**1. Introduction**

The global climate change influence negatively plant growth and development via the increase of the intensity of various abiotic stresses such as drought, chilling, salinity, waterlogging or flooding. Environmental stresses are one of the most threatening factors that can cause massive losses in agricultural crop production, ranging from 50–70% [1]. Plant biotechnology and engineering are promising

platform for exploring the unlimited potential of many various plants species [2]. In recent years, plant metabolism engineering provides successful pathways to increase the production of metabolites that can significantly counterattack the damages caused by diverse abiotic stresses [3]. To improve stress tolerance in plant, various metabolic engineering technologies were used to introduce or increase the synthesis of diverse osmolytes, secondary metabolites or hormones. The adaptation of various plant species to stressful environments can be managed through: (i) the identification of diverse mechanisms developed by plants to counterbalance abiotic stresses (ii) and the improvement of these processes in plants by metabolic engineering [4, 5]. Plant by-products including hormone (melatonin, MT) and osmoprotectant (glycine betaine, GB) that play a prominent roles in plant stress tolerance have been targeting in various plant species to counterattack environmental stresses. The clarification of the biosynthetic pathway of various plant compounds has provided the possibility to metabolically engineer new capabilities in plants as well as successfully engineer whole pathways into microbial systems [6]. Under environmental stresses plant is able to accumulate different molecules such as melatonin or glycine betaine to provide stress tolerance by counteracting with oxidative stress caused by drought, chilling, salinity or heavy metal stresses [7–9]. The protective properties of GB and MT in plant under abiotic stresses had made these substances targets for plant engineering resistance.

The natural biosynthesis of glycine betaine takes place in marine algae and various higher plant species belong to diverse families, counting the Gramineae, Malvaceae, Asteraceae, or Amaranthaceae [10–14]. Glycine betaine accumulation in non-accumulators and accumulators plant species under environmental stresses has long been a target for engineering stress resistance [15, 16]. The biosynthesis of glycine betaine passes by choline → betaine aldehyde → glycine betaine pathways. Most of the enzymes involving in these pathways such as choline monooxygenase (CMO) or betaine aldehyde dehydrogenase (BADH) have been identified, and genes for some of them have been cloned [4, 13].

Indeed, GB as a non-toxic molecule is biosynthesized through two phases of choline oxidation: the first step (Choline → betaine aldehyde) is catalyzed by CMO, and the second step (Betaine aldehyde → glycine betaine) is activated by BADH [13, 17]. The expression of CMO or BADH in tobacco has been done via the cDNA from two natural glycine betaine accumulators; spinach and sugar beet plants. The 35S promoter from plant virus, cauliflower mosaic virus which is a fundamental element of transgenic constructs in the majority of genetically modified plant species was used in transgenic tobacco to control the expression of cDNA for BADH pathway [18]. Also, a crucial tool in metabolism engineering of glycine betaine pathway is the use of a single gene codA from *Arthrobacter globiformis* which is involved in the synthesis of GB [19]. However, GB accumulation in transgenic plants depends on the capacity of endogenous choline uptake, the type of gene that catalyzes the GB biosynthetic pathway, and the localization of the transgene product in a particular cellular compartment [20].

Melatonin a plant hormone identified in a wide variety of animals and plants, has been extensively studied in plants for its properties to counteract with various environmental and biotic stresses [21, 22]. Transcriptome analyses indicated that melatonin primarily affects the pathways of plant hormone signal transduction and biosynthesis of secondary metabolites [23]. In plant the biosynthesis of melatonin is initiated with tryptophan which is converted in serotonin, and between the tryptophan and melatonin, the enzymes hydroxyindole-*O*-methyltransferase and caffeic acid O-methyltransferase (ASMT/COMT) catalyzed a reaction that produce an intermediate molecule named 5-methoxytryptamine [24–26]. The related enzymes involved in melatonin biosynthesis pathway have been targeted

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

to improve stress tolerance in diverse plant species. The over expression of COMT like gene (TaCOMT) in a transgenic *Arabidopsis* via various metabolic engineering techniques (cloning, transgenesis, genetics or promoter analysis) provided drought tolerance by increasing the concentration of melatonin [27]. Other enzymes such as serotonin N-acetyltransferase (MsSNAT) involve in melatonin biosynthesis have been targeted in rice [28] or *Arabidopsis* [29] to provide stress tolerance, either to clarify the role of melatonin in plant. This chapter will focus on the use of glycine betaine, spermidine and melatonin in plant metabolism engineering, particularly in stress engineering.
