Melatonin enhances thermotolerance of maize seedlings ( Zea mays L.) by modulating antioxidant defense, methylglyoxal detoxification, and osmoregulation systems

Melatonin (MT), derived from tryptophan, is an amazing signaling molecule with multiple functions in plants. Heat stress (HS) induced by high temperature is a major stress factor that limits metabolism, growth, development, and productivity of plants. However, whether MT could enhance the thermotolerance of maize seedlings and the underlying mechanisms is not completely known. In this study, treatment of maize seedlings with MT enhanced the survival percentage of maize seedlings under HS conditions, mitigated an increase in malondialdehyde (MDA, product of membrane lipid peroxidation) and electrolyte leakage, and improved tissue vitality compared with the control without MT treatment, indicating that MT treatment could enhance the theromotolerance of maize seedlings. To understand the mechanisms underlying MT-enhanced thermotolerance of maize seedlings, the antioxidant defense (guaiacol peroxidease: GPX; glutathione reductase: GR; catalase: CAT; ascorbic acid: AsA; and glutathione: GSH), methylglyoxal (MG) detoxification (glyoxalase I: Gly I; and glyoxalase II: Gly II), and osmoregulation (proline: Pro; trehalose: Tre; and total soluble sugar: TSS) systems were assayed. The results showed that MT treatment stimulated the activities of antioxidant enzymes (GPX, GR, and CAT) and MG detoxification enzymes (Gly I and Gly II), increased the contents of nonenzyme antioxidants (AsA and GSH) and osmolytes (Pro, Tre, and TSS) in maize seedlings under normal culture conditions, and maintained a higher abovementioned enzyme activity and antioxidant and osmolyte contents under HS conditions compared with the control. This work reported that MT could enhance the thermotolerance of maize seedlings by modulating the antioxidant defense, MG detoxification, and osmoregulation systems.