Abstract: Mitogen-activated protein (MAP) kinase (MAPK) cascades are highly conserved signaling modules that play crucial roles in plant disease resistance. Emerging evidence shows that MAPK cascades are potential targets of plant pathogen effectors. Bacterial leaf blight (BLB), caused by Xanthomonas oryzae pv. oryzae (Xoo), is a devastating disease in rice. Xoo secretes multiple effectors into host rice plants to suppress immune activation. XopY is a conserved effector across diverse Xoo strains; however, how XopY manipulates disease resistance and what its host targets are in rice remain largely unknown. Here, we report that XopY from the Xoo strain PXO86 interacts with the MAPK kinase kinase OsMAPKKK24 and RECEPTOR-LIKE CYTOPLASMIC KINASE 55 (OsRLCK55). OsRLCK55 phosphorylates OsMAPKKK24, and both contribute to rice BLB resistance by positively regulating early immune responses. XopY suppresses the activation of OsMAPKKK24 by enhancing its intramolecular interaction between the N- and C-terminal regions. The association of XopY with OsRLCK55 and OsMAPKKK24 also interferes with their interaction and suppresses OsRLCK55-mediated transphosphorylation of OsMAPKKK24, ultimately inhibiting MAPK activation and subsequent disease resistance. Taken together, these findings not only reveal that OsRLCK55 and OsMAPKKK24 are crucial for BLB resistance but also uncover a mechanism by which XopY compromises disease resistance by manipulating OsMAPKKK24 activation in rice.

Key words: bacterial leaf blight, disease resistance, MAPK, OsMAPKKK24, rice, XopY