Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Crown Gall

Friday, December 4, 2009

In the many years I have been writing these nature stories, I have come to one firm belief about nature. There is no such thing as a boring organism. Animal, plant, fungus, bacteria, virus – every living creature has a fascinating story. All it takes is a researcher to discover its secrets and a writer to make them known to the public.
So I decided to put this belief to the test. I would be on the lookout for a totally boring creature. One that has no flashy colors, no moving parts, no obvious life or death struggles. One that most everyone would pass by in the woods without giving it a second glance. The one I came up with was the crown gall, found on many plants in the rose family – roses, fruit trees, raspberries and grape vines.
The crown of a plant is the area where the roots join the stem right at the soil level. The crown gall is a tumor-like growth of the plant tissue in this area. Unlike some interesting galls caused by nematodes (a type of worm) or insects, cutting open a crown gall shows no organization of tissue. The plant just seems to grow crazy at this spot for no apparent reason. Inspecting a crown gall by hand reveals nothing about what might have caused this growth – seemingly nothing to interest a nature writer.

The reason that inspecting the crown gall by hand reveals nothing about this growth is because the organism that causes it is much too small to see without a microscope. And if it is often true that good things come in small packages, it is even more true that amazing creatures come in microscopic packages. In this case, the microscopic package is a bacterium Agrobacterium tumefasciens.

Consider the bacterium A tumifasciens. Like all bacteria, its whole body consists on only one cell. Humans, by contrast, consist of many trillions of cells. We have bone cells to support our body parts. Bacteria have no bones. We humans have muscle cells to move our bones. Bacteria have no bones to move, so no muscles. We have stomachs and intestines to digest our food. There are no stomachs or intestines in bacteria. We have eyes to see, noses to smell, nerve cells to touch. Bacteria don’t need sense organs to sample their environment. We have brains to coordinate our thoughts and actions. No brains and no thinking in bacteria. In fact with only one lone cell making up their entire body, you might be wondering what could a bacteria possibly be capable of accomplishing. It is this very idea that makes A tumifasciens anything but a boring creature. Consider what it accomplishes with only one cell to work with – and marvel at it.

A tumifasciens is a soil bacterium, capable of living and multiplying in the soil around the root system of its host plants. If the plant sustains an injury in the crown area, either through natural causes or grafting of cultivated plants, this wounded area releases chemicals that inadvertently (from the plant’s point of view) attract the bacteria. They move toward the injury, make contact with the plant and bind to the wound site. However, the bacterium does not enter the plant through the wound. Rather, it inserts a piece of its DNA into the plant cells.
DNA is a chemical that instructs cells what to do, what chemicals they must make. This piece of bacterial DNA in the plant cell acts like a secret agent. It tells the plant cells what chemicals it must make. The plant, not realizing what it is doing (but of course plants can’t realize anything anyway!) produces chemicals that are beneficial to the bacterium, not the plant!
As well as causing uncontrolled growth of the plant cells – resulting in the gall - the bacterial DNA also causes the plant to secrete certain chemicals that the bacterium feed on. So, in effect, a single celled A tumifasciens bacterium coerces a trillion-celled “higher organism” – the plant – to feed it. I would call that quite an accomplishment!

If I have succeeded in convincing you that amazing creatures come in microscopic packages, I want to take up a few more lines of your time to show you that truly amazing creatures come in much larger packages.
Human researchers have learned from A tumifasciens. They reason that if A tumifasciens can trick the plant into working for the bacterium, maybe the researchers can trick the bacterium into working for them! So they have inserted a piece of DNA into A tumifasciens. A tumifasciens then inserts the DNA into the plant. Now the plant contains DNA that the researchers are interested in. Now the plant will work for the researchers instead of the bacterium.
Using this technique, researchers have “genetically engineered” certain crop plants to make their own insecticide. These plants no longer need to be sprayed with dangerous insecticides to protect them from insect damage. Other plants are engineered to be resistant to herbicides – chemicals used to kill weeds. A farmer can weed his or her fields with an herbicide and kill all the plants except the crop plant.

In the end, it doesn’t matter how big or small an organism is – one cell or trillions of cells. All organisms have an interesting story to tell. It’s just a matter of being able to read the book.