Hypovirus‐Induced Phosphorylation of CpIre1 Modulates Unfolded Protein Response and Virulence in Cryphonectria parasitica

The chestnut blight fungus Cryphonectria parasitica and its hypovirus constitute a valuable model for investigating fungal pathogenesis and cross‐kingdom virus–host interplay. To investigate how hypovirus regulates protein function at the phosphorylation level in C. parasitica , we performed a comparative phosphoproteomic analysis in the fungus with or without Cryphonectria hypovirus 1 (CHV1) infection. Comparative profiling between the wild‐type (EP155) and hypovirus‐infected (EP155/CHV1‐EP713) strains revealed 700 differentially phosphorylated sites (174 upregulated, 526 downregulated). Among these, the serine 896 and 897 sites on the endoplasmic reticulum (ER) stress‐sensing protein CpIre1 drew our particular attention, as hypovirus‐induced phosphorylation targets. Western blot analysis showed that virus‐encoded p29, p40, and p48 proteins could promote CpIre1 phosphorylation. Site‐specific mutagenesis revealed that Ser‐896 and Ser‐897 are essential for CpIre1 phosphorylation, which regulates fungal phenotypic traits, virulence, and stress tolerance in C. parasitica . Reverse‐transcription‐quantitative PCR analysis of the ER stress marker genes CpHac1 and CpBip1 confirmed that CpIre1 and its phosphorylation are essential for a functional ER stress response. Notably, hypovirus replication was significantly impaired in phospho‐deficient CpIre1 mutants, showing about 40% reduction in viral RNA accumulation, whereas phospho‐mimic mutants maintained wild‐type levels of viral RNA. This indicates that efficient hypovirus accumulation requires functional phosphorylation of CpIre1. Our findings demonstrate that hypovirus‐induced phosphorylation of CpIre1 modulates fungal ER homeostasis, pathogenicity, and viral RNA accumulation, thereby revealing a mechanism through which the virus reprogrammes its host via targeted post‐translational modification.