Melatonin in Plant Growth and Signaling

In recent years, knowledge has been gained into the mechanisms of action of melatonin in plants and its regulation of morphogenesis. Melatonin accumulates in several organs, such as the root, stem, and leaves, and can be transported from the major site of synthesis in leaves to distant tissues through the vascular bundles, where it affects cell signaling in crosstalk with major phytohormones. The structural similarity of melatonin with indole-3-acetic acid (IAA) led some authors to suggest a potential auxinic effect in plant signal transduction, particularly root branching and stem elongation. However, its physiological roles throughout the life cycle of plants did not support an auxinic role, but in contrast suggest independent mechanisms of action for each molecule, in agreement with the recent discovery of the melatonin receptor CAND2 that differs from the auxin receptors. This chapter describes the recent roles of melatonin in seed germination, root architecture, shoot development, reproduction, and senescence, and the genes and proteins targeted by melatonin signaling. The function of melatonin in these processes goes beyond its function as an antioxidant, and their possible applications represent a valuable input to optimize plant productivity and confer protection against stressing growth conditions.