By David Stauth

Researchers at Oregon State University have developed a way to genetically engineer plants that have total resistance to crown gall disease, a pervasive, multi-million dollar problem that for decades has plagued the nursery and horticultural industries around the world.

The system has been tested with tobacco plants and apple trees and appears to provide virtually complete protection from this plant disease, which can cause unsightly tumors on plants, usually on their roots, and diminish plant productivity, affect their structural integrity and often force their replacement.

OSU scientists believe this genetic technology could be applicable to a wide variety of other fruit, nut, and ornamental trees and plants - everything from grapes to roses, apple trees and chrysanthemum - which can suffer impacts from crown gall disease.

The findings were just published in two professional journals, Plant Physiology and Molecular Breeding.

"Crown gall can be a disaster for nursery owners, and people have been trying to develop ways to address this problem for decades," said Walter Ream, a professor of microbiology at OSU. "The problem is serious enough that it's illegal to sell a plant that has been infected. But this technique should work on a wide variety of plants and it's reasonable to believe it will find at least some applications in agriculture."

The commercial use of this technology, Ream said, may be slowed somewhat by the cost of field trials and the significant regulatory hurdles that face any use of a genetically engineered plant. Some of the crops that it could be applied to, he said, are also niche markets, such as ornamental rhododendrons, roses, or nut trees.

Crown gall disease is caused by Agrobacterium tumefaciens, bacteria found in soils around the world that can genetically transform plant cells to grow as tumors. The bacteria are widespread. It's not unusual for a single gram of soil to harbor a million or more bacterial cells, and it's been found in native grasslands that have never been cultivated.

The bacteria usually infect a plant when it is wounded by some type of cut. The benign tumors produced by an infected plant are usually, but not always, found on the plant roots, and some of the unsightly "galls," or tumors, have grown to several feet in diameter in the branches of trees.

In Oregon and Washington, it's known that crown gall disease causes the destruction of at least $400,000 of nursery stock in an average year, but the losses may be far higher than that. That figure does not include losses in established orchards and vineyards.

In California, a typical walnut orchard loses 1-2 percent of its trees every year to crown gall, costing the walnut growers of the state at least $1 million a year. In one especially severe outbreak in an Oregon nursery, 14,000 fruit trees had to be destroyed in a single season.

Crown gall is a bacterial infection in plants that causes them to convert the amino acid tryptophan into an auxin, which is a hormone that can promote rooting and growth. Plants routinely make some auxins in normal development, but a crown gall infection causes the process to get out of control.

The "gene silencing" technique developed by OSU researchers to deal with this problem essentially tricks plants into sensing that they are being attacked by a virus, which they then destroy with their own defense systems.

According to Ream, this approach allows crown gall bacteria to infect a plant, inject tumor-inducing genes into the plant's DNA, and then begin to express RNA as the plant begins a biochemical process that would eventually lead to uncontrolled growth of a tumor. But the genetically engineered plants make double-stranded RNA, instead of the single-stranded RNA ordinarily produced. The plant recognizes the double-stranded RNA as a virus, which it has the capacity to destroy with its own natural defense systems. So even though the plant has been infected by crown gall bacteria, the process of tumor formation is interrupted before any damaging effects can occur.

"We've already demonstrated the efficacy of this approach with tobacco and apples, and other scientists have used it effectively on walnuts," Ream said. "It appears we can make this system work with most plants, and create varieties that are genetically resistant to the damaging effects of crown gall disease."

The commercial use of this technology may be facilitated, Ream said, by its use just on the root stocks of plants, which are often grafted with fruiting wood of various types above the root structure. This would prevent concerns about genetic drift of newly engineered plant characteristics, since the genetically changed part of the plant would play no role in its seed production, pollination or other reproductive systems.

This research was supported by the U.S. Department of Agriculture and the OSU Agricultural Research Foundation.