Fig. 5: Schematic summary illustrating XopD in planta functions.

At the late stage of infection, Xcc bacteria (purple rods) break through the plant’s xylem vessels and enter mesophyll apoplast space in symptomatic tissues, where they get access to the cells that are connected to the apoplast and the symplast, such as mesophyll cells, epidermal pavement cells and their neighbouring guard cells, and deliver effectors into these cells. In the XopD recipient cells (red), XopD promotes the expression of ABA-responsive genes (for example, RD29A) possibly by controlling functions of DELLA-interacting TFs via its SUMO protease activity, and activates OSCA1.1 expression correlating with demethylation of the OSCA1.1 promoter. The XopD-dependent promotion of ABA singling and OSCA1.1-mediated Ca²⁺ increase induces stable stomatal closure for establishing an aqueous apoplastic space that supports bacterial growth. Blue arrows indicate intercellular signals, such as Ca²⁺, miRNAs and hormones. The proposed XopD–DELLA–TF complex is shaded in grey.