Cytokinins (CK) receptor related to bacterial two-component regulators. 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. In the presence of cytokinin, feeds phosphate to phosphorelay-integrating histidine phosphotransfer protein (HPt) and activates subsequent cascade. Involved in meristems establishment in seedlings. Redundant negative regulator of drought and salt stress responses and abscisic acid (ABA) signaling. Together with AHK3, plays a negative regulatory role in cold stress signaling via inhibition of ABA response, occurring independently of the cold acclimation pathway. Redundant positive regulator of cytokinin signaling that regulates many developmental processes including seed germination, cell division, seed size, chlorophyll retention during leaf senescence, root repression and shoot promotion. Involved in alkamides (e.g. N-isobutyl decanamide) and N-acylethanolamides (NAE) signaling that control meristematic activity and differentiation processes during plant development. Contributes to vascular bundle formation and secondary growth in a cytokinin-dependent manner, probably by promoting the maintenance of mitotic activity and/or identity of procambial cells. Together with AHK4, required for growth and reproduction promotion stimulated by the endophytic fungus Piriformospora indica in a trans-zeatin-dependent manner. Required by the cytokinin-dependent flower development regulation pathway. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Activated by cytokinins to initiate phosphorelay signaling. Self-interacts. Interacts with AHK3, AHP1, AHP2, AHP3, AHP5, ATAF2, AT2S3, BETAA-AD, CYP20-2, DRP1A, HIR1, HIR2, PI4KB1, PI4KG5 and At4g12060. Expressed in roots, leaves and flowers, mostly in the vascular tissues. Present in seedlings. In seedlings, mainly localized in meristematic tissues (e.g. shoot apical meristem SAM, root tips, and growing leaf and lateral root primordia). Present in all the vasculature and the shoot apical meristem (SAM) of the adult plant. In flowers, localized in carpels and developing ovules. In the root tips, expressed in and near the vascular initial cells. Rapidly induced by dehydration, slightly induced by high salinity and abscisic acid (ABA). 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). Hypersensitivity to ABA, and strong drought and salinity tolerance. Slightly reduced sensitivity to cytokinin. More rapid germination, reduced requirement for light, and decreased far-red light sensitivity. Reduced sensitivity to N-isobutyl decanamide. Defects in procambium proliferation and absence of secondary growth. Enhanced freezing tolerance. Impaired meristematic development in seedlings. Disturbed cytokinin-mediated flower development abnormality. Truncated N-terminus. Truncated N-terminus.