Alfalfa MsCBL4 enhances calcium metabolism but not sodium transport in transgenic tobacco under salt and saline-alkali stress

Key message MsCBL4 expression in tobacco enhanced its salt and saline-alkali stress tolerance by regulating calcium accumulation in roots, indicating the important role of calcium metabolism in plant saline-alkali stress tolerance The calcineurin B-like (CBL) family of proteins play important roles in plant abiotic stress tolerance and signal transduction. CBL4 is known to participate in the Salt Overly Sensitive pathway; however, little is currently known regarding the mechanisms underlying the response of CBL4 to saline-alkali stress. In this study, we cloned and characterized the alfalfa MsCBL4 gene. We found that MsCBL4 showed the highest expression in root tissues and was induced by salt and saline-alkali stress, with the latter causing higher induction. Overexpression of MsCBL4 in tobacco enhanced salt and saline-alkali stress tolerance and reduced the Na+/K+ ratio in roots of transgenic lines. Salt (30 and 300 mM NaCl) and saline-alkali (30 mM NaHCO3) stress assays performed for MsCBL4 transgenic tobacco lines revealed a substantial influx of sodium ions in roots under saline-alkali stress and indicated that the expression of MsCBL4 had little influence on sodium ion content reduction. In contrast, in roots subjected to saline-alkali stress, calcium accumulation occurred and was significantly enhanced by the overexpression of MsCBL4. Physiological and biochemical analyses indicated that MsCBL4 plays an important role in saline-alkali stress tolerance via its influence on the regulation of calcium transport and accumulation. These results provide novel insights into the saline-alkali stress tolerance mechanisms of plants.