Flavonoid pathways

Study. Its flavonoid chemistry is particularly well documented, the enzymology and genetics of the flavonoid pathway are well understood, and the molecnlar biological tools are available. 

Fig. 5 Scheme of the flavonoid pathway leading to synthesis of proanthocyanidins. The enzymes involved in the pathway are shown as follows CHS = chalcone synthase CHI = chalcone isomerase F3H = flavanone-3B-hydroxylase DFR = dihydroflavonol-4-reductase LDOX = leucoanthocynidin dioxygenase LAR = leucoanthocyanidin reductase ANR = anthocyanidin reductase adapted from [27] and [28]... 

Flavanones In some cases, flavanones produced by CHI will accumulate to sizeable amounts instead of being diverted away to form flavonols, anthocyanins, and flavanols (see Fig. 5.4). These flavanone products, hesperetin and naringenin being the most common, are frequently encountered in citrus fruits and juices (USDA Flavonoids Database Release 2.1,2007). In most of these cases, essentially no flavonols or anthocyanins are encountered the flavonoid pathway is essentially blocked at the F3H step. 

INTRACELLULAR ORGANIZATION OF THE FLAVONOID PATHWAY AS A MEMBRANE-ASSOCIATED MULTIENZYME COMPLEX... 

WINKEL-SHIRLEY, B., Evidence for enzyme complexes in the phenylpropanoid and flavonoid pathways, Physiol. Plant., 1999,107, 142-149. 

To what extent is the response of cytosolic and plastidic isozymes of the shikimate pathway coordinated or coupled with one another and to alterations in expression of enzymes of the flavonoid and phenylpropanoid-pathway segments Some of the emerging information is given in Figure 6. Thus, light induction, well known to induce PAL and enzymes of the flavonoid pathway, also induces both DS-Mn and DS-Co in parsley cell cultures (49). However, only the cytosolic CM-2 (and not the plastidic CM-1) was induced. Fungal elicitor was reported to induce only DS-Mn—not DS-Co or either of the chorismate mutase isozymes (49). Previous studies... 

The key enzymes involved in the formation of the hydroxycinnamic acids (HCAs) from phenylalanine and malonyl-CoA are now discussed in detail, while later sections address the branches of the flavonoid pathway leading to anthocyanins, aurones, flavones, flavonols, PAs, and isotlavonoids. This is followed by brief reviews of the regulation of flavonoid biosynthesis and the use of flavonoid genes in plant biotechnology. To assist the reader. Figure 3.1 presents the carbon numbering for the various flavonoid types discussed. 

The flavonoid pathway contains many branch points at which enzymes may compete for substrate, depending on the spatial and temporal occurrences of the enzymes and any metabolite channeling effects. Altering the balance of the competing activities may alter the levels of the different enzyme products and their derivatives. Alternatively, when a potential substrate is accumulating in tissues, a rate-limiting step may be overcome by increasing levels of the required enzyme. 

Rosati, C. et al., Engineering of flower colour in forsythia by expression of two independently-transformed dihydroflavonol 4-reductase and anthocyanidin synthase genes of flavonoid pathway. Mol Breed, 12, 197, 2003. 

Bruce, W. et al.. Expression profiling of the maize flavonoid pathway genes controlled by estradiol-inducible transcription factors CRC and P. Plant Cell, 12, 65, 2000. 

Hutangura, P. et al.. Auxin induction is a trigger for root gall formation caused by root-knot nematodes in white clover and is associated with the activation of the flavonoid pathway, Aust. J. Plant Physiol, 26, 221, 1999. 

It is with great pleasure that we accepted the offer by CRC Press to assemble and edit this compilation of reviews on flavonoids and their properties and functions for the present volume. We considered the volume timely in that the last book of this general type, The Flavonoids — Advances in Research Since 1986 (edited by Jeffrey B. Harborne), appeared over a decade ago. Since then, advances in the flavonoid field have been nothing short of spectacular. These advances are particularly evident in the contributed chapters that cover the discovery of a variety of new flavonoids the application of advanced analytical techniques genetic manipulation of the flavonoid pathway improved understanding of flavonoid structures and physiological functions in plants and animals and, perhaps most importantly, the significance of flavonoids to human health. 

The biosynthesis of flavonoids, stilbenes, hydroxycinnamates, and phenolic acids involves a complex network of routes based principally on the shikimate, phenyl-propanoid, and flavonoid pathways (Figs. 1.35 and 1.36). These biosynthetic pathways constitute a complex biological regulatory network that has evolved in vascular plants during their successful transition on land and that ultimately is essential for their growth, development, and survival [Costa et al., 2003]. 

Figure 21.3 Models of R2R3-MYB, bHLH, and WDR transcriptional control of different branches of the flavonoid pathway. (Adapted from Quattrocchio et al., [2006] and Lepiniec et al., [2006].)...

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