Melatonin improves aluminum resistance in rice (Oryza Sativa) via a nitric oxide dependent pathway

Aluminum (Al) toxicity represents a significant constraint on crop growth in acidic soils. Al stress can stimulate the biosynthesis of melatonin (MT) in rice (Oryza sativa L.). The exogenous application of 20 μM MT markedly reduces Al accumulation in root tips and effectively mitigates Al stress. By enhancing the levels of root hemicellulose and down-regulating the expression of two genes sensitive to aluminum rhizotoxicity (OsSTAR1 and OsSTAR2), which function to obscure Al binding sites within the cell wall, MT significantly improves the fixation of Al by the cell wall. Furthermore, MT diminishes cytoplasmic entry of Al by reducing expression levels of nramp aluminum transporter 1 gene (OsNRAT1), while inducing expression of an aluminum-sensitive gene (OsALS1), thereby promoting vacuolar compartmentation to lower cytoplasmic Al concentration. Additionally, MT substantially decreases Nitric oxide (NO) accumulation induced by Al in roots. The alleviation effect exerted by MT on Al toxicity can be replicated through administration of the NO scavenger carboxyl-PTIO (c-PTIO). Moreover, the Al-sensitive phenotype exhibited in the osgsnor mutant, coupled with the absence of mitigation by MT, further substantiates that MT alleviates Al toxicity in rice primarily via regulation of NO content, potentially relying on NO signal transduction pathways. In summary, MT plays a pivotal role in modulating rice resistance to Al toxicity and stimulates multiple pathways to enhance tolerance against this abiotic stressor. This study provides new insights into understanding mechanisms underlying rice resistance to abiotic stresses.