## 3 Rice Phytoalexin Biosynthesis
### Biosynthesis of Rice Diterpenol Phytoaleoxins
4.2.3.x); DXR: 1-deoxy-D-xylulose 5-phosphate reductoisomerase (EC 1.1.1.267); DXS: 1-deoxy-D-xylulose 5-phosphate synthase (EC 2.2.1.7); FDP: farnesyl diphosphate; FPPS: farnesyl diphosphate synthase (EC 2.5.1.10); G3P: D-glyceraldehyde 3-phosphate; GPP: geranyl diphosphate; GPPS: geranyl diphosphate synthase (EC 2.5.1.29); GGPPS: geranylgeranyl diphosphate synthase (EC 2.5.1.29); HDR: (E)-4-hydroxy-3-methylbut-2-enyl diphosphate reductase; HDS: (E)-4-hydroxy-3-methylbut-2-enyl diphosphate synthase (EC 1.17.1.2); HMGR: 3-hydroxy-3-methylglutaryl-CoA reductase (EC 1.17.1.2); HMGS: 3-hydroxy-3-methylglutaryl-CoA synthase (EC 2.3.3.10); IPPI: isopentenyl diphosphate \(\Delta\)-isomerase (EC 5.3.3.2); IPP: isopentenyl diphosphate; MCT: 2-C-methyl-d-erythhtol 4-phosphate cytidyltransferase (EC 2.7.7.60); MDD: diphosphomevalonate decarboxylase (EC 4.1.1.33); MDS: 2-C-methyl-D-erythhtol 2,4-cyclodiphosphate synthase (EC 4.6.1.12); MTPS: monoterpene synthase (EC:4.2.3.-); MVK: mexulonate kinase (EC 2.7.1.185); MVAP: mexulonate 5-phosphate; MVAPP: mexulonate diphosphate; PMK: phosphomevalonate kinase (EC 2.7.4.2); STPS: sesquiterpene synthase (EC 4.2.3.49; 4.2.3.47; 3.1.7.6); TPS: terpene synthase (EC 4.2.3.47); TTPS: triterpene synthase (EC 5.4.99.-) (adapted from [107]). GGPP is the central precursor of all diterpenoids, including rice diterpenoid phytoalaxins (Figure 6). The IDS responsible for GGPP biosynthesis is GGPP synthase (GGPPS). The structural diversity of labdane-type diterpenoids arises from the pairwise activity of class I and class II diterpene synthases (diTPSs) which act sequentially to convert GGPP into distinct diterpenoid scaffolds [24; 26; 108] (Figure 6). Class II diTPSs catalyze the first step of rice diterpenoid biosynthesis, consisting of the conversion of (_E_,\(E\),_E_)-GGPP into the bicyclic premyl diphosphate intermediates (_55_,_95_,_108_)-copalyl diphosphate (_syn_-CPP) and (_55_,_95_,_105_)-copalyl diphosphate (_ent_-CPP). Two class II diTPSs belonging to the TPS-c subfamily [24; 109] are responsible for the biosynthesis of _syn_-CDP and _ent_-CDP, i.e., OsCPS4 and OsCPS1/2, respectively. Subsequently, class I diTPSs convert _syn_-CPP and _ent_-CPP via ionization of the diphosphate substituent and several cyclization and rearrangement reactions downstream of the carbocation intermediate [26]. Many genes encoding enzymes involved in the biosynthesis of rice diterpenoid phytoalaxins are arranged as biosynthetic gene clusters (BGCs). A BGC is made up of three or more non-homologous genes encoding enzymes involved in the same biosynthetic pathway and located close to each other on the same chromosome. The clustering of genes encoding protein complexes has been proposed to be a useful strategy for coordinating the regulation of component genes and providing an optimal proportion of gene products. Furthermore, the grouping of biosynthetic pathways could help reduce autotoxicity caused by the accumulation of intermediates. The co-inheritance of entire biosynthetic pathways promotes the evolution of common gene expression regulation mechanisms, accelerating  
Figure 6: Proposed pathway for the biosynthesis of rice diterpenoid phytoalaxins and gibberellins. The enzymes whose chloroplast localization is established are written in green. CPP: copalyl diphosphate; diTPS I: class I diterpene synthase; diTPS II: class II diterpene synthase; GGPP: geranylgeranyl diphosphate; OsCPS1: _ent_-copalyl diphosphate synthase (EC 5.5.1.13); OsCPS2 (OsCyc2): _ent_-copalyl diphosphate synthase (EC 5.5.1.13); OsCPS4 (OsCyc1): _syn_-copalyl diphosphate synthase (EC 5.5.1.14); OsKS1: _ent_-kaur-16-ene synthase (EC 4.2.3.19); OsKS4 (OsKS14): _syn_-pimara-7,15-diene synthase (EC 4.2.3.35); OsKS7 (OsKS1Z; OsDTC1): _ent_-cassa-12,15-diene synthase (EC 4.2.3.28); OsKS8 (OsKS18; OsK8; OsDTC2): stemar-13-ene synthase (EC 4.2.3.33); OsKS10 (OsKS10): _ent_-sandarcopimara-8(14),15-diene synthase (EC 4.2.3.29) (adapted from [110]). responses to environmental conditions and maximizing plant fitness [111; 112; 113].