The Ca2+-binding protein PCaP2 located on the plasma membrane is involved in root hair development as a possible signal transducer

Plasma membrane-associated Ca2+-binding protein2 (PCaP2) of Arabidopsis thaliana is a novel-type protein that binds to the Ca2+/calmodulin complex and phosphatidylinositol phosphates (PtdInsPs) as well as free Ca2+. Although the PCaP2 gene is predominantly expressed in root hair cells, it remains unknown how PCaP2 functions in root hair cells via binding to ligands. From biochemical analyses using purified PCaP2 and its variants, we found that the Nterminal basic domain with 23 amino acids (N23) is necessary and sufficient for binding to PtdInsPs and the Ca2+/calmodulin complex, and that the residual domain of PCaP2 binds to free Ca2+. In mutant analysis, a pcap2 knockdown line displayed longer root hairs than the wild-type. To examine the function of each domain in root hair cells, we over-expressed PCaP2 and its variants using the root hair cell-specific EXPANSINA7 promoter. Transgenic lines over-expressing PCaP2, PCaP2G2A (second glycine substituted by alanine) and 23PCaP2 (lacking the N23 domain) exhibited abnormal branched and bulbous root hair cells, while over-expression of the N23 domain suppressed root hair emergence and elongation. The N23 domain was necessary and sufficient for the plasma membrane localization of GFP-tagged PCaP2. These results suggest that the N23 domain of PCaP2 negatively regulates root hair tip growth via processing Ca2+ and PtdInsP signals on the plasma membrane, while the residual domain is involved in the polarization of cell expansion.