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

寄生被子植物是全世界农作物和森林生产力的主要制约因素。在农学上最重要的寄生虫是独脚金(Striga spp.)因为它们的宿主是边际农业地区的自给自足作物。本研究探讨了石蒜(Striga gesnerioides(Wild.)瓦特克及其豆科寄主植物作为研究寄生植物的模式：寄主相互作用。Gesnerioidas是分布最广的一种杂草，不同地区的分离物在形态特征和寄主专一性上存在差异。此外，不同寄主基因型对不同菌株的敏感性也存在差异。寄生在豌豆上的五个小种。Walp.]在西非，根据豌豆品种在田间和实验室测试条件下对寄生虫的不同抵抗能力，已经确定了该病毒的存在。这些比赛被指定为SG1到SG5。利用感病和抗病品种杂交的F_2代和高级自交系群体进行的遗传研究表明，小种特异性抗性由Rsg1、Rsg2等单一显性基因控制。寄生在豌豆以外的豆类寄主上的S.gesnerioid菌株也存在于非洲和美国东南部。其中一个这样的菌株(命名为FL1)只寄生毛木蓝。据推测，Fl1被引入佛罗里达州中部，当时被污染的寄主种子被无意中种植，作为磷矿开采地点土壤开垦和侵蚀控制战略的一部分。对寄生在西非豌豆上的五个小种的群体结构和群体内及群体间的遗传多样性的研究表明，这些小种之间有着密切的亲缘关系，但可以通过多态的分子特征加以区分。同样，有可能区分寄生在豌豆上的苦参种群和寄生在其他豆科植物上的种群，特别是寄生在印度支那上的非洲和美国品系。这里提出的实验试图确定是否存在与豌豆中鉴定的小种特异性抗性基因相对应的特定无毒基因，以及这些基因和/或存在于强毒寄生虫中的其他可遗传因素是否负责定义寄主范围和专一性。具体地说，我们测试了在植物-病原菌相互作用中运行的基因对基因机制是否在植物-寄主关联中发挥作用。利用来自贝宁不同地区的SG4菌株，仅在寄生豌豆品种b301的能力上不同，我们将证明分离的无毒基因存在于Gesnerioidas中。此外，利用被定义为代表每个小种的亲本植株，我们进行了Gesnerioides小种1(布基纳法索的SG1)、小种3(尼日利亚的SG3)和Fl1(来自佛罗里达)的正反对杂交，以建立F1杂交和F2和回交群体，这些群体可以用来确定这些个体和/或其他控制寄主范围和专一性的基因座的小种特异性无毒基因的遗传方式。通过对抗性和感病的豌豆和其他豆科植物种质(如木蓝)的寄主差异反应分析，我们将调查不同小种/品系的杂交是否具有改变的寄主范围或表现出改变的毒力水平。最后，在证明了存在特定于小种的无毒基因并确定了与寄生虫寄主范围和毒力有关的基因后，将进行研究，以确定与这些不同基因座连锁的分子标记。这项工作的长期目标是确定与AVR基因(S)连锁的标记，该标记对应于目前在豌豆中定义的每个小种特有的抗性基因。这项研究是最早的分子遗传学研究之一，旨在确定寄生植物的致病力和寄主偏好的可遗传成分，并将有助于我们理解植物与植物病原菌的相互作用。这些研究还具有更广泛的实际意义，因为寄生植物侵扰正在对世界各地的农业生产力构成越来越大的威胁。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，只有数量有限的训练有素的学生和资深科学家参与这些独特的植物-植物组合的研究。几乎没有年轻的美国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多的研究和培训机会。

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.