Emerging roles of melatonin in mitigating abiotic and biotic stresses of horticultural crops

Environmental stress and pathogen infection are the major limiting factors in worldwide horticulture production, resulting in huge yield loss. Melatonin (N-acetyl-5-methoxytryptamine) has gained a great interest in plant science research owing to its multifaceted role in plant systems. Melatonin mediated efficient reactive oxygen species scavenging and activation of antioxidant defence responses is the two major mechanisms to cope with most of the abiotic stresses and pathogen infections. Besides that, the activation of gene expression of stress-specific genes, antioxidant enzyme genes and pathogenesis-related genes make it a master regulator in plant defence responses under pathogen infection and environmental stresses. Additionally, hormonal cross-talk of melatonin with other phytohormones in regulating drought stress and virus infection gives an idea to explore the possibilities of such a mechanism in other stresses in diverse horticultural crops. The complementary effect of melatonin with fungicides is giving new hope in reducing the burden of exhaustive use of toxic chemicals in vegetables and fruit crops. Melatonin can play an essential role in combating sequential and simultaneous combined stresses in crop plants which are an emerging challenge these days. Our review highlighted the functions of melatonin in ameliorating major abiotic (heat, cold, drought, and salt) and biotic (fungi, virus, bacteria, and insects) stresses of horticultural crops and elucidated the resulting physiological and molecular mechanism. The versatile role of this biological molecule in anti-stress regulation showing successful cases of growth promotion and enhanced resistance makes it suitable for environment-friendly horticultural crop production and to ensure food safety.