Exogenous melatonin enhances salt secretion from salt glands by upregulating the expression of ion transporter and vesicle transport genes in Limonium bicolor

• Published in: BMC plant biology Vol. 20; no. 1; pp. 493 - 11
• Main Authors: Li, Junpeng, Yuan, Fang, Liu, Yanlu, Zhang, Mingjing, Liu, Yun, Zhao, Yang, Wang, Baoshan, Chen, Min
• Format: Journal Article
• Language: English
• Published: England BioMed Central Ltd 27.10.2020; BioMed Central; BMC
• Subjects: Abiotic stress; Antioxidants; Botanical research; Caryophyllidae; Environmental stress; Gene expression; Gene Expression Regulation, Plant - drug effects; Genes; Genes, Plant - drug effects; Genetic aspects; Halophytes; Homeostasis; Ion homeostasis; Ion transport; Ions; Limonium bicolor; Melatonin; Melatonin - pharmacology; Mesophyll; Physiological aspects; Plant growth; Plant Growth Regulators - pharmacology; Plant Leaves - metabolism; Plant resistance; Plumbaginaceae - drug effects; Plumbaginaceae - genetics; Plumbaginaceae - growth & development; Plumbaginaceae - metabolism; Pretreatment; Protein transport; Proteins; Salinity tolerance; Salt; Salt gland; Salt secretion; Salt stress (Botany); Salt tolerance; Salt Tolerance - drug effects; Salt Tolerance - genetics; Secretion; Seedlings; Sodium Channels - drug effects; Sodium Channels - metabolism; Sodium chloride; Up-Regulation; China; Salt gland; Melatonin; Limonium bicolor; Salt secretion; Ion homeostasis
• ISSN: 1471-2229; 1471-2229
• DOI: 10.1186/s12870-020-02703-x

Salt, a common environmental stress factor, inhibits plant growth and reduces yields. Melatonin is a pleiotropic molecule that regulates plant growth and can alleviate environmental stress in plants. All previous research on this topic has focused on the use of melatonin to improve the relatively low salt tolerance of glycophytes by promoting growth and enhancing antioxidant ability. It is unclear whether exogenous melatonin can increase the salt tolerance of halophytes, particularly recretohalophytes, by enhancing salt secretion from the salt glands. To examine the mechanisms of melatonin-mediated salt tolerance, we explored the effects of exogenous applications of melatonin on the secretion of salt from the salt glands of Limonium bicolor (a kind of recretohalophyte) seedlings and on the expression of associated genes. A pretreatment with 5 μM melatonin significantly improved the growth of L. bicolor seedlings under 300 mM NaCl. Furthermore, exogenous melatonin significantly increased the dry weight and endogenous melatonin content of L. bicolor. In addition, this treatment reduced the content of Na and Cl in leaves, but increased the K content. Both the salt secretion rate of the salt glands and the expression level of genes encoding ion transporters (LbHTK1, LbSOS1, LbPMA, and LbNHX1) and vesicular transport proteins (LbVAMP721, LbVAP27, and LbVAMP12) were significantly increased by exogenous melatonin treatment. These results indicate that melatonin improves the salt tolerance of the recretohalophyte L. bicolor via the upregulation of salt secretion by the salt glands. Our results showed that melatonin can upregulate the expression of genes encoding ion transporters and vesicle transport proteins to enhance salt secretion from the salt glands. Combining the results of the current study with previous research, we formulated a novel mechanism by which melatonin increases salt secretion in L. bicolor. Ions in mesophyll cells are transported to the salt glands through ion transporters located at the plasma membrane. After the ions enter the salt glands, they are transported to the collecting chamber adjacent to the secretory pore through vesicle transport and ions transporter and then are secreted from the secretory pore of salt glands, which maintain ionic homeostasis in the cells and alleviate NaCl-induced growth inhibition.