Exogenous Betaine Enhances Salt Tolerance of Glycyrrhiza uralensis through Multiple Pathways

Abstract

Background Glycyrrhiza uralensis Fisch. is a medicinal plant with high economic value. Despite the relatively strong salt tolerance of perennial individuals, its salt tolerance at the seedling growth stage is low. Improving its salt stress resistance at the seedling stage is the key to the cultivation of the plant using salinized land. In this study, we investigated the enhancement effect of exogenous glycine betaine (GB) on the growth of G. uralensis seedlings under NaCl stress conditions and analysed its physiological mechanism. Results The root growth of G. uralensis seedlings was severely inhibited under NaCl stress conditions, and the addition of GB could effectively alleviate the adverse effects of the salt stress; the application of 20 mM GB had the most significant alleviating effect. Compared with the NaCl-only treatment group, the application of 20 mM GB under NaCl stress conditions significantly increased total root length, total root surface area, and total root volume by 80.38%, 93.28%, and 175.61%, respectively, and obviously increased the GB content in its roots, stems, and leaves by 36.88%, 107.05%, and 21.63%, respectively, and their activity of betaine aldehyde dehydrogenase 2 (BADH2) was increased by 74.10%, 249.38%, and 150.60%, respectively. The GB-addition treatment significantly increased content of osmoregulatory substances (soluble proteins, sluble sugars and proline) in the roots and leave, markedly enhanced the activity of antioxidant enzymes and the content of antioxidant, and dramatically promoted salt secretion capacity of the leaves, which especially promoted the secretion of Na+. Conclusions In summary, the exogenous addition of GB is an effective way to significantly improve the salt tolerance of G. uralensis seedlings, and promotion of the synthesis of osmoregulatory substances, enhancement of antioxidant enzyme activities, reduction of membrane lipid peroxidation products, and discharge of excess salts in the body (especially the significant promotion of the secretion of Na+) are the critical reasons why exogenous addition of GB was able to effectively alleviate the NaCl damage of G. uralensis seedlings.