# Role of Melatonin to Enhance Phytoremediation Capacity
## 5 Conclusions and Expectations  
The application of melatonin on plants seems to be a useful option for cleaning toxic pollutants from the environment by improving phytoremediation processes. In this capacity, three aspects should be taken into account: (i) the inhibitory effect of metals or toxic agents on growth and other basic functions such as metabolism and photosynthesis, (ii) the ability of cells to mobilize, absorb, and sequester the said agents, and (iii) other adverse elements that usually accompany the presence of contaminants such as drought, salinity, extreme temperatures, etc. The beneficial effects of melatonin described above were seen to cover all three cases. The potential of melatonin to mobilize toxic metals, through phytorelatins, their transport, and sequestration adds to the general biostimulatory effect of melatonin on plants, resulting in a high degree of plant tolerance against toxic substances (Figure 2). Furthermore, the improvement in the absorption and metabolism of elements such as N, P, and S helps the process. Although data are still limited, it seems that the presence of several stressors (e.g., metals and drought) synergistically induces the response of melatonin biosynthesis, which reinforces the overall response of the system. Beyond the use of transgenic plants that overaccumulate melatonin, the application of exogenous melatonin or the induction of its biosynthesis through environmental elicitors can be excellent strategies for phytoremediation purposes. There are clear benefits to be had from further studying the applicability of melatonin for phytoremediation purposes. As regards plant species, more research into the biochemical and physiological aspects of melatonin in hyperaccumulator plants is indispensable, furthering our knowledge of the synergistic effect of abiotic stressors on endogenous melatonin levels and its phytoremediation capacity. Furthermore, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Plant Species** & **Abiotic Stressor** &
\begin{tabular}{c} **Increased Level of** \\ **Melatonin vs. Control** \\ \end{tabular} & **Reference** \\ \hline Alfalfa & Waterlogging & 2–4.5-fold & [113] \\ _Arabidopsis_ & Cold & 2-fold & [114] \\  & Heat & 2–5-fold & [115] \\  & NaCl, drought, cold & 3–6-fold & [116] \\  & Fe deficiency & 6-fold & [84] \\  & Drought & 4-fold \({}^{*}\) & [117] \\ Barley & Zn, NaCl, H\({}_{2}\)O\({}_{2}\) & 6-fold & [100] \\ Barley & Drought, cold & 2-fold & [118] \\ Bermudagrass & NaCl, drought, cold & 2-3-fold & [106,119] \\ Cassava & Bacterial blight & 1.2–4-fold \({}^{*}\) & [120] \\ Cherry & Field growth conditions & 10-fold & [103] \\ Grape & Field growth conditions & 15-fold & [102] \\ Lupin & Zn, NaCl, H\({}_{2}\)O\({}_{2}\) & 1.5–12-fold & [99] \\  & Cold, drought & 1.5–6-fold \({}^{*}\) & [121] \\ _Malus_ & Drought & 1.5–6-fold \({}^{*}\) & [122] \\ Rice & Cd & 6-fold & [122] \\ Cold, salt, drought, pathogen & 1.5–4.5-fold \({}^{*}\) & [123] \\ Ryegrass & Darkness & 2-fold & [124] \\ Sunflower & NaCl & 2–6-fold & [125] \\ Tomato & Field growth conditions & 10-fold & [101] \\  & Cd & 1.6–4-fold & [62] \\  & Cd & 2-fold & [60] \\  & Cd & 2–15-fold \({}^{*}\) & [63] \\  & High temperature & 2–15-fold \({}^{*}\) & [126] \\ _Vitis_ & NaCl & 5.5-fold & [127] \\  & Osrnotic & 1.5-fold & \\ Water hyacinth & Field growth conditions & 2-fold & [52] \\ Water melon & V & 4-fold \({}^{*}\) & [75] \\ \hline \hline \end{tabular} \({}^{*}\) Number of increments of one or more transcripts of melatonin biosynthesis enzymes due to stressor presence.  
\end{table} 
Table 2: Effects of abiotic stressors on the endogenous melatonin levels. the modes of application of exogenous melatonin through the roots and/or through the leaves should be studied, although its amphipathic nature already means that we know that it is well absorbed at both sites (rhizosphere and leaves) without the need for adjuvants and that it is easily transported throughout the plant.