Melatonin, a controversial compound in plants, is related to the transport of sodium and potassium ions and proline content

Melatonin, known primarily for its role in animals, also performs essential functions in plants, where it acts as a growth regulator, antioxidant, and modulator of stress responses. However, their role remains controversial because of the differences observed between species and experimental conditions. In the context of sodium‒potassium ion transport (Na⁺ / K⁺), melatonin contributes to maintaining ionic balance, especially under salt stress. It does this by stimulating the activity of ionic transporters, such as the anti-carrier Na⁺ / H⁺, which expels sodium from cells or stores it in vacuoles, and by promoting the retention of potassium, which is crucial for metabolic and photosynthetic functions. In addition, melatonin influences the levels of proline, a key amino acid involved in the response to abiotic stress. It promotes its accumulation by activating biosynthetic genes, such as P5CS, and by improving antioxidant metabolism, protecting cells against oxidative damage. Although its effects are promising, the variability of results in different plants and conditions has generated debates about its effectiveness and precise mechanisms. More research is needed to clarify its role in plant physiology.