Abstract
The results of recent intensive studies in Arabidopsis thaliana supported the view that the His-Asp phosphorelay is crucially involved in cytokinin signal transduction. This phosphorelay circuitry consists of 3 AHK histidine kinases, 5 AHP intermediates, ARR transcription factors and/or modulators (more than 20). However, clarification of its comprehensive picture in plants is at the very early stage. As a new approach to this end, we recently employed Arabidopsis T87 cultured cells, and showed that this cell line is versatile to study the cytokinin-mediated phosphorelay. Here we extended this approach, and made several interesting observations: cytokinin-dependent intracellular localization of AHP1, differential localization of ARR15 and ARR16. More interestingly, it was suggested that ARR15 is phosphorylated at a new site (not the canonical Asp site), and that this phosphorylation appears to be crucial for its nuclear-localization. These results from T87 cells will be discussed in the context of the cytokinin-mediated phosphorelay circuitry.
