Advances in flavonoid research since 1992

Abstract

Some of the recent advances in flavonoid research are reviewed. The role of anthocyanins and flavones in providing stable blue flower colours in the angiosperms is outlined. The contribution of leaf flavonoids to UV-B protection in plants is critically discussed. Advances in understanding the part played by flavonoids in warding off microbial infection and protecting plants from herbivory are described. The biological properties of flavonoids are considered in an evaluation of the medicinal and nutritional values of these compounds.

Introduction

Advances in flavonoid research over recent decades have been reviewed in a series of four volumes, beginning with Harborne et al. (1975) and culminating in Harborne (1994). Since then, reviews of new structures in the anthocyanin and flavonoid field and with the isoflavones have appeared (Donnelly and Boland, 1995, Harborne and Williams, 1995, Harborne and Williams, 1998) as well as accounts of isoprenylated flavonoids (Tahara and Ibrahim, 1995, Barron and Ibrahim, 1996). A volume of short research reports and reviews on flavonoids and bioflavonoids was published in 1995 (Antus et al., 1995). An introduction to flavonoids has been published (Bohm, 1999). The only other major work to appear recently has been “The Handbook of Natural Flavonoids” (Harborne and Baxter, 1999). This is essentially a listing of 6467 known flavonoid structures, with formulae, references and information on biological activities.

The purpose of the present review is to discuss recent developments in the biochemistry and medicinal aspects of the flavonoids. Much new information has accrued on the nature of the anthocyanin–flavone complexes that contribute to blue flower colour in several different plant families and it is appropriate to summarise these data here, since one of the best established functions of flavonoid pigments is in the production of flower colour and the provision of colours attractive to plant pollinators. By contrast with the very visible flavonoids in flower petals, the flavonoids present in leaves are completely hidden by the ubiquitous green of the chlorophylls. Nevertheless, there is increasing evidence that these flavonoids, particularly when they are located at the upper surface of the leaf or in the epidermal cells, have a role to play in the physiological survival of plants. Recent work on the UV-B protection provided by leaf flavonoids will be reviewed.

It is already well established that flavonoids make some contribution to disease resistance, either as constitutive antifungal agents or as phytoalexins. Some of the continuing research in this area will be mentioned. There is also increasing evidence that some flavonoids, and especially the flavolans or proanthocyanidins, provide defence against herbivory and some recent experiments in plant–animal interactions will also be mentioned.

Perhaps the most active area of flavonoid research at the present time is in the possible medicinal contribution that flavonoids make to human health. For example, the senior author contributed recently to a symposium entitled ‘Wake Up to the Flavonoids’ held in London, the proceedings of which are due to be published shortly. Recent research on the biological properties of flavonoids will therefore be a further subject of the present review.