# Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation
## 2 Structural Diversity and Roles of Rice Secondary Metabolites
### Terpenoids and Their Biological Functions
#### 2.3.2. Sesquiterpenoids and Their Biological Functions  
Sesquiterpenoids are also volatile components which contribute to the aroma quality of rice. They can be analyzed and identified by GC and GC-MS. The relative content of sesquiterpenoids was much lower, on average, than that of monoterepenoids in rice. Sesquiterpenoids are usually produced and released from wounds or microbe-infected sites. They act as signaling molecules that induce defense against tissue damage caused by herbivores or plant pathogens [67]. Rice sesquiterpenoids and their biological activities are listed in Table 4. Their structures are shown in Figure 4.  
Sesquiterpenes are biosynthesized from farnesyl diphosphate (FPP) by TPSs via the mexolonate (MVA) pathway in the cytoplasm. Rice terpene synthase 18 was found to localize in the cytoplasm and synthesized the sesquiterpenes (\(E\))-nerolidol (**139**) and (\(E\))-\(\beta\)-farnesene (**132**), whose amounts increased after JA treatment. (\(E\))-Nerolidol (**139**) had significant antibacterial activity against _Xoo_[68]. Rice sesquiterpenoids and their biological activities are listed in Table 4.  
Figure 3. Structures of the monoterepenoids identified in rice.  
_Molecules_**2018**, _23_, 3098  
**Table 4.** Sesquiterpenoids and their biological activities and functions.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **Rice Part Used for Isolation** & **Biological Activity and Function** & **Ref.** \\ \hline Abscisic acid (**120**) & Whole rice plant & Regulation of growth and development & [69] \\ (\(Z\))-\(\alpha\)-Bergamotene (**121**) & Leaves & - & [66] \\ \(\beta\)-Bisabolene (**122**) & Bran & - & [65] \\ (\(E\))-\(\gamma\)-Bisabolene (**123**) & Leaves & - & [66] \\ \(\alpha\)-Cadinene (**124**) & Leaves & - & [66] \\ \(\beta\)-Caryophyllene (**125**) & Leaves & - & [66] \\  & Bran & - & [65] \\ \(\alpha\)-Copaene (**126**) & Leaves & - & [66] \\  & Bran & - & [65] \\  & Seedlings & - & [63] \\ \(\alpha\)-Curcumene (**127**) & Leaves & & [66] \\ \(\gamma\)-Curcumene (**128**) & Leaves & & [66] \\ Cyclosativene (**129**) & Seedlings & - & [63] \\ \(\alpha\)-Elemene (**130**) & Bran & - & [65] \\ \(\beta\)-Elemene (**131**) & Seedlings & - & [63] \\ (\(E\))-\(\beta\)-Farnesene (**132**) & Leaves & - & [68] \\ Germacrene D (**133**) & Leaves & & [66] \\ \(\alpha\)-Gurjunene (**134**) & Bran & - & [65] \\ \(\beta\)-Gurjunene (**135**) & Leaves & & [66] \\ \(\alpha\)-Humulene (**136**) & Leaves & & [66] \\ Italicene (**137**) & Leaves & & [66] \\ \(\gamma\)-Muurolene (**138**) & Leaves & & [66] \\ (\(E\))-Nerolidol (**139**) & Leaves & Antibacterial activity against _Xoo_ & [68] \\ 7-_epi_-\(\alpha\)-Selinene (**140**) & Bran & - & [65] \\ Valencene (**141**) & Leaves & - & [66] \\ Viridiflorene (**142**) & Leaves & - & [66] \\ \(\alpha\)-Ylangene (**143**) & Bran & - & [65] \\ \(\alpha\)-Zingiberene (**144**) & Leaves & - & [66] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Sesquiterpenoids and their biological activities and functions.  
Figure 4: _Cont_.