Activator of the abscisic acid (ABA) signaling pathway that regulates numerous ABA responses, such as stomata closure in response to drought, darkness, high CO(2), plant pathogens, or decreases in atmospheric relative humidity (RH) (PubMed:30361234). Involved in the resistance to drought by avoiding water loss. Required for the stomata closure mediated by pathogen-associated molecular pattern (PAMPs) (e.g. flg22 and LPS) of pathogenic bacteria such as P.syringae pv. tomato (Pst) and E.coli O157:H7. As a plant defense process, stomata are closed transiently in order to limit invaders, but actively reopened by bacteria after a few hours; virulent strains (e.g. Pst DC3000) are more efficient than avirulent strains (e.g. Pst DC3000 AvrRpt2) in reopening stomata. Mediates the phosphorylation and activation of the S-type anion efflux channel SLAC1, and thus promotes stomata closure. Essential for stomatal closure in response to reactive oxygen species (ROS). Promotes MAPKKK18 activity upon abscisic acid (ABA) treatment (PubMed:26443375). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Kinase activity enhanced by ABA and low humidity. Repressed by PP2CA independently of its phosphatase activity. Probably inactivated by ABI1 (PubMed:12468729, PubMed:12514244, PubMed:19955427). Repressed by TOPP1 (PubMed:26943172). Negatively regulated by ABI2 (PubMed:22730405). Interacts with ABI1, PP2CA and SLAC1 (PubMed:16365038, PubMed:19874541, PubMed:19955405, PubMed:19955427). Interacts with B'ALPHA, B'BETA, B'DELTA, PP2AA2, PP2AA3, PP2A1 and PP2A2 (PubMed:26175513). Associates with MAPKKK18 within the nucleus (PubMed:26852793). Interacts with I-2, TOPP1 and TOPP2 (PubMed:26943172). Interacts with ABI2 (PubMed:22730405). Expressed in seedlings, leaves, flowers, stems, and roots, but restricted to guard cells and vascular tissue. In roots by ABA in an ABI1-dependent manner and by osmotic stress (e.g. sorbitol) in an ABI1-independent manner, but in leaves by low humidity (at protein level). Also induced by salt stress. Autophosphorylation on residues Ser-7, Ser-18, Ser-29, Ser-43, Ser-175 and/or Thr-176 (PubMed:30361234). Only the phosphorylation of Ser-175 is crucial for the kinase activity. The phosphorylation of Ser-43 may repress the ABA signaling pathway in absence of ABA. Impaired ozone-triggered rapid transient decrease (RTD) in stomatal conductance (PubMed:20128877). Impaired induction of MKKK18 activity after 90 minutes of abscisic acid (ABA) treatment (PubMed:26443375). Delayed CO(2)-mediated and darkness-induced and reduced abscisic acid (ABA)-triggered stomatal closure (PubMed:30361234). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.