546 Mini-review Received: 15 August 2008 Revised: 17 October 2008 Accepted: 17 October 2008 Published online in Wiley Interscience: 05 March 2009 (www.interscience.wiley.com) DOI 10.1002/ps.1731 New genetic opportunities from legume intercrops for controlling Striga spp. parasitic weeds Antony M Hooper, a* Ahmed Hassanali, b Keith Chamberlain, a Zeyaur Khan b and John A Pickett a Abstract In smallholder farming in East Africa, intercropping of maize with the cattle forage legume, Desmodium uncinatum Jacq., prevents parasitism by Striga hermonthica (Del.) Benth. (witchweed) through an allelopathic mechanism. Isoschaftoside, a di- C-glycosylflavone, isolated from the root extract and root exudate of Desmodium, interferes with in vitro radicle development of germinated Striga. The biosynthetic pathway of this class of compound is already mostly present in edible legumes and in cereals, so characterisation of the enzyme and genes that control C-glycosylflavone biosynthesis has the potential to create this protection mechanism in other agriculturally important plants. c  2009 Society of Chemical Industry Keywords: Desmodium uncinatum; Striga hermonthica; C-glycosylflavone; C-glycosyltransferase; Zea mays; allelopathy; weed control; isoschaftoside 1 INTRODUCTION Witchweeds, comprising parasitic plants in the Striga genus, also commonly called striga, are a major threat to the staple food crops of over 100 million people in Africa. 1 They germinate close to their host plants in response to specific chemical signals from the latter, of which the hydroquinone (sorgoleone) 2–4 and sesquiterpene lactones, especially strigolactones, 5–7 are typical. The radicle subsequently grows and, when approaching the host root cells, undergoes haustoriogenesis giving rise to the functional attachment organ through which parasitism is initiated. This review describes how the agricultural practice of intercropping Desmodium with maize in Striga-infested areas suppresses parasitism of maize and the authors’ investigations into the mechanism of suppression. The state of research into the allelopathic mechanism by which chemical agents mediate this process, which is known to work in the field, is explained, along with how these discoveries may be exploited in the future as studies on the biology and chemistry of the interaction move forward and offer molecular biological opportunities. 2 STRIGA INHIBITION BY DESMODIUM IN THE FIELD Intercropping maize with cattle forage legumes comprising Desmodium spp. [silverleaf D. uncinatum (Jacq.) (Fabaceae) and greenleaf D. intortum (Mill.)] reduces dramatically the infestation of maize and sorghum by Striga hermonthica (Del.) Benth. and is similarly effective against S. asiatica (L.) Kuntze. 8,9 As a consequence of this, not only is the maize yield increased but soil fertility is improved by nitrogen fixation and reduced soil erosion. 10 Further examination of the mechanism by which Desmodium prevents Striga parasitism on maize was conducted. Field plots were established with maize intercropped with D. uncinatum with and without added nitrogen fertiliser, maize monocrop with and without added nitrogen fertiliser and maize monocrop with artificial ground covering of maize stover with and without added nitrogen fertiliser (Fig. 1). 8 These data show that Desmodium is able to reduce Striga parasitism through ground shading (mimicked by stover covering) and also through nitrogen fixation (as shown by the experiments that added fertiliser), but significantly greater reduction of Striga parasitism by Desmodium cannot be explained by these physical and soil fertility factors alone, indicating an incremental effect through an allelopathic mechanism in addition to contributions from the other factors. The allelochemical-mediated effect was confirmed in pot experiments. Desmodium uncinatum plants were grown in pots on shelves, and water dripping from their root system was used to irrigate maize planted in Striga-infested soil. Desmodium was planted with and without Rhizobium inoculation, to compare the effect of fixed nitrogen and the source of root chemistry. Comparisons were made between maize plants grown in Striga- infested soil irrigated by water from pots containing Desmodium ∗ Correspondence to: Antony M Hooper, Biological Chemistry Department, Centre for Sustainable Pest and Disease Management, Rothamsted Research, Harpenden, Herts AL5 2JQ, UK. E-mail: tony.hooper@bbsrc.ac.uk a Biological Chemistry Department, Centre for Sustainable Pest and Disease Management, Rothamsted Research, Harpenden, Herts AL5 2JQ, UK b International Centre of Insect Physiology and Ecology, PO Box 30772, Nairobi, Kenya Pest Manag Sci 2009; 65: 546–552 www.soci.org c  2009 Society of Chemical Industry