# Role of Melatonin to Enhance Phytoremediation Capacity
## 3 Metals as a Severe Abiotic Stress and Effects Induced by Melatonin in Plants
\begin{table}
\begin{tabular}{l l c c c c} \hline \hline
**Stress Type** & **Plant Species** & **Melatonin Treatment (aM)** & **Effects Observed** & **Reference** \\ \hline Acid rain & Tomato & 100 & \(\uparrow\) tolerance, growth, chloroplast integrity, photosynthesis, antioxidant enzymes, 1 ROS, & [85] \\ Salinity, Fe-low & Proper & 100 & \(\uparrow\) growth, Chils, photosyathouse, trunk yield, Fe, K uptake, antioxidant enzymes & [85] \\ Fe-low & _Arabidopsis_ & 5 & \(\uparrow\) melatonin, Fe shoots and roots, Fe mobilization, NO, polyamines, \(\downarrow\) chloroplasts, Fe root & [84] \\ C-low & Tomato & 100 & \(\uparrow\) uptake, assimilation, transport and metabolism, peroxidase, redox homeostasis, & [85] \\ S-low & Tomato & 100 & \(\uparrow\) 1 ROS, DNA damage & [85] \\ N-low & Wheat & 1 & \(\uparrow\) N and nitrate, N absorption, N metabolism, growth, yield, in shoots and roots & [86] \\ N-excess & Cucumber & 100 & \(\uparrow\) tolerance, growth, NYR balance, Ca, \(\downarrow\) damage, nitrate, ammonium & [87] \\ Chemamic acid & Cucumber & 100 & \(\uparrow\) tolerance, growth, water and nutrient balance, hormonal balance & [88] \\ Influencial & Rice & MOE & \(\uparrow\) herbicide tolerance, Chils, antioxidant enzymes, 1 ROS, MDA & [89] \\ Huopoletide & Potato & 1-10 & \(\uparrow\) fungide tolerance, 1 ROS, potato tube light, mycel growth of \(P\)_infetrans_ & [90] \\ Paraquat & Pea & 50-200 & \(\uparrow\) Chils, porphyrin synthesis pathways, herbicide damage, cDNA blockchain & [91] \\ Cartendazzim & Tomato & 0.5 MOE & \(\uparrow\) fungide tolerance, antioxidant enzymes, ASA-CSH cycle, 1 ROS, MDA & [92] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._Selenocysteine had the most marked effect on melatonin biosynthesis. Se treatment increased the levels of glutathione (GSH) and phytochelathains, as well as the expression of GSH and phytochelating biosynthetic genes in non-silenced plants, but the effects of Se were lessened in _TDC_ silenced plants (low melatonin content) under Cd stress. Furthermore, Se and melatonin supplements significantly increased plant Cd tolerance, optimizing plant growth parameters. Thus, exogenous selenocysteine could ameliorate Cd phytotoxicity, but a basal level of endogenous melatonin is required for Se conferred Cd tolerance, which might enhance the detoxification of Cd (Table 1) [56]. Copper (Cu) salts have also been used to study the protective and biostimulatory effect of melatonin (Table 1). In addition to its known effect on the germination of seeds in the presence of Cu, which was one of the first roles of melatonin discovered [68; 93], low levels of melatonin provide greater tolerance of the presence of Cu in cucumber plants. Melatonin induces metabolic activity, especially glycolysis and the pentose phosphate pathway, to generate more ATP. Melatonin treatment broadly altered gene expression under Cu stress, increasing the levels of GSH and phytochelatin to chelate excess Cu and promoting cell wall trapping, retaining more Cu in the cell wall and vacuole [69]. Furthermore, some experiments have been made using zinc, aluminum, and lead in wheat, soybean, _Arabidopsis_, and maize (Table 1). Vanadium (V) adversely affects plant growth through drastic changes in cellular metabolism, including gene expression and ROS production. Higher levels of V affect root growth and the formation of lateral roots and provokes leaf chloroplasts. Vanadium is a chemical analogue of phosphorus (P) and alters the P absorption capacity of plants [94; 95]. In a recent study, melatonin pretreatment lowered leaf and stem V concentrations by reducing V transport from root to shoot in watermelon (Table 1). In V treated plants, melatonin also renewed plant growth and increased photosynthesis efficiency; the antioxidant enzyme pool was also improved. Taken together, this evidence underlines the protective role of melatonin in increasing V tolerance [75]. Boron (B) is an essential micronutrient for normal plant growth, but high concentrations are toxic for plants.