Mechanistic insights on melatonin-mediated plant growth regulation and hormonal cross-talk process in solanaceous vegetables

In horticultural crop production, the use of plant growth regulators has become a common practice for improvement of produce quality and mitigation of abiotic and biotic stresses. In solanaceous vegetable crops these growth regulators have been extensively used during all growth stages of seed sowing to harvesting and during postharvest storage. Recently, the indolamine compound melatonin (MEL; N-acetyl-5-methoxytryptamine) has emerged as a multifunctional and ubiquitous molecule that regulates germination, seedling growth, photosynthesis, transpiration and senescence along with alleviation of major abiotic and biotic stress faced by the solanaceous crops in field. The present review highlights the significant roles of ME with a special emphasis of MEL-mediated reactive species signalling and hormonal cross-talk that leads to a better plant growth and stresstolerant phenotypes. Under the stimulus of oxidative burst, endogenous MEL employs sophisticated antioxidant mechanisms to balance ROS (reactive oxygen species) generation via enzymatic and non-enzymatic antioxidants. The maintenance of redox homeostasis was also correlated with higher expressions of genes encoding antioxidant enzymes. Likewise, MEL either interacts with NO-biosynthetic enzymes or superoxide anion (O2 center dot-), forming ONOO- through a signal cascade which ultimately results in reactive nitrogen species regulation. Furthermore, the regulatory function of MEL in the modulation of metabolism of other plant hormones plays a crucial role under stress conditions. The hormonal cross-talk of MEL with other growth and development related hormones such as auxin, cytokinin, gibberellins, ethylene and abscisic acid as well as biotic stress hormonal network such as jasmonates, salicylates, brassinosteroids and polyamines are discussed with special emphasis on the regulation of gene expression.