Melatonin plays multifunctional role in horticultural crops against environmental stresses: A review

Melatonin (N-acetyl-5-methoxytryptamine) is a multifunctional nontoxic signaling molecule, universally distributed in different plant organs and accountable for invigorating numerous physiological mechanisms against various adverse environmental factors. In the current review, we revealed that the biosynthesis of melatonin occurred in plants by themselves and accumulation of melatonin vacillated distinctly by regulating its biosynthesis and metabolic pathways under stress conditions. its concentration varied from plant to plant and even organ to organ of plants. Endogenous or exogenously applied melatonin boosted the tolerance against several environmental stresses, such as cold, heavy metals, drought, salt, chilling injury and temperature through a direct scavenging of reactive oxygen species (ROS) and reactive nitrogen species (RNS) production by enhancing antioxidant enzymes activity, non-enzymatic antioxidants, and enzymes related to oxidized protein repair. In addition, melatonin acts as an important signaling molecule to induce protective mechanisms via up-regulating the expression of defense genes by activating the SA, GA and ABA dependent pathways. There are also several studies that melatonin promotes seed germination, seedling growth, root regeneration, root growth and photosynthesis rate of horticultural crops under the influence of several environmental stresses. However, our review emphasized the multiple roles of melatonin against environmental stresses in horticultural crops along with future research directions, which could be useful for improving the eco-friendly crop production and ensure food safety.