By Vicki Miller
IANR News Service

Revving up an amino acid that plants already contain might protect them from a host of environmental stresses, such as heat, salt, drought or herbicides, University of Nebraska-Lincoln research indicates.

University of Nebraska Plant Pathologist Marty Dickman and colleagues discovered a previously unrecognized protective power of proline, an amino acid, by chance while studying what regulates cell death in plants.

Proline is known for protecting plants against drought and salt stress by helping cells retain water. This research revealed a potentially much broader protective role for proline as a potent antioxidant that also inhibits cell death.

So far, the Institute of Agriculture and Natural Resources team has tested proline in the lab on an alfalfa fungus and a yeast with surprising results. When researchers added proline to the lab dishes containing the fungus or yeast, the organisms survived environmental stresses, including heat, ultraviolet light, salt, hydrogen peroxide and herbicide treatment.

While he's excited by the findings, which were published earlier this year in the Proceedings of the National Academies of Science, Dickman is cautious. He points out that what works in a fungus or yeast doesn't automatically translate to plants or animals.

"Our research suggests the potential value of proline as an antioxidant," Dickman said. "We want to be realistic but what we've seen so far suggests proline has a broader protective function than previously realized. That's motivation to expand our research into plants and animals."

Dickman's team made the proline discovery while working with a mutant form of a fungus that attacks alfalfa. The mutant contained a cancer-like gene that made it grow abnormally under certain conditions. In a nutritionally rich growth medium, the mutant grew normally but in a nutritionally sparse medium "it was messed up," he said.

The nutritionally rich growth medium was composed of vitamins and amino acids. Dickman wanted to know specifically which component restored normal growth in the mutant fungus. The team ruled out vitamins and a graduate student then tested 20 amino acids one at a time on the mutant fungus.

"We got lucky. One amino acid, proline, restored normal fungal growth," he said, "but we had no idea why."

To learn more, the IANR team grew the mutant fungus in the presence of known antioxidant compounds and then with proline. Antioxidants counteract cell damage caused by free radicals and other forms of toxic oxygen that damage cells through oxidation.

Proline had the same effect as the antioxidants. It restored normal growth and removed virtually all of the toxic oxygen. Scientists also found that proline prevented programmed cell death, or apotosis. In programmed cell death, cells essentially commit suicide as part of the cycle of cell replacement and disease or injury protection. When an organism is being attacked or stressed, damaged or old cells die to protect healthy cells.

In other experiments the team exposed a normal alfalfa fungus to life-threatening stressors including heat, salt, hydrogen peroxide or UV light. Without added proline, the fungus died. When it was added, the stressed fungus survived and programmed cell death ceased.

"We were pretty excited about this," Dickman said. "Even though this was in a fungus, we thought that if proline works, it could have a broader role than previously recognized."

To find out, Dickman tested proline in baker's yeast that had been treated with paraquat, a herbicide that kills plants by generating destructive toxic oxygen that kills cells. Results were the same: Yeast grown in the presence of paraquat died while yeast grown with paraquat and proline grew.

"We can impose various stresses from UV to paraquat and basically, what's lethal without proline grows just fine with it," Dickman said.

Dickman is beginning tests to determine whether proline has the same protective power in plants. He's also collaborating with UNL biochemist Don Becker on research to more specifically understand proline's protective mechanism.

If proline proves effective in further studies, "we might be able to generate plants with broad spectrum stress tolerance," Dickman said.

That's appealing because scientists might be able to simply boost the activity of an amino acid that's already present in plants to enhance their resistance to stresses. Typically in genetic engineering, scientists must look for to other organisms for genes with the desirable characteristics.

"A little bit of tweaking and we could have stress protection," Dickman said.

The National Science Foundation and the university's Redox Biology Center helped fund this IANR Agricultural Research Division research.