Cooh

COSCoA

Flavonoids Isoflavo noids

Suberin Lignin ' Cell Wall-bound phenolics

Figure 3 General phenylpropanoid metabolism.

Flavonoids Isoflavo noids

Suberin Lignin ' Cell Wall-bound phenolics

Figure 3 General phenylpropanoid metabolism.

The potential health interest in the majority of these metabolites lies in the fact that many of the simple and complex phenols that are present in foods have been shown to be powerful antioxidants in vitro (23) and demonstrate a wide range of potential benefits in animal experiments. They play an important role in the determination of flavor, color, and taste.

Clearly, given the importance of these metabolites to the plant, any proposed genetic modification must pay careful attention to a possible adverse impact on the plants' responses to environmental stress. For this reason, most of the successful attempts at up-regulating the production of a specific metabolite in the pathway have been where the gene is involved in the control of the end step in the biosynthesis rather than at earlier stages in the biosynthetic process. Careful attention also has to be given to the choice of promoters for correct temporal and spatial expression. For the strategy to succeed, promoters must direct the biosynthesis to edible storage organs or organelles. Up-regulation of expression in all parts of a plant often leads to adverse consequences (24).

A great deal is known about the biochemical events that occur in PP metabolism. An array of mutants are available in which steps in the pathway are blocked, especially in the pathways to flavonoid pigmentation. Complementary DNA (cDNA) and/or gene sequences are also available for the early steps in PP biosynthesis as well as in flavonoid biosynthesis (25).

PAL deaminates phenylalanine to cinnamate, which is subsequently hydroxylated to 4-hydroxycinnamic acid through the action of C4H. PAL is present constituitively, together with C4H and 4CL, in varying activities throughout plant development. The evidence suggests that these enzymes are highly regulated and exist in individual isoforms that may well be species and cell dependent.

Although PAL overexpression is likely to play a key role in increasing flux into total phenylpropanoid metabolism, it is probably too nonspecific and many biological functions would be affected. It has been found that overexpression of PAL in tobacco increases its resistance to microbial attack but decreases its resistance to insect larvae (26). Similar problems are likely to be found with the overexpression of other enzymes early in the pathway. The free acids that are synthesized through this pathway rarely accumulate to high levels within the plant and are usually conjugated to sugars, cell wall carbohydrates, or organic acids.

The texture of fruit and vegetables is closely related to the chemical and physical properties of the cell walls. The end point determinant of texture is related to the overall mechanical properties of the plant, which are determined by the structural characteristics of the cell wall, and the way in which these determine cell-to-cell adhesion (27). An important factor in tissue softening is the separation of individual cells. Fruit ripening generally results in the biochemical dissolution of the middle lamella pectic polysaccharides, which bind cells together. Thermal treatment can also cause (3-eliminative degradation of the pectic polymers involved in cell-cell adhesion.

Ripening and thermally induced cell separation occur predominantly in nonlignified, thin-walled tissues. Tissues that fail to soften are frequently the result of secondary thickening and associated lignification. However, some parenchyma-rich plant tissues with thin, nonlignified tissues soften only slowly on cooking. In any studies of the texture of fruit and vegetables it will be important to understand the factors that control the processes behind maturation within both the middle lamellar and the xylem tissues. The role of specific steps in the biosynthesis of these structures in fruit and vegetable textural characteristics is described further in Chap. 11.

A. Flavonoids and Isoflavonoids

Flavonoids, whose basic structure is shown in Fig. 4, form an important class of metabolites of the PP pathway. They show protective effects against cancer progression in experimental animals. In vivo and in vitro studies have shown a range of potentially beneficial effects such as antioxidant properties, and they are effective vasodilators and platelet disaggregators (28). They are present in significant amounts in many food plants. They also illustrate ex-

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