Molecular Genetic Analysis of Host Range in Crown Gall Tumors

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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.