The function of salt responsive promoter of Suaeda salsa SsCLCb explains why the species can accumulate large amount of nitrate from saline soils

Abstract

Suaeda salsa exhibits exceptional nitrate (NO₃⁻) absorption and accumulation capacity under saline conditions, making it an ideal candidate for phytoremediation of nitrate-enriched saline soils. However, the underlying molecular regulatory mechanism remains unclear. In this study, the expression of a chlorine channel gene SsCLCb in S. salsa leaves was upregulated 17-fold and 16-fold under 200 and 500 mM NaCl treatment, respectively. The primary root length of seedlings from Arabidopsis overexpressed SsCLCb lines were higher than the wild-type (WT) and AtCLCb mutant (atclcb) under salt treatment. Under 100 mM NaCl treatment, SsCLCb overexpression lines exhibited significantly improved growth and metabolic profiles compared to WT i.e., shoot fresh weight, dry weight, leaf NO₃⁻ and chlorophyll content increased by 7.5 %, 10 %, 19 % and 13.7 %, respectively. Histochemical staining showed lighter DAB and NBT deposition than those of WT, correlating with 17.8 % and 50.9 % decreased hydrogen peroxide and superoxide anion content, respectively. Malondialdehyde (MDA) content also declined by 47.1 %. The salt-responsive promoter ProSsCLCb exhibited strong transcriptional activation under salinity stress, as demonstrated by significantly intensified GUS staining in the leaves of S. salsa, tobacco and Arabidopsis under salinity. Notably, ProSsCLCb::SsCLCb (generated in WT background) transgenic Arabidopsis exhibited superior salt tolerance compared to 35S::SsCLCb lines (e.g., 27.5 % higher dry weight). In conclusion, SsCLCb and its promoter enhances NO₃⁻ accumulation in the leaves, thereby improving osmoregulation capacity and mitigating salt-induced oxidative stress. These results provide mechanistic insight into the ecological adaptation of S. salsa, revealing how the halophyte efficiently acquires and utilizes NO₃⁻ from saline environments.