The mechanism of suppression by the legume Desmodium uncinatum of the parasitic weed Striga

在撒哈拉以南的大草原农业中，非洲农民有一系列的害虫问题需要解决，但却无法获得现代杀虫剂。在控制害虫，特别是本地和引进的飞蛾的茎蛀幼虫的工作过程中，通过与驱避植物间作和与吸引成年飞蛾的植物诱捕作物间作，发现了一种间作对非洲独脚金（Striga）的显著控制。这些金鱼草科的寄生杂草对该地区1亿多人口的主要粮食作物构成重大威胁。控制寄生虫的间作作物是牛饲料豆科作物，通常称为假丝茅，当在农场中作为玉米和其他自给谷物（如高粱）的一对一间作种植时，寄生的斯特里加虫得到了控制，玉米的产量远远低于1吨/公顷，可以提高到5吨/公顷以上。实验证明，这种效果可以通过将水从铁丝茅的根部输送到含有斯特里加和玉米种子的土壤中来实现。此外，当假丝酵母生长在有营养物但没有土壤的水中时，水捕获了这种活性，然后这种活性可以转移到土壤中的斯特里加线虫和玉米种子上，再次赋予寄生虫控制作用。从水中分离出来的化学物质影响了这种控制，确定了影响发育的部分，这是通过测量寄生虫根生长的减少来记录的。从这些馏分中产生的化学物质已被鉴定出具有上述活性。然而，这些化学物质防止斯特riga感染的确切方式尚未确定。因此，本项目的目的是确定寄生菌生命周期中受寄生菌根分泌物中抑制化合物影响的阶段，纯化其中的一些化合物，并用它们来检验它们在田间抑制寄生菌的假设。最后，另一种在分子研究中常用的豆科植物——日本莲花的根分泌物，我们知道它也会产生一种抑制斯特里加菌根生长的根分泌物，将被检查以确定它们是否含有抑制斯特里加菌感染的化合物。虽然从农艺学上讲，日本豆科植物不适合实际控制Striga，但丰富的基因组信息和工具将使这种模式豆科植物的合成途径在未来得到阐明。这项工作将首次阐明有价值的杂草抑制是如何发生的，并将为英国农业控制杂草的尝试提供基础。

In sub-Saharan savannah agriculture, African farmers have a range of pest problems with which to contend and yet without access to modern pesticides. During the course of work on controlling insect pests, particularly the stem-boring larvae of indigenous and introduced moths, by intercropping with repellent plants and trap cropping with plants that attract the adult moths, an intercrop was found that provided dramatic control of the African witchweed, Striga species. These parasitic weeds in the snapdragon family are a major threat to the staple food crops of over 100 million people in the region. The parasite-controlling intercrops were cattle forage legumes, commonly called desmodium, and when grown on-farm as a one-to-one intercrop between maize and other subsistence cereals such as sorghum, the parasitic Striga was controlled to the extent that a yield of well under 1 tonne/hectare of maize could be raised to over 5 tonnes/hectare. Experimentally, it was demonstrated that the effect could be provided by passing water over the roots of desmodium and then into soil containing the Striga and maize seeds. Furthermore, when desmodium was grown in water with nutrients but without soil, the water captured the activity, which could then be transferred to the Striga and maize seeds in soil, and again conferred control of the parasite. Fractionation of the chemicals from the water effecting this control identified a fraction that affected development, which was recorded simply by measuring the reduction in radicle growth of the parasite. Chemicals from these this fractions accounting for the activities described, have been identified. However, the exact way in which these chemicals work to prevent Striga infestation has not been determined. Thus, the aim of this project is to identify the stage(s) of the Striga lifecycle affected by the inhibitory compounds present in desmodium root exudates, to purify some of these compounds and to use them to test the hypothesis that they are responsible for the suppression of Striga seen in the field. Finally, the root exudates of another legume, that is commonly used in molecular studies, Lotus japonicus, that we know also produces a root exudates that inhibits Striga radicle growth, will be examined to determine whether they contain compounds that suppress Striga infection. Although L. japonicus would not be agronomically suitable for practical control of Striga, the wealth of genomic information and tools available for this model legumes would enable the biosynthetic pathways involved in the synthesis of the novel flavones to be elucidated in the future. This work will provide the first elucidation of how a valuable weed suppression occurs and will provide the basis for attempts to control weeds in UK agriculture.