ARR1, a Transcription Factor for Genes Immediately Responsive to Cytokinins

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 -overexpressing plants and arr1 mutant plants, indicates that the phosphorelay to ARR1, probably from CRE1, constitutes an intracellular signal transduction occurring immediately after cytokinin perception.