Abstract
In the past few years, enormous progress has been made in understanding cytokinin perception and signalling. Three cytokinin receptor proteins, which are hybrid histidine kinases, have been identified in Arabidopsis. These receptors may transduce signals in a quantitative rheostat-like fashion, thus permitting long-lasting and continuously variable signalling that is directly dependent on the hormone concentration. Evidence has been provided that downstream signalling is transmitted through a His-to-Asp phospho-relay involving phosphotransmitter and response regulator proteins, typical of two-component systems. On the basis of mutant analysis, protein–protein interaction studies and target gene identification, a cellular network is emerging that links cytokinin activity to both developmental and physiological processes.