Rhizobium radiobacter is a common, rod-shaped bacterium that lives naturally in the soil. Although widely known for decades as Agrobacterium tumefaciens, modern classification has placed it within the Rhizobium genus. Its significance lies not only in its role as a plant pathogen but also in its unparalleled ability to transfer a segment of its genetic material into a host plant cell’s genome. This unique mechanism causes a serious plant disease, yet it has also become a powerful tool in modern plant biotechnology.
The Crown Gall Disease
The most recognizable symptom of infection by this bacterium is the formation of large, woody, and disorganized growths called galls. These tumors typically appear near the soil line on the crown of the plant or on the roots, which is why the condition is known as crown gall. Initially, the galls are smooth swellings, but as they grow, they develop a rough, warty, or cracked surface.
The disease affects a wide range of plants, including woody perennials and herbaceous species. Susceptible species include grapevines, roses, stone fruits such as peaches and almonds, and nut crops. Galls disrupt the plant’s vascular system, which is responsible for transporting water and nutrients. This interference leads to stunted growth, weakened plants, and a reduction in yield, causing financial losses in agricultural and nursery industries.
How the Bacterium Hijacks Plant DNA
Rhizobium radiobacter strains capable of causing crown gall possess a large piece of circular DNA called the Ti plasmid. The bacterium is attracted to a host plant when it is wounded, sensing chemical compounds released at the injury site. Once attached, the bacterium transfers a specific segment of the Ti plasmid, known as the T-DNA, into the plant cell.
The T-DNA moves into the plant cell nucleus, where it integrates permanently into the host plant’s chromosome. The T-DNA carries genes that are then expressed by the plant cell’s own machinery. These foreign bacterial genes code for enzymes that synthesize specific plant growth hormones, auxins and cytokinins.
The overproduction of these hormones causes the plant cells to divide and proliferate without control, forming the gall. The T-DNA also contains genes that force the plant cell to produce unique compounds called opines. These opines serve as a specialized food source that only the infecting Rhizobium bacterium can consume, creating a self-sustaining nutrient factory for the pathogen.
Managing and Preventing Infections
Controlling crown gall is challenging because the bacterium can survive in the soil for years and the galls are difficult to treat once established. Prevention relies on minimizing opportunities for the pathogen to enter the plant. Since the bacterium requires a wound to infect, growers must take precautions to avoid mechanical injury to susceptible plants during cultivation, pruning, and harvesting.
Sanitation practices are important in preventing the spread of the disease. Tools used for pruning or grafting should be disinfected between uses to prevent transferring the bacteria. New nursery stock should be inspected before planting, and any plants showing galls must be discarded immediately.
A biological control strategy involves introducing a non-pathogenic strain of R. radiobacter into the soil or onto the plant’s roots. This harmless strain competes with the disease-causing strains for attachment sites on the plant roots. By occupying the entry points, the non-pathogenic bacteria prevent the tumor-inducing strains from infecting the plant.
Using This Natural Process in Crop Engineering
The bacterium’s natural ability to insert its T-DNA into a plant genome is valuable in biotechnology. Scientists have repurposed this genetic transfer mechanism to create genetically modified crops. The process involves removing the tumor-causing genes from the Ti plasmid, creating a “disarmed” vector that is no longer pathogenic.
Researchers insert beneficial foreign genes, such as insect resistance or herbicide tolerance, into this modified T-DNA region. When the disarmed bacterium infects a target plant cell, it transfers the T-DNA containing the desired trait. The new gene integrates into the plant’s chromosome, and the plant cell can then be grown into a full crop.