In Vivo and In Vitro Characterization of the ARR11 Response Regulator Implicated in the His-to-Asp Phosphorelay Signal Transduction in Arabidopsis thaliana

In Arabidopsis thaliana, Histidine-to-Aspartate (His--> Asp) phosphorelay is a paradigm of a signaling system that is considered to be involved in response to plant hormones, including ethylene and cytokinin. In the current framework of His-->Asp phosphorelay in this higher plant, the type-B ARR (response regulator) family members appear to act as DNA-binding transcriptional regulators. Although Arabidopsis thaliana has 11 type-B ARR family members, except for ARR1 and ARR2, no biological information is available with regard to others. As the main objective of this study, we characterized another example, ARR11, in terms of not only its in vitro biochemical properties, but also its biological activity in plants. In plants, the ARR11 gene was expressed predominantly in roots. In vitro, ARR11 showed the ability to acquire a phosphoryl group from a histidine-containing phosphotransfer intermediate (AHP), and also it showed the ability to recognize a specific nucleotide sequence, GGATT. These in vitro results supported the view that ARR11 is indeed a DNA-binding transcription factor, the ability of which is most likely modulated by phosphorylation in its receiver domain. In vivo, when a C-terminal DNA-binding domain lacking the N-terminal phospho-accepting (or receiver) domain was aberrantly expressed, the resulting transgenic plants showed characteristic anomalies during development of apical parts. The observed anomalies included "unusual proliferation of tissues in cotyledons" and "outgrowth of adventitious shoots near cotyledons". These results with regard to the functions of ARR11 are mainly discussed in comparison with those of the previously characterized type-B response regulators.