Aluminum is the most abundant metal on earth and has a ubiquitous presence in the human environment in the form of products such as beverage cans, kitchen utensils and aircraft. However, aluminum (Al 3+) is very unfriendly to agriculture as it injures plant root cells and thus interferes with root growth and nutrient uptake in crops. Aluminum's harmful effects are worst under acidic conditions where it becomes soluble; in non- acid soils it is insoluble and thus less deleterious. More than one-third of the arable land in the world suffers from soil acidity and aluminum toxicity; low agricultural productivity in acid soils is directly attributable to the effects of aluminum.

The problem is most severe in the humid tropics. In Colombia, for instance, 70% of the agricultural land is acidic. Crops such as corn, field bean, soybean and cotton thus do not grow well in the tropics because of their high sensitivity to soil acidity. Corn grown under acid soils can suffer yield losses up to 80%. Agricultural lime is applied by farmers around the world to combat the problem, but liming is a recurring financial burden on resource-poor farmers and also contributes to run-off pollution.

A recent report from a research team in Mexico led by Luis Herrera-Estrella may provide a breakthrough to the aluminum problem in agriculture (1). By introducing a bacterial citrate synthase (CSb) gene into tobacco and papaya, the Mexican scientists have genetically engineered plants that are more tolerant to the insidious metal.

The strategy capitalized on the fact that some plants tolerate aluminum by releasing citric acid which binds to the metal making it difficult to enter plant roots. Transgenic plants expressing the CSb gene from Pseudomonas aeruginosa produced up to ten-fold more citrate in their roots and released four-fold more of the compound than control plants. When grown under extremely high aluminum and acidic conditions, transgenic CSb plants showed substantially lower root growth inhibition compared to the untransformed plants. Normal seeds failed to develop roots when germinated in the presence of high aluminum while transgenic CSb seeds showed a clear tolerance. Transgenic roots contained less aluminum in their tissues, possibly because the citrate synthase produced by these plants was preventing uptake.

Herrera-Estrella, who was among the first to develop transgenic plants in the early eighties, has already introduced the citrate synthase gene into two more important crops, rice and corn (2). If these two crops, along with tobacco and papaya, prove to be tolerant to aluminum with no reduction in their yield or growth under field conditions, the research will likely have a major impact on agriculture in the tropics. Soils that were once inhospitable may now be brought under cultivation (2). The technology certainly appears to have the potential to elevate agricultural productivity in developing countries where the devastating effects of aluminum are at their worst, and where the need to produce more food is most urgent. This new report provides another illustration of how basic biotechnology research is being used ingeniously to address practical problems of the real world.

References
1. J.M. de la Fuente, et al. 1997. Aluminum tolerance in transgenic plants by alteration of citrate synthesis. Science 276:1566-1568.

2. M. Barinaga, 1997. Making plants aluminum tolerant. Science 276:1497.

C. S. Prakash
Center for Plant Biotechnology Research
Tuskegee University
prakash@acd.tusk.edu