The ApHSF-ApCtf1β2-ApCUT3 Cascade Couples Host ROS Detoxification to Cuticle Penetration During Fungal Pathogenesis.

Successful infection by plant-pathogenic fungi requires both penetration of the host cuticle and tolerance of the reactive oxygen species (ROS) burst associated with host immunity. However, how these early infection events are temporally coordinated remains unclear. Here, we identified a three-tier transcriptional cascade, ApHSF-ApCtf1β2-ApCUT3, in Arthrinium phaeospermum, the causal agent of shoot blight in hybrid bamboo (Bambusa pervariabilis × grandis). Yeast one-hybrid, electrophoretic mobility shift and dual-luciferase assays showed that ApHSF directly binds the CTAGAA core motif in the ApCtf1β2 promoter and activates its transcription. Functional analyses further showed that ApHSF promotes detoxification of host-derived ROS by activating the fungal antioxidant system, whereas pharmacological suppression of host ROS accumulation substantially restored the pathogenicity defect of the ΔApHSF mutant. ApHSF neither bound nor independently activated the ApCUT3 promoter. Instead, yeast three-hybrid and combinatorial dual-luciferase assays showed that, under early oxidative stress, ApHSF acts as a cofactor to enhance ApCtf1β2-dependent activation of ApCUT3. In vivo fluorescence imaging further demonstrated that ROS detoxification alone was insufficient for full pathogenicity, which additionally required an intact ApCtf1β2-ApCUT3 module for cuticle penetration and colonization. Phos-tag immunoblotting, λ-protein phosphatase treatment and kinase inhibition assays showed that heat and oxidative stress induced phosphorylation of ApHSF and ApCtf1β2 and that these phosphorylation responses depended on p38 MAPK activity. By contrast, host cuticle-derived cues preferentially induced the ApCtf1β2-ApCUT3 module. Together, these findings reveal how a plant-pathogenic fungus integrates oxidative and cuticle-associated signals to coordinate early infection.