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

在撒哈拉以南的萨凡纳农业中，一项主要活动是种植玉米作为自给作物。农民资源贫乏，面临一系列问题，特别是虫害（蛀茎虫）和寄生杂草（striga）造成的产量损失。玉米蛀茎虫是本地和外来蛾类的幼虫，它们攻击玉米茎，造成广泛的损害，甚至导致植株倒塌。在控制害虫的工作过程中，通过与驱避性植物间作和与吸引成虫蛾远离玉米的植物诱捕，发现了对独脚金的显著控制的间作。这些寄生杂草属于金鱼草科，是该地区1亿多人主食作物的主要威胁。控制striga的间作作物是牛饲料豆类，通常称为desmodium，当在农场种植玉米和其他维生谷物（如高粱）之间的一对一间作时，寄生striga被控制到可以将远低于1吨/公顷的玉米产量提高到5吨/公顷以上的程度。为了表明这种保护是基于从山蚂蝗属的根系产生的化学物质，证明了可以通过使水流过山蚂蝗属的根，然后进入含有独脚金和玉米种子的土壤中来提供该效果。此外，当山蚂蝗生长在有营养但没有土壤的水中（水培）时，水捕获了活性，然后可以转移到土壤中的striga和玉米种子中，并再次控制寄生虫。已经研究了影响这种控制的水的化学成分，并将含量的馏分彼此分离。独脚金种子萌发后，产生胚根（根），胚根将发育成玉米寄生的附着器官。然而，一个特定的部分会干扰胚根的发育，因此我们已经确定了一种有效的胚根抑制剂。这种化合物的结构是一种二-C-糖基黄酮，称为isoschaftoside。许多植物具有产生黄酮的能力，然而产生异沙夫托糖苷所需的糖基化步骤是不寻常的。我们建议确定在山蚂蝗中进行这种糖基化的酶，通过寻找在根组织的蛋白质提取物的生物合成途径中的底物上的C-糖基转移酶活性。我们将通过使用标准蛋白质纯化技术跟踪该活性来纯化蛋白质，并通过确定其氨基酸序列来鉴定其结构。这一序列将使我们能够搜索可食用作物豆类的基因组，努力找到这种蛋白质，从而培育出可食用作物豆类，当间作时，也可以防止striga寄生玉米。此外，在植物中产生C-糖基转移酶蛋白的山蚂蝗基因本身为可食用作物豆科植物的遗传修饰提供了靶标，这将防止独脚金寄生玉米。这项工作将首次阐明如何利用有价值的杂草抑制机制，并为其他发展中国家和发达国家（如英国）控制农业杂草的尝试提供基础。

In sub-Saharan savannah agriculture, a primary activity is the cultivation of maize as a subsistence crop. The farmers are resource poor and have a range of problems with which to contend, in particular the loss of yield due to damage by insect pests (stem-borers) and parasitic weeds (striga). The insect stem-borers are the caterpillars of indigenous and introduced moths, and attack maize stems causing extensive damage and even the loss of the plant by collapse. During the course of work on controlling insect pests, by intercropping with repellent plants and trap cropping with plants that attract the adult moths away from the maize, an intercrop was found that provided dramatic control of striga. These parasitic weeds from the Striga genus are in the snapdragon family and are a major threat to the staple food crops of over 100 million people in the region. The striga-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. To show this protection was based on chemicals generated from the root system of desmodium, 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 (hydroponically), 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. The chemical composition from the water effecting this control has been studied and fractions of the content separated from each other. After the striga seed germinates, a radicle (root) is produced that will develop into the attachment organ for parasitism of maize. However, one particular fraction causes interference with the development of the radicle and from this we have identified a potent radicle inhibitor. The structure of this compound is a di-C-glycosylflavone called isoschaftoside. Many plants possess the ability to produce flavones, however the glycosylation steps required to produce isoschaftoside are unusual. We propose to identify the enzyme in desmodium that performs this glycosylation by searching for C-glycosyltransferase activity on substrates in the biosynthetic pathway in protein extracts of root tissue. We will purify the protein by following this activity using standard protein purification techniques and characterise its structure by determining its amino acid sequence. This sequence will allow us to search the genomes of edible crop legumes in an effort to find this protein and so breed edible crop legumes that, when intercropped, also prevent striga from parastising maize. In addition, the desmodium gene itself, which generates the C-glycosyltransferase protein in the plant, provides a target for genetic modification of an edible crop legume that will prevent striga parasitizing maize. This work will provide the first elucidation of how a valuable weed suppression mechanism can be utilised and could provide the basis for attempts to control weeds in agriculture in other developing countries and potentially also in developed countries, such as the UK.