# Biochemistry of Terpenes and Recent Advances in Plant Protection
## 1 Introduction
### Classification of Terpenes  
The isoprene unit that defines terpene consists of a head and a tail. According to [PERSON], terpenoids bond by head-tail linkage of isoprene units. Tetraterpenes, such as carotenoids, deviate from this rule by centrally forming a tail-tail (4-4) bond [18; 19]. Some terpenes have high vapor content and are therefore classified as volatile (VTs), e.g., hemipepenes, monoterpenes, and sesquiterpenes. Others are semi-volatile or non-volatile, such as diterpenes. The number of isoprene units on a terpene backbone generally accounts for their volatility. Fewer isoprene units are highly volatile terpenes [22]. Hemiterpenes are the simplest, with a single isoprene unit. The oxygen-containing derivatives of the isoprene unit form other hemiterpenes (Figure 1a). Hemiterpenes are being investigated as potential sources of biofuel [18]. Monoterpenes are highly diverse and occur in monocotyledonous and dicotyledonous angiosperms, fungi, bacteria, and gymnosperms. They have two isoprenoid units [23]. They are odoriferous compounds that partly account for the scent of many flowers and fruits. Approximately 18 rice monoterpenoids play varying roles, including defense against pathogens and pests [24]. Monoterpenes include acyclic, monocyclic, and bicyclic forms (Figure 1b). They are components of essential oil compounds that give plants aroma and flavor and are vital for a range of active ingredients for agricultural, pharmaceutical, cosmetic, and food applications. Pinenes, carveol, camphor, menthol, and limonene, for example, are active ingredients in a variety of industrial applications [25]. Figure 1: Structural forms of volatile terpen (structures were drawn and analyzed with ChemDraw software, version 20.0.0.41). **(a)** Hemiterpenes; **(b)** Monoterpenes; **(c)** Sesquiterpenes. Sesquiterpene is an abundant natural compound with 3-isoprene [26]. They share ring classification with monoterpenes, an exception being a few tricyclic terpen (Figure 1c). This terpene diversity arises from the arrangement of the 15-carbon skeletons, the layering of the functional groups, and the substituents on their backbone [27]. Some members are hydrocarbons (humulene, farnesene), aldehydes (farnesal and lepidozenal), oxygenated hydroxyl or carbonyl derivatives, and esters (torilin and ejaponines). Some are also alcohols, such as \(\delta\)-elemanol and \(\beta\)-germacrenol [28]. They are antimicrobial, antifungal, antitumor, and anti-inflammatory agents. They have wide plant defense applications against herbivores and are active constituents in the perfumery industry [29]. However, these potentials have largely remained unexplored in the biotechnology industry. Diterpenes are non-volatile C20 hydrocarbons derived from four isoprene units and are structurally diverse [18; 30]. They include linear, bicyclic, tetracyclic, pentacyclic, or macrocyclic forms (Figure 2a). Diterpenes are characterized by ployoxygenated keto and hydroxyl groups [26]. Diterpenes from various sources have exhibited inhibitory effects against pathogenic microbes, herbivore pests, and weeds. These promising biological activities place them among the essential agricultural secondary metabolites with potential in the production of biopsies [31]. Rice plants produce several phytoalexins and allelo-chemical diterpenes for protection against pathogens, pests, and weeds. The accumulation of these compounds, on the other hand, is low and can be increased by genetic alteration or the recruitment of exogenous elicitors. T triterpenes are derivatives of the C30 precursor, squalene, with over 20,000 known members. Most are members of the plant kingdom. However, bacteria and sea cucumbers produce defense-related triterpene glycosides [14]. Two sesquiterpene molecules form triterpenes by linking in a head-head fashion [32]. Cyclic triterpenes (1-5 rings) are the most significant members. They are primarily alcohols, aldehydes, or carboxylic acids [27]. A cyclopentane perhydrophenanthrene ring system defines sterols and phytosterols as triterpenes [32].