# Role of Melatonin to Enhance Phytoremediation Capacity
## 2 How Can Melatonin Contribute in a More Efficient Phytoremediation?  
Decarboxylation of aromatic amino acids by specific decarboxylases leads to the production of starter compounds for the biosynthesis of secondary metabolites involved in stress resilience mechanisms [28]. More in particular, plant tryptophan decarboxylase (TDC) converts tryptophan intoryptamine [28], the precursor of N-acetyl-5-methoxytryptamine, known commonly as melatonin [29]. The protective effect of melatonin against abiotic stress situations in plants has been widely studied. Melatonin acts as an effective free radical scavenger against harmful reactive molecules, both reactive oxygen (ROS) and reactive nitrogen (RNS) species, such as hydroxyl radical, superoxide anion, singlet oxygen, hydrogen peroxide, hypochlorous acid, nitric oxide, peroxynitrite anion, peroxynitrous acid, and lipid peroxyl radicals, among others. The excellent properties of melatonin as an in vivo antioxidant against ROS/RNS, the absence of pro-oxidant effects, and the cascade antioxidant effect of melatonin related compounds have been the objective of a great number of researchers [30]. Melatonin is a more effective antioxidant than vitamin C and E, with a scavenging activity 4-6 fold higher [31]. Furthermore, the amphipathic properties of melatonin permit it to scavenge free radicals in both hydrophilic and lipophilic media [30; 32; 33; 34; 35; 36; 37; 38]. This direct chemical action of melatonin with ROS and RNS has been referred to as a receptor independent action [39].  
In addition to this direct action, melatonin is capable of inducing many changes in gene expression. Melatonin changes the expression of a great number of gene elements in different physiological situations both in plant and animal cells [40; 41; 42]. Among the most studied melatonin mediated aspects in plants are the responses to abiotic stress (heat, cold, drought, salinity, alkalinity, heavy metals, and other toxic agents such as herbicides, fungicides, diverse contaminants) and biotic stress (fungi, virus, and bacteria). Furthermore, processes such as foliar senescence, growth and development, germination, rooting induction, flowering, parthenocarpy, fruit set, and fruit ripening were studied. Others aspects, such as photosynthesis and its regulation, primary and secondary metabolism, including osmoregulation, and the regulation of plant hormones (auxin, gibberellins, cytokins, abscisic acid, ethylene, jasmonates, salicylic acid, polyamines, brassinosteroids, strigolactones) have been also analyzed [41; 43; 44; 45; 46; 47; 48; 49].  
In general, melatonin induces several gene expression changes in plants that result in a biostimulating response [41]. Due to the diversity of its actions, melatonin has been proposedas a plant master regulator, but also as a new plant hormone since its receptor (PMRR1) has been identified in _Arabidopsis_[41; 50]. Melatonin acts as a main regulator of the redox network in plants, controlling directly and indirectly the ROS/RNS levels and the gene expression of many factors through the nitric oxide signaling cascade, among other pathways [51]. As a result, melatonin regulates redox network homeostasis, balancing several ROS and RNS and related key enzyme expressions (NOS-like, NR, RbOHs, ASA-GSH cycle, antioxidant enzymes). It is also one of the main intermediates in many cellular and physiological responses, as is depicted in the integrative model of Figure 1.