SOS signals calling for immediate help from nearby animals can help plants to fend off attacking herbivores, new research published in Science has revealed.
When plant tissues are damaged they rapidly release fragrant chemical mixtures called Green Leaf Volatiles (GLVs). These are the chemical equivalent of a plant screaming in pain and serve to attract a range of predators, which use these signals to track down plant-eating prey. 
That much was known. But what has now become clear is that some plants can also make highly specific emergency calls for help when attacked by certain pests.
Max Planck Institute for Chemical Ecology, Germany based researchers Silke Allman, Ian Baldwin and colleagues found that when the leaves of tobacco plants came into contact with the saliva of the tobacco hornworm caterpillar, the mixture of chemicals released by damaged leaves was dramatically different compared with control leaves sprayed only with water.
The researchers then tested artificially-recreated mixtures of the smells that had been released by the leaves in each condition. In these experiments the substances were added to tobacco leaves and the numbers of arriving caterpillar-predators were counted. The researchers found that leaves treated to resemble plants under caterpillar attack attracted significantly more predators.
A component of the caterpillar's saliva, the team found, was triggering the rapid production of the emergency signal. By understanding its function better, the researchers anticipate that it might be possible to develop "green" pesticides that will enable plants to mobilise a bespoke army of predators to take out whatever is making a meal of them.
So next time you smell that summery bouquet of freshly mown lawn, you might also want to wonder what message the grass is really trying to convey, and who that message might be for...
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