene approaches, we searched for genes whose expression is regulated by phenolics. The C. heterostrophus genome contains a cluster of genes for metabolism of phenolics (β-ketoadipate pathway). The toxicity of a series of phenolics correlated well with GFP:ChAP1 nuclearization. Expression of CCHD1, encoding an intradiol dioxygenase, inversely correlated with ChAP1 nuclearization, suggesting that unmetabolized phenolics could cause a stress response. Structure-activity relationships of a series of phenolics showed different requirements for activating the two signaling pathways postulated: one leading to metabolism of phenolics, and another stress-related. The structure-activity data suggest specific receptors, and set the stage for their identification. Focusing on the group of compounds that cause a stress response, we found that ferulic acid (FA), but not caffeic acid, caused membrane damage as shown by permeability to trypan blue, and hyphal shrinkage. Mutants in the MAPKs Hog1 and Chk1 are hypersensitive to growth on toxic phenolic compounds, and exposure to FA caused strong dephosphorylation of both Hog1 and Chk1. Mutants in the third MAPK Mps1 showed similar sensitivity to WT and with exposure to FA, Mps1 was phosphorylated. These results show involvement of the three known fungal MAPK pathways in the stress response to phenolics.
