Bettina Stadelhofer

Background: The Arabidopsis response regulator 22 (ARR22) is one of two members of a recently defined novel group of two-component system (TCS) elements. TCSs are stimulus perception and response modules of prokaryotic origin, which signal by a His-to-Asp phosphorelay mechanism. In plants, TCS regulators are involved in hormone response pathways, such as those for cytokinin and ethylene. While the functions of the other TCS elements in Arabidopsis, such as histidine kinases (AHKs), histidine-containing phosphotransfer proteins (AHPs) and A-type and B-type ARRs are becoming evident, the role of ARR22 is poorly understood. Results: We present evidence that ARR22 is a preferentially cytoplasmic protein, exclusively expressed in the chalaza of developing seeds. ARR22 specifically interacts with AHP2, AHP3 and AHP5 in yeast and living plant cells. Two new loss-of-function alleles, arr22-2 and arr22-3, were isolated and characterized. With respect to their morphology and metabolite status, no significant difference in the developing seeds of the arr22 mutants was observed compared to wild type. The genetic complementation of the arr22 mutants with a genomic ARR22 fragment resulted in plants (arr22/gARR22) with a pleiotropic phenotype of different penetrance. This phenotype was not observed when the phosphorylatable Asp74 of ARR22 was changed to either a dominant-active Glu or a dominant-inactive Asn. The phenotype of the arr22/gARR22 plants was comparable to that of multiple ahk, ahp and B-type arr mutants. Conclusion: Our results favor the model that ARR22 acts as a phospho-histidine phosphatase on specific AHPs in the cytoplasm of Arabidopsis chalaza cells. The lack of any aberrant morphological and metabolite phenotype in the seeds of the arr22 mutants indicates that ARR22 is probably primarily responsible for the fine tuning of specific branches of chalaza-based TCS signalling. Even when slightly mis-expressed, ARR22 interferes with hormone homeostasis in non-chalaza tissues. Our data indicate that the chromatin status might play a crucial role in maintaining the chalaza-restricted expression of ARR22.

Like many other plant species, Arabidopsis uses arginine (Arg) as a storage and transport form of nitrogen, and proline (Pro) as a compatible solute in the defence against abiotic stresses causing water deprivation. Arg catabolism produces ornithine (Orn) inside mitochondria, which was discussed controversially as a precursor for Pro biosynthesis, alternative to glutamate (Glu).