Common bond

By Andy Coghlan IMMUNE systems in plants are switched on and fortified by nitric oxide, a gas long known to do the same job in animals. In mammals, nitric oxide plays a key role in the transmission of nerve signals, but its main task is to orchestrate the first line of defence against infections. Now, two groups of botanists have independently shown that nitric oxide also plays a key role in the immune systems of plants. This suggests that plants and animals inherited the core components of their immune systems from a common evolutionary ancestor. “We’ve discovered plants utilise nitric oxide much as animals do to turn on the immune system,” says Daniel Klessig of Rutgers University in Piscataway, New Jersey. “It also appears that many of the critical components the animal system uses to regulate nitric oxide signals are also used in plant defence systems.” Klessig and his colleagues made their discovery by comparing normal tobacco plants with strains that are unusually resistant to the tobacco mosaic virus, which causes spots on tobacco leaves. When they exposed the resistant strain to the virus and examined plant tissue, the researchers found five times as much nitric oxide synthase, the enzyme that manufactures nitric oxide, as in the normal strains. This strongly indicates that nitric oxide plays an important role in the immune system. To follow up the result, Klessig’s team dosed the leaves and individual cells of tobacco plants with nitric oxide synthase from rats, together with a substance that drives the enzymes. They found that this activated a gene called Pathogenesis related-1 (PR-1), which regulates plant immunity ( Proceedings of the National Academy of Sciences, vol 95, p 10 328). These findings are complemented by those from a separate study by Chris Lamb and his colleagues at the Salk Institute in La Jolla, California, which showed that in soya bean cells, nitric oxide sets off a chain of biochemical commands almost identical to that tripped by nitric oxide in neutrophils, white blood cells that engulf and destroy invading bacteria. The cells do this with reactive oxygen intermediates, powerful chemical agents such as hydrogen peroxide and superoxide ions, which are thought to deliver the killer punch. Lamb, working with a team at the Noble Foundation in Ardmore, Oklahoma, also showed that nitric oxide is the vital missing link in the “hypersensitive disease” response, where infected plant cells commit suicide kamikaze-fashion in order to stamp out pockets of disease before it can overrun the whole plant (Nature, vol 394,
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