Potential for producing increased levels of isoflavones in transgenic plants.

Abstract

Abstract Isoflavones are a subgroup of isoflavonoids, a group of flavonoids found almost exclusively in Leguminosae. The main isoflavones accumulating in legumes are daidzein and genistein and 4-0-methylated formononetin and biochanin A. Isoflavones and derived isoflavonoids play important roles in plant protection against pathogens, as signalling molecules in nitrogen fixing legume-rhizobia associations and can be beneficial for human health. Considerable research efforts aim to introduce isoflavones into non-leguminous plants through genetic transformation. The biosynthesis of isoflavones requires the activity of isoflavone synthase (IFS) and transformation of non-leguminous plants with IFS demonstrated that it is, in principle, possible to introduce the biosynthesis of genistein into plants not normally synthesizing this compound. However, isoflavone concentrations are typically at least an order of magnitude lower than those found in soybean seeds or red clover leaves. Major limiting factors are insufficient flux through naringenin, the general precursor of (iso)flavonoids and in particular competition between flavonoid specific enzymes and isoflavonoid specific IFS. Attempts to increase isoflavone concentrations beyond the natural variation in soybean seeds through overexpression of IFS have also been met with limited success and recently identified transcriptional regulators of the isoflavonoid biosynthetic pathway might present a more promising option. All reviewed studies focused on the manipulation of genes encoding proteins directly involved in the (iso)flavonoid pathway and we propose that engineering strategies introducing or enhancing the biosynthesis of isoflavones might have to consider wider metabolic network interactions such as the link of (iso)flavonoid precursors with protein, lipid and carotenoid biosynthetic pathways.