Pest versus Pest
The parasitic weed, broomrape (Orobanche aegyptiaca), attaches to the root of such vegetable crops as tomato, potato, bean, and sunflower. With no need for leaves of its own, it produces only a floral shoot above ground. Meanwhile, its host is barely able to survive, much less be productive. Now, the defense mechanism of another pest - the fly - may provide a weapon against parasitic weeds. Researchers from Virginia Tech in the United States and the Agricultural Research Organization (ARO) of Israel will likely create a buzz of fascination when they present their results at the 227th national meeting of the American Chemical Society.
Broomrape is very disruptive throughout the Middle East and Africa, as well as in some parts of Europe. Plant breeders have been trying for decades to breed crops that will resist the weed. Egyptian broomrape was certainly a logical target for the efforts of Noureddine Hamamouch of Morocco, a doctoral student in the Department of Plant Pathology, Physiology, and Weed Science (PPWS) at Virginia Tech, and genetic engineering was a logical strategy. But the toxin he decided to experiment with, an antibacterial peptide that is part of the defense arsenal of the flesh fly (Sarcophaga peregrina), was a matter of luck, says PPWS professor James Westwood. Westwood's colleague, Radi Aly, of the Weed Science Department at Newe Ya'ar Research Center of ARO had been working with the fly peptide, sarcotoxin, as part of another, unrelated project. "He had it on hand and just tried it to see what would happen.”
The model plant for the research is tobacco, which Virginia Tech researchers have used for other transgenic projects. At around the time Aly realized he had a potential toxin in hand, Westwood's group had just identified a gene promoter that switched on specifically in response to the parasite. The two groups joined forces to maximize the impact of their strategy. Hamamouch and Aly linked the parasite-induced promoter to the sarcotoxin gene and introduced the final product into the tobacco genome using Agrobacterium-mediated transformation. The introduced gene was thus silent in the healthy tobacco plant but turned on when it sensed an invading parasite.
Results have been variable. In some instances, the broomrape planted with the treated tobacco perished. In other instances, it faltered to different degrees, while the host plant yielded better than untreated tobacco that was also sharing space with broomrape. The researchers have also demonstrated that broomrape sucks up macromolecules far bigger than the sarcotoxin peptide along with water and nutrients from the host. "We suspect the toxin moves into the parasite and disrupts its growth," says Westwood.
The goal now is to determine how the new peptide works and how to make it more effective. "We think we need higher levels of expression to get complete resistance. We think that the peptide degrades rapidly, so we need to stabilize it so it lasts longer." The effectiveness of a fly-defense antibacterial peptide is not entirely serendipity. Westwood explains that flies must have defense systems to protect themselves from microbes - considering their life styles. "They carry defenses with a broad spectrum of activity. Sarcotoxin attacks the membranes of many different bacteria, but is relatively safe for higher organisms. It is interesting that it also is effective against parasitic plants and we want to understand the mechanism."
The researchers also demonstrated that the toxin is produced only where the parasite attacks the host. "It is produced at the injury site in great numbers and the parasite is like a vacuum cleaner - taking in as much as it can. So it accumulates more of the peptide than remains with the host." (Virginia Tech Release via Agnet, 3/29/04).
