**Abstract**

Metabolic engineering in plant can be describe as a tool using molecular biological technologies which promotes enzymatic reactions that can enhance the biosynthesis of existing compounds such as glycine betaine (GB) in plant species that are able to accumulate GB, or produce news compounds like GB in non-accumulators plants. Moreover we can include to these definition, the mediation in the degradation of diverse compounds in plant organism. For decades, one of the most popular ideas in metabolic engineering literature is the idea that the improvement of gly betaine or melatonin accumulation in plant under environmental stress can be the main window to ameliorate stress tolerance in diverse plant species. A challenging problem in this domain is the integration of different molecular technologies like transgenesis, enzyme kinetics, promoter analysis, biochemistry and genetics, protein sorting, cloning or comparative physiology to reach that objective. A large number of approaches have been developed over the last few decades in metabolic engineering to overcome this problem. Therefore, we examine some previous work and propose some understanding about the use of metabolic engineering in plant stress tolerance. Moreover, this chapter will focus on melatonin (Hormone) and gly betaine (Osmolyte) biosynthesis pathways in engineering stress resistance.

**Keywords:** metabolic engineering, biosynthesis, molecular, abiotic stress, stress tolerance
