Agrobacterium-plant cell DNA transport: have virulence proteins, will travel.

Agrobacterium fumefaciens (Agrobacterium) is a soil phytopathogen that genetically transforms host cells, causing crown gall tumors, an agronomically important disease that affects most dicotyledonous plants. In nature, these tumors are formed at the soil-air junction, the so-called crown of the plant. The Agrobacterium-plant cell interaction is the only known natural example of DNA transport between kingdoms. In this process, DNA is transported from wild-type Agrobacterium into the plant cell nucleus. Expression of this transferred DNA (T-DNA) results in neoplastic growths (tumors) on the host plant. The wild-type T-DNA carries genes involved in the synthesis of plant growth hormones and the production of opines, tumor-specific compounds formed by the condensation of an amino acid with a keto acid or a sugar. It is the production of growth hormones in the transformed host cells that induces the formation of tumors. These tumors then synthesize opines, a major carbon and nitrogen source for Agrobacterium. Agrobacteria are usually classified based on the type of opines specified by the bacterial T-DNA, the most common strains being octopine or nopaline specific (Hooykaas and Beijersbergen, 1994). Opine import into and the subsequent catabolism within the bacterial cell require specialized enzymes. Because these enzymes are encoded by the Agrobacterium tumor-inducing (Ti) plasmid, practically no other soil microorganisms can metabolize opines, creating a favorable biological niche for Agrobacterium. Three genetic components of Agrobacterium are required for plant cell transformation (Figure 1). The first component is the T-DNA, which is actually transported from the bacterium to the plant cell. The T-DNA is a discrete segment of DNA located on the 200-kb Ti plasmid of Agrobacterium; it is delineated by two 25-bp imperfect direct repeats known as the T-DNA borders. The second component is the 35-kb virulence (vir) region, also located on the Ti plasmid, which is composed of Seven major loci (virA, virB, virC, virD, vir€, viG, and vir/+). The protein products of these genes, termed virulence (Vir) proteins, respond to the specific compounds secreted by the wounded plant (see below) to generate a copy of the T-DNA