A genetically engineered tomato plant that thrives in salty irrigation water and may hold the key to one of agriculture's greatest dilemmas has been developed by plant biologists at the University of California, Davis, and the University of Toronto.

As the first truly salt-tolerant crop, these tomatoes offer hope that other crops can also be genetically modified for planting in many areas of the world that have salty irrigation water and salt-damaged soils.

"Since environmental stress due to salinity is one of the most serious factors limiting the productivity of crops, this innovation will have significant implications for agriculture worldwide," said Eduardo Blumwald, who led the research team that discovered the salt-tolerance gene. The research, much of which was done at the University of Toronto, continues in the UC Davis Department of Pomology.

The most recent findings by Blumwald and Hong-Xia Zhang, a postdoctoral fellow at the University of Toronto, will be published July 31 in the August issue of the journal Nature Biotechnology.

Worldwide an estimated 24.7 million acres (10 million hectares) -- about one-fifth the area of California -- of once agriculturally productive land are being lost annually because of irrigation-induced salinity, according to the U.S. Department of Agriculture. Crop production is limited by salinity on 40 percent of the world's irrigated land and on 25 percent of irrigated land in the United States.

This progressive loss of farmable land is on a collision course with the expanding global population, which over the next 30 years is expected to require an increase in food production of 20 percent in developed countries and 60 percent in developing nations.

Although scientists have been trying to develop salt-tolerant crop varieties using selective breeding techniques throughout the past century, none of those efforts has proven successful.

Crop irrigation is an age-old practice that allows farmers to be less dependent on seasonal rainfall and the uncertainties of the weather. However, irrigation also increases the salinity of soils and water by depositing in the fields soluble salts such as sodium, calcium, magnesium, potassium, sulfate and chloride that the water has picked up from the soils and rocks it has passed through. Eventually these salts accumulate in the irrigated soils at levels that decrease the vigor and productivity of the crops grown there.

Salty irrigation water wreaks havoc on most plants by upsetting their ability to take in water through their root cells. In fact, if salt concentrations in the soil are very high, flow of water into the plant is actually reversed and the plant dehydrates and dies as water is drawn out of its cells.

To counter this effect, Blumwald and Zhang genetically engineered tomato plants that produce higher levels of a naturally occurring protein known as a "transport protein." The gene that controls increased production of the transport protein was taken from Arabidopsis, a relative of the cabbage that is commonly used in plant research.

The transport protein uses energy available in the cells to move salt -- in the form of sodium ions -- into compartments within the cells called vacuoles. Once the salt is stashed inside the vacuoles it is isolated from the rest of the cell and unable to interfere with the plant's normal biochemical activity.

These genetically engineered salt-tolerant plants actually remove salt from the soil. And because their salt-storing activity occurs only in the plants' leaves, the quality of the tomato fruit is maintained.

Blumwald and colleagues have demonstrated that the genetically engineered tomato plants grow and produce fruit even in irrigation water that is about 50 times saltier than normal. The plants were irrigated with water having a salt concentration of 200 mM sodium chloride; this is more than one third as salty as seawater, which is about 530 mM sodium chloride.

The tomato discovery is a continuation of Blumwald's research on salt tolerance in plants. In 1999 he and colleagues announced that they had discovered the gene that governs production of the transport protein and genetically engineered salt tolerance in the Arabidopsis plant. They published their findings in the Aug. 20, 1999, issue of the journal Science.

The transgenic tomato plants were grown in greenhouses at the University of Toronto. Blumwald hopes to continue the research at UC Davis, including field trials in salt-damaged soils. He projects that, with proper funding, it would be possible to develop commercially useful salt-tolerant tomato plants within three years.

Blumwald's transgenic salt-tolerance research was funded by the Natural Sciences and Engineering Research Council of Canada and by the Will W. Lester Endowment from the University of California.