Devastating parasitic weed may be felled by toxin borrowed from flies
The parasitic weed, broomrape, attaches to the root of such vegetable crops as tomato, potato, beans, and sunflowers. 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.
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 in Anaheim, Calif., March 28-April 1.
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. Genetic engineering to create resistant crops is the latest strategy.
Egyptian broomrape, or Orobanche aegyptiaca, was certainly a logical target for the efforts of Noureddine Hamamouch of Morocco, a doctoral student in plant pathology, physiology, and weed science (PPWS) at Virginia Tech. And genetic engineering was the 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.
But would Egyptian broomrape be repelled by this souped-up off shoot of an antibacterial peptide from a flesh fly? Yes.
But the results are somewhat uneven. In some instances, the broomrape planted with the treated tobacco perished. In other instances, it faltered to different degrees, while the host plant produced better than untreated tobacco that also was sharing space with broomrape.
The researchers also have demonstrated that broomrape does suck 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."
Why doesn't it also attack the host? The researchers 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."
The paper, "Engineering crop resistance to parasitic weeds (AGFD 28)" will be presented by Westwood at 9:20 a.m. Monday, March 29, as part of the symposium on natural products for pest management from 8:15 a.m. to 12:30 p.m. in the Hilton's Pacific Ballroom B. Authors are Hamamouch, Aly, Virginia Tech PPWS professor Carole L. Cramer, and Westwood.
Abstract:
Parasitic weeds of the genus Orobanche (broomrapes) pose a severe problem for agriculture because they are difficult to control and highly destructive to crops. Our objective is to engineer resistance to parasites based on host expression of sarcotoxin IA, an antibacterial peptide originally isolated from the flesh fly (Sarcophaga peregrina). The gene encoding this peptide was fused to a parasite-inducible promoter and transformed into tobacco. The resulting transgenic plants exhibited greater biomass accumulation and reduced levels of parasitism as compared to non-transgenic plants when grown in Orobanche-inoculated soil. Initial investigations into the mechanism of sarcotoxin toxicity to the parasite have documented the host-parasite translocation of protein and macromolecules that are larger than sarcotoxin IA (4 kDa), so it is likely that sarcotoxin IA accumulates in, and acts directly on, the parasite. Sarcotoxin IA, and potentially other macromolecules, may form part of an effective host defense against parasitic weeds.
For more information, contact:
Dr. Noureddine Hamamouch, who has received his PhD. from Virginia Tech, nhamamou@vt.edu
Dr. James L. Westwood at Virginia Tech, westwood@vt.edu, (540) 231-7604.
Dr. Radi Aly, Newe Ya'ar Research Center, ARO, radi@volcani.agri.gov.il, 972-4-6539514
