Ectopic expression of CsTGase enhances salt tolerance by regulating polyamine biosynthesis, antioxidant activities and Na+/K+ homeostasis in transgenic tobacco

Transglutaminases (TGases), mediators of the transamidation of specific proteins by polyamines (PA), play critical roles in PA metabolism in animals, but their functions and regulatory mechanisms are largely unknown in plants. In this study, we demonstrated that TGase from cucumber played a protective role in the regulation of PA metabolism under salt stress. The expression of TGase was induced by salt stress in cucumber. Ectopic overexpression of cucumber TGase in tobacco conferred enhanced tolerance to salt stress based on both external symptoms and membrane integrity. Overexpression lines maintained high levels of PAs under salt stress, suggesting that PAs played a vital role in TGase-induced salt tolerance. In contrast, the levels of Na+ content in the wild-type (WT) plants increased, while they decreased in the overexpression plants. The expression levels of several genes related to ion exchange enhanced, and the Na+/K+ ratio decreased by increased TGase activity under salt stress. The activities of the proton-pump ATPase (H+-ATPase), vacuolar H+-ATPase (V-ATPase) and vacuolar H+-pyrophosphatase (PPase) were higher in the overexpression lines than in WT plants under salt stress. Moreover, the malondialdehyde (MDA) and H2O2 contents were significantly lower in the overexpression lines than in WT plants, accompanied by increased antioxidant enzyme activity. Taken together, these findings demonstrate that TGase plays protective roles in response to salt stress, which may promote plant survival by regulating PA metabolism and the Na+/K+ balance under salt stress.