# Role of Melatonin to Enhance Phytoremediation Capacity
## 3 Metals as a Severe Abiotic Stress and Effects Induced by Melatonin in Plants  
The metal removal efficiency of plants is highly related to their growth rate, tolerance to high levels of metals, and adaptability to different environments. Melatonin is widely distributed among different plants, including hyperaccumulators such as water hyacinth, sunflower, mustard plants, radish, etc. [43], which suggests a probable role in phytoremediation, especially due to its high levels. Table 1 shows several studies with metals and the effects observed by exogenous melatonin treatment. Furthermore, several studies with transgenic plants overexpressing melatonin biosynthesis genes, which extra-accumulate endogenous melatonin, have been made [44]. Several metals and non-metal elements have been used to study the resistance, tolerance, mobilization, accumulation, and diverse metabolic and physiological changes that occur in plants treated with melatonin compared with control plants. The first report related to phytoremediation and melatonin was published by [PERSON] and co-workers (2007), who observed high contents of melatonin and melatonin by-products in water hyacinth [52]. The capacity of these plants to tolerate high levels of toxic pollutants was proposed. In a related study, the tolerance of pea plants to copper (Cu) significantly improved after supplementation with melatonin [53]. Cadmium (Cd) has been the most widely assayed heavy metal in melatonin treated plants of several different species (Table 1). In general, melatonin treatment increases Cd tolerance, plant growth, and photosynthesis efficiency compared with non-treated plants. Furthermore, their water content and ion homeostasis were improved. Antioxidant enzymes increased and the ASA-GSH cycle improved, while ROS and MDA contents were reduced, optimizing the redox balance. Cd transport increased with the phytochelatin content in alfalfa and tomato, where the Cd content tended to accumulate in different locations (roots, stems, leaves, shoots). When the effects of melatonin on growth and Cd uptake in _Malachium aquaticum_ (Cd accumulator) and _Galinsoga parviflora_ (hyperaccumulator) were studied, melatonin application significantly increased the Cd content in both plants in a concentration dependent form, suggesting melatonin could improve phytoremediation [54]. In _Cyphomandra betacea_, a South American fruit tree, 50 \(\upmu\)M melatonin increased the Cd contents in stems, leaves, and shoots in  
Figure 1: Model of the redox network regulated by melatonin and the effect of abiotic stressors. a soil cultivation experiment. Low levels of melatonin were seen to promote the growth of _C. betacea_ seedlings and their Cd accumulation capacity [55]. Melatonin treated plants, in a combination of Cd with selenium (Se) or Zn, also showed increased plant growth and higher antioxidative defenses than control plants. In an interesting experiment with tomato plants that over-produced melatonin, pre-treatment with different forms of Se significantly induced the biosynthesis of melatonin and its precursors (tryttophan, tryptamine, and serotonin) [56].