Hac1-independent functions of Ire1 drive morphogenesis in Candida albicans through distinct transcriptional programs.

The pathogenic yeast Candida albicans relies on morphogenesis - the transition from spherical yeast to filamentous hyphal forms - for infection. Although morphogenesis requires Ire1, a transmembrane protein that canonically initiates the unfolded protein response (UPR) through HAC1 mRNA splicing, the specific mechanisms linking Ire1 to filamentation remain unclear. Using transcriptome analysis, we found that the Ire1-dependent transcriptional response driving morphogenesis was fundamentally distinct from the canonical UPR response to proteotoxic stress, with minimal overlap between programs. Morphogenesis was associated with only limited HAC1 splicing compared to the robust splicing seen during proteotoxic stress, and HAC1 deletion only partially impaired filamentation, unlike the near-complete loss observed with IRE1 deletion. These findings establish that Ire1 regulates hyphal development through previously uncharacterized HAC1-independent pathways. We identify cell wall integrity as a key HAC1-independent mechanism, with Ire1 - but not Hac1 - being essential for cell wall stress tolerance and upregulation of cell wall biosynthesis genes during filamentation. Our data also reveal Ire1-dependent decreases in transcripts encoding secretory proteins during both proteotoxic stress and morphogenesis, consistent with a possible role for Ire1-mediated mRNA degradation in these processes. Given the essential role of Ire1 in pathogenesis and extensive development of Ire1-targeting compounds for mammalian systems, our findings position Ire1 as a highly promising druggable target for novel antifungal therapeutics and development of fungal-specific inhibitors.