Phenylpropanoid pathway, stilbenes

Studies have shown that phenylpropanoid metabolism can be stimulated by ozone. The activity of PAL increased in soybean [91], Scots pine (Pinus sylvestris L.) [92], and parsley (Petroselinum crispum L.) [93] soon after treatment with 150-200 nmol O3 mol 1. Rapid increases in transcript levels for PAL in response to ozone have been observed in parsley [93], Arabidopsis thaliana L. Heynhold [94] and tobacco (Nicoticma tabacum L.) [95]. Transcript levels for 4-coumarate CoA ligase (4CL), the last enzyme in the general phenylpropanoid pathway, increased commensurately with PAL transcripts in ozone-treated parsley seedlings [93]. Phenolic compunds reported to accumulate in leaf tissue in response to ozone include hydroxycinnamic acids, salicylic acid, stilbenes, flavonoids, furanocoumarins, acetophenones, and proanthocyanidins [85, 92, 93, 96, 97]. 

It has been noted that the chemical diversity of plant phenolics is as vast as the plant diversity itself. Most plant phenolics are derived directly from the shikimic acid (simple benzoic acids), shikimate (phenylpropanoid) pathway, or a combination of shikimate and acetate (phenylpropanoid-acetate) pathways. Products of each of these pathways undergo additional structural elaborations that result in a vast array of plant phenolics such as simple benzoic acid and ciimamic acid derivatives, monolig-nols, lignans and lignin, phenylpropenes, coumarins, stilbenes, flavonoids, anthocyanidins, and isollavonoids. 

Efforts to increase the production of trans-R for commercial apphcations have focused on heterologous expression of the phenylpropanoid and stilbene pathways in S. cerevisiae and E. coli, although as the stilbene pathway does not exist in yeast or bacteria, the entire functional pathway needs to be introduced. Thus, when in both systems, 4cl from N. tabacum and sts from V. vinifera were expressed, 16 and 6 mg trans-R were produced in E. coli and yeast cells, respectively. However, the... 

Resveratrol biosynthesis branches from the phenylpropanoid pathway. The resveratrol biosynthesis pathway consists of four enzymesrphenylalanine ammonia lyase (PAL), cinnamic acid 4-hydroxylase (C4H), 4-coumarate CoA ligase (4CL), and stilbene synthase (STS). The first two enzymes of the pathway, PAL and C4H, convert phenylalanine into /)-coumaric acid. The third enzyme, 4CL, attaches /)-coumaric acid to the pantetheine group of coenzyme-A (CoA) to produce 4-coumaroyl-CoA. The fourth enzyme, STS, catalyzes the condensation of resveratrol from one molecule of 4-coumaroyl-CoA and three molecules of malonyl-CoA, which originate from fatty acid biosynthesis. TAL is homologous to PAL and converts the amino acid tyrosine directly into / -coumaric acid. Methylated resveratrol derivatives of pinostilbene and pterostilbene are produced by resveratrol O-methyltransferase (ROMT) from resveratrol [135] (Figure 10.10). 

The biosynthesis of the stilbenoids, including 1, has been previously reviewed. Briefly, the synthesis of 1 is dependent upon a single key enzyme known as stilbene synthase or resveratrol synthase as part of a mixed phenylpropanoid-polyketide pathway [2,3,4,5,6] (Fig. (1)). Stilbene synthase catalyzes the formation of 1 through the condensation of one p-coumaroyl CoA and three malonyl CoA molecules, both of which are ubiquitous intermediary plant metabolites. 

The biosynthesis of flavonoids, stilbenes, hydroxycinnamates and phenolic acids involves a complex network of routes based principally on the shikimate, phenylpropanoid and flavonoid pathways (Figures 1.15-1.17). An overview of these pathways will be discussed... 

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