Expression and functional analysis of putative vacuolar Ca 2+ -transporters (CAXs and ACAs) in roots of salt tolerant and sensitive rice cultivars

Vacuolar Ca 2+ -transporters could play an important role for salt tolerance in rice ( Oryza sativa L.) root. Here, we compared the expression profiles of putative vacuolar cation/H + exchanger (CAX) and calmodulin-regulated autoinhibited Ca 2+ -ATPase (ACA) in rice roots of salt tolerant cv. Pokkali and salt sensitive cv. IR29. In addition to five putative vacuolar CAX genes in the rice genome, a new CAX gene ( OsCAX4 ) has been annotated. In the present study, we isolated the OsCAX4 gene and showed that its encoded protein possesses a unique transmembrane structure and is potentially involved in transporting not only Ca 2+ but also Mn 2+ and Cu 2+ . These six OsCAX genes differed in their mRNA expression pattern in roots of tolerant versus sensitive rice cultivars exposed to salt stress. For example, OsCAX4 showed abundant expression in IR29 (sensitive) upon prolonged salt stress. The mRNA expression profile of four putative vacuolar Ca 2+ -ATPases ( OsACA4 - 7 ) was also examined. Under control conditions, the mRNA levels of OsACA4 , OsACA 5, and OsACA7 were relatively high and similar among IR29 and Pokkali. Upon salt stress, only OsACA4 showed first a decrease in its expression in Pokkali (tolerant), followed by a significant increase. Based on these results, a role of vacuolar Ca 2+ transporter for salt tolerance in rice root was discussed.