Flavonols in plant

Flavonoid analyses are mostly concentrating on plants which are of either pharmaceutical interest or of commercial value. In addition, flavonoids are important factors in biological interactions between living organisms. This is best illustrated by the last review Advances in flavonoid research since 1992 focusing on these topics. In contrast, mere distribution studies or chemosystematically oriented compilations are rare (e.g., on Asteraceae). Naturally, the presently known distribution of flavones and flavonols in plants reflects the current scientific interests, and hence the interpretation of their chemodiversity must be made with caution. 

Although the majority of flavones and flavonols in plants occur as glycosides or other bound forms, small amounts of aglycones frequently are present and occasionally represent a sizable proportion of the total flavonoid compounds present in or on the plant. Flavonoid aglycones frequently are methylated or esterified and the mixture is lipophilic. In many plants, these compounds are the products of epidermal glandular trichomes (Rodriguez et al., 1984 Wollenweber and Dietz, 1981). Some highly methylated flavonoids are toxic to mammals and other animals. 

The diversity, ubiquity, and bioactivity of flavones and flavonols make these compounds of interest to a wide variety of research. It is stiU unknown exactly how these compounds exert their influence on growth, auxin transport, and other biologically relevant interactions in the plant kingdom. It is obvious that considerably more research is required to dissect out the complexities involved with flavone and flavonol functions in the animal kingdoms. There is a need to improve research on mechanisms of bioactivity in animals and plants. The advances in genomics, proteomics, and metabolomics provide new approaches to define the role of flavones and flavonols in plant development and to explore their potential application in agriculture and medicine. 

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. 

Coe, E.H., McCormick, S., and Modena, S.A., White pollen in maize, J. Hered., 83, 11, 1981. Mo, Y., Nagel, C., and Taylor, L.P., Biochemical complementation of chalcone synthase mutants defines a role for flavonols in functional pollen, Proc. Nat. Acad. Scl USA, 89, 7213, 1992. Ylstra, B. et al., Flavonols stimulate development, germination, and tube growth of tobacco pollen, Plant Physiol, 100, 902, 1992. 

Two classes of dimeric anthocyanins isolated from plants (section 10.2.6) have been identified in plants for the first time. One class includes pigments where an anthocyanin and a flavone or flavonol are linked to each end of a dicarboxylic acyl unit. The other class includes four different catechins linked covalently to pelargonidin 3-glucoside. During the last decade, seven new desoxyanthocyanidins and a novel type of anthocyanidin called P)Tanoanthocyanidins have been reported (Section 10.2.2). Toward the end of the 20th century, several color-stable 4-substituted anthocyanins, pyranoanthocyanins, were discovered in small amounts in red wine and grape pomace.Recently, similar compounds have been isolated from extracts of petals of Rosa hybrida cv. M me Violet, scales of red onion, and strawberries. About 94% of the new anthocyanins in the period of this review are based on only six anthocyanidins (Table 10.2). 

One of the hypotheses regarding evolutionary aspects of flavonoid diversification concerns the concept of flavonol accumulation in basal Angiosperms versus flavone accumulation in advanced families. Recently, some further efforts have been made towards defining the flavone/flavonols ratio in Dicotyledonae and their relation to lignification, indicating an increased tendency towards flavonol accumulation in lignified plants, whereas herbaceous species tend to accumulate more of the flavones. From the presented entries, it appears that flavone derivatives are more abundant in Lamiaceae than flavonols. In the Asteraceae, however, more data concern the flavonols. Both families are more or less herbaceous and members of the more advanced Angiosperms. 

Markham, K.R., Flavones, flavonols and their glycosides, in Methods in Plant Biochemistry, Dey, P.M. and Harbome, J.B., Series Editors, Vol. 1, Plant Phenolics, Harborne, J.B., Ed., Academic Press, London, 1989. 

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

Flavonols are present in plant foods mainly in the leaves and in the outer parts of plants with quercetin and kaempferol the most common ones. Quercetin and its glycoside are ubiquitous in fruits and vegetables. Conversely, kaempferol and myricetin are less distributed (Table 7) [21-23]. 

One of the main flavonols studied is quercetin. Indirect evidence for the presence of quercetin conjugates in humans was obtained by Manach et al [73], who found the presence of quercetin in plasma after the consumption of a complex meal rich in plant products only after P-glucuronidase and sulfatase treatment. Furthermore, the authors reported... 

Flavonols that accumulate in plant tissues are almost always in the form of glycosylated conjugates. The main flavonols in onions are quercetin- 4 -O-glucoside and quercetin-3,4 -0-,diglucoside with smaller amounts of isorham-netin-4 -0-glucoside (Fig. 1.3) [Mullen et al., 2004]. 

Figure 8.1 Structures of main flavonol aglycones and quercetin glycosides present in plant foods.

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