A type-B response regulator drives the expression of the hydroxymethylbutenyl diphosphate synthase gene in periwinkle

In plant cytokinin (CK) signaling, type-B response regulators (RRs) act as major players in orchestrating the transcriptome changes in response to CK. However, their direct targets are poorly known. The identification of putative type-ARR1 motifs located within the promoter of the CM-responsive hydroxyl methyl butenyl diphosphate synthase (HDS) gene from the methyl erythritol phosphate (MEP) pathway prompted us to investigate the ability of a previously isolated periwinkle type-B RR (CrRR5) that presents high homologies with ARR1 to interact with the promoter. Electrophoretic mobility shift assays (EMSAs) demonstrated that the CrRR5 DNA-binding domain binds specifically type-ARR1 motifs within the HDS promoter. We also established through yellow fluorescent protein (YFP) imaging the targeting of CrRR5 into cell nucleus in accordance with its putative function of transcription factor. In transient assays performed on periwinkle cells cultivated with CM, overexpression of the full-length CrRR5 or a truncated CrRR5 engineering a constitutive active form (35S: Delta DDK) did not affect the HDS promoter activity that reached a threshold. By contrast, in absence of CM, overexpression of CrRR5 Delta DDK enhanced promoter activity up to the threshold level observed in cells grown with CK. Our results strongly suggest that CrRR5 directly transactivates the HDS promoter. CrRR5 is the first identified transcription factor mediating the CM signaling that targets a gene from the MEP pathway involved in isoprenoid metabolism. Moreover, CrRR5 could play a role in a regulatory mechanism controlling CM homeostasis in periwinkle cells. (C) 2012 Elsevier GmbH. All rights reserved.