# Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation
## 4 Metabolic Regulation of Secondary Metabolites
### Metabolic Regulation by Abiotic Stresses
#### 4.1.2 Metabolic Regulation by Oligosaccharides  
Oligosaccharides derived from fungal and plant cell wall polysaccharides are a class of well characterized elicitors that can induce not only accumulation of secondary metabolites but also MAPK activation, ROS generation, and defense-related enzyme activities at a very low concentration, triggering plant innate immunity [162; 163; 164; 165; 166; 167; 168]. Studies were performed on the structure-activity relationships of the oligosaccharidesadries, the characterization of the corresponding receptors, and the analysis of signal transduction cascades and elicitor-responsive genes. Early studies focused on the inducible effects of oligosaccharidesadries on rice phytoplankton. Their elicitation mechanisms have been partially revealed in recent years.  
A mixture of chitin fragments obtained from the cell walls of the rice pathogen _Fusarium moniliforme_ through either the action of constitutive rice chitinases or partial acid hydrolysis was active to induce diterpeneoid phytoalexin biosynthesis in rice cell cultures in suspension [169]. \(N\)-Acetylchitooligosaccharides larger than hexaose induced the formation of momilactones A (**148**) and B (**149**) as well as oryzaeins A (**163**), B (**164**), and D (**166**) at concentrations of \(10^{-9}\)-\(10^{-6}\) M [170].  
\(\beta\)-glucan fragments (oligosaccharides) from the cell walls of the rice blast fungus _M. oryzae_ had the ability to elicit phytoalexin (i.e., momilactone A) biosynthesis in suspension-cultured rice cells. The potent elicitor glucopentaose, namely, tetraglucosyl glucitol, was purified from the digestion of the glucan by an endo-\(\beta\)-(\(1\)\(\rightarrow\)\(3\))-glucanase. Interestingly, the obtained tetraglucosyl glucitol from _M. oryzae_ did not induce phytoalexin biosynthesis in the soybean cotyledon cells, indicating differences in the recognition of gluco-oligosaccharide elicitor signals in these two plants [171]. In addition, two purified oligosaccharidesacharide elicitors, \(N\)-acetylchitoheptase and tetraglucosyl glucitol, derived from _M. oryzae_ cell walls, synergistically activated the biosynthesis of phytoalexin in suspension-cultured rice cells. Inhibition experiments for the binding of the radio labeled \(N\)-acetylchitooligosaccharide elicitor to the plasma membrane from rice cells indicated that the two elicitors are recognized by different receptors [172]. Rice cells recognize oligosaccharides for defense signaling mainly through plasma membrane receptors [173; 174].  
Diterpenoid phytoalexins are synthesized through the plasidic MEP pathway in rice. OsTGAP1, a basic leucine zipper transcription factor, which is induced by the fungal chitin oligosaccharidesacharide elicitor, was identified as a key regulator of the coordinated expression of the clustered biosynthetic genes for diterpeneoid phytoalexin production in rice [138]. The overexpression of the bZIP transcription factor OsbZIP9 resulted in the suppression of the chitin oligosaccharidesacharide-inducible expression of diterpeneoid phytoalexin biosynthetic genes, and thus caused a decrease in the accumulation of diterpeneoid phytoalexin in rice cells. OsbZIP9 is considered a negative regulator of rice diterpeneoid phytoalexin production [175].