What is Agrobacterium?

Agrobacterium is a naturally occurring soil bacterium with the unique ability to transfer part of its DNA into plant cells. It is a phytopathogen (organism causing disease to plants) that causes the crown gall disease.

The crown gall disease causes abnormal growths (or galls) on roots, twigs, and branches. Usually, Agrobacterium does not seriously harm plants unless galls occur in the root crown of young plants, where they may become stunted and subject to wind breakage and drought stress.

A tumor-inducing (Ti) plasmid harbored by Agrobacterium is responsible for inducing the galls on roots and crowns of the plants.

Diagram of the crown gall disease and the armed Ti plasmid of agrobacterium

What is a plasmid?

A plasmid is a small, extrachromosomal DNA molecule within the bacterium that is physically separated from chromosomal DNA and can replicate independently.

labeled agrobacterium bacterial cell with Ti plasmid

Components of the wild Agrobacterium.

The plasmid responsible for plant infection is composed of a transfer-DNA (T-DNA) region, which carries the oncogenes (e.g., auxin, cytokinin and opine genes) to induce the tumor-like cell growth on the plants.

The opines are amino acid derivatives used by the bacterium as a source of carbon.

Close to this T-DNA, there are the virulence genes (vir genes) required for T-DNA transfer.

labeled Ti plasmid with T-DNA region, nopaline / octapine region, and virulence gene region

Components of Agrobacterium Ti-plasmid.

Agrobacterium used as a vector in plant biotechnology

After the discovery of Agrobacterium, researchers explored and understood the mechanism of how wild and pathogenic Agrobacterium strains transferred DNA into the plants.

The Agrobacterium tumor-inducing plasmid, or Ti-plasmid, does the DNA transfer. Later, to use Agrobacterium as a "tool" for plant genetic engineering, scientists modified the T-DNA region by removing the oncogenes to avoid the tumorigenicity to avoid getting sick plants while introducing a gene of interest. So, researchers created a disarmed (nonpathogenic) Agrobacterium strains and used it as a vector.

In cloning, a vector is a DNA molecule used as a vehicle to artificially carry foreign genetic material into another cell, where it can be replicated and expressed. Thus, a vector allows you to create transgenic organisms (not only plants) with improved traits (e.g., disease resistance), or understand a gene or protein function.

Today, many agriculturally and horticulturally important species are routinely transformed using this bacterium, and the list of Agrobacterium-mediated transformation susceptible species grows at an accelerated rate.


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Written by: Adriana Gallego, PhD.