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Successful control of reproduction could help address concerns about use of engineered trees

Forestry scientists have found a way to arrest the development of flowers in poplar trees, paving the way for control of the unintentional spread of engineered or non-native tree species.

With this method, researchers raise the possibility of developing trees as crops for biofuel and other industrial purposes while preventing them from becoming established in nearby forests.

“Our goal isn’t to make reproductively modified trees just to have that trait,” said Amy Klocko, postdoctoral scientist in the College of Forestry at Oregon State University. “It’s to prevent genetically modified or non-native trees from spreading, either to wild forests or to other plantations. This would help alleviate concerns over gene flow, whether for scientific or ethical reasons.”

Klocko is the lead author of a paper published in Nature Biotechnology, reporting the results of more than a decade of research. She and her colleagues used a technique known as RNA interference to suppress a gene that is known to play a central role in the development of flowers in poplars and many other plants.

The gene, which scientists call LEAFY, is still present in the trees, but RNA interference acts like a brake to slow down the gene’s activity. Scientists grew trees containing the gene-slowing technology in experimental field trials authorized by the U.S. Department of Agriculture in the Willamette Valley. Before the trees flowered, researchers collected hundreds of small twigs containing flower buds and studied the flowers that emerged in the laboratory.

“We noticed that some of the reproductive parts were tiny (and undeveloped),” said Klocko. “And we wondered if the flowers would have that same feature when they opened in the plantation. And they did.”

By studying genetic activity in those trees, researchers then showed that the undeveloped flowers could be traced to the impact of RNA interference on the LEAFY gene.

In the future, the finding could be applied to commercial plantations of fast-growing hybrid poplars, which are not genetically engineered in the United States. Other reproductively modified trees — such as bananas, seedless oranges and many ornamentals — are commonly grown in agriculture and landscaping, but these trees have been produced using conventional forms of breeding such as hybridization and intentionally induced changes to DNA.

“People have made pollen-free male plants before, including trees,” said Steve Strauss, an OSU distinguished professor of forestry and a co-author on the paper. “But the approach we used is based on detailed knowledge of the genes that direct the production of flowers in nature, and the trees are designed to be completely incapable of producing pollen or seeds. We’ve turned down a gene that is essential for all flowering.”

The use of RNA interference to change the expression of LEAFY is the first time genetic engineering technology has been used to produce a seedless forest tree of any kind, Strauss noted.

Strauss, Klocko and their team are analyzing tree growth rates and other characteristics to see if slowing down LEAFY has consequences beyond flower development. Their data show that the trees are identical in appearance and do not differ in growth rates from unmodified trees. They are hopeful that by modifying flowering, researchers might ultimately increase wood production.

The researchers are also studying similar gene containment mechanisms in apple, sweetgum and eucalypt trees. “The principle applies everywhere,” said Strauss.

Scientists have known for more than two decades that LEAFY is key to flower development. This study is the first to show what would happen in a tree with modified LEAFY activity and grown in the field over several years.

“We are hopeful that this technology, or the related technology of gene editing applied to this gene, will reduce tensions and regulatory obstacles to the use of highly productive, genetically engineered or exotic trees,” said Strauss.

“There is no question that advanced genetic engineering methods, used responsibly, can increase productivity and sustainability of plantations, but the question of when and if to allow gene dispersal is a real point of contention for both scientists and society. We hope this technology helps us to get beyond this longstanding concern.”

Support for the research came from the USDA, the National Science Foundation, the J. Frank Schmidt Charitable Foundation and the Tree Genomics and Biosafety Research Cooperative, a two-decade-old consortium of university and tree growing industries based at OSU.

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