What Is Agrobacterium rhizogenes and How Is It Used?

Agrobacterium rhizogenes is a soil-dwelling, Gram-negative bacterium known for its ability to transfer genetic material to plants. This process causes hairy root disease, the prolific growth of roots from the infection site. This capacity for genetic transformation makes the bacterium a valuable tool in plant science and biotechnology for modifying plant traits and producing useful compounds.

The Infection Mechanism of Agrobacterium rhizogenes

The infection process begins when Agrobacterium rhizogenes detects phenolic compounds, such as acetosyringone, which are released by plants at a wound site. These chemical signals attract the bacterium and activate a series of genes necessary for infection. Central to this mechanism is a large plasmid called the Ri (root-inducing) plasmid. This plasmid contains a specific segment of DNA, known as T-DNA (transfer DNA), which is excised and prepared for transfer into the plant cell.

Once inside, the T-DNA is transported to the nucleus and integrates into the host plant’s genome, resulting in a permanent genetic modification. The T-DNA carries the rol (root loci) genes, including rolA, rolB, and rolC, which disrupt the plant’s normal hormonal balance. For example, the rolB gene significantly increases the sensitivity of plant cells to auxin, a plant hormone that promotes root formation. The combined expression of these genes stimulates the rapid and uncontrolled proliferation of root cells.

Characteristics and Establishment of Hairy Root Cultures

Roots induced by Agrobacterium rhizogenes infection are visually and functionally distinct from normal plant roots. They exhibit a highly branched growth pattern, forming a dense mat of fine roots that are not responsive to gravity (agravitropic). This morphology maximizes the surface area for nutrient absorption and allows the bacterium to manipulate the plant into producing specialized nutrients called opines, which only the bacterium can metabolize.

Another feature is their rapid growth rate, which surpasses that of untransformed roots. They can also be cultivated in vitro on a nutrient medium without the addition of external plant hormones. This hormonal autotrophy is due to the production of auxins and cytokinins directed by the integrated bacterial genes.

To establish a hairy root culture, the induced roots are excised from the host plant and placed in a sterile liquid or solid medium. These cultures can be maintained indefinitely under controlled laboratory conditions at a temperature of 25-28°C and a neutral pH.

Biotechnological Exploitation of Hairy Roots

The properties of hairy root cultures make them useful in biotechnology. Primary applications include:

• Production of secondary metabolites. Hairy root cultures act as biological factories for plant-derived chemicals used in pharmaceuticals, flavorings, and pigments, offering a controlled system for large-scale production.
• Phytoremediation. The extensive and fast-growing root systems can absorb and metabolize pollutants, helping to clean up contaminated soil and water by sequestering heavy metals and degrading organic contaminants.
• Functional genomics. Scientists can introduce a gene of interest into the T-DNA to quickly generate transgenic roots, allowing them to study the gene’s effect on root development, nutrient uptake, or microbial interactions.
• Creation of transgenic plants. Plants regenerated from hairy roots can inherit the modified root systems, potentially leading to improved traits like drought tolerance or more efficient nutrient acquisition.

Agrobacterium rhizogenes Versus Agrobacterium tumefaciens

Agrobacterium rhizogenes is often compared to its close relative, Agrobacterium tumefaciens, as both can genetically engineer plants. The main distinction is the plasmid they carry and the resulting disease. A. rhizogenes has the Ri (root-inducing) plasmid, causing hairy root disease, while A. tumefaciens has the Ti (tumor-inducing) plasmid, which causes crown gall disease, an undifferentiated mass of tissue.

The genetic basis for these outcomes is on their respective T-DNAs. The Ri plasmid’s T-DNA carries rol genes that alter hormone sensitivity to promote organized root structures. The Ti plasmid’s T-DNA contains genes that directly code for auxin and cytokinin synthesis, and their overproduction leads to disorganized tumors.

These differences dictate their biotechnological uses. A. rhizogenes is used for applications involving differentiated root cultures, while A. tumefaciens is more broadly used for creating transgenic plants without causing disease.