The ApCtf1β2‐ApCUT3 Module Mediates Host Cuticle Penetration and Modulates Immune Responses Thereby Influencing Blight Resistance in Bambusa pervariabilis × Dendrocalamopsis grandis

ABSTRACT Arthrinium phaeospermum is a globally distributed pathogen with a broad host range and is one of the principal causal agents of blight on Bambusa pervariabilis × Dendrocalamopsis grandis (hybrid bamboo). Cutinase Transcription Factor 1 (Ctf1) facilitates degradation of the plant cuticle, as well as early infection and colonisation; however, the transcriptional regulation and pathogenic mechanisms of the Ctf1β subfamily remain unclear. We found that the ApCtf1β family contains conserved core domains and variable terminal regions. Through synteny analysis combined with interaction transcriptomics, we identified ApCtf1β2 as a key pathogenicity gene that is significantly upregulated during infection. Functional analyses revealed that ApCtf1β2 is involved in fatty acid metabolism, stress responses, and sporulation, and that Δ ApCtf1β2 significantly reduces virulence. Integrated CUT Yeast One‐Hybrid (Y1H), Electrophoretic Mobility Shift Assay (EMSA), and Dual luciferase assay (DLR) confirmed that ApCtf1β2 directly activates ApCUT3 expression by binding to the GCC(n5)CGG motif in the ApCUT3 promoter. ApCUT3, a secreted cutinase, possesses a functional signal peptide, localises to extranuclear compartments, and plays a pivotal role in degrading the host cuticle. Single‐gene knockouts (Δ ApCtf1β2 or Δ ApCUT3 ) each attenuate pathogenicity, whereas the double knockout (Δ ApCtf1β2 / ApCUT3 ) exhibits a pronounced hypovirulent phenotype with synergistic interaction. Hosts infected with the double mutant maintain relatively intact cuticles, exhibit substantially weakened JA and SA defence signalling, and display markedly reduced defence enzyme activities. In summary, the ApCtf1β2‐ApCUT3 module promotes fungal infection by transcriptionally activating ApCUT3 , a key cutin‐degrading enzyme, thereby compromising the host physical barrier and indirectly modulating host defence responses through Damage‐Associated Molecular Patterns (DAMPs) release. These findings provide a theoretical basis for targeted control of fungal diseases.