|After being subjected to a pathogen, the tomato plant on the left wilts and dies, while the harpin-treated one remains robust. Photo by Kent Loeffler/Cornell University.Copyright © Cornell University A high-resolution copy of this photo (1500x1305 pixels, 253K) is available here.|
"Treating plants with the harpin protein signals the plant to turn on its natural defense systems," says Steven V. Beer, Cornell professor of plant pathology and one of the protein's discoverers in 1991. "The plant must be treated before the pathogen attacks, and it takes several days for the plant's system to mobilize its own defenses."
The protein combined with other ingredients will be sold under the name Messenger(tm) by Eden Bioscience Corp., Bothell, Wash., under license from the Cornell Research Foundation, which aids in the development of Cornell-discovered technology. Since entering into a licensing agreement in 1995, Eden has conducted over 500 field trials of the product on about 45 crops in four countries.
Curiously, the protein is derived from a plant pathogen, Erwinia amylovora, the bacterium responsible for fire blight, a scourge in Northeast fruit orchards since the 18th century. The bacterium attacks apple and pear trees and many ornamentals in the rose family, leaving blackened branches, trunks, leaves, flowers and fruit.
While the bacterial blight is ruinous for plants, its protein derivative is quite the opposite. "In fact, the range of its beneficial effects is rather surprising," says Alan Collmer, Cornell professor of plant pathology and a co-developer of the protein.
Agricultural scientists had long sought the chemical basis of a defense response in plants called the hypersensitive reaction, which develops in the few cells in direct contact with an invading pathogen in the plant's intercellular spaces. By using a technique called molecular mutagenesis, Eva Steinberger (Cornell Ph.D., 1988) and David Bauer (Cornell Ph.D., 1990), two of
Beer's graduate students, identified a number of hrp (pronounced "harp") genes of E. amylovora. These genes are involved both in fire blight infection and in the development of the hypersensitive response, essentially the suicide of plant cells attempting to thwart disease.
The protein product of a particular hrp gene is harpin, identified by researchers in Beer's laboratory. For example, when harpin is placed in a few of the intercellular spaces of tobacco, tomato or geranium leaves, the plant cells collapse and die within 24 hours, as if bacteria had been introduced. The collapsed plant cells immobilize the bacterial cells, preventing the spread of further infection.
The discovery of harpin was spearheaded by Zhongmin Wei, then a postdoctoral fellow and research associate in Beer's lab and now vice president for research at Eden Bioscience. Former Cornell researchers Ron Laby and Cathy Zumoff participated in identifying the harpin protein, together with Bauer and Sheng Yang He, then researchers in Collmer's lab. Their findings were reported in a cover story in the journal Science (1992, Vol. 257, pp 85-88) titled "Harpin elicitor of the hypersensitive response produced by plant pathogen Erwinia amylovora."
In addition to the hypersensitive response the plant pathologists found that treating plants with harpin induced a second response called systemic acquired resistance, or SAR. This response provides protection against a broad range of pathogens. When Wei attempted to infect harpin-treated tobacco plants with bacterial or viral pathogens of tobacco, he found that the plants rejected the pathogens. While SAR previously had been described in the scientific literature, the scientists were surprised that the harpin protein spurred systemic resistance with no adverse effects.
Researchers at Cornell and Eden then noticed the protein's third important attribute: Harpin-treated plants grew larger and faster than plants not treated with the protein, suggesting possibly higher yields and earlier maturity. These effects later were confirmed by Eden researchers in field tests in the Northwest, Florida, California, Mexico and several states in the Southeast. Growers found that harpin accelerated ripening and improved yields on plants like cotton, citrus, peppers and tomatoes.
Finally Thomas A. Zitter, Cornell professor of plant pathology, found the protein's fourth major attribute. While performing a field trial on pepper plants in 1996, he noted that the harpin-treated plants were less damaged by insects than control plants not treated with the protein. "That study alerted us to the possibility that harpin induces resistance to insects," says Beer.
"It's a really peculiar protein," Beer says. "When the isolated protein from the bacteria is applied to many sorts of plants, pathogen resistance develops and other beneficial effects occur too."