The plant hormone cytokinin is involved in many processes in the plant, including cell division, seed germination, photomorphogenesis, shoot and root development, leaf senescence and seed set. The model for cytokinin signaling is similar to a two-component phosphorelay with which bacteria sense and respond to environmental stimuli. The cytokinin receptors are Hybrid Histidine Kinases that autophosphorylate on a conserved histidine residue in response to cytokinin binding. The phosphoryl group is transferred via an intermediate Histidine Phosphotransfer Protein to a conserved aspartate residue on the receiver domain of a Response Regulator.

Members of the Response Regulator protein family in Arabidopsis thaliana (ARRs) contain the conserved N-terminal receiver domain required for phosphorylation by two-component elements and can be classified into three groups based on sequence similarity and protein structure: type-A, type-B and type-C ARRs. The ten type-A ARRs are rapidly up-regulated by cytokinin treatment but their sequences do not predict known outputs. The eleven type-B ARRs have DNA binding and transactivating activity and are positive activators of cytokinin-regulated transcription. The pair of type-C ARRs are less similar in sequence to the two other groups of ARRs, are not transcriptionally regulated by cytokinin and do not have transcriptional activity.

In order to study the role of type-A ARR in cytokinin signaling and development, I have isolated multiple type-A arr loss-of function mutants up to a septuple arr3,4,5,6,7,8,9 mutant. Type-A arr mutants exhibit additive hypersensitivity to cytokinin, indicating that type-A ARRs play overlapping roles in negatively regulating cytokinin response. Subsets of type-A arr mutants show specific responses consistent with their patterns of expression. In particular, a subset of type-A ARRs interact with the meristem maintenance gene WUSCHEL to modulate shoot meristem activity. To further investigate the role of phosphorelay on type-A ARR function, I constructed site-directed mutants targeting the conserved aspartate phosphorylation site and tested their functions in planta. My results indicate that type-A ARR proteins are activated by phosphorylation and are likely to function by phospho-dependent interactions, with implications for functional specification.