Cytokinin receptor related to bacterial two-component regulators. Functions as a histidine kinase and transmits the stress signal to a downstream MAPK cascade. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Highly expressed in young leaves and spikelets, and at lower levels in roots, mature leaves and stems. By isopentenyladenine (iP), cis-zeatin (cZ) and dihydrozeatin (DHZ) (PubMed:22642989). Induced by cytokinin in roots (PubMed:17408920). Histidine-containing phosphotransfer domain (HPt) contains an active histidine that mediates the phosphotransfer. Activation probably requires a transfer of a phosphate group between a His in the transmitter domain and an Asp of the receiver domain. Semi-dwarf, chlorina, late heading and low fertility phenotypes.