Science / Medicine : Plant Pesticides : Control Without Chemicals : Scientists Use Gene Alterations to Make Crops Resistant to Infestation
Gary Reed has been spying on his potato plants. What he sees comes as a welcome surprise. In one 50-row section of his 1.6-acre plot, a number of beetles have become transfixed. “They’re afraid to move, afraid to fly and afraid to eat,” the Oregon State University entomologist said.
Adjoining this thriving growth, Reed has cultivated another area. Here the beetles have engaged in their normal activity--munching the green leaves and leaving behind only spindly stalks. The beetles have had a heyday chomping away at the free lunch.
Both plots are free of chemical pesticides. The difference between the two: The vibrant green bundles have been genetically engineered to resist beetle infestation.
Scientists have incorporated insecticidal genes into the plants’ DNA. Taken from the bacterium Bacillus thuringiensis, the gene produces a protein that is toxic to beetles, caterpillars and flies, though harmless to humans and beneficial insects.
B. thuringiensis, commonly referred to as Bt, has been sprayed by farmers as a benign insecticide for 30 years. When attacking bugs gnaw at the plant leaves, they also ingest a protein called Bt toxin. The protein wreaks havoc on the insects’ digestive systems. The little critters stop eating and die of starvation.
But because Bt is an expensive pesticide, it can be used only on high-value crops. Furthermore, the Bt toxin is degraded rapidly in the environment and washed off by rainfall.
For these reasons, botanists would like to incorporate it into the plant, reducing the cost of use and providing permanent protection throughout the lifetime of the plant. That is precisely what Reed has achieved in his potato plots.
Plant biology is undergoing a revolution more profound than any since the monk Gregor Mendel discovered the principles of the inheritance of genes 125 years ago. Molecular biologists are modifying agricultural crops to give them a variety of new characteristics, including resistance to pests and herbicides, tolerance to salt and drought, and increased nutritional properties.
They are also developing plants, such as tobacco, that will manufacture drugs that can be isolated inexpensively after the crop is harvested. Already, major chemical and biotechnology companies have conducted more than 120 field trials of genetically engineered plants in the United States, and the numbers are increasing sharply each year.
But one of the principal focuses has been on providing increased resistance to insects, because of the public’s growing intolerance of the use of chemical pesticides and the mammoth size of the pesticide industry--$1 billion per year in the United States and $5 billion worldwide. The biotechnology companies hope to grab a significant share of that market, enticing farmers to buy seeds that might cost 50% more than normal, but that would save several times the extra cost in reduced use of pesticides. Much of that research focuses on Bt.
In the mid-1980s, researchers at Plant Genetic Systems in Belgium successfully inserted the gene for the Bt toxin into a tobacco plant. “And lo and behold,” said Ron Muessen, director of plant biotechnology research at Northrup King, “if you put caterpillars on the plants, they would start to eat it and die.”
But years of work still lay ahead. Tobacco had been used only as a model. It was plants like potatoes, tomatoes and corn that would benefit from the Bt toxin--plants that suffered specifically from caterpillar and beetle damage.
Since then, numerous companies have succeeded in transferring the Bt toxin gene into the desired tomato, cotton and corn plants. The plants stave off predators without harming beneficial insects in the process. They also seem to produce no adverse environmental effects. Industry analysts predict that the first genetically engineered crops, such as cotton, could be approved by the government by 1995.
Some companies have tried to circumvent the cumbersome regulatory process by using genetic engineering techniques on the pesticide rather than the plant itself and are therefore more likely to win speedy approval.
Scientists at Mycogen Corp. in San Diego and Ecogen in Langhorne, Pa., are attempting to make sprayed-on Bt toxin persist longer.
At Mycogen, molecular biologists have inserted the Bt toxin genes into another bacterium, Pseudomonas fluorescens. The altered bacteria, grown in laboratory vats, produce large quantities of Bt toxin. When the bacteria are killed, their outer membranes form “biocapsules” that retain the toxin and prevent it from being degraded. The dead bacteria are sprayed onto plants, where they provide protection for weeks instead of days.
Mycogen makes two versions of the product, both of which were approved by the U. S. Environmental Protection Agency this summer. One, called MVP, kills the diamondback moth and other caterpillar pests that attack cabbage, broccoli, lettuce and other vegetables. The second, called M-Trak, attacks the Colorado potato beetle, which attacks potatoes, tomatoes and eggplant. The world market for pesticides for these crops alone is $400 million, according to the company.
This approach could have a much wider impact. Scientists have identified up to 9,000 varying strains of Bt, each with varying toxicity and different targets. While scientists assumed Bt only destroyed a short list of insects, Mycogen has recently identified particular Bts that poison other organisms, including nematodes and flatworms.
Ecogen has capitalized on this breadth of toxins by inserting genes for several different Bt toxins into one strain of Bt. In that manner, you can target caterpillars and beetles with a single insecticidal spray using the same product.
“We discovered a strain of Bt that had good activity against the Colorado potato beetle,” Ecogen’s John E. Davies says. “We mated that beetle-active strain with a caterpillar-active strain and came up with a strain that makes two different toxins, one that controls the beetle and one that controls a specific caterpillar--European corn borer.”
Rather than inserting a single gene into the new Bt, Ecogen’s scientists introduce a large chunk of DNA carrying the desired gene within it. Their process resembles traditional breeding techniques more than today’s precise genetic engineering. The result: Ecogen has had no trouble in the regulatory game obtaining approval for their genetically “altered,” rather than “engineered” product.
One other company has taken a third course, combining elements from both approaches. Crop Genetics International of Hanover, Md., has developed a plant “vaccine.” Scientists there insert the Bt toxin gene into a bacterium, called Cxc endophyte, which normally lives inside Bermuda grass.
Laboratory and field tests have shown that if the altered bacterium, which the company calls InCide, is inoculated into corn seed, it multiplies and carries the Bt toxin throughout the corn plant’s stalks, leaves and roots.
The primary advantage of this approach, compared to engineering the Bt gene into the plant itself, is that the Bt toxin does not enter the corn kernels, so that there is no question that the kernels are safe to eat. Crop Genetics hopes to begin marketing InCide for corn within the next year and is developing similar pesticides for use on other crops, including wheat, rice, soybeans and cotton.
Why would it be dangerous to consume Bt toxin in a plant if the topical pesticide itself is deemed safe? Most researchers in the industrial sector assume that Bt toxin does not pose any harm to humans. Rebecca Goldburg of the Environmental Defense Fund is not convinced. She explains that the toxin produced by plants varies slightly from the original synthesized by Bt.
While enthusiasm for Bt is strong, none of the developers imagine that biological insect control will ever put the chemical companies out of business. “There have been claims that genetic engineering is going to let us eliminate chemical pesticides,” Muessen said. “That’s not going to happen. The number and complexity of pest problems that growers face is sufficiently daunting that we’re not going to completely abandon one set of tools.”
Genetic Pesticides: A Look at One Process
The advent of genetic engineering and a distaste for chemical pesticides has sparked research into biological control of pests. Here is a look at one of the basic strategies, in which a gene from the bacterium Bacillus thuringiensis-- which produces an insecticidal protein--is incorporated into a plant’s DNA, providing it with its own natural insect repellant:
1. The gene coding for an insecticidal protein is taken from the Bacillus thuringiensis bacterium and inserted into a plasmid (a ring of DNA) from another bacterium.
2. The plasmid, which now contains the insecticidal protein gene, is returned to the bacterium. The bacterium serves as a factory for replicating the Bacillus thuringiensis gene.
3. After copies have been made, the desired gene is removed from the plasmid and spliced into an Agrobacterium plasmid, a soil microorganism that can insert DNA into plant cells.
4. The Agrobacterium then ferries the Bacillus thuringiensis gene into the DNA of cultured plant cells.
5. Plants grown from these cultured cells will produce the same insecticidal protein that is made by the Bacillus thuringiensis.
