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Tuesday, 25 December, 2001, 12:45 GMT
'Natural genetic engineer' laid bare
Corn, Cereon
A corn (maize) plant that has been modified using Agrobacterium
It has to be one of the cleverest lifeforms on the planet - and scientists have just been given the best handle yet on its inner workings.

Agrobacterium tumefaciens is a microbe that infects plants; it gives them tumours that start producing the chemicals on which the microbe likes to feed.

The bacterium can gorge itself on this nutrient supply because it has evolved the unique ability to transfer DNA into plants and change their behaviour - a skill that has led some to dub it the "natural genetic engineer".

It's a tricky little bug

Dr Steve Slater, Cereon Genomics
Researchers have hijacked A. tumefaciens to make plants do more useful things than just turn themselves into microbial dinner tables. The bacterium has helped plant scientists create new crops that grow larger, are resistant to disease and even produce drugs in their leaves.

And such modifications should become easier and more sophisticated now that the complete genetic code of A. tumefaciens itself has been cracked.

Unusual genome

"It's a tricky little bug," said Dr Steve Slater, from Cereon Genomics, who led the decoding project. "It causes the plant to make a safe home for it and then engineers the plant to feed it," he told BBC News Online.

The DNA sequence of the bacterium, known as strain 58, has been deposited in a public database for all to see and use, and in a December issue of the journal Science, researchers detailed what they knew about the organism's life code.

Greenhouse, Monsanto
Plant scientists hope their work can be made more efficient
A. tumefaciens has about 5,500 genes distributed on two circular structures known as plasmids and on two larger chromosomes - one of these is also circular; the other is linear in shape. This arrangement is quite unusual in bacteria.

Scientists hope the new information will give them a better insight into the bacterium's remarkable infectious process and also tell them something about the evolution of disease in plants.

"Knowledge gleaned from the genome sequence of A. tumefaciens could be key to understanding the evolution of plant-microbe relationships," said University of Washington (UW) microbiologist Dr Derek Wood.

Wounded plants

"This Agrobacterium and its cousin, Sinorhizobium meliloti, will let us study the separation of closely related organisms into divergent pathogenic and symbiotic lifestyles."

Dr Slater said it was extraordinary that a microbe should carry around genetic material that could be used to control the cells of a higher organism.

This is the only case in nature where DNA and proteins from bacteria are transferred into plants or animals

Eugene Nester, University of Washington
"The thing I think is really astounding is that the genes are expressed in the plant cell; the bacterium is carrying around genes that have transcriptional control elements that are eukaryotic-type - are plant-type. It's really amazing."

Agrobacterium has a long history in the laboratory. It was way back in 1907 that scientists first showed that the bacterium was responsible for the plant disease called crown gall, which takes the form of a clump of undifferentiated cells that grow at the site of a wound.

When, in the 1940s and 1950s, researchers could see these galls, or tumours, growing on plants even after all traces of the bacterium had been removed, they knew it must be because Agrobacterium had in some way transformed the plants' cells.

But it was another 20 to 30 years before they could show Agrobacterium was actually inserting segments of DNA - called tDNA - into the plants' genomes to alter the behaviour of those cells.

Important crops

Scientists could see that if they replaced the tDNA with code that led not to tumours but more useful traits, they would have an efficient tool to create a new generation of biotech crops.

And although they have been doing just this for some time now, it is only in the last few years that scientists have managed to make the process work efficiently in the major food crops - wheat, maize and rice, etc.

Magazine, Science
Corn (maize) ranks with wheat and rice as one of the world's chief food crops
"This is the only case in nature where DNA and proteins from bacteria are transferred into plants or animals," said Eugene Nester, a UW professor of microbiology and long-time leader in A. tumefaciens research.

"At one time, many scientists said it couldn't be done. But in fact, normal tobacco plants contain the A. tumefaciens DNA."

And Dr Slater added: "What the full sequence gives us is the ability to look for additional genes that may be involved in the tDNA transfer and identify ways that we might be able to enhance the process and make it more efficient."

See also:

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The biological goldrush
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