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 spikelets and at lower levels in roots, young leaves, mature leaves and stems. Highly induced by isopentenyladenine (iP). Induced by 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.