The ROP2-RIC7 pathway negatively regulates light-induced stomatal opening by inhibiting exocyst subunit Exo70B1 in Arabidopsis

Stomata are the tiny valves on the plant surface that mediate gas exchange between the plant and its environment. Stomatal opening needs to be tightly regulated to facilitate CO2 uptake and prevent excess water loss. Plant Rho-type (ROP) GTPase 2 (ROP2) is a molecular component of the system that negatively regulates light-induced stomatal opening. Previously, ROP-interactive Cdc42- and Rac-interactive binding motif-containing protein 7 (RIC7) was suggested to function downstream of ROP2. However, the underlying molecular mechanism remains unknown. To understand the mechanism by which RIC7 regulates light-induced stomatal opening, we analyzed the stomatal responses of ric7 mutant Arabidopsis plants and identified the target protein of RIC7 using a yeast two-hybrid screen. Light-induced stomatal opening was promoted by ric7 knockout, whereas it was inhibited by RIC7 overexpression, indicating that RIC7 negatively regulates stomatal opening in Arabidopsis. RIC7 interacted with exocyst subunit Exo70 family protein B1 (Exo70B1), a component of the vesicle trafficking machinery. RIC7 and Exo70B1 localized to the plasma membrane region under light or constitutively active ROP2 conditions. The knockout mutant of Exo70B1 and ric7/exo70b1 exhibited retarded light-induced stomatal opening. Our results suggest that ROP2 and RIC7 suppress excess stomatal opening by inhibiting Exo70B1, which most likely participates in the vesicle trafficking required for light-induced stomatal opening.