Mite and fungus: foe and friend?

Mite and fungus: Foe and friend?

If you were a plant, being attacked byboth spider mites and leaf wilt fungus would spell double trouble, right? Not necessarily, according to a recent study of plants' resistance to pathogens.

Prior exposure to one makes a planttwice as likely to ward off attack by the other, say Richard Karban, Rodney Adamchak and William C. Schnathorst from the University of California at Davis, in a report in the Feb. 6 SCIENCE.

Previous research by other scientistshas shown that inoculating cotton seedlings with the fungus causing verticillium wilt protected against later fungal attacks. Other experiments found that a similar resistance could be induced against spider mites if there was prior leaf damage by the insect. But the Davis results are evidence that these very different species can provoke an inter-species, systemic resistance in plants.

The data do not dispel the mysteries ofplant "immunity,' however. "People have assumed that changes [causing resistance] are defenses on the part of the plant,' Karban told SCIENCE NEWS. "However, the change need not be an adaptive effect, but simply a detrimental change that affects the plant and the things that attack it. These plant changes may not be anywhere near as [defined] as a [animal immune systems]. I really don't have a clue as to how it works.'

The possible mechanisms of plant resistanceare many and varied. The plant may produce a chemical that directly affects the plant pathogen. For example, alfalfa under attack by insects produces a chemical that inhibits the insects' digestion (SN: 5/25/85, p.327). Or, suggest Karban and his co-workers, the first species attacking the plant "may deplete the most desirable nutrients or plant parts.' Another possibility is that pathogens may activate dormant plant metabolites.

Whatever the mechanism, the commercialpotential of inducing "immunity' is already blooming. Karban points out that European growers routinely inoculate tomato plants with viruses to elicit resistance and decrease losses.

In recent years, work on induced resistancein plants to herbivores has become "fashionable,' Karban says. But, he adds, it also has become controversial: "Most other scientists are skeptical of these sorts of interactions. At this time, there's not a whole lot of scientific evidence that these interactions exist. . . . But their possible consequences, especially if we can manipulate them, would be fantastic.'