# Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation
## 2 Structural Diversity and Roles of Rice Secondary Metabolites
### Terpenoids and Their Biological Functions
#### 2.3.1 Monoterpenoids and Their Biological Functions  
Monoterpenoids are mainly volatile compounds which confer rice its good aroma character. They can be extracted from the headspace of some rice bran samples by solid-phase microextraction (SPME). At least 18 monoterponoids have been identified in rice. Their names and biological activities are listed in Table 3. Their structures are shown in Figure 3.  
Rice monoterponoids are synthesized by various types of terpene synthases (TPSs), such as OsTPS20 and OsTPS24. These TPSs contain a transit peptide for localization in the chloroplasts where monoterpenes are biosynthesized from geranyl diphosphate (GPP) by TPSs via the 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway. TPSs can be induced by jasmonic acid (JA). The amount of \(\gamma\)-terpinene (**117**) increased after JA treatment. \(\gamma\)-Terpinene (**117**) had significant antibacterial activity against _Xoo_. However, it did not show significant antifungal activity against the rice blast pathogen. The antibacterial mechanism of \(\gamma\)-terpinene (**117**) against _Xoo_ involved damage to bacterial cell membranes [62].  
Monoterpenes (\(S\))-limonene (**107**), myrcene (**111**), \(\alpha\)-pinene (**113**), sabinene (**115**), \(\alpha\)-terpinene (**116**), and \(\alpha\)-thujene (**119**) were detected from one-week-old _Xoo_-infected rice seedlings by the method of solid-phase microextraction-GC-MS. However, only (\(S\))-limonene (**107**) severely inhibited _Xoo_ growth, which suggests that (\(S\))-limonene (**107**) plays a significant role in suppressing _Xoo_ growth in rice seedlings [63].  
Many volatile monoterponoids including linalool (**108**) were accumulated in response to the exogenous application of JA. The xpression of linalool synthase gene was upregulated by JA. Vapour treatment with linalool (**108**) induced resistance to _Xoo_. The transgenic rice plants overexpressing linalool synthase gene were more resistant to _Xoo_, which suggests that linalool (**108**) plays an important role in JA-induced resistance to _Xoo_[64].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **Rice Part Used for Isolation** & **Biological Activity and Function** & **Ref.** \\ \hline Camphene (**102**) & Bran & - & [65] \\ Camphor (**103**) & Bran & - & [65] \\ Carveol (**104**) & Bran & - & [65] \\
1,4-Cineol (**105**) & Bran & - & [65] \\ Fenchyl acetate (**106**) & Bran & - & [65] \\ (\(S\))-Limonene (**107**) & Leaves & - & [66] \\  & Bran & - & [65] \\  & Seedlings & Antibacterial activity on _Xoo_ & [63] \\ Linalool (**108**) & Leaves & - & [66] \\  & Leaves & Resistance induction to _Xoo_ & [64] \\ _cis_-Linalool oxide (**109**) & Bran & - & [65] \\ _trans_-Linalool oxide (**110**) & Bran & - & [65] \\ Myrcene (**111**) & Seedlings & - & [63] \\  & Bran & - & [65] \\ _trans_-\(\beta\)-Ocimene (**112**) & Bran & - & [65] \\ \(\alpha\)-Pinene (**113**) & Seedlings & - & [63] \\ \(\beta\)-Pinene (**114**) & Bran & - & [65] \\ Sabinene (**115**) & Seedlings & - & [63] \\  & Bran & - & [65] \\ \(\alpha\)-Terpinene (**116**) & Seedlings & - & [63] \\ \(\gamma\)-Terpinene (**117**) & Leaves & Antibacterial activity on _Xoo_ & [62] \\ Terpinen-4-ol (**118**) & Bran & - & [65] \\ \(\alpha\)-Thujene (**119**) & Seedlings & - & [63] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Monoterpenoids and their biological activities.