Localization of the Arabidopsis histidine phosphotransfer proteins is independent of cytokinin.

Cytokinins are a class of mitogenic plant hormones that influence shoot and root growth, vascular and photomorphogenic development, leaf senescence, and many other aspects of plant growth and development. The Arabidopsis histidine phosphotransfer proteins (AHPs) play an important role in cytokinin signaling by bridging the perception of cytokinins by plasma-membrane receptors to the activation of cytokinin-responsive transcription factors. Based on previous microscopic observations, a model was developed in which the AHPs were thought to relocalize from the cytosol into the nucleus in response to exogenous cytokinin. However, analysis and quantification of the intracellular distribution of AHPs in both protoplasts and intact transgenic plants revealed that the subcellular localization of the AHPs is persistently nucleo-cytosolic and non-responsive to the state of the cytokinin response pathway. Here, we review and extend these findings and discuss their implications.