Biotechnology: Toting Up the Early Harvest of Transgenic Plants
Science Volume 282
According to this story, after centuries of improving their crop plants and domestic animals the old-fashioned way - by breeding in desirable traits - agricultural scientists took a big step in the early 1980s, by adopting the tools of modern molecular biology to introduce genes into plants and animals for the traits they wanted. Some 15 years after the first such gene transfers, 700 researchers and policy-makers from 30 countries attended the Second Agricultural Biotechnology International Conference, held last summer in Saskatoon, Canada, to assess the fruits of their past labors and look ahead.
The story says that several major crop plants, including corn, oilseed canola, soybean, and cotton, have been engineered with genes that make them resistant to insect pests or to the herbicide glyphosate, so that the weedkiller doesn't threaten the crop.
Such transgenic plants have met opposition in many European countries because of fears that they may be unsafe for the consumer, damage the environment, or lead to further, costly surpluses (Science, 7 August, p. 768). But they are winning acceptance in other countries, including the United States, Argentina, China, and Canada. During this past growing season, at least 30 million hectares worldwide were planted with the modified crops. As a result, more than one-half of the world soybean harvest and about one-third of the corn harvest now comes from plants engineered with genes for herbicide or disease resistance. These commodities find their way into hundreds of foods, such as breakfast cereals, cooking oils, corn syrup, soft drinks, and candies.
Anatole Krattiger, executive director of the International Service for the Acquisition of Agribiotech Applications in Ithaca, New York was quoted as saying, "The speed of commercialization of agribiotech applications has taken many by surprise," adding that for industrialized nations, agbiotech can increase the efficiency of producing existing crops by reducing the need for pesticide applications and other costly treatments; in developing, food-short nations, it can increase yields, essentially without the cost of additional inputs, such as pesticides. And a few genetically modified plants that promise entirely new products - including some that make ingestible vaccines for human diseases and at least one, sweet potato, with an improved protein content - are moving through the pipeline.
The story adds that researchers are, however, running into trouble in their efforts to transform conventional crops into factories for high-value novel products, such as a "natural" cotton/polyester blend grown by cotton plants, or for substances traditionally supplied by synthetic chemistry, such as plastics. And sometimes even successful genetic transformations can be stymied by practical concerns.
For example, the Flavr-Savr tomato, genetically engineered by the biotech firm Calgene Inc. of Davis, California, with a so-called "antisense" gene that slows down the activity of polygalacturonase, an enzyme that degrades cell walls. By inhibiting rotting, this change allows the fruit to ripen on the vine instead of being picked green and hard. But the Flavr-Savr tomato had to be pulled from the market, mainly because conventional tomato-picking and packing equipment damages the soft, naturally ripened vine fruit.
The story says that the modifications that have worked best are the simplest:those that can be accomplished by introducing just one or a few foreign genes into a plant, with minimal effects on its physiology. For example, researchers made plants resistant to the herbicide glyphosate by transforming them with a natural bacterial enzyme that is highly resistant to the herbicide, while insect- resistant plants are created by adding the gene for one of the toxins produced by Bacillus thuringiensis, a type of bacterium that infects and kills insects. This can pay off economically. In 1997, in U.S. corn belt states, the story says that corn transformed to express a BT protein had a 7% increase in yield per acre, bringing the farmer, on average, an increased net return per acre of $16.88. Buoyed by these successes, researchers are now expanding their efforts.
One goal is to use genetic engineering to enhance food quality. Early indications are that some of these attempts will work, particularly those involving the manipulation of only one gene. C. S. Prakash of Tuskegee University in Alabama was cited as describing progress in improving the quality of the proteins made by sweet potato, an important, easy-to-grow food crop in areas such as the poorer countries of the tropics, where high-quality protein foods may be hard to come by. Prakash inserted into sweet potato plants a synthetic gene coding for a storage protein that has a high content of the so-called essential amino acids, ones that the human body can't make for itself.
Early on, Prakash worried that the energy drain imposed by synthesis of the foreign protein would reduce the harvests of the transgenic sweet potato plants, but his fears proved unwarranted. Although the protein content of two strains of the genetically engineered plants increased by 2.5- to 5-fold, the first field trials in Alabama during the summer of 1997 showed, if anything, a slight increase in yields. The story says that the transgenic plants produced between 64 and 68 bushels per hectare, compared to the control plants' 61 bushels per hectare.
These transgenic potatoes are not yet in commercial production, as researchers are just beginning to assess their nutritional quality. A first feeding trial on hamsters looks promising, though. Animals fed the transgenic, high-protein potatoes weighed 56% more than controls after 28 days and showed no evidence of any toxic effects.
The story notes that Joseph Hirschberg of the Hebrew University in Jerusalem
has induced tobacco plants to make a carotenoid pigment called astaxanthin,
which can be used to tint flowers, farm-raised shrimp, and salmon and,
when fed to chickens, can color egg yolks a vibrant orange. Currently,
astaxanthin is extracted from seashells or synthesized chemically and
carries a price tag of about $2600 per kilogram. But Hirschberg's efforts
may help bring that price down.
The story goes on to say that more recently, Monsanto polymer biochemist Ken Gruys and his colleagues induced both Arabidopsis and canola to produce a copolymer of PHB and poly-3-hydroxyvalerate (PHBV) by transferring four genes into the plants. This material, which is much more pliable than PHB, has been produced commercially by fermentation. Biochemist William Page of the University of Alberta was cited as saying that, however, even if these efforts succeed, extracting the plastic is likely to be difficult and that "the savings of producing polyester in the field may be lost in extraction."
The story also notes that how plants regulate gene expression is one
of the big issues researchers need to grapple with, especially if they
try to develop improved plant varieties by tinkering with large gene clusters,
such as the packets of genes that direct nitrogen fixation or photosynthesis.
So far, researchers have done little with these systems, because, says
BTI president Charles Arntzen, "they're too complicated." But that doesn't
mean researchers have given up - just that they will need to understand
the systems better to try to identify modifications that might improve