Three type-B response regulators, ARR1, ARR10 and ARR12, play essential but redundant roles in cytokinin signal transduction throughout the life cycle of Arabidopsis thaliana.

Arabidopsis thaliana has 11 members belonging to the typical type-B ARR (authentic response regulator) family. Among them, seven highly homologous members appear also to be conserved in rice (Oryza sativa), but others are not. It was suggested that these seven ARRs are commonly implicated as DNA-binding transcription factors in the phosphorelay-mediated cytokinin signal transduction network in higher plants. To gain an insight into the functions of the cytokinin-associated type-B ARRs, we previously investigated an arr1 arr10 arr12 triple mutant and reported that it exhibited stunted growth and abnormality in vascular development. Based on this fact, here we attempted to characterize the mutant intensively with reference to cytokinin-associated phenotypes through the life cycle. We showed that the observed cytokinin-associated phenotypes of arr1 arr10 arr12 were very severe and highly analogous to those observed for certain ahk2 ahk3 ahk4/cre1 triple mutants, which have virtually no cytokinin receptor to propagate the phosphorelay signal transduction network. Among the seven ARR members belonging to the cytokinin-associated type-B ARR subfamily, it was thus suggested that ARR1, ARR10 and ARR12 together play essential (or general) roles in cytokinin signal transduction. It is therefore conceivable that the other type-B ARRs (ARR2, ARR11, ARR14 and ARR18) might play more specific roles spatially and temporally in plants.