Functions as an osmosensor histidine kinase that detects water stress 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. Positive regulator of drought and salt stress responses, and abscisic acid (ABA) signaling. Confers drought tolerance, probably by regulating levels of ABA accumulation. Plays a redundant role in regulating plant growth and development. Required for the regulation of desiccation processes during seed formation. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Interacts with AHP2, depending of the phosphorylation state of Asp-1075 in the receiver domain, but probably not with AHP1 and AHP3. Mostly expressed in roots, and, to a lower extent, in stems, leaves and flowers. Up-regulated in response to changes in external osmolarity, low temperature, cytokinin (CK) treatment, and dehydration. Autophosphorylated predominantly on His residues. Activation probably requires a transfer of a phosphate group between a His in the transmitter domain and an Asp of the receiver domain (By similarity). Insensitivity to ABA, but hypersensitivity to drought and high salinity stresses. Loss of seed viability over time dur to altered desiccation.