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Aluminum Tolerance Engineered in Plants

C. S. Prakash Prakash at ACD.TUSK.EDU
Wed Jul 2 12:41:07 EST 1997

A short article commenting on a recent SCIENCE paper
(ISB News Report, July 1997, http://www.nbiap.vt.edu)

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.

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 at acd.tusk.edu

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