An ankyrin repeat protein is involved in anthocyanin biosynthesis in Arabidopsis

The ankyrin domain is one of the most common protein motifs in eukaryotic proteins. Repeated ankyrin domains are ubiquitous and their mediation of protein-protein interactions is involved in a number of physiological and developmental responses such as the cell cycle, signal transduction and cell differentiation. A novel putative phytochrome-interacting ankyrin repeat protein 2 (PIA2) containing three repeated ankyrin domains was identified in Arabidopsis. An in vitro pull-down and phosphorylation assay revealed that PIA2 is phosphorylated and interacts directly with oat phytochrome A. The N-terminal domain of PIA2 was specifically phosphorylated, whereas interactions between the domains of PIA2 and phytochrome A had no Pr/Pfr preference. PIA2 was ubiquitously expressed in most tissues and was localized in both the nucleus and the cytoplasm independent of treatment with light of specific wavelengths. Anthocyanin accumulation in seedlings grown under far-red light, a typical phenotype of wild-type plants, was reduced in a loss-of-function mutant of PIA2 (pia2), whereas anthocyanin accumulation was increased in an overexpressing plant (PIA2-OX). The gene expression of UDP-flavonoid-3'-glucosyl-transferase (UF3GT), a major enzyme in the anthocyanin biosynthesis processes, was decreased in pia2 knockout plants suggesting that decreased anthocyanin was because of the decreased expression of UF3GT. Our results suggest that PIA2 plays a role in the anthocyanin biosynthesis during seedling development as a novel phytochrome-interacting protein.