## 1 Introduction  
Terpenoids are the most chemically, physically, and functionally complex family of natural chemicals found in living creatures, with over 80,000 compounds identified to date, and many more expected to exist [1]. They are the most structurally varied group of plant-derived natural compounds, and are economically significant due to their use in a variety of industrial products, including pharmaceuticals, flavoring agents, insecticides, antimicrobial agents, and perfumes [2]. They play an important role in plant-environment, plant-plant, plant-insect, and plant-animal interactions in nature [3]. Many terpenoids have a strong link to primary metabolism (e.g., phytol, the plant hormone gibberell, and carotenoid pigments), while others are common secondary metabolites in plants [4]. Terpene biochemistry and chemistry have been studied for over a century, mainly in plants [5]. Recently, genes that encode enzymes and regulators engaged in terpene biosynthesis have been discovered, and their genomic location, mode of expression, and long-term evolution have been investigated [6]. Despite their considerable structural variations, all terpenoids are derived from the universal isoprene C5 building blocks. The terpenoid backbone is synthesized from two precursors: IPP (isopentenyl pyrophosphate), and its isomer dimethylallyl pyrophosphate (DMAPP). Terpene biosynthesis is a complex mechanism involving two independent biosynthetic pathways. The cytosolic mexolonate (MVA) pathway is found in most eukaryotes (all mammals, the cytosol and mitochondria of plants, fungi), archea, and some bacteria [7, 8]. They are utilized in the production of bigger compounds, such as sesquiterpenes (C15), triterpenes (C30), sterols (C27-29), and dolichols (C40-50). The methyleryththird phosphate (MEP) pathway is found in plant chloroplasts,bacteria, algae, and cyanobacteria. This pathway predominantly produces mono-terpenes (C10), diterpenes (C20), and tetraterpenes (C40). Unlike most microbial organisms, both pathways operate in plants--the MEP pathway in the chloroplast and the MVA pathway in the cytoplasm--and the labor division between them represents a complex array of chemicals that control the development and growth of plants, and interact with plants and their surroundings to control these interactions [9]. Over recent years, efforts to generate large amounts of monoterpenes and sesquiterpenes in transgenic plants proved effective. Many plant species have been genetically modified, with the overexpression of terpene synthase under constitutive promoters being the most common. Plants with overexpressed linalool synthase genes that were produced include tomato, petunia, _Arabidopsis_, potato, and carnation [10; 11; 12; 13; 14]. Such plants generated and released linalool and its glycosylated or hydroxylated derivatives. Similarly, \(\alpha\)-pinene, \(\gamma\)-terpinene, and limonene synthases were shown to alter the terpenoid profile of tobacco and mint plants [15]. The overexpression of gene encoding enzymes from various stages of the MEP pathway may result in even higher levels of terpenoid precursors (DXR and HDR) [16]. The genes encoding enzymes that modify the monoterpene composition have also been effectively overexpressed or knocked down in mint and tobacco. Sesquiterpene production in transgenic plants is more difficult than monoterpene production. In an effort to engineer sesquiterpenes in plants using terpene synthases, tobacco plants were transformed with _Artemisia annua_ amorpha-4,11-diene synthase and fungal trichodiene synthase on either side, but this resulted in only a low-level yield [17; 18]. Owing to their traditionally known pharmacological importance, terpenoids have at-attracted attention from plant breeders, biochemists, and pharmacologists, who have exploited them for their diverse metabolite/chemical profile.