Updates on the model and the evolution of cytokinin signaling

• Novel subcellular localization for cytokinin receptors and phosphotransfer proteins.
• Both types of response regulators are regulated via protein degradation.
• Novel interaction of the cytokinin signaling pathway with others (e.g. auxin, nitric oxide) on the posttranscriptional level.
• Cytokinin circuitry genes seemed to be introduced via endosymbiosis.

Cytokinins represent a class of phytohormones, which are key players not only in many processes important for plant growth and development, but also in the response to changes in their environment. The model for the cytokinin signaling pathway was established at the turn of the last century and many experiments confirmed its validity. In recent years several changes and extensions to the model were necessary to accommodate new findings concerning its components, such as subcellular localization, selective protein degradation and new modes of cross talk. In addition phylogenetic analyses of components of the cytokinin circuitry started to reveal the origin and evolution of the cytokinin regulatory system.