Determinants of Virulence and Host Specificity in the Parasitic Angiosperm Striga Gesnerioides

被子植物寄生是世界农作物和森林生产力的主要制约因素？宽在农学上最重要的寄生虫是独脚金（独脚金属）（Striga spp.）因为它们的寄主是边际农业地区的自给作物。本研究探讨了独脚金（Striga gesnerioides（Willd.）Vatke及其豆科寄主植物作为研究寄生植物与寄主相互作用的模式。S.苦苣苔属是分布最广的独脚金属植物，来自不同地区的分离物可通过其形态特征和宿主特异性的变化来区分。此外，不同寄主基因型对不同分离株的敏感性也存在差异。S. gesnerioides寄生在豇豆[Vigna unguiculata（L.）Walp.]在西非，根据豇豆品种在田间和实验室试验条件下对寄生虫的不同抵抗能力，已经鉴定出了这种寄生虫。这些种族被指定为SG1至SG5。利用感、抗独脚金品种杂交F_2代和高代自交群体的遗传研究表明，小种特异性抗独脚金。Gesnerioides菌株受单显性基因控制，命名为Rsg1、Rsg2等。寄生在除豇豆以外的豆科宿主上的gesnerioides也存在于非洲以及美国东南部。一个这样的菌株（命名为FL 1）仅寄生毛木蓝。FL1大概是引入到中央佛罗里达，当污染的主机种子无意中种植的土壤复垦和侵蚀控制策略的一部分，在磷酸盐矿区。对五个小种的群体结构和居群内及居群间遗传多样性进行了研究。gesnerioides寄生在西非的豇豆表明，种族是密切相关的，但可区分的多态性分子特征。类似地，可以区分S. gesnerioides寄生在豇豆和那些寄生在其他豆类物种，特别是非洲和美国的S。gesnerioides寄生在Indogofera上。 本实验旨在确定S. gesnerioides，对应于在豇豆中鉴定的种族特异性抗性基因，如果这些基因和/或其他遗传因子存在于有毒寄生虫负责定义主机范围和特异性。具体来说，我们测试是否基因对基因机制在植物病原体相互作用中发挥作用的Striga主机协会。利用来自贝宁不同地点的SG4分离株，其唯一不同之处在于它们寄生豇豆品种B301的能力，我们将证明分离的无毒基因存在于S。苦苣苔属。此外，使用定义为每个种族代表的亲本植物，我们在S.的个体之间进行了互惠成对杂交。gesnerioides小种1（SG 1，来自布基纳法索）、小种3（SG 3，来自尼日利亚）和FL 1（来自佛罗里达），以建立F1杂种和F2和回交群体，其可用于确定这些个体中小种特异性无毒基因和/或参与控制宿主范围和特异性的其它基因座的遗传模式。 利用对抗性和敏感豇豆和其他豆类胚芽（例如，我们将研究不同小种/品系的杂交种是否具有改变的宿主范围或表现出改变的毒力水平。最后，已经证明了种族特异性无毒基因的存在，并确定了参与指定寄生虫宿主范围和毒力的位点，将进行研究，以确定与这些不同位点相关的分子标记。这项工作的长期目标是确定标记连锁的Avr基因（S）对应于每个种族？目前在豇豆中确定的特定抗性基因。这项拟议的调查构成了第一个分子遗传学研究，旨在确定在寄生植物的毒力和寄主偏好的遗传成分之一，并将大大有助于我们了解植物-植物病原体相互作用。这些研究还具有更广泛的实际重要性，因为寄生植物侵染对全世界农业生产力的威胁越来越大。事实上，这些寄生虫造成的大部分损害发生在发展中国家，那里的粮食稳定是一个主要问题。目前只有少数训练有素的学生和资深科学家参与这些独特的植物-植物关联的研究。很少有美国年轻科学家有处理非洲农业和粮食安全问题的经验。我们的工作涉及美国和西非科学家之间的合作，并将为来自美国和发展中国家的学生和博士后科学家提供更好的研究和培训机会。

Parasitic angiosperms are a major constraint to crop and forest productivity world?wide. Among the most agronomically important parasites are the witchweeds (Striga spp.) because their hosts are subsistence crops in regions of marginal agriculture. This study examines the interaction between Striga gesnerioides (Willd.) Vatke and its legume host plants as a model for studying parasitic plant: host interactions. S. gesnerioides is the most widely distributed of the "witchweeds" and isolates from various locations are distinguishable by variations in their morphological characteristics and host specificity. In addition, difference exists among host genotypes in their susceptibility to different isolates of the parasite. Five races of S. gesnerioides parasitic on cowpea [Vigna unguiculata (L.) Walp.] have been identified in West Africa based on the ability of cowpea cultivars to differentially resist parasitism under field and laboratory test conditions. These races are designated SG1 to SG5. Inheritance studies using F2 and advanced inbred populations derived from crosses of Striga-susceptible and -resistant cultivars showed that race-specific resistance to S. gesnerioides is controlled by single dominant genes designated as Rsg1, Rsg2, etc. Strains of S. gesnerioides parasitic on legume hosts other than cowpea also exist in Africa, as well as the southeastern part of the United States. One such strain (designated FL1) parasitizes only Indigofera hirsuta. FL1 was presumably introduced into Central Florida, when contaminated host seeds were inadvertently planted as part of a soil reclamation and erosion control strategy at phosphate mining sites. Studies of population structure and genetic diversity within and among populations of the five races of S. gesnerioides parasitic on cowpea in West Africa indicate that the races are closely related, but distinguishable by polymorphic molecular characteristics. Similarly, it is possible to distinguish between populations of S. gesnerioides parasitic on cowpea and those parasitic on other legume species, in particular, the African and US strains of S. gesnerioides parasitic on Indogofera. The experiments proposed here seek to determine whether specific avirulence genes exist in S. gesnerioides that correspond to the race-specific resistance genes identified in cowpea and if these genes and/or other heritable factors present in virulent parasites are responsible for defining host range and specificity. Specifically, we test whether the gene-for-gene mechanism operating in plant-pathogen interactions is functioning in Striga-host associations. Using isolates of SG4 from different locations in Benin differing solely in their ability to parasitize the cowpea cultivar B301, we will demonstrate that discrete avirulence genes exist in S. gesnerioides. In addition, using parental plants defined as representative of each race, we have carried out reciprocal pair-wise crosses between individuals from S. gesnerioides race 1 (SG1 from Burkina Faso), race 3 (SG3 from Nigeria) and FL1 (from Florida) in order to establish F1 hybrids and F2 and backcross populations that can be used to determine the mode of inheritance of the race-specific avirulence genes in these individuals and/or other loci involved in controlling host range and specificity. Using host differential response assays on resistant and susceptible cowpea and other legume germplasms (e.g., Indigofera) we will investigate whether hybrids of the various races/strains have a modified host range or exhibit altered levels of virulence. Finally, having demonstrated the existence of race-specific avirulence genes and identified loci involved in specifying parasite host range and virulence, studies will be undertaken to identify molecular markers linked to these different loci. The long-term goal of this work is to identify markers linked to the Avr gene(s) corresponding to each of the race?specific resistance genes currently defined in cowpea. This proposed investigation constitutes one of the first molecular genetic studies aimed at identifying heritable components of virulence and host preference in parasitic plants and will contribute significantly to our understanding of plant-plant pathogen interactions. These studies also have a broader practical importance, since parasitic plant infestations are an expanding threat to agricultural productivity worldwide. In fact, most of the damage wrought by these parasites occurs in developing nations, where food stability is a major concern. There are currently a limited number of well-trained students and senior scientists involved in the study of these unique plant-plant associations. Few young American scientists have experience dealing with agriculture and food security issues in Africa. Our work involves cooperation between scientists in the US and West Africa and will provide enhanced research and training opportunities for students and post-doctoral scientists from the US and from developing nations.