Abstract
The discovery of cytokinin receptors of Arabidopsis thaliana ten years ago was a milestone in plant hormone research. Since then, research has yielded insights into the biochemical properties and functions of these sensor histidine kinases. Their affinities to both trans-zeatin and isopentenyladenine are in the low nM range. Cytokinin ribosides, cis-zeatin and thidiazuron were established as compounds with genuine cytokinin activity and the first cytokinin antagonists were identified. Numerous functions of cytokinin receptors in plant development, as well as in the plant's responses to the environment, have been elucidated and are summarized. Finally, we address the question how the receptors have evolved during plant evolution.