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Transglutaminase (TGase) is inextricably associated with plant growth and development. However, the mechanism by which TGase enhances salt tolerance of higher plants under salt stress is poorly understood. In this study, we investigated the effects of NaCl stress and exogenous o-phenanthroline (o-Phen, a metalloprotease inhibitor) on TGase activity, chlorophyll fluorescence parameters, carbohydrates contents, the reactive oxygen species (ROS) scavenging system, and endogenous polyamines (PAs) contents of salt-sensitive 'Jinyou No. 4′ and salt-tolerant 'Inbred Line 9930' cucumber. Salt stress significantly inhibited plant growth of the two cultivars, as well as hindered carbohydrates transport, which was more evident in the salt-sensitive cultivar. TGase activity and expression, ROS scavenging capacity, and bound PAs content were up-regulated by salt stress to some extent, which was more distinct in the salt-tolerant cucumber cultivar. However, o-Phen treatment significantly inhibited TGase expression, and further decreased plant growth and the actual photochemical efficiency of photosystem II in the two cultivars. In addition, application of o-Phen significantly decreased endogenous PAs content in leaves of 'Jinyou No. 4' and 'Inbred Line 9930' seedlings by 9.60% and 42.32% under NaCl stress, respectively. These results suggested that high activity of TGase increases the salt stress tolerance of cucumber plants by increasing endogenous PAs content and ROS scavenging capacity, and promoting carbon assimilation and photosynthetic products. • Salt stress caused plant growth retardation as well as inhibited transportation of carbohydrates both in the two cucumber cultivars. • Salt tolerant cucumber variety showed higher transglutaminase (TGase) activity as well as increasing bound polyamines (PAs) content and reactive oxygen scavenging capacity. • o-Phen treatment reduced accumulation of bound PAs. • The o-Phen treatment reduced photochemical efficiency of PSII of the two cucumber cultivars. [ABSTRACT FROM AUTHOR]

In this study, we used two cucumber ( Cucumis sativus L.) cultivars with different salt tolerances, Jinyou NO.4 (salt-sensitive) and Inbred line 9930 (salt-tolerant), to investigate the effects of salt stress (75 mM of NaCl) on photosynthesis and antioxidant capacity in cucumber leaves. The results showed that the plant height, fresh weight, and dry weight of the seedlings from the two cultivars were decreased under salt stress. The restrained degree of growth parameters induced by NaCl was more serious in Jinyou NO.4 than in Inbred line 9930. Salt stress caused significant decreases in photosynthetic pigment content, chl a / b , Pn, Gs and Tr in the leaves of the two cultivars. Ci content was increased in Jinyou NO.4, and it was decreased in Inbred line 9930. Moreover, Fv/Fm, ΦPSII, qP, NPQ, and the mRNA expression levels of RbcL , RbcS and RCA were decreased in the two cucumber leaves under salt stress, and the effects of salt stress on Jinyou NO.4 were more serious. Meanwhile, the activities and encoding gene expression levels of SOD, POD, CAT, and APX were increased before being decreased under salt stress. These expression levels were greater in Inbred line 9930 than in Jinyou NO.4. NaCl stress-induced TGase activity, gene expression and bound PAs contents were higher in Inbred line 9930. Salt stress increased the MDA content and O 2 ∙－ generation rate in cucumber leaves, especially in Jinyou NO.4. These results suggested that salt-tolerant cultivars could enhance salinity tolerance by regulating photosynthetic efficiency and antioxidant capacity compared with the salt-sensitive cultivar. This could alleviate salt damage to cucumber seedlings. [ABSTRACT FROM AUTHOR]

Transglutaminase (TGase) is a regulator of posttranslational modification of protein that provides physiological protection against diverse environmental stresses in plants. Nonetheless, the mechanisms of TGase-mediated salt tolerance remain largely unknown. Here, we found that the transcription of cucumber TGase (CsTGase) was induced in response to light and during leaf development, and the CsTGase protein was expressed in the chloroplast and the cell wall. The overexpression of the CsTGase gene effectively ameliorated salt-induced photoinhibition in tobacco plants, increased the levels of chloroplast polyamines (PAs) and enhanced the abundance of D1 and D2 proteins. TGase also induced the expression of photosynthesis related genes and remodeling of thylakoids under normal conditions. However, salt stress treatment reduced the photosynthesis rate, PSII and PSI related genes expression, D1 and D2 proteins in wild-type (WT) plants, while these effects were alleviated in CsTGase overexpression plants. Taken together, our results indicate that TGase-dependent PA signaling protects the proteins of thylakoids, which plays a critical role in plant response to salt stress. Thus, overexpression of TGase may be an effective strategy for enhancing resistance to salt stress of salt-sensitive crops in agricultural production.

• This study was the first to use the transgenic pathways to analyze the effect of transglutaminase on the salt tolerance. • This work demonstrate that TGase may promote plant survival by regulating PAs metabolism and Na+/K+ balance under salt stress. • This study lays the foundations for cucumber salinity improvements in the future. 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 H 2 O 2 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. [ABSTRACT FROM AUTHOR]

Transglutaminase (TGase) is a regulator of posttranslational modification of protein that provides physiological protection against diverse environmental stresses in plants. Nonetheless, the mechanisms of TGase-mediated salt tolerance remain largely unknown. Here, we found that the transcription of cucumber TGase (CsTGase) was induced in response to light and during leaf development, and the CsTGase protein was expressed in the chloroplast and the cell wall. The overexpression of the CsTGase gene effectively ameliorated salt-induced photoinhibition in tobacco plants, increased the levels of chloroplast polyamines (PAs) and enhanced the abundance of D1 and D2 proteins. TGase also induced the expression of photosynthesis related genes and remodeling of thylakoids under normal conditions. However, salt stress treatment reduced the photosynthesis rate, PSII and PSI related genes expression, D1 and D2 proteins in wild-type (WT) plants, while these effects were alleviated in CsTGase overexpression plants. Taken together, our results indicate that TGase-dependent PA signaling protects the proteins of thylakoids, which plays a critical role in plant response to salt stress. Thus, overexpression of TGase may be an effective strategy for enhancing resistance to salt stress of salt-sensitive crops in agricultural production. [ABSTRACT FROM AUTHOR]