Could function as a HCO(3)(-) -sensing component in the CO(2) signaling pathway in guard cells. Acts as an upsteam repressor of HT1. Plays a role in stomatal response to CO(2). Interacts with BCA4 and HT1. Preferentially expressed in guard cells. Defective in stomatal responses to Co(2). Showed more rapid leaf expansion under high CO(2) condition as compared with the wild type. Belongs to the multi antimicrobial extrusion (MATE) (TC 2.A.66.1) family.