Melatonin-priming ameliorates aluminum accumulation and toxicity in rice through enhancing aluminum exclusion and maintaining redox homeostasis.

Seed priming can effectively enhance the plant's ability to withstand stress during subsequent growth and development; however, the role of melatonin-priming in attenuating aluminum (Al) toxicity remain unknown. In this study, 10, 50 and 100 μM melatonin were selected for rice seed priming to investigate the protective effects and potential mechanisms of melatonin against Al toxicity. Al stress inhibited seed germination by induction of abscisic acid (ABA) accumulation and reduction of α-amylase activity. However, melatonin-priming substantially rescued the Al-induced poor germination of seeds, as evidenced by less ABA content and higher α-amylase activity. Compared to no priming under Al stress, melatonin-priming significantly increased root elongation and plant fresh weight of rice seedlings by 135.1% and 39.4%, respectively. Melatonin-priming scavenged Al-induced superoxide anion (O·) and hydrogen peroxide (HO) bursts by activating the antioxidant enzymes (superoxide dismutase and catalase) and antioxidants (ascorbate and glutathione) in root tips, thereby reducing malondialdehyde (MDA) and callose levels and ultimately mitigating oxidative damage. Furthermore, melatonin-priming enhanced Al resistance by inhibiting Al uptake into the symplast through increased citric acid secretion. The decrease of Al deposition in the cell wall was attributed to melatonin-stimulated reduction of cell wall pectin and hemicellulose contents under Al stress. Collectively, these findings reveal a positive role of melatonin-priming in alleviating Al toxicity in plants.