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 AHK2, 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. Can interact with isoprenoid-type cytokinins trans-zeatin (tZ and tZR), cis-zeatin (cZ), dihydrozeatin (DZ), buta-2,3-dienyladenine (HA-8), penta-2,3-dienyladenine (HA-1), 4-methyl-penta-2,3-dienyladenine (HA-10), 4-hydroxy-2-butynyladenine (RM1), 2-propynyladenine (RM3), 2-butynyladenine (RM6), and cytokinin ribosides and ribotides. Together with AHK4, involved in the cytokinin-dependent responses to Pi starvation and sucrose stresses. Promotes cytokinin-mediated leaf longevity through a specific phosphorylation of the response regulator ARR2. 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. Plays a role in the cytokinin-mediated repression of the iron uptake pathway. 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. This cytokinin-mediated activation is repressed by the trans-zeatin antagonists 6-(2-hydroxy-3-methylbenzylamino)purine (PI-55) and 6-(2,5-Dihydroxybenzylamino)purine (LGR-991). Optimum pH to bind cytokinin is about 8.5 at 0 degrees Celsius. Cytokinin-binding is stable at 0 degrees Celsius but transient at 37 degrees Celsius. Interacts with AHK2, AHK4, AHP1, AHP2, AHP3, AHP5 and At5g43560. The plasma membrane localization is supported by the over-expression of an AHK3-GFP fusion protein. Mostly expressed in leaves and flowers, and, to a lower extent, in roots, stems, and siliques, especially 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 the root tips, strongest expression in the procambium. Rapidly induced by dehydration, high salinity and cold stresses. 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. Reduced sensitivity to cytokinin (mostly in roots). More rapid germination, reduced requirement for light, and decreased far-red light sensitivity. Early senescence promoted by darkness. Reduced sensitivity to N-isobutyl decanamide. Defects in procambium proliferation and absence of secondary growth. Enhanced freezing tolerance. Impaired benzyladenine (6-BA)-mediated repression of the iron uptake pathway. Disturbed cytokinin-mediated flower development abnormality. Impaired meristematic development in seedlings. 'Oresara' means 'long living' in Korean.