Mechanism of Melatonin in Alleviating Aluminum Toxicity in Plants: A Review

Aluminum (Al) toxicity is a major limiting factor for plant growth and development in acidic soils. Melatonin, a plant growth regulator and signaling molecule, enhances resistance to multiple stresses. Recent studies show that melatonin alleviates Al toxicity through several complementary mechanisms. Here, we first outline the physiological and molecular impacts of Al stress and the external and internal strategies plants use to cope with it. We then summarize melatonin biosynthesis and its broader roles in stress adaptation. We focus on recent advances in melatonin-mediated mitigation of Al toxicity, highlighting four principal mechanisms: (i) the activation of antioxidant defense systems, (ii) the stimulation of organic acid anion exudation that chelates Al in the rhizosphere, (iii) the modification of cell wall composition to reduce Al binding sites, and (iv) the promotion of intracellular Al sequestration. We also discuss the crosstalk between melatonin and nitric oxide, as well as interactions with phytohormone signaling. Collectively, this review comprehensively synthesizes the current understanding regarding the role of melatonin in alleviating Al toxicity in plants, offering a promising strategy for crop production in acidic environments.