PH signaling in Sclerotinia sclerotiorum: Identification of a pacC/RIM1 Homolog

Sclerotinia sclerotiorum acidifies its ambient environment by producing oxa lic acid. This production of oxalic acid during plant infection has been im plicated as a primary determinant of pathogenicity in this and other phytop athogenic fungi. We found that ambient pH conditions affect multiple proces ses in S. sclerotiorum. Exposure to increasing alkaline ambient pH increase d the oxalic acid accumulation independent of carbon source, sclerotial dev elopment was favored by acidic ambient pH conditions but inhibited by neutr al ambient pH, and transcripts encoding the endopolygalacturonase gene pg1 accumulated maximally under acidic culture conditions. We cloned a putative transcription factor-encoding gene, pad, that may participate in a molecul ar signaling pathway for regulating gene expression in response to ambient pH, The three zinc finger domains of the predicted Pad protein are similar in sequence and organization to the zinc finger domains of the A. nidulans pH-responsive transcription factor PacC, The promoter of pad contains eight PacC consensus binding sites, suggesting that this gene, like its homologs , is autoregulated, Consistent with this suggestion, the accumulation of pa c1 transcripts paralleled increases in ambient pH. Pad was determined to be a functional homolog of PacC by complementation of an A. nidulans pacC-nul l strain with pad. Our results suggest that ambient pH is a regulatory cue for processes linked to pathogenicity, development, and virulence and that these processes may be under the molecular regulation of a conserved pH-dep endent signaling pathway analogous to that in the nonpathogenic fungus A. n idulans.