Ectopic overexpression of a cotton plastidial Na+ transporter GhBASS5 impairs salt tolerance in Arabidopsis via increasing Na+ loading and accumulation.

GhBASS5 is a member of the bile acid sodium symporter (BASS) gene family from cotton and a plastid-localized Na+ transporter that negatively regulates salt tolerance of plants. Soil salinization is a major constraint on global cotton production, and Na+ is the most dominant toxic ion in salinity stress. Hence, insights into the identities and properties of transporters that catalyze Na+ movement between different tissues and within the cell compartments are vital to understand the salt-tolerant mechanisms of plants. Here, we identified the GhBASS5 gene, a member of the bile acid sodium symporter (BASS) gene family from cotton, served as a plastidic Na+ transporter. GhBASS5 encodes a membrane protein localized in the plastid envelope. It was highly expressed in cotton roots and predominantly existed in the vascular cylinder. Heterogenous expression of GhBASS5 in Arabidopsis chloroplasts promoted Na+ uptake into chloroplasts, which contributed to an increased cytoplasmic Na+ concentration. And GhBASS5-overexpressed transgenic plants showed an increase in Na+ translocation from roots to shoots and an elevated Na+ content in both roots and shoots, but a dramatic decrease in the Na+ efflux from root tissues and the K+/Na+ ratio, especially under salt stress conditions. Furthermore, overexpressing GhBASS5 greatly damaged plastid functions and enhanced salt sensitivity in transgenic Arabidopsis when compared with wild-type plants under salt stress. Additionally, the salt-responsive transporter genes that regulate K+/Na+ homeostasis were dramatically expressed in GhBASS5-overexpressed lines, especially under salt stress conditions. Taken together, our results suggest that GhBASS5 is a plastid-localized Na+ transporter, and high expression of GhBASS5 impairs salt tolerance of plants via increasing Na+ transportation and accumulation at both cell and tissue levels.