Functions as a histidine kinase and transmits the stress signal to a downstream MAPK cascade. This protein undergoes an ATP-dependent autophosphorylation at a conserved histidine residue in the kinase core, and a phosphoryl group is then transferred to a conserved aspartate residue in the receiver domain. Negative regulator of the ETR1-dependent abscisic acid (ABA) and ethylene signaling pathway that inhibits the root elongation. Promotes stomatal closure. Regulates stomatal opening by integrating multiple signals via hydrogen peroxide H(2)O(2) homeostasis in guard cells in an ABA-independent manner. May contribute to basal defense mechanisms by closing stomata in the presence of bacterial pathogens. Regulates both hormone levels and ROS production in response to stress. Required for full immunity to bacterial pathogen and necrotrophic fungus. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Interacts with AHP1, APH2, APH3, APH5 and APH6, but not with APH4. Present in light-grown but not in etiolated seedlings. Mostly expressed in roots flowers and siliques, and, to a lower extent, in stems and leaves, especially in guard cells. By H(2)O(2). Hypersensitive to abscisic acid (ABA) and ethylene (ACC) in roots. Reduced stomatal closure in response to H(2)O(2), bacterial pathogen associated molecular pattern (PAMP) flagellin, Pseudomonas syringae, darkness, nitric oxide and ethylene, but normal closure in response to ABA and the peptide elf. Increased sensitivity to pathogens and increased tolerance to high salinity.