T-DNA of the Agrobacterium Ti and Ri Plasmids as Vectors

Abstract

Crown gall and hairy root disease are incited by Agrobacterium tumefaciens strains carrying large virulence plasmids (Zaenen et al., 1974; Van Larebeke et al., 1974, 1975; Watson et al., 1975; White & Nester, 1980a). These are groups of related plasmids and have been designated Ti (tumor-inducing) and Ri (root-inducing) plasmids (Sciaky et al., 1978; Chilton et al., 1982), based on morphology of the infected plant tissue. The pathogenic bacteria genetically transform the host plant cells by an unknown mechanism that results in incorporation of a specific part of the virulence plasmid into the host plant genome (Chilton et al., 1977, 1982; White et al., 1982). This foreign DNA element, called T-DNA (transferred DNA), is covalently joined to host plant nuclear DNA (Chilton et al., 1980; Willmitzer et al., 1980; Yadav et al., 1980; Zambryski et al., 1980). T-DNA contains genes that function in the transformed plant cells, producing polyadenylated transcripts (Drummond et al., 1977; Gelvin et al., 1981, 1982; Gurley et al., 1979; Willmitzer et al., 1981, 1982; Bevan and Chilton, 1982). The morphology of tumor cells is affected when specific transcripts are inactivated by transposon insertion mutagenesis (Ooms et al., 1981; Garfinkel et al., 1981; Leemans et al., 1982). The auxin and cytokinin autotrophy of tobacco crown gall tumor cells can be eliminated individually by specific T-DNA mutations (Barton et al., 1982; Binns et al., 1982). An additional genetic function encoded by T-DNA is the synthesis by tumor tissue of novel metabolites, called opines, catalyzed by enzymes encoded by mapped T-DNA genes (Holsters et al., 1980; Garfinkel et al., 1981; Murai and Kemp, 1982; Schroder et al., 1981). Because the opines serve as specific catabolic substrates for the inciting Agrobacterium strain (Petit et al., 1970; Tepfer and Tempe, 1981), the transformation of thehost plant cells can be rationalized as an example of genetic engineering by a prokaryotic organism.