Researchers have long been curious as to just how a plantwide resistance is signalled in plants even though they lack cells that signal and fight infection.
Now, the team led by Sang-Wook Park and Daniel F. Klessig, an adjunct professor in plant pathology at Cornell, has found that this happens because of the aspirin-like compound called methyl salicylate that alerts a plant's immune system to shift into high gear.
This phenomenon is called systemic acquired resistance and is known to require the movement of a signal from the site of infection to uninfected parts of the plant.
Methyl salicylate is a modified form of salicylic acid (SA), which has been used for centuries to relieve fever, pain and inflammation, first through the use of willow bark and, since 1889, with aspirin, still the most widely used drug worldwide.
SA has already been identified as a defence-signalling molecule in plants.
As a part of their study the researchers used plants in which the enzyme alicylic acid-binding protein 2 (SABP2) function was either normal, turned off or mutated in the infected leaves or the upper, uninfected leaves.
In 2003 and 2005 it had been found that SABP2 is required for systemic acquired resistance and converts methyl salicylate (which is biologically inactive as it fails to induce immune responses) into SA, which is biologically active.
Klessig's group revealed that SABP2 must be active in the upper, uninfected leaves for systemic acquired resistance to develop properly. By contrast, binding to SA must inactivate SABP2 in the infected leaves.
"This inactivation allows methyl salicylate to build up. It then flows through the phloem, or food-conducting 'tubes' to the uninfected tissue, where SABP2 converts it back into active SA, which can now turn on the plant's defences," Klessig said.
Klessig added that the finding could have wide consequences, boosting crop production and reducing pesticide use.
"By finally identifying a signal that moves from an infection site to activate defences throughout the plant, as well as the enzymes that regulate the level of this signal, we may be in a position to alter the signal in a way that enhances a plant's ability to defend itself," Klessig added.
The study has been issued in the journal Science.