Hac1-independent functions of Ire1 drive morphogenesis in <i>Candida albicans</i> through distinct transcriptional programs

The pathogenic yeast Candida albicans relies on morphogenesis-the transition from spherical yeast to filamentous hyphal forms-for infection. While 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 is fundamentally distinct from the canonical UPR response to proteotoxic stress, with minimal overlap between programs. Morphogenesis is associated with only limited HAC1 splicing compared to robust splicing during proteotoxic stress, and HAC1 deletion only partially impairs 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-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 Ire1's essential role 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.