Diverse Roles ofAgrobacteriumTi Plasmid-Borne Genes in the Formation and Colonization of Plant Tumors

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This chapter describes the various steps in plant colonization, including binding of the bacteria to host cells, recognition of diffusible host-released chemical signals, and processing and transfer of oncogenic DNA (T-DNA). It includes a discussion on the utilization of tumor-released compounds called opines and on the horizontal transfer of the Ti plasmid. Species of Agrobacterium have been classified primarily by their phytopathogenic properties, which are largely due to differences in plasmid content. With respect to chromosomally encoded properties, A. rubi and A. vitis are well-defined separate taxa whereas A. tumefaciens, A. radiobacter, and A. rhizogenes are not. The most thoroughly studied A. tumefaciens strains contain either octopine-type or nopaline-type Ti plasmids. Virtually all genes found on Ti plasmids play direct or indirect roles in some aspect of crown gall tumorigenesis or tumor colonization. VirD4 is absolutely required for transfer and pilus formation. Some members of the vir regulon are not essential for tumorigenesis and may play other roles in pathogenesis. Since Ti plasmids encode both TraI and TraR, each conjugal donor takes a census of other donors rather than of recipients. It is a challenge even to speculate about the adaptive value of such a system. The observation that two opines regulate conjugation in opposite ways is equally perplexing.

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