Fungi grow over a relatively wide pH range and adapt to extracellular pH through a genetic regulatory system mediated by a key component PacC, which is a pH transcription regulator. The cellulase production of the filamentous fungi Trichoderma reesei is sensitive to ambient pH. To investigate the connection between cellulase expression regulation and ambient pH, an ortholog of Aspergillus nidulans pacC, Trpac1, was identified and functionally characterized using a target gene deletion strategy. Deleting Trpac1 dramatically increased the cellulase production and the transcription levels of the major cellulase genes at neutral pH, which suggested Trpac1 is involved in the regulation of cellulase production. It was further observed that the expression levels of transcription factors xyr1 and ace2 also increased in the Delta Trpac1 mutant at neutral pH. In addition, the Delta Trpac1 mutant exhibited conidiation defects under neutral and alkaline pH. These results implied that Trpac1 in involved in growth and development process and cellulase gene expression in T. reesei. (C) 2014 Elsevier Inc. All rights reserved.