Environmental and genetic regulation of white-opaque switching in Candida tropicalis

Phenotypic switching is a strategy by which microbial organisms adapt to environmental changes. The human fungal pathogens, Candida albicans and Candida tropicalis, are closely related species and capable of undergoing morphological transitions. C. albicans primarily exists in human or warm-blooded animals as a commensal, whereas C. tropicalis not only exists as a commensal but also is widely distributed in the environment. In this study, we describe the environmental and genetic regulatory mechanisms of white-opaque switching in C. tropicalis, which is associated with virulence and sexual mating. A comparative study with C. albicans demonstrated that C. tropicalis responds to environmental stimuli, such as elevated CO2 levels and pH changes, in opposite manners. An acidic pH and elevated CO2 levels promote the opaque phenotype in C. albicans but have an opposite effect in C. tropicalis, whereas alkaline pH conditions facilitate white-to-opaque switching and sexual mating in C. tropicalis. The conserved Rim101-mediated pH sensing and Ras1-cAMP/PKA signaling pathways are involved in this regulation. By screening an overexpression library of transcription factors, we identified 26 white-opaque regulators, including WOR1, AHR1, EFG1, CUP9, BCR1 and SFL2. Transcriptional analysis indicated that the pH sensing and Ras1-cAMP/PKA signaling pathways and transcriptional regulators coordinately regulate white-to-opaque switching.