# Biosynthesis of Edible Terpenoids: Hosts and Applications
## 1 Introduction  
Terpenoids, natural products formed by the head-to-tail linkage of isoprene units, are widely found in organisms, including plants, animals, microorganisms, and marine life. These compounds function as critical secondary metabolites within living organisms. The diversity in their skeleton structures and functional groups confers a broad spectrum of biological activities and physicochemical properties to these compounds, ranging from simple monoterenes to complex polyterpenes (Figure 1). Terpenoids have substantial application prospects and market demand in fields such as medicine, food, and daily chemicals [1, 2, 3]. Particularly in the food industry, terpenoid compounds have diverse applications [4]. For example, limonene and menthol are employed as edible fragrances due to their unique flavors and tastes. Carnostic acid and \(\alpha\)-pinene exhibit potential as food preservatives owing to their antioxidative and antibacterial properties. Retinol and vitamin K are frequently used as nutritional enhancers in the food industry due to their roles in vision, hemagglutination, and bone metabolism. _Ganoderma_ triterpenoids and sclareol are widely used as medicinal health food for their capabilities in hypolipemic and antitumor activities. With the increasing attention to food safety, terpenoids are increasingly appliedin the food industry due to their natural origin [5]. Hence, natural terpenoids hold immense potential for application. However, the low content and uneven distribution of terpenoids in natural plants require the use of complex extraction and separation techniques, resulting in higher extraction costs. Furthermore, their synthesis and accumulation are influenced by various environmental factors such as geographical location and season, making it challenging to produce on a large scale continuously. While chemical synthesis of terpenoids offers scalability, it is encumbered with high raw material costs, intricate processes, and issues related to chiral recognition and stereoselectivity [6]. Microbial cell factories represent an innovative production approach that employs synthetic biology techniques to integrate key enzyme genes of the target biosynthetic pathway into heterologous hosts, such as _Escherichia coli_ and _Saccharomyces cerevisiae_. It allows for the direct and efficient production of valuable natural or non-natural chemicals through microbial fermentation, such as amino acids, organic acids, fatty acid derivatives, polyketides, terpenoid compounds, and so on [7]. This production approach has advantages such as a broad spectrum of raw materials, environmentally friendly processing, stereospecific products with high yield potential, and stable year-round production to meet market demands, irrespective of seasons or regions. _Phaffia rhodozyma_ naturally produces astaxanthin. Through chemical and ultraviolet radiation, the resulting strain _P. rhodozyma_ CBS 6938 achieved an astaxanthin yield 50 times higher than the wild type, thereby reaching commercial levels of production [8]. [PERSON] et al. redesigned the central carbon metabolism to improve the carbon efficiency and cofactor balance in an engineered famesene-producing _S. cerevisiae_. The titer of famesene reached 130 g/L, achieving industrial production [9]. Therefore, the production of edible terpenes using microbial cell factories has become a hot topic in current research [10]. The biosynthetic pathway of terpenoids is a complex and sophisticated process, including precursor synthesis, skeleton synthesis, and subsequent modification steps [11] (Figure 2). There are two isoprenoid precursor biosynthetic pathways in nature: the mexionate pathway (MVA pathway), which utilizes acetyl-CoA in the cytoplasm as the starting substrate, and the methylerythritol 4-phosphate pathway (MEP pathway), which uses 3-phosphoglycerate and pyruvate in plastids as the starting substrates (Figure 2a) [12; 13].