Towards incorporating the biosynthetic transformation required for Striga inhibition from Desmodium into edible legume intercrops.

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Desmodium uncinatum, when intercropped with maize and other cereals in soil with a high level of seeds of the parasite Striga hermonthica (Scrophulariaceae), gives yield increases of 5-fold and when repeatedly used, helps to reduce the Striga seed bank. Activity has been demonstrated to be in both root exudate and extracts of D. uncinatum and acts without the need for soil or rhizobial inoculation. Such suppressive activity is not present in the root exudates of other commonly used intercrops such as cowpea, beans, soybean. Bioassay-guided fractionation of D. uncinatum root exudate, from hydroponic culture, or extract has identified, by NMR spectroscopy, lipophilic components conferring high germination of the Striga seed and hydrophilic components that cause inhibition of the rate of elongation of the Striga radicle. The main inhibitory compound identified is an unusual di-C-linked glycoside of a flavonoid. D. uncinatum is a legume that can be used as cattle forage but edible crop legumes, such as cowpea, do not show this protection mechanism despite possessing the biosynthetic pathway to flavones. The ability to biosynthesise the active component requires this glycosylation process. We propose to isolate the C-glycosyltransferase (CGT) responsible for biosynthesising the active component from D. uncinatum and using amino acid sequence data, search for genes in sequenced legumes, in particular cowpea, that confer the same biosynthetic transformation. This provides a basis for breeding the protection pathway into an edible crop legume. In addition, the C-glycosyltransferase gene will be a target for genetic modification of cowpea to produce an edible legume intercrop that possesses the Striga protection mechanism.