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Protein points the way to salt-tolerant crops, Purdue scientists say

WEST LAFAYETTE, Ind. – High salt levels found in one-third of the world's cropland causes reduced yields and poor growing conditions.

Now, a team of Purdue University scientists have discovered the protein and the gene responsible for allowing salt to enter plants. The breakthrough is expected to lead to plants that are resistant to the salt found in saline soils and groundwater.

Ray Bressan, professor of horticulture, says, "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. We didn’t know the beginning of the story.

"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 the U.S. Department of Agriculture'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 National Science Foundation funded the research. The Purdue Research Foundation has filed a provisional patent on the gene.

Writer: Steve Tally, (765) 494-9809; tally@aes.purdue.edu

Sources:

Ray Bressan, (765) 494-1336; bressan@hort.purdue.edu

Ana Rus, (765) 494-1315; rus@hort.purdue.edu

Paul M. Hasegawa, (765) 494-1315; paul.m.hasegawa@purdue.edu

Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; http://www.agriculture.purdue.edu/AgComm/public/agnews/

Related Web sites:
Research activities of Bressan and Hasegawa
University of Arizona site on salt tolerance in plants

NOTE TO JOURNALISTS: Mike Hasegawa is on sabbatical in Japan until April. Until his return, he is responding to e-mail sent to his Purdue account, paul.m.hasegawa@purdue.edu.


ABSTRACT

AtHKT1 is a salt tolerance determinant
that controls Na+ entry into plants roots

Ana Rus, Shuji Yokoi, Altanbadralt Sharkhuu, Tracie K. Matsumoto, Hisashi Koiwa, Ray A. Bressan, and Paul M. Hasegawa, Center for Plant Environmental Stress Physiology, Purdue University; Muppala Reddy, Central Salt and Marine Chemicals Research Institute, Bhavanager, India; Byeong-ha Lee, Jian-Kang Zhu, Department of Plant Sciences, University of Arizona.

Two Arabidopsis thaliana extragenic mutations that suppress NaCl hypersensitivity of the sos3-1 mutant were identified in a screen of a T-DNA insertion population in the genetic background of Col-O gl1 sos3-1. Analysis of the genome sequence in the region flanking the T-DNA left border indicated that sos3-1 hkt1-1 and sos3-1 hkt1-2 plants have allelic mutations in AtHKT1. AtHKT1 mRNA is more abundant in roots than shoots of wild-type plants but is not detected in plants of either mutant, indicating that this gene is inactivated by the mutations. hkt1-1 and hkt1-2 mutations can suppress to an equivalent extent the Nab sensitivity of sos3-1 seedlings and reduce the intracellular accumulation of this cytotoxicion. Moreover, sos3-1 hkt1-1 and sos3-1 hkt1-2 seedlings are able to maintain[K+]int in medium supplemented with NaCl and exhibit a substantially higher intracellular ratio of K+/Na++ than the sos3-1 mutant. Furthermore, the hkt1 mutations abrogate the growth inhibition of the sos3-1 mutant that is caused by K+ deficiency on culture medium with low Ca2+ (0.15mM) and <200 ÁM K+. Interestingly, the capacity of hkt1 mutations to suppress the Na+ hypersensitivity of the sos3-1 mutant is reduced substantially when seedlings are grown in medium with low Ca2+ (0.15 mM). These results indicate that AtHKT1 is a salt tolerance determinant that controls Na+ entry and high affinity K+ uptake. The hkt1 mutations have revealed the existence of another Na+ influx system(s) whose activity is reduced by high [Ca2+]ext.


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