# Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation
## 2 Structural Diversity and Roles of Rice Secondary Metabolites
### Phenolic Acids and Their Biological Functions  
Rice phenolic acids can be classified as soluble-free, soluble-conjugated, and insoluble-bound forms. The insoluble-bound phenolic acids covalently bound to structural components of cells like cellulose, hemicellulose, lignin, pectin, rod-shaped structural proteins, etc. [19]. The distribution of rice phenolic acids exhibits varietal differences, and rice bran has the highest total phenolic acid content among four different fractions of whole rice grain [20; 21]. Overall, \(p\)-hydroxybenzoic acid (**2**), caffe acid (**7**), protocatechic acid (**10**), ferulic acid (**17**/**19**), sinapic acid (**27**), syringe acid (**30**), and vanillic acid (**32**) are present in the whole rice grain, and ferulic acid (**17**/**19**) is the most abundant phenolic acid in the insoluble-bound fraction [22]. Normally, the pigmented rice contains phenolic acids with a larger structural diversity and in higher content than the non-pigmented rice [2; 13]. About 32 phenolic acid analogues have be identified in rice. Rice phenolic acids and their biological activities are listed in Table 1. The structures of rice phenolic acids are shown in Figure 1. Most rice phenolic acids have antioxidant activities, though some of them have not been evaluated individually but only mixed with other rice phenolic acids [20]. [PERSON] et al. revealed that the main rice phenolic acids are protecatechic acid (**10**), \(p\)-coumaric acid (**15**), ferulic acid (**17**/**19**), sinapic acid (**27**), and vanillic acid (**32**) [23]. [PERSON] et al. investigated eight rice varieties of _O. sativa_ sp. _japonica_ and _O. sativa_ sp. _indica_ planted in different areas of China for their phenolic acids distribution by using UPLC-MS method. A total of 12 phenolic compounds were identified in all rice varieties. Protocatechic acid (**10**), ferulic acid (**17**/**19**), gallic acid (**24**), and syringic acid (**30**) were the dominant phenolic compounds in rice bran, while \(p\)-hydroxybenzaldehyde (**1**) was the main phenolic acid in rice husk. Bran and husk fractions provide more than 90% of phenolic acids and antioxidant activity of the whole rice plant. In addition, the rice subspecies _japonica_ has significant higher phenolic acids content and antioxidant activity than the _indica_ subspecies [24]. Ferulic acid (**17**/**19**) has also been found as the major phenolic compound in black rice bran, indicating the potential use of black rice bran as a natural source of antioxidants [25]. Phenolic acids are considered to be natural antioxidants, being able to scavenge free radicals that may increase oxidative stress and potentially damage large biological molecules such as lipids, proteins, and nucleic acids [26]. Therefore, the phenolic acid content was positively correlated with rice antioxidant capacity [27]. The development and utilization of phenolic acid analogues from rice husk and bran are important for improving the functionality of rice by-products. Some phenolic acids are released from rice roots as allelochemicals. The main phenolic acids in the root exudates were identified as \(p\)-hydroxybenzoic (**2**), caffeic (**7**), \(p\)-coumaric (**15**), syringic (**30**), and vanillic (**32**) acids [28]. As these phenolic acids are released at relatively low concentrations in the soil and other plant species have a high level of tolerance against phenolic acids, they are considered the least important allelochemicals in rice [8; 29; 30]. _Molecules_**2018**, _23_, 3098