The role of melatonin in regulating horticultural crop production under various abiotic stresses

Horticultural crops are very important component of our food chain, and their production is highly sensitive to different soil related abiotic stresses (SRAS). These stresses reduce plant growth and development by reducing seed germination (SG), seedling establishment, photosynthesis, reproduction, thus reduce fruit yield and quality. Nonetheless application of plant growth regulators has been shown a very promising technique to confer SRAS tolerance in horticultural crops. Melatonin (Mel)-a high conserved and stress regulatory molecule regulates a plethora of stress adaptive responses in plants. This paper summarizes the current state of knowledge of Mel involvement in regulating stress adaptive responses under SRAS. The major topic covered in this review included: (i) the effects of exogenous Mel application on seed germination, photosynthesis, flowering, and fruit yield under different SRAS, (ii) Mel crosstalk with stress regulatory mechanism to confer stress tolerance in horticultural crops. Latter topic further discussed the role of Mel in regulating (i) redox regulation, (ii) ionic homeostasis, and (iii) hormonal regulation. We show that Mel regulates stress adaptive responses and improves plant growth by allowing the direct scavenging of ROS, up-regulation of H+-ATPase activity and vacuolar sequestration of toxic ions in cytosol and activation of ROS-Ca2+ hub, which latter prevent the negative effects of ROS and membrane depolarization on the ROS-activated and voltage-activated ion channels. Such Mel mediated desensitization of ion channels may play a crucial role in reducing stress induced efflux of mineral ions such as K, Zn, Mg from cytosol, thereby improving plant growth. Moreover, Mel may operate in tandem with polyamines to regulate the biosynthesis of different phytohormones. However, studies are required to reveal the molecular identity of genes relating to Mel-polyamine tandem and to light signaling that could directly regulate phytohormone biosynthesis and the interaction of Mel with light signaling in regulating SG.