# Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation
## 4 Metabolic Regulation of Secondary Metabolites
### Metabolic Regulation by Abiotic Stresses  
#### 4.1.1 Metabolic Regulation by Phytohormones  
Secondary metabolite biosynthesis can be mediated by phytohormones such as ABA), JA, cytokins (CKs), salicylic acid (SA), ethylene (ET), and their conjugates [17; 153].  
JA, which is in the form of jasmonates, is a plant hormone which induces the biosynthesis of many secondary metabolites which play roles in plant-environment interactions [154]. JA can induce rice defense responses and plays an important role as a signal transducer for phytoalexin production in stress (e.g., CuCl\({}_{2}\), oligosaccharides, phytotoxins)-stimulated rice leaves through gene activation. For example, the endogenous level of JA increased rapidly in CuCl\({}_{2}\)-stimulated rice leaves, and exogenously applied JA caused a large amount of phytoplankton production in rice leaves [126]. As far as we know, JA can induced the biosynthesis of momilactones A (**148**) and B (**149**) as well as of \(\gamma\)-terpinene (**117**) [62].  
The basic leucine zipper transcription factor OsTGAP1 acts as a regulator of the coordinated production of diterpenoid phytoalexins in cultured rice cells. The inductive expression of _OstGAP1_ under JA treatment was only observed in a root-specific manner, consistent with the JA-inducible expressions of the biosynthetic genes of diterpenoid phytoalexins in roots [155].  
In addition, the amino acid conjugates of jasmonic acid \(N\)-[(\(-\))-jasmanoyl]-S-isoleucine (JA-Ile) and \(N\)-[(\(-\))-jasmanoyl]-S-phenylalanine were found to elicit the production of sakuraneetin (**81**) in rice leaves. The elicitation was considered to arise from the induction of naringenin 7-O-methyltransferasease, a key enzyme in sakuraneetin biosynthesis [156]. Very interestingly, JA-Ile was not required for diterpenoid production in blast pathogen-infected or CuCl\({}_{2}\)-treated rice leaves [157].  
The treatment with natural and synthetic CKs induced the production of diterpenoid phytoalexins in rice leaves and suspension-cultured cells [158]. However, CK treatment inhibited JA-inducible sakuraneetin (**81**) production in rice leaves [127]. On the other hand, exogenous root applications of SA promoted the accumulation of oryzaekins and momilactone A (**148**) in the leaves [159]. A synergistic crosstalk of CK and SA signaling was also reported, showing that 0.1 mM CKs with benzothiadiazole (BTH), a plant activator that enhances SA signaling pathway, induced a several-fold enhancement of momilactone and phytocassane biosynthetic genes [160]. In addition, it was reported that the treatment of wounded rice leaves with methionine, the precursor of ET, induced the accumulation of sakuraneetin (**81**) and momilactone A (**148**). Tiron, a free radical scavenger, counteracted the induction of both sakuraneetin (**81**) and momilactone A (**148**) production in methionine-treated leaves, indicating that active oxygen species might be important in methionine-induced production of phytoalexins. However, ET treatment of wounded leaves induced the production of sakuraneetin but not of momilactone A (**148**), suggesting that the induction of diterpenoid phytoalexin production by methionine was not regulated by ET alone [128]. In the susceptible rice cultivar Dorella, the bakanae pathogen (_Fusarium fujikuroi_) induced the production of gibberellin and abscisic acid and inhibited jasmonic acid production, and phytoalexin content in rice was very low [153].  
In addition, ethylene-inducing xylanase from _Thichoderma viride_ was a potent elicitor of immune responses in a variety of plant species, such as tobacco (_Nicotiana tabacum_), tomato (_Solanum lycopersicum_), and rice. This enzyme induced the expression of defense-related genes involved in the biosynthesis of phytoalexins (i.e., momilactones and phytocassanes) by a cation channel OsTPC1 in suspension-cultured rice cells [161].