Experimental evidence for a function as cytokinin receptors for two members of the newly identified subfamily. AHK4, PpCHK4, MpCHK1, and AHK4 (T301I) were tested in a bacterial complementation assay with different cytokinin and adenine concentrations. Ade, Adenine; BA, benzyladenine; cZ, cis-zeatin; iP, isopentenyl; tZ, trans-zeatin.