In This Issue:
Transgenic Cotton with Improved Polyester-like Fiber
Herbicide-Resistant Crops Offer Hope in Fight Against Parasitic Weeds
Transgenic Animals as Organ Donors
Technology Acquisition: Agrevo Makes a Move
Grant and Funding Information on the Internet
To develop the polyester-cotton, Dr. Maliyakal John and Dr. Greg Keller of Agracetus employed phaB and phaC genes isolated from the bacterium Alcaligenes eutrophus, which encode key enzymes necessary for the production of the biodegradable thermoplastic polyhydroxybutyrate (PHB). Earlier, Dr. John's research group identified two genes from cotton fibers that are developmentally regulated and tissue-specific in expression (2, 3). The gene E6 is expressed during early fiber development in elongating fibers while FbL2A is expressed during late fiber development and is associated with active secondary wall formation when cellulose deposition occurs. John and Keller hooked these fiber-specific promoters to the coding region of the phaB gene, while phaC was placed under the control of a constitutive CaMV 35S promoter. Nearly 14,000 cotton seeds were subjected to the electrical discharge particle bombardment procedure to eventually produce eight transgenic cotton plants. The cotton fibers isolated from transgenic plants grown in the greenhouse appeared normal in texture but when tested using epiflourescence and transmission electron microscopy, the PHB could be clearly observed as clusters of granules in transgenic fibers. The presence of these plastic granules in fibers were further confirmed with high pressure liquid chromatography and gas chromatography-mass spectrometry.
Agracetus scientists then analyzed the cotton fibers for their thermal properties and found that these transgenic fibers and the fabric made from their yarn "conducted less heat, cooled down slower and took up more heat than the conventional cotton fiber". The new fibers had 12% higher heat uptake than the control fibers and with noticeably lower thermal conductivity. Thus, the transgenic cotton fiber with an introduced polyester gene had insulation and warmth qualities reminiscent of acrylic, suggesting potential applications in winter clothing.
Dr. John, a native of India who heads the Fiber Technology Division at Agracetus, concedes that changes in the thermal properties of transgenic fibers are still small and it is necessary to achieve much higher expression of the PHB genes to make an impact on fiber product applications. Nevertheless, this research represents another new chapter in agricultural research where the potential of biotechnology is being increasingly harnessed to add value to farming and develop improved consumer products.
References
(1) M. E. John and G. Keller. 1996. Metabolic pathway engineering
in cotton: Biosynthesis of polyhydroxybutyrate in fiber cells.
Proc. Natl. Acad. Sci. USA. 93:12768-12773.
(2) M. E. John and L. J. Crow. 1992. Gene expression in cotton
(Gossypium hirsutum L.) fiber: Cloning of the mRNAs. Proc. Natl.
Acad. Sci. USA. 89:5769-5773.
(3) J. A. Rinehart, M. W. Petersen and M. E. John. 1996.
Tissue-specific and developmental regulation of cotton gene
FbL2A: Demonstration of promoter activity in transgenic plants.
Plant Physiol. 112: 1331-1341.
C. S. Prakash
Center for Plant Biotechnology Research
Tuskegee University
prakash@acd.tusk.edu
Parasitic weeds are flowering plants that have adapted a specialized life style whereby they make a physical attachment to another plant and then live either partially or wholly off the resources produced by that host plant. This adaptation has occurred in many plant families --- parasitic plants are not uncommon, though we rarely notice them because they tend to be small and live inconspicuously on their hosts. Perhaps the two most familiar types in the U.S. are the mistletoes, which parasitize tree branches, and the dodders, which grow in twiney shoots that parasitize stems of many plant species. However, two types of root parasites, the witchweeds and broomrapes, cause destruction and crop loss throughout Africa, the Middle East, Southern Europe, and wherever else in the world they have established infestations.
These parasites are among the most difficult weeds to control because of their close association with the crop host plant. They grow attached to the host root and don't produce an above-ground structure until they are nearly ready to flower. By this time, they have often caused significant damage by stealing much of the water and photosynthates the host needs for its own growth. In this way they do not present an accessible target for mechanical weed control or direct spraying of herbicides. Generally, the herbicides available that will kill the parasite will also destroy the host crop, so farmers are limited to either the costly and dangerous chore of killing the parasite seeds by soil fumigation, or to giving up and planting alternative crops that are not hosts for the parasites.
Thus, it was a significant breakthrough when Israeli scientist Danny Joel and colleagues reported that crops engineered for herbicide resistance were able to withstand herbicide treatment while the parasite plants were killed (1). They showed that canola resistant to glyphosate was freed of broomrape parasitism following glyphosate treatment. The glyphosate molecule is readily translocated in plants, so after application to the host foliage, it is taken into the translocation stream and dispersed throughout the plant. Since the broomrape parasite taps into the hosts phloem to obtain its food, in effect it was made to "drink" the glyphosate along with its dinner. This principle worked equally well for chlorsulfuron- and asulam-resistant tobacco plants.
However, the mechanism of herbicide resistance was very important. The plants engineered for resistance to glyphosate, chlorsulfuron, and asulam were resistant because changes had been made to the enzymes that would normally be inhibited by these herbicides. In contrast, tomato plants that had been made resistant to glufosinate by addition of an enzyme capable of metabolizing the herbicide were just as susceptible to broomrape attack following glufosinate treatment as were untreated control plants. In this case the host plant avoided injury to itself by degrading the glufosinate, but the result was that the herbicide was rendered nontoxic before it reached the parasite.
In order for this strategy to be used in controlling parasitic weeds it will be necessary to engineer herbicide resistance into the appropriate host crops. For broomrape this would require creating resistant lines of sunflower, broad bean, and a range of vegetable crops including (but not limited to) tomato, potato, cabbage, cucumber, parsley and carrot. These are relatively low acreage crops, so industry may be reluctant to commit the labor and expense, and deal with potential risks, in order to transform all host crops. For the witchweeds, which primarily attack cereals (corn, sorghum, and millet), this should not pose such a problem.
Indeed, a corn hybrid with resistance to the herbicides imazapyr and imazethapyr is commercially available and was recently used by G. Abayo and coworkers in Kenya to test the ability of these herbicides to control witchweed (2). Their approach was unusual, however, in that rather than apply the herbicides as a foliar spray, they added one ml of herbicide solution along with each seed in the planting hole. The imazapyr treatment reduced the number of emerged witchweed plants per plot by over 90% at 8 weeks and 50% at 12 weeks after planting, and greatly reduced seed production by those parasites that did emerge. This resulted in a tripling of crop yield relative to untreated plants.
Although the mechanism of how this treatment works has not been proven, it is most likely due to the herbicide being taken into the seed or germinating seedling and transferred to the parasite as described above. The effect is almost like a temporary vaccination for the crop, with a small amount of herbicide absorbed into, and circulating through the young developing plant, providing protection against any parasite that makes a vascular connection with the host. Because witchweed growing on a young host causes more damage than those growing on an older host, this delay of parasitization on the corn plants can greatly increase yields.
It is a cruel irony that the people who must face the most severe challenge in controlling parasitic weeds are some of the world's poorest subsistence farmers. The beauty of a novel technique such as applying an herbicide dose directly with corn seed is that the farmer need not buy expensive herbicide spraying equipment nor be skilled in its use. In this case, creative herbicide application combined with biotechnology can offer hope in alleviating crop devastation caused by these unique plants.
References
1) Joel, D., Y. Kleifeld, D. Losner-Goshen, G. Herzlinger, and J. Gressel. 1995. Nature 374:220-221.
2) Abayo, G.O., J.K. Ransom, J. Gressel, and G.D. Odhiambo.
1996. pp. 761-768 in: M.T. Moreno, J.L. Cubero, D. Berner, D.M.
Joel, L.J. Musselman, and C. Parker (eds.). Advances in
Parasitic Plant Research, Cordoba, Spain.
Jim Westwood
International Research and Development
Virginia Tech
westwood@vt.edu
Increasing public awareness about the importance of organ donation has not effectively increased the supply of organs to meet the demand. As an alternative approach, xenotransplantation or the transfer of organs between species has been proposed as a possible solution to alleviating the shortage of transplantable organs. As with any organ transplant, whether it be human-human or animal-human, the major medical obstacle that must be overcome is hyperacute rejection of the transplant by the host immune system. The complement system, which is a series of proteins that provides first line defense against foreign organisms or tissues, initiates a cascade of events that leads to the destruction of the foreign material in a matter of minutes. The presence of complement masking or shield proteins prevents the complement system from attacking a person's own cells. To prevent rejection of animal organs in humans, researchers are developing transgenic animals that express human shield proteins on the surface of their organs. These genetically modified organs should in theory escape the destructive effects of the complement system when transplanted into a human.
Imutran (Cambridge, U.K.) and DNX (Princeton, NJ) are two of the leading companies developing transgenic animals as organ donors. Pigs are the favored model for these transgenic studies because the size, anatomy and physiology of pig organs are compatible with humans. Also, there are very few swine diseases that can be transmitted to humans. Imutran has successfully produced transgenic pigs that express the human shield protein, decay accelerating factor (DAF). Transfer of DAF-expressing pig hearts into monkeys under severe immunosuppression showed an increase in survival time of the transplant. DNX has also produced transgenic pigs expressing shield proteins and likewise has demonstrated a delay in the onset of hyperacute rejection of the genetically modified organ. Although these results show promise in mitigating hyperacute rejection by the complement system, further technical obstacles need to be overcome. For example, the xenograft must still survive later attack from other components of the immune system.
This research raises a number of scientific and ethical considerations. Should transgenic animals be created as a source of organs? Proponents claim that harvesting an animal for its organs is not any different than the current practice of harvesting animal tissues for food. Because a pig's lifespan is shorter than a human's, would a pig organ be genetically programmed to senesce sooner than the human body in which it was transplanted? Would the public accept animal organs for transplantation? In a survey of attitudes of Australian nurses towards organ donation, two thirds were opposed to the use of animal organs for transplant. So although successful xenotransplantation may represent a breakthrough for medical science, the procedure will be of limited value if people are unwilling to accept animal organs.
Eric A. Wong
Department of Animal and Poultry Sciences
Virginia Tech
ewong@vt.edu
The Nature article points to PGS's position in the Bacillus thuringiensis (Bt)-related insect resistant plant arena as a major attractant of PGS. AgrEvo is hoping that access to PGS patents and technology related to Bt will help it gain upwards of 15 percent market share of an estimated $6 billion global market for genetically modified plants by the year 2005. AgrEvo inherits PGS's current patent infringement suites against Ciba Seeds and Mycogen Plant Science related to Bt.
The significant price that AgrEvo was willing to pay for PGS opens the door for other plant biotechnology firms to shop their technologies, as some have started to do including the European firm Mogen. Demonstrated commercial success of plant biotechnology products will also continue to enhance the value of firms working in agbiotech, and likely increase the cash flowing into the sector as investors recognize the opportunities for ample returns on investment.
References
1. Ward, M. PGS-AgrEvo deal stirs up plant biotechnology. Nature Biotechnology, Vol. 14, No. 10, October 1996, p. 1210
William O. Bullock
Institute for Biotechnology Information
http://www.biotechinfo.com
The FEDIX conducts a daily search of grant announcements and automatically emails you with any 'hits' that match your profile. One could also conduct a search on the FEDIX web site for current grant announcements from the participating agencies or browse by 'agency' or 'subject'. The FEDIX also provides links to the web sites of many federal agencies including the USDA/CSREES http://www.reeusda.gov). The National Science Foundation does not participate in the FEDIX program, but you can visit the NSF site at http://www.nsf.gov and click on the 'Program Areas' to learn about their grant programs.
C. S. Prakash
Center for Plant Biotechnology Research
Tuskegee University
prakash@acd.tusk.edu
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