March 5, 2010

Standford study in legumes could reduce fertiliser use

Excess nitrogen from fertiliser runoff into rivers causes algae blooms that create oxygen-depleted dead zones but findings by Standford researchers indicate that the mechanisms of legumes may blunt those negative effects.

"We have discovered a new biological process, by which leguminous plants control behaviour of symbiotic bacteria," said molecular biologist Sharon Long. "These plants have a specialised protein processing system that generates specific protein signals. These were hitherto unknown, but it turns out they are critical to cause nitrogen fixation."

The ability of legumes to capture nitrogen from the air and turn it into plant food, or "fix" it, also leaves the soil enriched through the plant matter left after harvesting, creating a natural fertiliserfor other crops, which is the basis for crop rotation. Alternating legumes such as soy with other crops has been a major component of agriculture around the world for thousands of years. Yet until recently, little was known about how nitrogen fixation worked, or why some legumes are efficient at fixing nitrogen and others poor.

The key part of the process that Long''s research group uncovered is a plant gene that triggers a critical chemical signal. Without the signal, no nitrogen gets fixed by the bacteria.

The beneficial bacteria in question reside inside the nodules of legumes such as peas, beans, alfalfa and clover, where they pluck molecules of nitrogen from air in the soil and turn it into ammonia, which feeds the plant. It sounds simple, but it is a complicated and poorly understood process. Only bacteria that contain a special enzyme are capable of this sort of "nitrogen fixing" using airborne nitrogen -- no other type of living organism can do it. All other plants have to get their nutrients from using already fixed nitrogen in the soil.

The legume that Long''s team worked with is called barrel medic, a forage plant similar to alfalfa. They tracked down the newly discovered gene by studying mutant plants that were failing to produce healthy nodules on their roots.

While bacteria inside normal nodules will thrive, in the defective nodules of this plant those bacteria cannot provide the benefit they are wired to deliver. Long said that the mutant "contained perfectly good bacteria, but was making these lousy nodules."

By comparing the genome of the mutant plants with normal plants, the group found a gene that was missing from the mutants. Suspecting that gene might be the culprit, the researchers took a functional version of the gene from normal plants and put it into the mutants. The mutant legumes then began fixing nitrogen the same as normal ones.

Since 1960, the use of nitrogen fertiliser in the United States has roughly quadrupled, as has the price per tonne, according to the USDA. Prices have been driven up by the rising cost of natural gas used to manufacture the fertiliser.

"That might make things more expensive for American farmers and increase food prices for consumers, but this is going to wipe out people in developing countries, whose soils are perhaps most in need of fertilisers," Long said. "This is a crucial issue. And nitrogen fixation is a key to sustainability."

Costs aside, the production of chemical fertiliser also adds to the problem of global warming, both by way of the fossil fuels used in production of chemical fertiliser and through the impact of leftover fertiliser that degrades into nitrous oxide, a highly potent greenhouse gas.

With the planet''s ever-growing population, Long said there is going to be increased need to keep productivity going on lands that are starting to become marginal because of drought, temperature or salinity problems, among others.

"The rhizobium bacteria are a critical partner in whether that kind of extension of serviceable land can occur," she said. "In order for us to take existing symbioses and help make them better, optimise them for being productive even when conditions start to deteriorate, tools such as understanding how to improve nitrogen fixing in legumes are crucial."