# Role of Melatonin to Enhance Phytoremediation Capacity
## 4 Abiotic Stressors Induce a Melatonin Burst that Activates Anti-Stress Responses  
Endogenous melatonin levels change with environmental conditions of plant growth. Melatonin is accumulated as a protective molecule in response to different environmental abiotic stressors, such as water deficit and waterlogging, cold and heat, UV radiation, soil heavy chemicals and related, among others [41, 48, 99]. Table 2 shows some studies on the increase of endogenous melatonin levels by the presence of different abiotic stressors. Thus, the expression of the biosynthesis enzyme transcripts (TDC, SNAT, ASMT, and COMT genes) occurs in stress situations, producing a burst in the levels of endogenous melatonin. The global influence of environmental factors on the melatonin levels of plant organs was clearly demonstrated in barley, tomato, and lupin plants by [PERSON] and co-workers [99, 100, 101]. This effect was previously suggested in water hyacinth plants [52], and later corroborated in grape berry skin [102] and cherry fruits [103]. Salinity, cold, drought, and heavy metals have been the abiotic environmental agents most frequently studied as inducers of melatonin biosynthesis in plants, although attack by pathogens also induces the biosynthesis of melatonin (Table 2). This interesting response of stressed tissues clearly induces tolerance, fortifying the redox network against ROS and RNS and upregulating the expression of stress specific response genes. All this relieves the inhibitory processes due to stress and reinforces plant growth and critical processes such as photosynthesis, water economy, metabolism, etc. [41, 51, 104, 105]. The remediation efficiency is directly correlated to higher biomass and an improved tolerance of plants to toxic pollutants. Studies made with melatonin suggested that the stimulatory effects of melatonin on biomass and antioxidative defense machinery are reinforced by a strong primary and secondary metabolism and also by plant hormone stress responses. Melatonin acts as a biostimulator and/or protector of photosynthesis and the stomatal apparatus, upregulating many elements of photosystems, thylakoid electron transporters, and ATP-ase genes. Melatonin also optimizes stomata functionality (e.g., by causing increased stomata opening) in adverse conditions through the regulation of guard cell anion channel proteins and dehydrins, all of which increase CO\({}_{2}\) availability. In the Calvin cycle, melatonin regulates the expression of RuBisCO elements, glyceraldehyde-3-phosphate dehydrogenases, and interconversion carbohydrate enzymes. In addition, elements of the ASC-GSH cycle, TCA cycle, and myo-inositol and fatty acid biosynthesis pathways are also regulated by melatonin. As regards osmoregulation, higher levels of proline, carbohydrates (glucose, maltose, fructose, sucrose, and trehalose), and a multitude of amino acids and organic acids in melatonin treated plants had beneficial effects under abiotic stress conditions. Thus, relevant changes in carbohydrate, lipid, amino acid, nitrogen, phosphorus, and sulfur metabolism indicate the beneficial physiological processes that occur in melatonin treated plants during abiotic stress [86, 87, 106, 107, 108, 109]. Melatonin is also involved in secondary metabolism, where it induces anthocyanin biosynthesis and flavonoids [110, 111] and also regulates steps in the carotenoid biosynthesis [76, 112]. Finally, melatonin regulates the expression of multiple elements (enzymes, receptors, and transcription factors) in the biosynthesis, catabolism, and signaling of auxin, gibberellins, cytokins, ABA, ethylene, jasmonic acid, SA, brassinosterol, strigolactones, and polyamines [41, 46, 48] (Figure 1).