Receptor kinase acting as an early component in abscisic acid (ABA) signaling (PubMed:22730405). Required for darkness, ABA, high CO(2) and hydrogen peroxide (H(2)O(2)) induction of S-type anion currents in guard cells leading to stomatal closure, possibly via the phosphorylation and activation of the anion channel SLAC1 and as a scaffolding component (PubMed:22730405, PubMed:27694184, PubMed:30361234). Seems to act in parallel with SRK2E/OST1 in the ABA signaling pathway which regulates stomatal movement (PubMed:22730405). Binds ATP (PubMed:30361234). Involved in the local and/or systemic stomatal responses (e.g. stomatal closure) to light stress (PubMed:29463779). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Negatively regulated by ABI2. Interacts with SLAC1 (via N-terminus) (PubMed:22730405, PubMed:30361234). Binds to ABI2, but not ABI1 (PubMed:22730405). Interacts with CPK3 (PubMed:30361234). Expressed in guard cells and in the vasculature of roots and leaves. The protein kinase domain is predicted to be catalytically inactive. Phosphorylated by HT1; this phosphorylation is inhibited by MPK12 and MPK4. No visible phenotype under normal growth conditions but early wilting (PubMed:22730405). Increased sensitivity to drought stress due to impaired stomatal closure and increased water loss (PubMed:22730405). Abolished CO(2)-mediated and darkness-induced stomatal closure (PubMed:27694184). Defective abscisic acid (ABA) and hydrogen peroxide (H(2)O(2)) induction of stomatal closure associated with an impaired activation of S-type anion currents in guard cells (PubMed:22730405). Impaired stomatal closure after treatment with methyl jasmonate (MeJA), salicylic acid (SA) and flagellin 22 (Flg22) (PubMed:29463779). Altered ABA-mediated inhibition of light-induced stomatal opening (PubMed:22730405). Apoplastic ROS-sensitive plants exhibiting severe tissue damage when exposed to ozone (O3) (PubMed:30361234). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. Exhibits protein kinase activity according to PubMed:22730405, but in contradiction, described as an inactive pseudokinase by PubMed:30361234. Truncated N-terminus.