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.
