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Sunshine fuels a bacterial relationship.

Sunshine fuels a bacterial relationship

Scientists have identified a bacterium that lives with and "fixes" nitrogen for certain plants, but uses the sun's energy instead of the plant's to survive. The discovery, which highlights nature's unexplored biological diversity, could someday be used to increase the yield of certain crops, says Ralph W.F. Hardy, president of the Ithaca, N.Y.-based Boyce Thompson Institute for Plant Research, where the work was done.

Although other photosynthetic, nitrogen-fixing bacteria exist, Hardy says these are the first such bacteria known to form symbiotic relationships with leguminous plants, a family including soybeans, alfalfa and peanuts. He is continuing the work of plant physiologist Allan R.J. Eaglesham, who first isolated the novel bacterium and has since moved to Enichem Americas, a research facility in Monmouth Junction, N.J. The bacterium, tentatively named Photorhizobium thompsonum, belongs to the genus Rhizobium, whose members form nofules or swellings on the roots of leguminous plants and supply the plants with usable nitrogen.

Root-living rhizobia draw carbohydrates and other energy-supplying materials from the plant, decreasing the amount of seed or vegetation the plant produces. They use about 12 pounds of plant material for every pound of nitrogen they fix, Hardy says. Although rhizobia normally appear on roots, under certain conditions they live on stems instead. For instance, stem nodules may appear on plants in flooded areas, where water cuts off oxygen and nitrogen needed for root nodules.

If made to live on stems, Photorhizobium thompsonum uses sunlight instead of plant resources for energy. "The plant material saved could be redirected in producing seed or forage," Hardy suggests. Photosynthetic rhizobia provide "exciting [agricultural] potential that seems realistic based on what we know about these [leguminous' plants, but the reality remains to be proven."

The researchers discovered the bacteria serendipitously after transplanting some plants dying from nitrogen deficiency into some sand they took from Virginia. After flooding the dying plants' roots to make nofules appear on the stems, they found the nodules were produced by a bacterium not known to be living in the sand.

Plant-living bacteria are a more cost-effective and environmentally sound way to provide plants with nitrogen than are synthetic nitrogen fertilizers, Hardy says. Fertilizers require huge capital and labor costs and are inefficiently used by the plant, which takes up only about half the nitrogen. Unused fertilizer runs off or seeps into the soil, polluting the environment. In contrast, rhizobia deliver nitrogen directly to the plant, Hardy says.

In order to use these bacteria to increase the yield of leguminous crops, scientists will need a better understanding of stem nodulation -- specifically, how to make stem nodules in plants that do not have them. "We now do this experimentally by flooding the roots, but undoubtedly there are other ways," Hardy says.

In addition to exploring how the bacteria form stem nodules, Hardy is trying to discover what gives this rhizobium its special characteristics, whether other photosynthetic rhizobia exist and when such organisms appeared in evolution.
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Author:Wickelgren, Ingrid
Publication:Science News
Date:Jan 21, 1989
Words:497
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