ReviewBiosynthesis of flavonoids and effects of stress
Introduction
Flavonoids are ubiquitous plant secondary products that are best known as the characteristic red, blue, and purple anthocyanin pigments of plant tissues [1]. These compounds serve essential functions in plant reproduction by recruiting pollinators and seed dispersers. They are also responsible for the beautiful display of fall color in many plant species, which has recently been suggested to protect leaf cells from photo-oxidative damage, thereby enhancing the efficiency of nutrient retrieval during senescence [2•]. In fact, flavonoids are a remarkably diverse group of secondary products (Fig. 1) with a vast array of biological functions, including apparent roles in stress protection. The flavonols may be among the most important flavonoids in this regard; they are the most ancient and widespread of the flavonoids, synthesized even in mosses and ferns, and have a wide range of potent physiological activities [3]. Progress continues to be made in understanding the roles of flavonoids in stress protection, as well as in defining the mechanisms that control the amounts and varieties of flavonoids that are produced in plants in response to diverse environmental cues [4].
Section snippets
Flavonoids and stress protection
The ultra-violet (UV)-absorbing characteristics of flavonoids have long been considered to be evidence for the role of flavonoids in UV protection. Indeed, flavanoids are often present in the epidermal cell layers of leaves and in tissues that are susceptible to UV light, such as pollen and the apical meristem. The first direct evidence in support of a role for flavanoids in UV protection came from experiments with Arabidopsis mutants, which showed that lesions in chalcone synthase (CHS) or
Transcriptional regulation of the flavonoid pathway
One important avenue to understanding the role of flavonoids in the stress response is to understand how the expression of the biosynthetic pathway is regulated. This system represents one of the oldest examples of coordinated gene and enzyme regulation in response to environmental and developmental factors [24]. A great deal has been learned from studies in a variety of plant species, primarily about transcriptional regulation, although evidence for other types of control also exists. Recent
Enzymology of flavonoid biosynthesis
Many, although not all, of the enzymes of flavonoid biosynthesis are encoded by small gene families. The functional significance of this redundancy has been the subject of substantial interest over the years. Kimura et al. [32•] reported recently that licorice (Glycyrrhiza echinata) contains two CHI isozymes that can use both 6′-hydroxychalcone and 6′-deoxychalcone, and therefore are likely to be involved in the legume-specific isoflavonoid pathway (Fig. 1). One of these genes is induced by
Conclusions
The accumulation of anthocyanin pigments in vegetative tissues is a hallmark of plant stress, yet the role that flavonoids play in the stress response is still poorly understood. In many cases, these compounds may provide antioxidant activity as part of a general stress response, which may also explain their health-promoting qualities in animals. However, there is also evidence that flavonoids may function in plants to screen harmful radiation, bind phytotoxins, and help to regulate the stress
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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