Agrobactium tumefacienser has recently gained much notoriety among rosarians for the simple reason it causes the benign but the ugly disease called “crown gall” (click on photo for a close-up). SCIENCE |
Agrobacterium tumefaciens--a villain or a savior? Crown Gall on Roses
by Dr. Lakshmi Sridharan, Science Editor
Agrobactium tumefacienser has recently gained much notoriety among rosarians for the simple reason it causes the benign but the ugly disease called “crown gall” (click on photo for a close-up).
To quote one rosarian -- “ It is screaming for attention!” A few months ago, Sun Flare, which Clair Martin had planted en masse in large beds at the entry to the famous Tea Room in the Huntington Botanical garden, became a mass victim of Agrobacterium tumefaciens. They are now gone, replaced by beds of healthy, new Sun Flare.
INFECTION:
Agrobacterium tumefaciens is present in the soil all over the garden. It infects nearly 10,000 species of plants, causing what is known as crown gall disease. The galls are tumor-like enlargements. While the disease is harmful to some plants, it is not usually a major threat to roses; unlike the most dreaded attackers: black spot, downy mildew, powdery mildew, etc. The crown gall bacteria gain entry most particularly through wounds. The villain is not exactly the bacterium, but the plasmid it carries within it. A plasmid is an extra-chromosomal element that replicates independently of the bacterial chromosome. (You may be interested in knowing that antibiotic resistance in human bacterial pathogen is also due to the presence of plasmids within these bacteria.) The bacterium simply provides ‘shuttle service’ to the plasmid. Agrobacterium carries a plasmid called ‘Ti’ plasmid, which carries the tumor-causing gene. When the bacterium establishes itself in the plant tissue, it transfers the Ti plasmid DNA into its host -- the only bacterium known to do so. It is this foreign DNA that causes the galls or tumors. The tumor-causing gene induces uncontrolled cell division which leads to the production of the unsightly tumors; the crown gall. Rough, abnormal galls develop on roots or trunks. Galls are not hard, but soft and spongy. The centers of older galls decay. The infected plants become stunted, and older plants often develop secondary wood rots. The bacteria survive in the gall tissue and in the soil. They enter only through wounds.
CONTROL:
Handle plants with care to avoid injury as much as possible, both during planting and then during the entire life of the plant. Galltrol and Norbac are preparations of the biological control agent Agrobacterium radiobacter-84, effective only as a preventive treatment. Dip the roots prior to planting. It does not eradicate existing galls. When dealing with a microbial pathogen, remember that they can develop new strains resistant to any controlling agent, chemical or biological. Note that strains of A. tumefaciens have been reported as resistant to Galltrol and Norbac. Another option, however, is that you may surgically remove existing galls. Apply Gallex immediately after the removal of galls. Carefully follow label instructions for exposing crown and roots and removing large galls.
Fellow rosarians! Do not simply send A. tumefaciens to the gallows. Exploit this natural genetic engineering agent to your advantage. Scientists were able to snip out the tumor-inducing genes and replace them with genes of their choice. When introduced into a plant tissue culture, new genes are incorporated into plant’s genetic makeup.
Disease-resistant genes isolated from a huge number of plants show a high degree of DNA sequence homology. A number of these genes have been cloned, hence are readily available for developing disease-resistant roses. One can use them as probes for identifying disease-resistant genes in Rugosa roses, isolating the resistant genes from Rugosa roses, and using Agrobacterium as a vector for introducing the resistant genes into other roses.
Unless rosegrowers demand disease-resistant roses and refuse to buy susceptible roses, the world rose industry will not invest money into this kind of research as they should.
SCIENCE |