Redox Regulation of an AP-1-Like Transcription Factor, YapA, in the Fungal Symbiont Epichloë festucae

One of the central regulators of oxidative stress inSaccharomyces cerevisiae is Yap1, a bZIP transcription factor of the AP-1 family. In unstressed cells, Yap1 is reduced and cytoplasmic, but in response to oxidative stress, it becomes oxidized and accumulates in the nucleus. To date, there have been no reports on the role of AP-1-like transcription factors in symbiotic fungi. An ortholog of Yap1, named YapA, was identified in the genome of the grass symbiontEpichloë festucae and shown to complement anS. cerevisiae Δyap1 mutant. Hyphae of theE. festucae ΔyapA strain were sensitive to menadione and diamide but resistant to H2 O2 , KO2 , andtert -butyl hydroperoxide (t -BOOH). In contrast, conidia of the ΔyapA strain were very sensitive to H2 O2 and failed to germinate. Using a PcatA-eGFP degron-tagged reporter, YapA was shown to be required for expression of a spore-specific catalase gene,catA . Although YapA-EGFP localized to the nucleus in response to host reactive oxygen species during seedling infection, there was no difference in whole-plant and cellular phenotypes of plants infected with the ΔyapA strain compared to the wild-type strain. Homologs of theS. cerevisiae andSchizosaccharomyces pombe redox-sensing proteins (Gpx3 and Tpx1, respectively) did not act as redox sensors for YapA inE. festucae . In response to oxidative stress, YapA-EGFP localized to the nuclei ofE. festucae ΔgpxC , ΔtpxA , and ΔgpxC ΔtpxA cells to the same degree as that in wild-type cells. These results show thatE. festucae has a robust system for countering oxidative stress in culture andin planta but that Gpx3- or Tpx1-like thiol peroxidases are dispensable for activation of YapA.