The redox-sensitive transcription factor ChAP1 [Cochliobolus heterostrophus YAP1 (Yeast Activator Protein 1) orthologue] of C. heterostrophus is required for oxidative stress tolerance. It is not known, however, to what extent the intracellular redox state changes on exposure of the fungus to oxidants, and whether ChAP1 is involved in the return of the cell to redox homeostasis. In order to answer these questions, we expressed a ratiometric redox-sensitive fluorescent protein sensor, pHyper, in C. heterostrophus. The fluorescence ratio was sensitive to extracellular hydrogen peroxide (H2O2) concentrations that had been shown previously to inhibit the germination of conidia and growth of the pathogen in culture. chap1 mutants showed a slower return to redox homeostasis than the wild-type on exposure to H2O2. Plant extracts that mimic oxidants in their ability to promote nuclear retention of ChAP1 reduced, rather than oxidized, the fungal cells. This result is consistent with other data suggesting that ChAP1 responds to plant-derived signals other than oxidants. pHyper should be a useful reporter of the intracellular redox state in filamentous fungi.
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