Brassica juncea plant cadmium resistance 1 protein (BjPCR1) facilitates the radial transport of calcium in the root

Abstract

Calcium (Ca) is an important structural component of plant cell walls and an intracellular messenger in plants and animals. Therefore, plants tightly control the balance of Ca by regulating Ca uptake and its transfer from cell to cell and organ to organ. Here, we propose that Brassica juncea PCR1 (PCR1), a member of the plant cadmium resistance (PCR) protein family in Indian mustard, is a Ca2+ efflux transporter that is required for the efficient radial transfer of Ca2+ in the root and is implicated in the translocation of Ca to the shoot. Knock-down lines of BjPCR1 were greatly stunted and translocated less Ca to the shoot than did the corresponding WT. The localization of BjPCR1 to the plasma membrane and the preferential expression of BjPCR1 in the root epidermal cells of WT plants suggest that BjPCR1 antisense plants could not efficiently transfer Ca2+ from the root epidermis to the cells located inside the root. Protoplasts isolated from BjPCR1 antisense lines had lower Ca2+ efflux activity than did those of the WT, and membrane vesicles isolated from BjPCR1-expressing yeast exhibited increased Ca2+ transport activity. Inhibitor studies, together with theoretical considerations, indicate that BjPCR1 exports one Ca2+ in exchange for three protons. Root hair-specific expression of BjPCR1 in Arabidopsis results in plants that exhibit increased Ca2+ resistance and translocation. In conclusion, our data support the hypothesis that BjPCR1 is an exporter required for the translocation of Ca2+ from the root epidermis to the inner cells, and ultimately to the shoot.