Regulatory Role of Melatonin in the Redox Network of Plants and Plant Hormone Relationship in Stress

Reactive oxygen and nitrogen species (ROS and RNS) have received increasing interest on the part of researchers over the years. These chemical species of a radical nature are characterized by their dual cellular action: on the one hand they are highly reactive chemical species that can alter the structure of macromolecules, thereby generating dysfunctions, and on the other hand, they are used by the cell as messengers. Under stress conditions, both ROS and RNS act as indicators in various physiological processes such as germination, growth and development, photosynthesis and yield, senescence, and others. For its part, melatonin (N-acetyl-5-methoxytryptamine) is the a hormone synthesized in the pineal gland of animals with numerous cellular and physiological roles. Since its discovery in plants in 1995, it has been widely studied, and numerous functions have been attributed to it in vascular plants. In addition to its recognized role as a universal antioxidant, other relevant functions in plants include its protective role in stress situations (biotic and abiotic), its rhizogenic- and vegetative growth effects, and its protective role in leaf senescence and in the photosynthetic and stomatic apparatus. Also studied have been its action as an osmoregulator agent and metabolic corrector in different stresses, its effects on climacteric and non-climacteric fruits, its effects on seed germination, among others. One of the most outstanding aspects is the implication of melatonin as a multi-signal molecule in plants. The role of melatonin as a gene expression regulator in many physiological situations is studied in this chapter. Its relationship with central elements of the plant redox network such as ROS and RNS, and the regulation of important elements is discussed. The relationship of melatonin with plant hormones such as gibberellins, auxin, abscisic acid, cytokinins, ethylene, brassinosteroids, salicylic- and jasmonic acids and polyamines is also discussed.