Cytokinins (CK) receptor related to bacterial two-component regulators. Binds also the synthetic urea-type cytokinin thidiazuron, a potent defoliant and herbicide. 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. In the absence of cytokinin, removes phosphate from HPt proteins, decreasing the system phosphoload. Involved in meristems establishment in seedlings. Acts as a redundant negative regulator of drought and salt stress responses, and abscisic acid (ABA) signaling in a cytokinin-dependent manner. Required to set vascular asymmetric cell divisions that establish phloem and procambium cell lines. 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), isopentenyladenine (iP), and isopentenyladenosine (iPR), the meta hydroxylated derivative of benzyladenine m-topolin, 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-butynyladenine (RM6), and to a lower extent, with cis-zeatin (cZ), zeatin riboside and dihydrozeatin (DZ). Together with AHK3, involved in the cytokinin-dependent responses to Pi starvation and sucrose stresses. Required for the formation of auxin-transporting vascular tissues in the hypocotyl, and primary and lateral roots, but not in adventitious roots, thus leading to auxin basipetal transport that regulates root development and branching. Involved in alkamides (e.g. N-isobutyl decanamide) and N-acylethanolamides (NAE) signaling that control meristematic activity and differentiation processes during plant development. Prevents the uptake of sulfate by mediating cytokinin-dependent down-regulation of high-affinity sulfate transporters (e.g. SULTR1;1 and SULTR1;2) expression in roots. Together with AHK2, required for growth and reproduction promotion stimulated by the endophytic fungus Piriformospora indica in a trans-zeatin-dependent manner. Required to trigger the phytotoxic effect of the snapdragon (Antirrhinum majus) flowers volatile organic compound (VOC) methyl benzoate (MB). Plays a role in the cytokinin-mediated repression of the iron uptake pathway. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate 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 7-8.5 at 0 degrees Celsius. Cytokinin-binding is more stable at 0 degrees Celsius than at 20 and 37 degrees Celsius. Homodimer. Interacts with AHP1, AHP2, AHP3, AHP5, AHK3, AMPD, FBR12, WNK5 and At4g15630. Mostly expressed in roots, specifically in the vascular cylinder and pericycle, and, to a lower extent, in leaves and flowers. Present in seedlings. Expressed specifically in the vasculature since the early stages of embryogenesis. At the globular stage of embryogenesis, detected in the four innermost cells, which are the precursors of the vascular tissue. During the heart, torpedo, and nearly mature stages, expressed in the procambium of the cotyledon shoulders, prospective hypocotyl, and embryonic root. In seedlings, mainly localized in meristematic tissues (e.g. shoot apical meristem SAM, root tips, and growing leaf and lateral root primordia), especially in vasculature. 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 the central cylinder. Rapidly induced by dehydration. Down-regulated by Pi starvation and induced by cytokinins. 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. Reduced sensitivity to cytokinin (mostly in shoots). Narrow vascular cylinder composed mainly of protoxylem cell files, with no apparent metaxylem or phloem. Hypersensitivity to ABA. Strong drought and salinity tolerance only in the presence of CK. Reduced cytokinin repression of several Pi starvation-responsive genes and increased sucrose sensitivity. More rapid germination, reduced requirement for light, and decreased far-red light sensitivity. Reduced sensitivity to N-isobutyl decanamide. Impaired benzyladenine (6-BA)-mediated repression of the iron uptake pathway. Impaired meristematic development in seedlings.