**3.2 Melatonin involve in abiotic stress tolerance**

Melatonin is well know as a hormone which can significantly increase the plant survival rates, photosynthetic efficiency and antioxidant activities in plant under environmental stress [74, 78]. For these reasons, many studies were focused on the effects of exogenous melatonin on various plant species under abiotic stress. Indeed, exogenous melatonin could stimulate the biosynthesis of cold tolerance agents and contribute to increase the plant growth and development under cold stress [79]. As show **Table 3**, the alleviation of environmental stresses by melatonin has been investigated in many plant species: under drought (Zea mays) [89], under heavy metal (Caryaca thayensis) [90], under chilling stress (*Cynodon dactylon*) [91] and under salinity (*Cucumis sativus*) [82]. Compared to glycine betaine genetic engineering in plant under stress, the use of melatonin in transgenic plant to provide stress tolerance is fewer. However, there is several studies that focused on the over expression of melatonin via metabolic engineering (**Table 3**).

In Transgenic Arabidopsis the over expression of N-acetyltransferase gene increased salt tolerance via the increase in autophagy, and the reestablishment of redox and ion homeostasis [29]. Furthermore, increase of over-expressing N-acetyltransferase gene enhances the endogenous content in transgenic rice that provoked pleiotropic phenotypes, including enhanced seedling growth, delayed flowering, and low grain yield [28].


#### **Table 3.**

*Reported roles of MT exogenously applied and in transgenic plant under abiotic stresses.*


**Table 4.**

*Overview of MT metabolic engineering in diverse plants.*

#### **3.3 Melatonin in plant metabolism engineering**

Previous studies using genetic engineering (transgenic plant) in various plants species with low or high MT accumulation has been achieved to determined the role of MT in plant growth regulation, stress tolerance or MT function in plant (**Table 4**). Indeed it was reported the implication of MT in seed germination, root development, fruit ripening, senescence, yield, circadian rhythm and plant homeostasis [98]. Ectopic over-expression (transgenesis) of human serotonin N-acetyltransferase increased endogenous melatonin that allowed transgenic rice

*Insights into Metabolic Engineering of the Biosynthesis of Glycine Betaine and Melatonin… DOI: http://dx.doi.org/10.5772/intechopen.97770*

seedlings to face chilling stress [80]. The increase of endogenous melatonin in various transgenic plant organisms compared to the wild type has been reported in *Arabidopsis thaliana* [29], in *Lycopersicum esculentum* [88] or in *Medicago sativa* [92].

Most of the studies in MT transgenesis are based on the ability of *Agrobacterium* to transfer DNA to plant cells by genetic engineering (**Table 4**). Indeed *Agrobacterium tumefaciens* is a widespread naturally occurring soil bacterium which demonstrated a great ability to introduce new genetic material into diverse plant cell species [99]. The *Agrobacterium*-mediated transformation process can be resumed in this following line: 1- Isolation of the targeted genes → 2- development of a functional transgenic construct → 3- insertion of the transgene → 4- introduction of the T-DNA-containing-plasmid into *Agrobacterium* → 5- mixture of the transformed *Agrobacterium* with plant cells → 6- regeneration of the transformed cells into transgenic plant → 7- testing for trait performance or transgene expression [99–101]. The catalytic activities of different enzymes involved in MT metabolic engineering have been elucidated in various species. The catalytic activity of Acetylserotonin O-methyltransferase (EC: 2.1.1.4) encoded by *ASMT* gene in *Homo sapiens* is done by this following line: (*N*-acetylserotonin + *S*-adenosyl-L-methionine = H+ + melatonin + *S*-adenosyl-L-homocysteine) [102]. The catalytic activity of Serotonin N-acetyltransferase (EC: 2.3.1.87) from *Ovis aries* (Sheep) encoded by *AANAT* gene is done by this reaction: (2-arylethylamine + acetyl-CoA = CoA + H+ + *N*-acetyl-2-arylethylamine) [103]. Moreover the catalytic activity of Caffeic acid 3-O-methyltransferase (EC: 2.1.1.68) implicated in many MT genetic engineering manipulations has been decoded in Medicago sativa (Alfalfa): ((*E*)-caffeate + *S*-adenosyl-L-methionine = (*E*)-ferulate + H+ + *S*-adenosyl-Lhomocysteine) [104].

The elucidations of these reactions and techniques provided a huge benefit to increase the use of those compounds in metabolic engineering. There are others areas to explore and clarify to shed light the use of melatonin or glycine betaine metabolic engineering.
