Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
3/17/03
Contacts:
Barbara Ambruzs, Plant Pathology, (515) 294-1741, ambruzsb@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning March 21
It Takes Gall to Harness Bacteria
By Barbara Ambruzs
Extension Plant Pathologist
Iowa State University Extension
During the past few years there's been a heavy focus on biotechnology, molecular biology and genetic engineering. The tools of these fields can help us identify desirable DNA and move it from one organism to another.
This seems like a new and exciting process, yet this phenomenon frequently takes place in nature. Microorganisms, such as some bacteria, developed this skill a very long time ago. The bacterium Agrobacterium tumefaciens (A. tumefaciens) is an example.
Plants infected with A. tumefaciens can be recognized by galls (tumor-like growths) that can be as large as several inches across. They usually form on the crown (the part of the plant just above the soil line) of infected plants, hence the name, crown gall. However, they also may form on roots, stems, and branches.
A. tumefaciens can enter plants through wounds made by insects, machinery, pruning, growth cracks, freeze damage, or anything else that makes an opening in the plant. In fact, a wound or natural opening is necessary since bacteria can't get into plants on their own. Once inside certain plants, such as rose, euonymus, poplar, apple, cherry and just about every other non-grass species, A. tumefaciens begins to multiply.
When the bacterial cells become quite numerous, A. tumefaciens does a very sneaky thing. It transfers some of its DNA into the cells of the plant. The bacterial DNA tells the plant to make hormones that regulate plant growth. The hormones cause plant cells to abnormally enlarge and divide (reproduce) uncontrollably, resulting in the formation of galls. High numbers of bacterial cells are in the outer part of galls. As galls age the outer edges turn dark, dry out, and typically fall off into the soil. The bacterium survives winter either in the soil or in an infected plant.
Since bacterial infected plants are easy to work with, scientists thought they could study the plant tumors to learn about cancer in humans and other animals. Although this route did not lead to a solution to the cancer mystery, a lot of information was learned about A. tumefaciens. One of the most important discoveries was that galls continued to grow and develop even without the presence of the bacterium. This meant that the bacterium did something to change or transform plant DNA. Actually, part of the bacterial DNA stayed in the plant even after the disease organism was long gone. A. tumefaciens was genetically engineering or transforming plants in nature.
Some people understood what a powerful tool A. tumefaciens could become. Essentially, it became a "workhorse" in many labs, being used as a delivery service for getting selected DNA into plants without causing the crown gall disease. This system works well, but remember, grasses don't typically get the disease. Different methods of delivering DNA were eventually developed for plants that A. tumefaciens doesn't infect.
You may be wondering, "What does all this have to do with gardening?" The crown gall bacterium can infect many of our garden plants. However, crown gall isn't a big problem at the present time. Even if plants are diseased, most often the tops appear normal, but if there is a serious infection, galls may interfere with water uptake and nutrient flow. These plants might be stunted, fruit development could be impaired, or leaves may be off color mimicking fertility problems.
The best control for crown gall is avoidance. Inspect plants before buying them to be sure they are free of galls. The bacterium can live in the soil for a few years, even without plants. Always be careful not to injure your plants since A. tumefaciens needs an opening to get in. Pruning out galls is an option if they are not on the main stem or below the ground. Take care to clean tools with a solution of one part bleach to ten parts water. A biological control treatment is available commercially, as well. It's actually another species of Agrobacterium called A. radiobacter. It produces an antibiotic that works against A. tumefaciens.
If you see one of these galls, the bacterium may no longer be in the plant. A. tumefaciens might have already reproduced, delivered its message to plant cells and fallen back to the soil. It will rest waiting for a fresh wound on a plant so it can start its life cycle all over again.
-30-
ml: isugarden
|
Editors: A color photo, suitable for publication, is available at right. Click on the thumbnail photo to go to the fullsized photo. The fullsize photo is 188K. Caption: Crown gall |
 |
Extension programs are available to all without regard to race, color, national origin, religion, sex, age, or disability.