Serine/threonine/tyrosine kinase involved in the control of stomatal movement in response to CO(2) (PubMed:16518390, PubMed:27694184, PubMed:27923039, PubMed:30361234). Functions as a major negative regulator of CO(2)-induced stomatal closing (PubMed:16518390). Does not seem to be involved in stomatal closure in response to abscisic acid (ABA) or light (PubMed:16518390). Involved in the control of red light-induced stomatal opening (PubMed:26192339). Is epistatic to SRK2E/OST1 function during stomatal responses to red light and altered CO(2) (PubMed:26192339). Phosphorylates SRK2E/OST1 and GHR1 to prevents SRK2E/OST1- and GHR1-induced activation of SLAC1, thus preventing stomatal closure (PubMed:25599916, PubMed:27694184, PubMed:30361234). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Inhibited by MPK4 and MPK12. Interacts with DTX56 (PubMed:25599916). Binds to MPK4 and MPK12 (PubMed:27694184, PubMed:27923039). Mainly localizes in guard cells. Expressed at low level in leaves, stems, roots and flowers. Autophosphorylated. 'Suu' means 'mouth' in both Estonian and Finnish. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.