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A Virtuous Infection

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[Photo of virus  entering carrot cell]

Scanning electron microscopy reveals several Agrobacterium tumefaciens as they begin to infect a carrot cell. In the process, the bacteria's genetic material will enter the plant cell. Source: A. G. Matthysse, K. V. Holmes, R. H. G. Gurlitz


Genetic engineering turned out to be relatively easy in animal or bacterial cells. Plants, however, initially appeared to be much harder. Fundamental research on a common soil bacterium and its interaction with the plants it infects unexpectedly showed the way. Now genes introduced into plants with the help of that bacterium are poised to bring huge economic and environmental benefits to U.S. agriculture.

The soil bacterium is called Agrobacterium. It infects nearly 10,000 species of plants, causing what is known as crown gall disease. The galls are tumorlike enlargements, and while the disease is harmful to some plants, it is not usually considered a major threat to crops and so is not worth a targeted research effort. Nonetheless, it attracted the curiosity of agricultural scientists in universities and government laboratories as a possible model for cancer; this research led to some fascinating discoveries.

What makes Agrobacterium unique is that when it infects a plant, it transfers a tiny bit of its genetic material, its DNA, into its host--the only bacterium known to do so. It is this foreign DNA that causes the galls or tumors. Further research uncovered the details and showed how to take advantage of this natural genetic engineering agent. The genetic material that causes the galls resides in a small circular ring, or plasmid, of bacterial DNA, a portion of which is incorporated into the plant cell's chromosomes. Scientists were able to snip out the tumor-inducing genes and replace them with genes of choice. Then, when the altered Agrobacterium infects a plant, the new genes are incorporated into its genetic makeup.

Such controlled, virtuous infections have become the method of choice for genetic engineering of many important commercial crops. Genes inserted with this method have led to spoilage-resistant tomatoes, insect-resistant cotton, and a host of experimental varieties of soybeans, rapeseed, poplar trees, and roses. Fresh tomatoes constitute a $4 billion market in the United States; a spoilage-resistant variety that can be ripened longer on the vine for greater flavor is one of the first genetically-engineered products in supermarkets. Cotton, the fifth largest U.S. crop, is also the largest user of pesticides, so that insect-resistant cotton will save farmers money and reduce environmental risks.

Although Agrobacterium is no longer the only genetic engineering tool in the plant scientist's arsenal, the fundamental research that uncovered its secrets helped launch the agricultural biotechnology industry. Biotechnology is expected to help produce safer, more nutritious foods and other agricultural products, create crop cultivars needed to cope with changing climates and pathogens, and make feasible alternative farming techniques that can conserve or reclaim fragile soils.

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