Arsenic Resistance Protein BcArs2 Modulates Endogenous ROS Homeostasis in Infection Cushions to Enhance the Virulence of Botrytis cinerea Toward Host Plants

Botrytis cinerea triggers destructive grey mould with substantial economic losses, yet the functions of numerous pathogenicity genes remain largely unclear. Here, by screening a T‐DNA insertion library, we identified BcARS2 , a key component of the Microprocessor complex. BcARS2 is constitutively expressed, localizes to the nucleus, and modulates gene expression. The Δ Bcars2 mutants exhibited impaired vegetative growth, enhanced conidiophore sporulation, increased conidiation, reduced sclerotial size, and increased sensitivity to multiple stresses. Pathogenicity assays showed that different inoculation methods led to distinct virulence phenotypes in Δ Bcars2 mutants. Microscopic observations revealed that although the Δ Bcars2 mutant formed normal appressoria and successfully invaded host cells, its invasive hyphae exhibited limited expansion. Furthermore, exogenous cAMP did not restore the impaired development of infection cushions (ICs), resulting in defective infection. Multitranscriptome analyses demonstrated that BcARS2 positively regulates BcPDI1 and BcCGF1 , two key genes responsible for reactive oxygen species (ROS) homeostasis and highly expressed during the IC developmental stage. Correspondingly, deletion of BcARS2 upregulated catalase gene expression, elevated catalase activity, reduced H 2 O 2 accumulation, disturbed ROS balance, and ultimately inhibited IC formation. In addition, most toxin biosynthesis‐related genes, which are normally upregulated during IC development, were significantly downregulated in the Δ Bcars2 mutant. Collectively, BcARS2 modulates the virulence of B. cinerea by regulating IC development, vegetative hyphal growth, and stress responses.