# Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation
## 2 Structural Diversity and Roles of Rice Secondary Metabolites
### Phenolic Acids and Their Biological Functions
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **Rice Part Used for Isolation** & \multicolumn{2}{c}{**Biological Activity and**} \\  & **Function** & **Ref.** \\ \hline \(p\)-Hydroxypenzaldehyde (**1**) & Husk and bran & - & [24] \\  & Bran & Antioxidant activity & [31] \\ \(p\)-Hydroxypenzoic acid (**2**) & Root exudate & Allelopathic effect & [30] \\  & Husk and bran & - & [24] \\ \(p\)-Hydroxy methyl benzoate & Bran & Antioxidant activity & [31] \\ \(p\)-Hydroxyphenyl acetaldehyde (**3**) & Husk and bran & - & [24] \\ \(p\)-Hydroxy phenyl acetic acid (**5**) & Husk and bran & - & [24] \\ \(2\)-Hydroxy 5-[(35)-3-hydroxybutyl] & & & Inhibitory activity on \\ phenyl \(\beta\)-d-glucose (HHPG) (**6**) & & & \\ Caffeic acid (**7**) & Endosperm and bran/embryo of _indica_ variety & Antioxidant activity & [20] \\  & Root exudate & Allelopathic effect & [30] \\  & Husk and bran & - & [24] \\ Methyl caffeine (**8**) & Bran & Antioxidant activity & [31] \\ Caffeoyl quinic acid methyl ester (**9**) & Grains of brown rice & - & [33] \\ Protocatechuric acid (**10**) & Endosperm and bran/embryo of _indica_ variety & Antioxidant activity & [20] \\ Chlorogenic acid (**11**) & Endosperm and bran/embryo of _indica_ variety & Antioxidant activity & [20] \\ Cinnamic acid (**12**) & Husk and bran & - & [24] \\ \(o\)-Cournaric acid (**13**) & Endosperm and bran/embryo of _indica_ variety & Antioxidant activity & [20] \\  & Root exudate & Allelopathic effect & [30] \\ \(m\)-Cournaric acid (**14**) & Grains & - & [23] \\ \(p\)-Cournaric acid (**15**) & Grains of brown rice & - & [33] \\ \(3\)-\(O\)-\(p\)-Cournacyl quinic acid (**16**) & Grains of brown rice & - & [33] \\ Leaves & - & [34] \\ _trans_-Ferulic acid (**17**) & Endosperm and bran/embryo of _indica_ variety & Antioxidant activity & [20] \\  & Grains & - & [23] \\  & Black rice bran & Antioxidant activity & [25] \\  & Husk and bran & - & [24] \\  & Bran & Antioxidant activity & [31] \\ _trans_-Ferulic acid methyl ester (**18**) & Bran & Antioxidant activity & [31] \\ _cis_-Ferulic acid (**19**) & Bran & Antioxidant activity & [31] \\ _cis_-Ferulic acid methyl ester (**20**) & Bran & Antioxidant activity & [31] \\
1,3-\(O\)-Diferuloylglycerol (**21**) & Leaves & - & [34] \\
1-\(O\)-Feruloyl-\(\beta\)-d-glucose (**22**) & Leaves & - & [34] \\
3-\(O\)-Feruloylquinic acid (**23**) & Leaves & - & [34] \\ Gallic acid (**24**) & Endosperm and bran/embryo of _indica_ variety & Antioxidant activity & [20] \\  & Husk and bran & - & [24] \\ \(m\)-Salicylic acid (**25**) & Grains of brown rice & - & [33] \\ Salicylic acid & Leaves & - & [34] \\
2-\(O\)-\(\beta\)-d-glucopyranoside (**26**) & Grains & - & [23] \\ Sinapic acid (**27**) & Leaves & - & [34] \\
1-\(O\)-Sinapoyl-\(\beta\)-d-glucose (**28**) & Leaves & - & [34] \\ Springaldehyde (**29**) & Grains of brown rice & - & [33] \\ Springic acid (**30**) & Endosperm and bran/embryo of _indica_ variety & Antioxidant activity & [20] \\  & Root exudate & Allelopathic effect & [30] \\  & Husk and bran & - & [24] \\  & Husk and bran & - & [24] \\  & Grains of brown rice & - & [33] \\  & Husk and bran & - & [24] \\  & Grains of brown rice & - & [33] \\ Vanillic aldehyde (**31**) & Bran & Antioxidant activity & [31] \\ Vanillic acid (**32**) & Root exudate & Allelopathic effect & [30] \\  & Husk and bran & - & [24] \\  & Grains of brown rice & - & [33] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Phenolic acids and their biological activities. Figure 1: _Cont_.