Pests and diseases

Take a field of rye and watch what happens when the plants are infected with a fungus. The fungus sweeps through the field and plants die. But not all of them. The question is: why does one plant succumb to a fungal disease while a second plant growing near it resists? Researchers have found that, in many cases, the difference between resistance and susceptibility is simply the rate of the plant's response. If a plant can respond to the first attack of fungi rapidly, then it can resist further damage.

Using this knowledge, farmers can now inoculate their crops against some fungal diseases. Slow-respond-ing plants are given a head start by being deliberately infected with disarmed fungi — the same principle used to vaccinate children against infectious diseases. The plants' response to these harmless versions prepares them to resist virulent forms that may arrive later.

Crops have also been inoculated against viral diseases by giving them copies of viral genes that limit the ability of invading viruses to replicate inside the plant cells. This approach induces resistance to diseases that were previously unmanageable, and opens new possibilities for effectively controlling viral diseases before they become established.

Fungal diseases of fruits, vegetables, and grains alone can cost growers billions of dollars annually. New fungal-resisting genes can now be inserted into corn using a gene gun — an instrument that literally shoots tiny bullets of microscopic metal particles coated with genes. It shoots genes into clusters of cells, which are then stimulated to multiply and grow into complete plants.

Figure 4.2 A researcher uses a gene gun to introduce new genes into an organism. DNA is coated onto microscopic gold or tungsten pellets that are propelled by the particle gun into plant or animal tissues that are in a petri dish (inset).

Traditionally, disease resistance was developed in crop strains through selective breeding of naturally resistant individuals. The process is now made much faster by cloning the genes responsible for resistance and inserting them into other plants, cutting the time needed to develop new strains from about 12 years to only two or three years. Once a resistant strain is established, the genes will persist in future generations through normal breeding methods. This technique has been used to culture barley plants with resistance to yellow dwarf virus.

On paper, these strategies tend to sound foolproof. But the tactics used to combat diseases must be as sophisticated and flexible as nature itself if they are to succeed for very long. Viruses, in particular, can quickly develop mutants that sidestep host resistance based on a single gene. To avoid this potential failure of their work, scientists plan to insert different genes for viral resistance into different plants and then crossbreed them so that offspring have more than one route of resistance and viruses will have a harder time evolving ways around their varied defenses. This approach combines the latest in biotechnology with the much older and still very valuable technique of crossbreeding to maintain genetic diversity. It is a common error to suppose that genetic engineering will replace the need for plant breeders.

One of the lessons taught by the widespread pest spraying programs of the '50s and '60s is that simplistic approaches to controlling pests or diseases don't last. Insect pests, with their rapid rates of reproduction, can quickly evolve resistance to toxic sprays while the buildup of the same poisons causes populations of their natural predators to decline. The result is a rebound of organisms that are much harder to get rid of. For all the power offered by biotechnology, ultimate success will still depend on the degree to which we understand the natural systems we want to manipulate. Co-opting nature is a better idea than opposing it. To that end, many agricultural scientists are investigating the potential of biological control methods.

Since the beginning of the century, the U.S. has imported and released approximately 800 natural enemies of insect pests, and about 40 percent of these continue to provide some level of pest control. How can biotechnology help give these predators and diseases of pests a helping hand?

When nibbled by insects, many plants release chemicals that drift through the air. Some predatory insects that feed on the plant nibblers use these chemical signals to zoom in on infested plants for an insect meal. For example, corn leaves chewed by beet armyworm caterpillars put out volatile compounds that draw the attention of parasitic wasps, which lay their eggs in the caterpillars. The wasps are very discriminating, ignoring similar odors released when leaves are damaged mechanically, for example by mowing. By analyzing the chemical molecules that predatory insects use to find their prey, researchers could engineer crops to produce stronger signals when attacked, attracting higher populations of the pests' natural enemies.

Instead of using chemicals to call in the cavalry, other plants use them directly for defense. Many secrete their unpleasant chemicals through forests of tiny, hollow hairs covering their leaves and stems. If you've ever grabbed a stinging nettle, you'll know how this works. Plant strategies vary. Some plant hairs produce sticky sugars that act like flypapers, trapping insects landing on the plant. Others, including citrus plants, tomatoes, and aromatic herbs like sage, thyme, and mint, produce chemicals that either repel or poison insects. Decoding the genetic control of these protective chemicals could potentially let bioengineers alter the quantity or type of chemicals produced, or add a built-in defense to other plants. These natural defenses, boosted by genetic engineering, may be all the pest protection that some plants need.

As well as causing billions of dollars' worth of direct damage to crops by feeding on them, many sucking insect pests, such as whiteflies, aphids, and leafhoppers, transmit viruses and bacteria that cause devastating plant diseases. One new approach to controlling these particular pests depends on the fact that sucking insects have vital symbiotic bacteria in their bodies. The bacteria provide essential amino acids to their insect hosts, benefiting them in the same sort of way that symbiotic bacteria in the stomachs of cows help the cows digest grass. By exploring ways to inactivate these little-studied microbes, either by manipulating their genes or by engineering an antimicrobial agent into the plants the insects feed on, scientists would have a powerful way of indirectly controlling the pests.

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