## 3 Rice Phytoalexin Biosynthesis
### Biosynthesis of Rice Flavonoid Phytoalexins  
Sakurantetin is a flavonoid belonging to the group of methoxylated flavanones. Like most plant-based flavonoids, Sakurantetin derives from the aromatic amino acid phenylalanine (Phe). In plants, the precursor of aromatic amino acids is chorismate, the end product of the shikimate pathway (Figure 11). Phe is then converted to _p_-coumaroyl-CoA via the phenylpropanol pathway. Chalcone synthase (CHS) catalyzes the condensation of three malonyl-CoA molecules with one _p_-coumaroyl-CoA molecule to form the open-chain flavonoid naringenin chalclone, which is converted to naringenin by chalcone isomerase (CHI). Naringenin is then transformed into Sakurantetin by S-adenosyl-l-methionine-dependent naringenin 7-O-methyltransferasearse (OsNOMT). OsNOMT can either be activated by UV or fungal infection [70; 72]. The 7-O-methylation appears to be crucial for the antifungal activity of Sakurantetin, as naringenin itself does not exhibit significant toxic effects against rice fungal pathogens such as _M. grisea_[70]. Since naringenin is the common biosynthetic intermediate for several flavonoids, OsNOMT is a key player at the branch point between flavonoid and Sakurantetin biosynthesis (Figure 11). OsNOMT was purified from UV-treated leaves of the _oscont1_ rice mutant, and the corresponding gene was identified by [PERSON] et al. [140]. Several putative flavonoid O-methyltransferasearse genes (_OMTs_) have been identified in the rice genome, and thus others may be involved in sakurantetin biosynthesis.  
Figure 10: Proposed pathway for the biosynthesis of oryzaekins A–E. Enzymes encoded by genes located on chromosomes 2, 4, 6, and 12 (c2, c4, c6, c12) are highlighted in green, yellow, blue, and pink, respectively. A, B, C: oryzaekin A, B, and C, respectively. CYP701A8: _ent_-sandaracopimaradiene 3-hydroxylase (EC 1.14.14.70); CYP76M6: oryzaekin E synthase (EC 1.14.14.122); CYP76M8: oryzaekin D synthase (EC 1.14.14.112; 1.14.14.123); GGPP: geranyl geranyl diphosphate; OsCPS4: _sync_-copalyl diphosphate synthase (EC 5.5.1.14); OsKS10: _ent_-sandaracopimara-8(14),15-diene synthase (EC 4.2.3.29) [13; 138].  
Phenylamides are formed by the conjugation of phenolic acid-CoAs (e.g., \(p\)-coumaroyl-CoA, _trans_-cinnamoyl-CoA, and feruloyl-CoA) with arylamines [94; 100; 141] (Figure 12). Tyramine, tryptamine, and its derivative serotonin are the aryl monoamines found in rice phenylamide phytoalexins [74; 97; 98; 100]. Tyramine and tryptamine result from the decarboxylation of the aromatic amino acids tryptophan (Try) and tyrosine (Tyr) catalyzed by Try decarboxylase (TDC) and Tyr decarboxylase (TYDC), respectively. Tyr is synthesized downstream of the shikimate pathway, which also provides precursors to the other aromatic amino acids Phe and Try [142]. Serotonin (5-hydroxytryptamine) is synthesized from tryptamine by tryptamine 5-hydroxylase (T5H). Additional details regarding the pathways involved in the biosynthesis of rice phenolic phytoalexins and their induction by abiotic and biotic stress can be found in [PERSON] and [PERSON] [21].  
Figure 11: Biosynthesis of sakuranetin. ACC: acetyl-CoA carboxylase (EC 6.4.1.2); CHI: chalcogen isomerase (EC 5.5.1.6); CHS: chalcone synthase (EC 2.3.1.74); OsNOMT: naringenin 7-O-methyltransferase(EC 2.1.1.232); SAH: adenosyl-L-homocysteine; SAM: S-adenosyl-L-methionine. Multiple arrows indicate multiple biosynthetic steps, while direct synthesis is indicated by single arrows.