Localization of the Arabidopsis histidine phosphotransfer proteins is independent of cytokinin

Abstract

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.

Figure 1

Analysis of the subcellular distribution of AHP4-GFP in wild-type Arabidopsis mesophyll protoplasts. (A–D) Representative images of GFP fluorescence signals from protoplasts transfected with plasmids encoding AHP4-GFP fusion proteins that have been treated with either water (control) or 1 µM trans-zeatin for 1 hr. (E) Quantification of data from (A–D). Values are the mean of three independent experiments, error bars indicate SE. Scale bars in (A and C) are 50 µm, scale bars in (B and D) are 25 µm. Methods are as described.

Figure 2

Potential implications of these observations. (A) AHPs may act as phosphatases and remove phosphoryl groups from ARRs. In this model, AHPs would serve to dampen the cytokinin signal if exogenous cytokinin levels were reduced. (B) Cytosolic-localized AHPs may donate phosphoryl groups to cytosolic-localized ARRs or other unkown cytoplasmic targets. (C) Unknown signals could modify AHPs to prevent or enhance AHP entry into the nucleus. As shown, AHPs movement is inhibited, perhaps resulting in reduced sensitivity to cytokinin. Filled arrowheads indicate cytokinin binding, open arrowheads indicate transfer of phosphoryl groups, dashed arrows indicate potential phosphotransfer to AHKs, bidirectional gray arrows indicate AHP movement into and out of the nucleus, and blocked arrowheads indicate inhibition of AHP movement.