# Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation
## 2 Structural Diversity and Roles of Rice Secondary Metabolites
### Flavonoids and Their Biological Functions  
According to the structural features, rice flavonoids can be classified as flavones (33-65), flavonols (66-77), flavanones (66-77), flavanones (or dihydroflavones, 78-83), flavonols (84-87), flavanols (88, 89), and anthocyanins (90-101), along with their glycosides. Rice flavonoids mainly have antioxidant properties, though some of them have not been evaluated for their antioxidant activities [10]. Among them, anthocyanins are mainly distributed in pigmented rice plants [2]. Rice flavonoids and their biological activities are listed in Table 2. Their structures are shown in Figure 2.  
Two apigenin \(C\)-glycos schatfoside (45) and isoschatfoside (46) were identified in whole rice leaves [35] and phloem [36]. The contents of both flavones were higher in the phloem of an insect-resistant rice variety than in a susceptible variety, which suggested that schaftoside (45) and isoschatfoside (46) in rice act as an antifeedant against brown planthopper (_Nilaparata lugens_) [37]. Flavones 56-64 belong to flavonoligrans. Both tricin 4\({}^{\prime}\)-\(O\)-(_eryth_ro-\(\beta\)-guiaacylglycerglycerol) ether (57) and tricin 4\({}^{\prime}\)-\(O\)-(_threo_-\(\beta\)-guiaacylglycerolglycerol) ether (58) from Njavara rice bran had cytotoxic activity and induced apoptosis in multiple tumor cells by themitochondrial pathway, which indicated their possible role as potential cytotoxic agents against cancer cells [38]. The flavonoids in rice include aglycones (i.e., quercetin, kaempfer and tricin) and their glycosides. Eight flavonoids, i.e., brassicin (66), isorhamtein-4\({}^{\prime}\)-\(O\)-\(\beta\)-\(D\)-glucopyranoside (67), brassicin-4\({}^{\prime}\)-\(O\)-\(\beta\)-\(D\)-glucopyranoside (68), isorhamtein-7-\(O\)-\(\beta\)-\(D\)-cellobioside (69), 3\({}^{\prime}\)-\(O\)-methyltaxifolia (84), 3\({}^{\prime}\)-\(O\)-methyltaxifolia-7-\(O\)-\(\beta\)-\(D\)-glucopyranoside (86), and 3\({}^{\prime}\)-\(O\)-methyltaxifolia-5-\(O\)-\(\beta\)-\(D\)-glucopyranoside (87), were isolated from _Oryza sativa_ sp. _japonica_ c.v. Hwa-Young. This cultivar has a high flavonoid content in the seeds and, particularly, in the endosperm tissue [39]. Two flavones, i.e., \(O\)-glycosides 5,4\({}^{\prime}\)-dihydroxy-3\({}^{\prime}\),5\({}^{\prime}\)-dimethoxy-7-\(O\)-\(\beta\)-glucopyranosideflavovne (37) and 7,4\({}^{\prime}\)-dihydroxy-3\({}^{\prime}\),5\({}^{\prime}\)-dimethoxy-5-\(O\)-\(\beta\)-glucopyranosideflavone (38), were identified in allelopathic rice seedlings. Only their aglycone, 5,7,4\({}^{\prime}\)-trihydroxy-3\({}^{\prime}\),5\({}^{\prime}\)-dimethoxyyflavone (36), was found in the soil. These two flavone \(O\)-glycosides were exuded from the rice roots to the rhizosphere and were then transformed into their aglycone forms, which showed an allelopathic effect on associated weeds and microbes [40,41]. Sakuranetin (81) is a flavanone-type phytoalexin in rice active against plant pathogens. Naringenin (79) is considered the biosynthetic precursor of sakuranetin (81) in rice. The bioconversion of naringenin (79) into sakuranetin (81) is catalyzed by naringenin 7-\(O\)-methyltransferase (OsNOMT)  
Figure 1: Structures of the phenolic acids isolated from rice. in rice leaves [42]. The antifungal activity of Sakuraneetin (**81**) was found to be higher than that of naringenin (**79**) [43; 44]. Very interestingly, Sakuraneetin (**81**) can be detoxificated into naringenin (**79**) and sterbin (**83**) by the rice blast pathogen _Magnaporthe oryzae_[44] and can also be detoxificated into naringenin (**79**), naringenin 7-O-\(\beta\)-d-xylopyranoside (**80**), and sakuraneetin 4\({}^{\prime}\)-O-\(\beta\)-d-xylopyranoside (**82**) by the rice sheath blight pathogen _Rhizoctonia solani_[45].