Molecular Genetic Analysis of Host Range in Crown Gall Tumors

农杆菌侵染双子叶植物可能是分子水平上了解最多的植物致病反应。然而，世界上许多非常重要的农作物，如玉米和水稻，都是单子叶植物，而且它们中的大多数对农杆菌的感染具有明显的抗性。对介导农杆菌宿主范围特性的因素的了解可能使其宿主范围扩大到包括迄今为止难以转化的植物。这样的认识也将对植物细胞转化的机制提供相当大的见解。我们将研究一个单一的系统，即葡萄球菌-葡萄系统。农杆菌的肿瘤诱导质粒（Ti）含有一个小区域，即T-DNA，当它被转移到受感染植物细胞的核DNA中并稳定地整合时，导致肿瘤的生长。在Ti质粒的另一个区域，vir区域发生突变，可以完全消除毒力。vir区包含至少6个操纵子，除virA操纵子外，没有一个是在没有植物细胞的情况下表达的。受伤的植物细胞分泌激活其他5个操纵子的分子。为了解释vir基因在宿主范围内的作用，需要确定不同的植物是否在不同的农杆菌菌株中合成了不同的激活vir基因的分子。将构建不同大小的质粒，以观察它们是否以不同的效率转移。还将尝试构建具有低宿主范围和宽宿主范围T-DNA基因不同组合的农杆菌菌株。这些菌株将被测试其诱导肿瘤和过敏的能力。从中华人民共和国葡萄肿瘤中分离的大约20株农杆菌将被筛选，因为其中许多菌株具有有限的宿主范围。将尝试检测在低标准低宿主范围菌株中发现的这些分离株中的特定基因，例如病毒a；确定是否可以通过添加从宽寄主范围菌株中选择的基因来扩大其寄主范围。这些研究将使我们更清楚地了解农杆菌宿主范围确定的分子方面，以及植物细胞转化早期阶段与宿主范围特异性相关的方面。

The infection of dicotyledonous plants by Agrobacterium is perhaps the best understood plant pathogenic response at the molecular level. However, many of the world's very important agricultural crops such as corn and rice are monocotyledonous and most of them are apparently resistant to infection by Agrobacterium. An understanding of the factors which mediate host range properties of Agrobacterium may enable the extension of its host range to include plants which thus far are refractory to transformation. Such understanding will also provide considerable insight into the mechanism of plant cell transformation. A single system will be studied, the A. tumefaciens - grapevine system. The tumor inducing plasmid (Ti) of Agrobacterium contains a small region, the T-DNA, which, when transferred into and stably integrated within the nuclear DNA of the infected plant cell results in the growth of a tumor. Mutations in another region of the Ti plasmid, the vir region, can abolish virulence completely. The vir region contains at least 6 operons, none of which, except for the virA operon, is expressed in the absence of plant cells. Wounded plant cells secrete molecules which activate the other 5 operons. To explain the role of the vir genes in host range, it will be determined whether different plants synthesize different molecules that activate vir genes in different strains of Agrobacterium. Plasmids of different sizes will be constructed to see if they are transferred with different efficiencies. Attempts will also be made to construct strains of Agrobacterium with different combinations of low host range and wide host range T-DNA genes. These strains will be tested for their ability to induce tumors and hypersensitivity. Approximately 20 strains of Agrobacterium isolated from grapevine tumors in the Peoples Rupublic of China will be screened since many of these strains have a limit host range. Attempts will be made to detect specific genes, e.g., virA, in these isolates, which are found in the low standard low host range strains; to determine whether of not their host range can be expanded by the addition of selected genes from the wide host range strain. These studies should result in a clearer insight into the molecular aspects of host range determination in Agrobacterium as well as into aspects of the early stages of plant cell transformation specifically related to host range.