Cytokinins are a class of phytohormones involved in various physiological events of plants. The Arabidopsis sensor histidine kinase CRE1 was recently reported to be a cytokinin receptor. We used a steroid-inducible system to show that the transcription factor-type response regulator ARR1 directs transcriptional activation of the ARR6 gene, which responds to cytokinins without de novo protein synthesis. This fact, together with characteristics of ARR1-over-expressing plants and arr1 mutant plants, indicates that the phosphorelay to ARR1, probably from CRE1, constitutes an intracelular signal transduction occurring immediately after cytokinin perception.
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