Scientists have engineered tobacco and papaya plants to resist one of the world's least known enemies of agriculture: aluminum. The findings, reported in today's issue of Science,* could lead to a technique for boosting crop yields in soils with high aluminum concentrations, a problem that afflicts 30% to 40% of the world's arable lands, especially those in developing countries.
The most common metal in soils, aluminum is normally tied up in insoluble compounds, but is released into ground water when soils become acidic. This can happen, for example, when heavy tropical rains leach alkaline materials from the soils. The released aluminum can then slip into the cells of plant roots, where it poisons cell metabolism and prevents healthy root growth. Indeed, for some important crops like corn, it is second only to drought as an impediment to crop yields, reducing production by up to 80%.
In an effort to create aluminum-tolerant crops that could boost these yields without extra cost, molecular biologist Luis Herrera-Estrella and his team at the Center for Research and Advanced Studies of the National Polytechnic Institute in Irapuato, Mexico, and his colleagues built on knowledge that some naturally aluminum-resistant plant strains have roots that secrete citric or malic acid, organic acids which bind to aluminum and prevent it from entering the roots.
They introduced a bacterial gene for citrate synthase, the enzyme that makes citric acid, into two plant species: tobacco and papaya, an important crop in tropical Mexico that is highly sensitive to aluminum. The plants carrying the citrate synthase gene secreted five to six times more citrate from their roots than control plants did. And that extra citrate translated into aluminum tolerance: The citrate-producing plants could grow well in aluminum concentrations 10-fold higher than those tolerated by control plants.
That degree of tolerance could well allow some crop plants to be planted where they couldn't grow before. Mexico's $97 million annual papaya crop, for example, comes from 20,000 hectares of land in the tropics, says José Garzon of the National Institute of Forestry, Livestock, and Agricultural Research in Celaya, Mexico. That crop could be expanded, he says, if the new aluminum-tolerant plants could be grown on some of the 3 million hectares of tropical Mexican land where aluminum toxicity has prevented papaya cultivation.