Revolutionary Bacteria

One of the great triumphs of modern agriculture is the Green Revolution: the development of lush new strains of wheat, rice and other cereals that have made the difference between starvation and survival for millions of people. Yet a drawback to the new high-yield plants is that they require large quantities of expensive nitrogen-rich fertilizers that drain into ponds and lakes. There, the fertilizers cause explosive growth of algae and make the water unfit for drinking and other uses.

Now, scientists at Britain's University of Sussex have developed a genetic engineering technique that may eventually reduce the dependence on artificial fertilizers. In search of an alternative, Molecular Biologists John Postgate and R.A. Dixon turned to nitrogen-fixing bacteria found in the soil and on the roots of plants. Capable of taking nitrogen from the atmosphere, these bacteria make it available to the plants as nitrates and other nitrogen compounds. Nitrogen-fixing bacteria, however, satisfy only part of the nitrogen needs of the new plants; the rest must come from natural or artificial sources. Was there any way of increasing the nitrogen supplied by bacteria and thus lessening the amounts required from the earth?

Conjugation. An answer, Postgate and Dixon reasoned, might lie in the multitudes of ordinary soil bacteria. If these organisms could be supplied with genes that conferred the nitrogen-fixing ability, the nitrogen provided to plants could be greatly increased. That might be done by crossbreeding soil bacteria with the nitrogen-fixing variety.

It would be no easy task. Two different species of bacteria would first have to be induced to conjugate. This is a primitive form of sexual reproduction in which two bacterial cells occasionally meet and exchange genetic material prior to their more common asexual form of reproduction, called fission (in which a single cell elongates, splits and produces two genetically identical offspring).

In Nature, Postgate and Dixon describe their strategy to encourage conjugation. First they "fertilized" nitrogen-fixing bacteria with a DNA "sex factor" from the common intestinal bacteria Escherichia coll, a non-nitrogen-fixing species. Then they mixed the nitrogen-fixers (now compatible with E. coli) with a strain of E. coli that has a particularly useful characteristic: unlike most bacteria, it can incorporate genetic material from another species. Out of the laboratory-induced union came a small but significant number of hybrid offspring with nitrogen-fixing ability. What is more, some of these crossbreeds could pass on the crucial nitrogen-fixing genes to future generations through ordinary fission.

The British experiment could have far-reaching implications. If a similar feat of genetic engineering can now be used on ordinary soil bacteria, high-yield grains will gain an important new source of nitrogen, thereby greatly diminishing the need for fertilizers.

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