## 2 Plant Secondary Metabolites and Their Biological Roles
### Terpenoids  
Terpenoids or isoprenoids are one of the most structurally diverse naturally occurring PSMs, with the main skeleton consisting of five-carbon isopentyl units, called 2-methyl-1,3-butadiene, or isoprene. Terpenes contain only isoprene units, while terpenoids have additional functional groups, such as ketone or heterocyclic and hydroxyl rings. Based on structural construction, terpenoids can be considered as two types, aliphatic (e.g., geraniol) and cyclic (e.g., limonene) terpenoids. Since terpenoids contain many isoprene units, they are divided into various groups, as described below (Figure 1):  
* Monoterpenoids--two isoprene units (C-10 carbon atoms,)--e.g., linalool;
* Sesquiterpenoids--three isoprene units (C-15 carbon atoms)--e.g., \(\beta\)-aryophyllene;
* Diterpenoids--four isoprene units (C-20 carbon atoms)--e.g., abiotic acid;
* Sesterterpenoids--five isoprene units (C-25 carbon atoms)--e.g., ophiobolin A;
* Triterpenoids--six isoprene units (C-30 carbon atoms)--e.g., ganoteric acid;
* Tetraterpenoids--eight isoprene units (C-40 carbon atoms)--e.g., \(\alpha\)-carotene;
* Polyterpenoids--more than eight isoprene units (\(>\)C-40 carbon atoms)--e.g., trans-1/\(A\)-polyisoprene. Terpenoids are formed from the mevalonate pathway inside cytosol or the 2-C-methyl-D-erythmitrol-4-phosphate (MEP) pathway inside the plastid [31]. The biosynthetic precursors of terpen include geranyl diphosphate (GPP) for monoterepenes; farnesyl diphosphate (FPP) for sesquiterpenenes; and geranylgeranyl diphosphate (GGPP) for diterpenes [32]. An in-depth discussion on terpenoids' biosynthetic pathway and structural diversity is covered by [PERSON] et al. (2005) and [PERSON] et al. (2014) in their review [31; 32]. More than 30,000 terpen have been reported to date [33]. They are mostly phytohormones (e.g., gibberellins), photosynthetic pigments (e.g., phytol, carotenoids such as \(\alpha\)-carotene and \(\beta\)-carotene), and carriers (e.g., ubiquinone, plastoquinone) in the electron chain transport systems [34; 35]. The role of terpenoids is to protect plants directly (e.g., releasing phytoalexins after pathogen attacks) or indirectly by producing mixtures of volatile organic compounds (VOCs) to attract carnivores of their herbivores [36]. Phytoalexins are antimicrobial compounds produced after microbes' challenge plants, and it is reviewed in-depth by [PERSON] et al. [37]. VOCs include terpenoids (isoprene or hemiterpenoids and monotereponoids), alkanes, alkenes, carbonyls, alcohols, esters, ethers, and acids [18]. VOCs are involved in plant-plant or plant-insect interactions, but some terpenoids act as lipid-soluble antioxidants inducing resistance to stress [38]. Several terpenoids have shown defensive roles against biotic and abiotic stresses in plants. [PERSON] et al. (2015) [39] and [PERSON] et al. (2016) [40] reported antioxidant activities of terpen (sesquiterpene b-caryophyllene, and monoterepenes 1,8-cineole and \(\alpha\)-pinene, respectively), suggesting their function in overcoming abiotic-induced oxidative stress. Terpenoids are also reported to protect plants from photodamage and oxidative stress by supporting photorespiration [41]. Carotenoids are the best-known terpenoidy involved in photoprotection [42]. Other examples of defensive terpen in plants are triterpen glycosides (or saponins) such as \(\alpha\)-tomatine in the fruits and leaves of tomatoes [43] and avenacin in oat (_Avena sativa_) roots [44]. Avenacin and \(\alpha\)-tomatine are important preformed antimicrobial compounds, commonly referred to as phytoanticipins, and they have defensive roles against microbial attacks [37]. Saponins are another group of compounds under terpenoids with triterpenes or steroidal aglycones linked to one or more sugar chains [45], but some saponins such as steroidal glycoalkaloids have a nitrogen atom in their aglycone chemical structure [45; 46]. Similar to many other PSMs, the amount and distribution of saponins in plants are influenced by season, biotic and abiotic stresses, and plant developmental stage.