Stagonosporopsis cucurbitacearum Effector Protein ScAA9 Contributes to Virulence and Triggers Plant Immune Responses

Fungal pathogens secrete cell wall-degrading enzymes that either act as virulence factors or are perceived by plants to trigger immunity responses. Stagonosporopsis cucurbitacearum causes gummy stem blight, a devastating disease affecting cucurbit crops worldwide. Here, we identified a lytic polysaccharide monooxygenase (LPMO) of the AA9 family, ScAA9, in S. cucurbitacearum and investigated its roles in fungal virulence and host immune manipulation. ScAA9 is a secreted protein that is highly expressed during early infection and contributes to full virulence but not growth, development and abiotic stress responses. The virulence role of ScAA9 is independent of its enzymatic activity. ScAA9 elicits cell death across various plant species, including cucurbit plants. This cell death-inducing ability does not require its LPMO activity and disulphide bonds but is determined by a 96-amino-acid fragment ScAA9^NC3. NbBAK1 and NbCERK1 but not NbSOBIR1, NbEDS1 and NbNDR1 are required for ScAA9- or ScAA9^NC3-triggered cell death in Nicotiana benthamiana. ScAA9 and ScAA9^NC3 trigger characteristic immune responses, including reactive oxygen species bursts, callose deposition and defence gene upregulation in N. benthamiana and watermelon. Furthermore, pretreatment with ScAA9 or ScAA9^NC3 and overexpression of ScAA9 confer enhanced resistance of watermelon plants against multiple fungal and bacterial diseases. These findings reveal the dual roles of ScAA9 as both a virulence factor for pathogenesis and a PAMP for host immune manipulation, highlighting the complex interplay between S. cucurbitacearum and its cucurbit hosts.