Ectopic expression of the phosphomimic mutant version of Arabidopsis response regulator 1 promotes a constitutive cytokinin response phenotype

BACKGROUND: Cytokinins control numerous plant developmental processes, including meristem formation and activity, nutrient distribution, senescence timing and responses to both the abiotic and biotic environments. Cytokinin signaling leads to the activation of type-B response regulators (RRBs), Myb-like transcription factors that are activated by the phosphorylation of a conserved aspartate residue in their response receiver domain. Consistent with this, overexpression of RRBs does not substantially alter plant development, but instead leads to cytokinin hypersensitivity. RESULTS: Here we present comparative analysis of plants overexpressing Arabidopsis RRB 1 (ARR1) or a phosphomimic ARR1(D94E) mutant in which the conserved aspartate-94 (D94) is replaced by the phosphomimic residue glutamate (E). The D94E substitution causes a 100-fold increase in response activation and instigates developmental and physiological changes that characterize wild-type plants treated with cytokinins or transgenic plants with increased cytokinin content. CONCLUSION: The current model of cytokinin signaling emphasizes the essential role of conserved aspartate residue phosphorylation of RRBs in promoting cytokinin responses. Our comparative analyses of developmental and physiological traits of ARR1 and ARR1(D94E) overexpressing plants revealed that the ARR1(D94E) protein is indeed a constitutive and wide-spectrum cytokinin response activator.