Functions as two-component phosphorelay mediators between cytokinin sensor histidine kinases and response regulators (B-type ARRs). Plays an important role in propagating cytokinin signal transduction through the multistep His-to-Asp phosphorelay. Interacts with the B-type response regulators ARR1, ARR2 and ARR10. Binds to AHK1, AHK2, AHK3, AHK4, AHK5, ETR1 and CKI1. Strongly expressed in flowers and roots. Detected also in leaves, siliques and stems. By salt, cold and drought stress. Histidine-containing phosphotransfer domain (HPt) contains an active histidine that mediates the phosphotransfer. Two-component system major event consists of a His-to-Asp phosphorelay between a sensor histidine kinase (HK) and a response regulator (RR). In plants, the His-to-Asp phosphorelay involves an additional intermediate named Histidine-containing phosphotransfer protein (HPt). This multistep phosphorelay consists of a His-Asp-His-Asp sequential transfer of a phosphate group between first an His and an Asp of the HK protein, followed by the transfer to a conserved His of the HPt protein and finally the transfer to an Asp in the receiver domain of the RR protein. May be due to an intron retention.