Review Melatonin: A Small Molecule but Important for Salt Stress Tolerance in Plants Haoshuang Zhan 1,†, Xiaojun Nie 1,†, Ting Zhang 1, Shuang Li 1, Xiaoyu Wang 1, Xianghong Du 1, Wei Tong 1,* and Weining Song 1,2,* 1 State Key Laboratory of Crop Stress Biology in Arid Areas, College of Agronomy and Yangling Branch of China Wheat Improvement Center, Northwest A&F University, Yangling 712100, China;
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[email protected] (X.D.) 2 ICARDA-NWSUAF Joint Research Center for Agriculture Research in Arid Areas, Yangling 712100, China * Corresponding authors:
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[email protected] (W.S.); Tel.: +86-29-8708-2984 (W.S.); Fax: +86-29-8708-2203 (W.S.) † These authors contributed equally to this work. Received: 1 January 2019; Accepted: 4 February 2019; Published: 7 February 2019 Abstract: Salt stress is one of the most serious limiting factors in worldwide agricultural production, resulting in huge annual yield loss. Since 1995, melatonin (N-acetyl-5-methoxytryptamine)—an ancient multi-functional molecule in eukaryotes and prokaryotes—has been extensively validated as a regulator of plant growth and development, as well as various stress responses, especially its crucial role in plant salt tolerance. Salt stress and exogenous melatonin lead to an increase in endogenous melatonin levels, partly via the phyto-melatonin receptor CAND2/PMTR1. Melatonin plays important roles, as a free radical scavenger and antioxidant, in the improvement of antioxidant systems under salt stress.