Melatonin ameliorates aluminum toxicity through enhancing aluminum exclusion and reestablishing redox homeostasis in roots of wheat

Melatonin is a universal regulator modulating plant development and responses to abiotic stresses. The alteration and potential roles of melatonin in mediating aluminum (Al) tolerance were investigated in two wheat genotypes differing in Al resistance. Using the high-resolution mass spectrometry, we observed that melatonin contents in Xi Aimai-1 were 1.7-fold higher than that in Yangmai-5. Application of melatonin conferred Al resistance in both genotypes. Melatonin treatment scavenged reactive oxygen species (ROS) accumulation and alleviated Al-induced oxidative damage to lipids and proteins by stimulating antioxidant enzymes and augmenting antioxidants. Additionally, melatonin treatment decreased root tip-Al contents by 19.0% and 15.5% in Xi Aimai-1 and Yangmai-5, respectively. Malate efflux, however, was not altered by melatonin under Al stress. The amount of cell wall polysaccharide and pectin methylesterase activity was significantly increased by Al treatment; but suppressed by melatonin. Melatonin synthesis inhibitor, p-CPA, significantly increased the amount of the Al binding in cell walls of the tolerant genotype, whereas exogenous melatonin decreased cell wall Al content in the sensitive genotype. These results suggest that melatonin alleviated Al toxicity through augmenting antioxidants and inducing antioxidant enzymes to control ROS and enhancing exclusion of Al from root apex by altering cell wall polysaccharides in wheat.