Use of animal forage legumes in the Desmodium genus on-farm against parasitic weeds, including Striga hermonthica, led to the discovery of novel chemical signals underpinning this antagonistic role. Bioassay-guided fractionation identified di-C-glycosylated apigenins, such as isoschaftoside, as related to the powerful and eradicant control of S. hermonthica when cereals, e.g. maize, sorghum, millets and rain-fed rice, are intercropped with Desmodium spp. Drought-tolerant Desmodium spp. are already being exploited extensively on-farm to deal with constraints of climate change. Using genes already known from related pathways in rice, though not involved in weed control, the early committed steps in the biosynthesis of isoschaftoside can be replicated by heterologous gene expression in cowpea, Vigna unguiculata. However, for the second C-glycosylation, which in isoschaftoside involves arabinose, the gene has not been fully characterised, but is highly functionally expressed in the drought-tolerant Desmodium incarnum. The intercropping delivery system is extremely effective for resource-poor faming and gives eradicant control of S. hermonthica by a mechanism involving suicidal germination, in which the seed bank of the parasite is eliminated after only a few years. The weed-controlling chemistry needs to be delivered directly into the rhizosphere, at the point at which the parasite normally attaches to the host roots, and so intercropping is highly effective. For this to be delivered without use of plants in the Desmodium genus, which are unique in producing this chemistry, then the pathway would need to be completed in an alternative intercrop such as cowpea or in the cereal itself. These approaches are currently being attempted.

New opportunities for controlling parasitic weeds with chemistry from antagonistic plants

254th National Meeting (2017)

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