Agrobacterium tumefaciens

Agrobacterium tumefaciens is a rod-shaped, Gram-negative bacterium found in soil.

It is the primary cause of crown gall disease, which creates tumors on plants. This bacterium affects over 140 species of eudicots. These include important agricultural crops like walnuts, grape vines, and stone fruits. It also impacts sugar beets, horseradish, and rhubarb. Because these tumors are persistent, the bacterium is a major economic concern for perennial crops.

The bacterium operates through a complex mechanism involving a Ti plasmid. This is a tumor-inducing plasmid that is 200 kilobase pairs long. The plasmid contains T-DNA, or transfer DNA, and the genes required to move it. To begin an infection, the bacteria use flagella to swim through the soil. They move toward chemical signals called photoassimilates in the rhizosphere. Some strains use chemotaxis to find plant wounds. They look for specific chemical exudates like sugars and acetosyringone.

Once the bacteria reach a wound, they must attach to the plant cell. This is a two-step process involving cellulose fibrils. These fibrils anchor the bacteria to the wounded cell and to each other. This helps the bacteria form a microcolony. A calcium-dependent protein called rhicadhesin also helps the bacteria stick to the cell wall. The bacteria use specific proteins to recognize the plant signals. The VirA protein detects phenolic compounds, while the ChvE protein recognizes sugars. These proteins then trigger the activation of at least 25 vir genes on the Ti plasmid.

To transfer the DNA, the bacterium creates a structure called a T-pilus. This is part of a type IV secretion mechanism. When the bacteria detect acetosyringone, 11 genes in the VirB operon are activated. These genes produce the subunits for the T-pilus. The T-DNA must first be cut out of the circular plasmid. A complex made of VirD1 and VirD2 nicks the DNA at the border sequences. The VirD2 protein attaches to the 5' end of the DNA. This helps target the DNA to the secretion system. Because the T-pilus channel is narrow, the VirD2 protein must partially unfold to pass through.

After the T-DNA enters the plant cell, it must reach the nucleus. In the plant cytoplasm, the T-DNA becomes coated with VirE2 proteins. These proteins are exported through the secretion system independently. The T-DNA uses nuclear localization signals to move through the nuclear pore complex. Proteins called importin alpha and importin beta help facilitate this transfer. Once inside the nucleus, the T-DNA integrates into the plant genome at a semi-random location. A protein called VIP2 may help target the T-DNA to areas of active chromatin.

The integrated T-DNA changes the plant's biology in two main ways. First, it forces the plant to produce hormones called auxin and cytokinin. The T-DNA uses the IAM pathway to make auxin. Since many plants cannot regulate this specific pathway, the plant produces auxin constantly. This, along with cytokinin, causes rapid cell division and the formation of galls. Second, the T-DNA forces the plant to create opines. Opines are specialized amino acid derivatives. While most organisms cannot use them, Agrobacterium uses opines as a source of nitrogen.

Historically, the classification of these bacteria has been quite confusing. Before 1980, scientists grouped them by the symptoms they caused. They used names like A. radiobacter for avirulent species and A. rhizogenes for hairy root disease. However, researchers later discovered that symptoms depend on the Ti plasmid, not the species itself. By 2000, scientists used the "biovar" concept based on metabolic traits. In 2001, researchers successfully sequenced the genome of the C58 strain. This strain was originally isolated from a cherry tree. The C58 genome is unique because it contains both a circular and a linear chromosome.

Today, Agrobacterium is a vital tool in biotechnology. Scientists use a process called Agrobacterium-mediated transformation. This allows them to deliver specific DNA sequences into plant cells. By using binary vectors, researchers can engineer plant genomes with precision. This technology is used to study plant growth and create new crop varieties. While it began as a study of a plant pathogen, it has become a cornerstone of modern genetic engineering.