| Seed germination is a complex and critical process in the life cycle of higher plants.it is involved in seeding establishment and plant growth. By definition, seed germination commences with the uptake of water and is completed with emergence of the radicle from the seed coat. After the uptake of water by seeds, a series of physical and metabolic events during germination are started including the protein synthesis, posttranslational modification and regulation of phytohormone on seed germination.Melatonin (N-acetyl-5-methoxytryptamine) is an indole amine which was long thought to be a classic hormone in animals. Although Melatonin has now been found to exist commonly in plants, the biological functions of melatonin in plants are still in its infancy. In addition to absorbing melatonin from the culture medium, plants can also synthesize melatonin. Evidences indicate that exogenously applied melatonin can improve the tolerance of plants to abiotic stresses. In this study, we investigated the effect of exogenous melatonin on seed germination under salt stress. The possible mechanisms of melatonin promoting seed germination under salt stress are also researched by proteomic analysis. The mainly results are documented as follows:(1) Exogenous melatonin treatment (1μmol/L) significantly promoted the seed germination under salt stress. Salt stress induces the accumulation of reactive oxygen species (ROS) in cucumber seeds. Melatonin-treated seeds showed relatively low levels of H2O2,-OH and O2-. As a good endogenous free radical scavenger, melatonin can decrease the ROS. Melatonin also increases the activity of SOD, POD, and CAT by inducing gene expression of CsCu-ZnSOD, CsFe-ZnSOD, CsPOD, and CsCAT under high salinity. So, melatonin can alleviate ROS damages and keep the balance of ROS.(2) It is well known that both ABA catabolism and GA biosynthesis play prominent roles in seed germination. Melatonin regulates seed germination through positively up-regulating GA biosynthesis and ABA catabolism. In this study, melatonin significantly upregulated ABA catabolism genes (CsCYP707A1 and CsCYP707A2) and downregulated ABA biosynthesis genes (CsNECD2), resulting in a rapid decrease of ABA content during the early stage of germination. At the same time, melatonin also positively upregulated GA biosynthesis genes (CsGA20ox and CsGA3ox), contributing to a significant increase of GA (especially GA4) content.(3) Proteomic data analysis showed that exogenous melatonin upregulated proteins that are related to cell elongation and cell division,such as tubulin and actin. Melatonin also downregulated the storage proteins, such as globulin-like proteins. Our data showed that melatonin was involved in responses to stress. A large amount of proteins including HSP,14-3-3 proteins, antioxidant enzymes and ubiquitin proteasome were regulated by melatonin. It is indicated that melatonin played a role in the signal perception, ROS elimination, stress tolerance and the modification and degradation of damaged proteins.(4) Proteomic data showed that melatonin regulated the energy metabolism pathways. The lipid was hydrolyzed and then the fatty acids are passed to glyoxylate cycle. The key enzymes of glyoxylate cycle, Isocitrate lyase and Malate synthase, were upregulated by melatonin. Melatonin also increased the activities of a, P-amylase and the degradation of the starch. About nine enzymes involved in the glycolysis, citric acid cycle which contributing to ATP production were regulated by melatonin. It indicated that melatonin was involved in energy production and promoted the seed germination under salt stress. |
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