High salt levels found in one-third of the world’s cropland causes reduced yields and poor growing conditions.

Purdue University scientists have just discovered what’s responsible for allowing salt – which lowers yields – to enter plants. The breakthrough is in the identification of the protein and the gene responsible for the high salt levels. It’s expected to lead to plants resistant to the salt found in saline soils and groundwater.

"As long as people have been working on salinity toxicity – over many decades and in thousands of scientific papers written on the subject – no one knew the most fundamental thing about it, which is how sodium gets into plants," says Ray Bressan, a Purdue horticulturist.

"So this is the first piece of work that shows what protein is responsible. There have been biochemical experiments that showed that this protein had the potential to be a sodium transporter, but there was no evidence that it was actually involved in tolerance to sodium toxicity in plants."

Salt toxicity in crops is a problem in areas where irrigation is used extensively, such as on high-value crops in California. According to researchers at USDA’s George E. Brown Jr. Salinity Laboratory, up to 25 million acres of land are lost because of salinity caused by irrigation each year.

Salinity also is a problem in areas with saline groundwater, such as is found in Egypt and Israel. In some areas, soil salinity is so high that crops can’t be grown. Despite decades of plant breeding efforts, researchers have not been able to develop more than a few salt-resistant plants.

"A second reason that this research is important is that we also discovered more about how the protein functions," Bressan says. "We discovered another entry system for sodium. This explains why controlling this entry system didn’t allow us to make completely salt-tolerant plants. They’re more tolerant, but not completely. But now we have important clues about how this works.

"When we’ve identified all of the salt-tolerance genes of plants, we’ll be able to control them, and we’ll be able to create salt-tolerant crops. Now we see the light at the end of the tunnel."

Mike Hasegawa, professor of horticulture and the principal investigator on the research, says this information fills in holes in scientists' understanding of how plants work.

"This is a significant discovery that answers one of the major questions in this field," Hasegawa says. "It is now clear that despite the fact that salt is toxic to plants, they have a specific transport system for the uptake of salt. This means that, in saline environments, plants have developed a way to cope with high salt levels instead of avoiding them."

The protein, which has the unwieldy scientific name of AtHKT1, is thought to act as a transporter in plant tissue, binding with the salt ion and ferrying it into the plant cells.

Genes are the blueprints for proteins in living things. When a gene is activated, or expressed, proteins are manufactured. To confirm that the protein AtHKT1 was involved in salt transport in plants, postdoctoral researcher Ana Rus used Arabidopsis thaliana, a type of wild mustard commonly used in plant experiments.

Rus searched for disabled genes in the plant that would cause a special salt-sensitive strain of Arabidopsis to become as resistant as regular Arabidopsis plants. After screening more than 65,000 plant lines, she found a double mutant plant that took up less salt and grew as quickly as normal plants. She then isolated the gene responsible for this action, and discovered that in the double mutant plant the gene that produces AtHKT1 had been disabled, or, in the jargon of scientists, knocked out.

Further research found that, at high salt concentrations, plant growth still declined, indicating that salt uptake is a complex system with multiple genes involved.

"What makes study of salt uptake so difficult is it depends on many genes, but we will continue our experiments to find these other genes," Rus says. "Another question we have is why is this gene responsible for sodium uptake when sodium has no value to the plant. What other functions it has aren’t known. But now that we have these plants with the knocked-out genes we can work on that, too."

The findings were announced in a paper in the Nov. 20 issue of the

Proceedings of the National Academy of Science. The research also won the top research presentation award at an October international conference on salinity, sponsored by the Juan March Institute for Study and Research in Madrid, Spain. The proceedings of that meeting will be published next spring by the scientific journal European Molecular Biology Organizations Reports.

The National Science Foundation funded the research. The Purdue Research Foundation has filed a provisional patent on the gene.