# Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation
## 2 Structural Diversity and Roles of Rice Secondary Metabolites
### Terpenoids and Their Biological Functions
#### 2.3.3. Diterpenoids and Their Biological Functions  
Almost all rice diterpenoids are members of the labdane-related superfamily, which includes not only phytohormone gibberellins (GAs) but also phytoalaxins (i.e., phytocassanes, oryzalides, and oryzaeins), participate in the defense against pathogens, and are allelochemicals (i.e., momilactone B) inhibiting the growth of other plant species. Rice diterpenoids and their biological activities are listed in Table 5. Their structures are shown in Figure 5.  
The major endogenous GA in rice was identified as GA\({}_{19}\) (**147**). Other GA analogs are GA\({}_{1}\) (**145**) and GA\({}_{4}\) (**146**). The level of active Gas, such as GA\({}_{1}\) (**145**), may be regulated by the rate of biosynthesis of GA\({}_{19}\) (**147**) or its metabolic conversion [70]. Up to now, 37 diterpenoid-type phytoalexin analogues have been identified from rice plants. They have been further classified into five subtypes according to their biosynthetic pathways and structural characters [17]. The first one (**148-153**) is the pimaradiene type which mainly includes momilactones A (**148**) and B (**149**) [71] and 9\(\beta\)-pimara-7,15-diene-3\(\beta\),6\(\beta\),19-triol (**153**) [72]. The second subtype (**154-173**) is the _ent_-sandaracopimaradiene type which mainly includes oryzaeins A\(\sim\)F (**163-168**) [73-79]. The third one is the stemarene type that contains oryzaealexin S (**174**) [44] and stemar-13-en-2\(\alpha\)-ol (**175**) [72]. The fourth one is the _ent_-cassadiene type, containing phytocassanes A\(\sim\)F (**176-181**) [72, 80-82]. The fifth one is the casbene type, including 5-deoxy-_ent_-10-oxodeprssin (**182**) [83], 5-dihydro-_ent_-10-oxodepresssin (**183**) [83], and _ent_-10-oxodepressin (**184**) [84]. Figure 4: Structures of the sesquiterpenoids isolated from rice. A few oryzalide-related compounds were isolated from the leaves of a cultivar resistant to the _Xoo_. They were identified as _ent_-15,16-epoxy-2,3-dihydroxy-kaurane (**154**) [85], _ent_-2,3,15-trihydroxy-kaurane (**155**) [85], _ent_-15,16-epoxy-kauran-3-one (**156**) [85], oryzadione (**157**) [86], _ent_-15,16-epoxy-3\(\beta\)-hydroxy-kauran-2-one (**158**) [86], _ent_-15,16-epoxy-3-oxa-kauran-2-one (**159**) [86], _ent_-15,16-epoxy-3\(\beta\)-myristoyloxy-kauran-2-one (**160**) [86], _ent_-15,16-epoxy-3\(\alpha\)-palmitoyloxy-kauran-2-one (**161**) [86], _ent_-15,16-epoxy-3\(\beta\)-palmitoyloxy-kauran-2-one (**162**) [86], oryzalide A (**163**) [87,88], oryzalide B (**164**) [88], oryzalide acid A (**169**) [88], and oryzalic acid B (**170**) [85]. In contrast to typical diterpene phytoalexins, the accumulation of oryzalide-related comounds is only moderately induced by _Xoo_ infection [89]. Three compounds, i.e., 9\(\beta\)-pimara-7,15-diene-3\(\beta\),6\(\beta\),19-triol (**153**), stemar-13-en-2\(\alpha\)-ol (**175**), and phytocassane F (**181**) were accumulated following an infection by the rice blast pathogen _M. oryzae_. 9\(\beta\)-pimara-7,15-diene-3\(\beta\),6\(\beta\),19-triol (**153**) and stemar-13-en-2\(\alpha\)-ol (**175**) exhibited weak antifungal activity and may be the biosynthetic intermediates of rice phytoalexins momilactones and oryzalexin S (**174**), respectively. Phytocassane F (**181**) exhibited relatively high inhibitory activity against the mycelial growth of _M. oryzae_, to the same extent as the known phytoplankton phytocassane A (**176**) [72]. Some diterpenoids such as momilactones A (**148**) and B (**149**) have their obvious allebotathic effects. Momilactones A (**148**) and B (**149**) mainly distribute in rice husks, leaves, seedlings, and straw. They function as either rice defense systems against pathogens and insects or growth inhibitors in seed dormancy [90]. Both momilacontes A (**148**) and B (**149**) inhibited the growth of barnyard grass (_Echinochla crus-galli_) and _Echinochla colonum_, the most noxious weeds in rice field, at concentrations greater than 1 and 10 \(\upmu\)M, respectively.