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Sick plant? Take two aspirin and....

Thanks to the immune system, diseases such as measles make people sick just once, and then the body develops a lifelong resistance to the infection. Plants, too, possess similar defenses. Something called "systemic acquired resistance" allows plants to fight off subsequent attacks not only by the original pathogen but by other infections as well.

For years scientists have suspected that salicylic acid, a chemical similar to aspirin's active ingredient, helped plants develop this resistance. New research has confirmed that hunch and shown that the amount of this chemical in a plant determines the degree to which it fights off current and future infections. Thomas Gaffney and his colleagues at Ciba-Geigy in Research Triangle Park, N.C., and Basel, Switzerland, report their findings in the Aug. 6 Science.

To assess the importance of salicylic acid, the researchers created "transgenic" tobacco plants. The plants contain a bacterial gene, which codes for a protein that breaks down salicylic acid. The researchers infected three lower leaves of these plants with a tobacco virus. A week later, they observed brown spots at the site of infection and measured the amount of the bacterial gene, its protein, and salicylic acid.

At the same time, the investigators infected plants that lacked the gene. Leaves from those plants increased their salicylic acid production 185-fold.

In contrast, infected leaves from plants with a lot of this protein had just two to three times more salicylic acid, the group reports. Moderate amounts of the bacterial protein led to moderate increases in salicylic acid.

Next, the researchers put virus on the upper leaves of these plants and, six days later, measured the size of the brown spots that developed. They observed that the less salicylic acid the plant possessed, the larger the spots.

"It was nice that everything correlated so well," says Leslie Friedrich, a geneticist at Ciba-Geigy. Also, plants with a lot of the bacterial protein developed larger brown spots at the site of the first infection than did the unmodified plants. This indicates that salicylic acid plays a role in neutralizing the initial viral attack, she and her colleagues suggest.

Ultimately, the researchers would like to understand enough about how systemic acquired resistance develops to create a "vaccine" that would mobilize this defense system prior to any infection, she adds.
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Title Annotation:salicylic acid in plants helps them fight diseases
Author:Pennisi, Elizabeth
Publication:Science News
Article Type:Brief Article
Date:Aug 14, 1993
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