Isoflavonoid pathways

Figure 21.2 Schematic representation of the general phenylpropanoid, flavonoid, and isoflavonoid pathways in plants (see text for legends).

Shimada N, Akashi T, Aoki T, Ayabe S. 2000. Induction of isoflavonoid pathway in the model legume Lotus japonicus Molecular characterization of enzymes involved in phytoalexin biosynthesis. Plant Sci 160 37-47. 

Harrison M.H., Dixon R.A. Spatial patterns of expression of flavonoid/isoflavonoid pathway genes during interactions between roots of Medicago truncatula and the mycorrhizal fungus Glomus versiforme. Plant J 1994 6 9-20. 

Control of glyceollln formation appears to involve large coordinate Increases in the activities of several enzymes of phenylpropanold metabolism. Despite our limited knowledge of isoflavonoid biosynthesis, evidence reported here favors the idea that control of glyceollln biosynthesis is dependent on the interplay of general phenylpropanold metabolism and its more specialized branch, the so far mainly vmknown isoflavonoid pathway. 

Feedback inhibition can be limited by optimizing both upstream and downstream enzyme expression such that the inhibitor does not significantly accumulate. In instances where a metabolite inhibits an enzyme in the isoflavonoid pathway, enzyme mutagenesis can alter the structural interaction between the enzyme and its inhibitor to block the inhibition mechanism. Recently, allosteric feedback inhibition of a tomato peel 4CL by naringenin 23, a product several steps downstream, was significantly reduced through directed evolution in E. coli [181]. 

DIXON, R.A., HARRISON, M.J., PAIVA, N.L., The isoflavonoid phytoalexin pathway from enzymes to genes to transcription factors, Physiol. Plant., 1995, 93, 385-392. 

There are many branches to the flavonoid biosynthetic pathways, with the best characterized being those leading to the colored anthocyanins and proanthocyanidins (PAs) and the generally colorless flavones, flavonols, and isoflavonoids. Genes or cDNAs have now been identified for all the core steps leading to anthocyanin, flavone, and flavonol formation, as well as many steps of the isoflavonoid branch, allowing extensive analysis of the encoded enzymes (Table 3.1). In addition, several DNA sequences are available for the modification enzymes that produce the variety of structures known within each class of compound. 

Akashi T, Sawada Y, Shimada N, Sakurai N, Aoki T, Ayabe S. 2003. cDNA cloning and biochemical characterization of S-adenosyl-L-methionine 2,7,4 -trihydroxyiso-flavanone 4 -0-methyltransferase, a critical enzyme of the legume isoflavonoid phytoalexin pathway. Plant Cell Physiol 44 103-112. 

Farag MA, Huhman DV, Dixon RA, Sumner LW. 2008. Metabolomics reveals novel pathways and differential mechanistic and elicitor-specific responses in phenylpropanoid and isoflavonoid biosynthesis in Medicago truncatula cell cultures. Plant Physiol 146 387 402. 

In either of the proposed pathways, salicylic acid is synthesised from tram-cinnamic acid. This is an intriguing observation and may provide a clue as to how and why the induction of SAR is tightly linked to the formation of a necrotic lesion. When plants react hypersensitively to pathogen attack, many biochemical changes occur, including the induction of phenylpropanoid biosynthesis. In bean, as well as other plants, this induction seems to be at least partly caused by an increase in the synthesis of phenylalanine ammonium lyase and other enzymes involved in the biosynthesis of isoflavonoid phytoalexins, flavonoid pigments and... 

The branch pathway for anthocyanin biosynthesis starts with the enzymatic reduction of dihydrofiavonols to their corresponding flavan 3,4-diols (leucoanthocyanidins) by substrate-specific dihydroflavonol 4-reductases (DFR). Flavan 3,4-diols are the immediate precursors for the synthesis of catechins and proanthocyanidins. Catechins are formed by enzymatic reduction of the flavan 3,4-diols in the presence of NADPH to leucoanthocyanidins, which are subsequently converted to anthocyanidins by the 2-oxoglutarate-dependant dioxygenase, anthocyanidin synthase. Further glycosylation, methylation, and/or acylation of the latter lead to the formation of the more stable, colored anthocyanins (Scheme 1.1). The details of the individual steps involved in flavonoid and isoflavonoid biosynthesis, including the biochemistry and molecular biology of the enzymes involved, have recently appeared in two excellent reviews.7,8... 

Fig. 4 Biosynthesis of isoflavonoids (pterocarpans) via isoflavones from phenylpropanoid pathway...

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