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Engineering Disease-resistant American Elm Trees

From the SUNY College of Environmental Science and Forestry

American elm trees (Ulmus americana) were once very common as forest and landscape trees in the eastern U.S. Their graceful vase shape and wide canopy made them especially popular for lining city streets. However, almost all mature American elm trees have been killed in the last 50 years by Dutch elm disease, which is caused by a fungus, Ophiostoma novo-ulmi, introduced from Asia. Elm bark beetles spread the fungus when they feed or lay eggs under the bark of an elm tree. The fungus grows into the xylem vessels of the tree, blocking water transport, which leads to wilting and eventual death.

An elm-lined street in Detroit in 1971(top image), and the same veiw in1984 after dutch elm disease pandemic (many trees gone)Researchers and horticulturalists are exploring various ways to treat and prevent Dutch elm disease. Mature American elms can be injected with fungicides to prevent infection, and beetle populations can be reduced with chemical sprays. Both of these methods are expensive and labor-intense, and insecticides can have harmful effects on other organisms (including humans). Traditional horticultural breeding programs have produced several lines of disease-tolerant hybrid elm trees, but many of these trees don’t have the shape and attractive characteristics of pure American elms. Furthermore, even though hybrids can tolerate Dutch elm disease, most of them are not truly resistant and may also be susceptible to other microbial tree diseases.

Dr. William Powell, along with his colleagues and graduate students at ESF, has designed and tested small antimicrobial peptides that can enhance a tree’s resistance to fungal diseases. Proteins of this type are called cationic antimicrobial peptides, which are a common class of disease-fighting proteins found in many plants and animals. Expression of this protein will be targeted to the fungus-infected tissues with a vascular-specific promoter cloned from the American chestnut. The first step in transforming the elm involves putting this gene construct into Agrobacterium, a soil bacterium that can naturally transfer a portion of its DNA into a plant host. The Agrobacterium is then induced to transfer that gene into American elm tissue. This transformed tissue is selected and cultured to produce new shoots, which are propagated in tissue culture, rooted, and finally potted in soil to grow into new trees. These trees are currently being grown in the greenhouse and tested for their resistance to Dutch elm disease. The American elm was once a common and pleasant part of our city streets, and with continued research, it may be once again.

Recommended Reading:

D'Arcy, C. J. (2000). "Plant Disease Lessons: Dutch Elm Disease." The Plant Health Instructor. www.apsnet.org/education/LessonsPlantPath/DutchElm/Top.htm

Gartland, J. S., C.M. Brasier, T.M. Fenning, R. Birch, K.M.A Gartland (2001). "Ri-plasmid Mediated Transformation and Regeneration of Ulmus procera (English Elm)." Plant Growth Regulation 33(2): 123-129.

Osusky, M., G. Zhou, L. Osuska, R. E. Hancock, W. W. Kay, and S. Misra (2000). "Transgenic plants expressing cationic peptide chimeras exhibit broad-spectrum resistance to phytopathogens." Nature Biotechnology 18(11): 1162 - 1166.

Prepared by Andy Newhouse, M.S. candidate under Dr. William Powell, Department of Environmental and Forest Biology. SUNY College of Environmental Science and Forestry. June 2004.