Agrobacterium tumefaciens and the plant: the David and Goliath of modern genetics.

Since the first reports on GM crops in the 1980s (Bevan, 1984), the amount of debate surrounding the generation of transgenic plants and the political and economical ramifications of their development have seldom been far from the public eye. It could be said that attention on the use of transgenically modified crop plants is substantially greater than on any other area of plant science. To date, the majority of commercially produced GM crops have been created by biolistically delivering transgenes into embryonic tissue (Padgette et al., 1995; James, 2002). However, as our understanding of the mechanisms of T-DNA integration grows and further information is reported on the proteins involved, the biolistic approach may well be superseded by a more sophisticated and “natural” solution to the problem. Agrobacterium tumefaciens is a soil-borne bacterium that, in nature, is capable of inserting a defined fragment of its DNA into the genome of dicotyledonous plants (for review, see Tzfira and Citovsky, 2002; Gelvin, 2003). It has been the focus of research for a wide spectrum of biologists, from bacteriologists to molecular biologists to botanists, for a number of years. Moreover, the unique process by which it delivers trans-kingdom DNA into host plant cells has become a staple in almost all undergraduate plant science courses. However, it is important to remember that A. tumefaciens remains at the cutting edge of plant science. Our understanding of the molecular mechanisms of DNA integration has grown immeasurably in recent years, and in a climate of intense political debate surrounding the future of transgenic crops, the future role of A. tumefaciens in the controlled creation of transgenic crops or simply as the model type IV secretion system and as a research tool would appear to hang in the balance. The aim of this review is to discuss recent advances and future prospects in the field of A. tumefaciensmediated plant transformation (for more detailed review, see Tzfira and Citovsky, 2002; Gelvin, 2003). To achieve this end, an overview is provided of the molecular mechanisms involved in T-DNA generation, transfer, and integration in the host genome, together with an outline of the most recent developments in the identification of plant proteins interacting with T-DNA. These data provided insights into the value of Agrobacterium both as a model system and of its potential future importance in the area of transgenic crop production. This leads, in the second part of the review, to discussion of the controversy surrounding the release of GM crop plants into the environment.