Some injured plants can change the way that their neighbours taste, new research shows -- an insight that could help scientists to develop the next generation of pesticides.

Many plants have a 'neighbourhood watch' scheme. Under threat, they alert nearby plants to imminent danger. They use different alarm calls depending on the nature of the threat, be it the jaws of hungry insects or gardening shears.

Lima bean (Phaseolus lunatus) leaves attacked by spider mites (Tetranychus uticae) release chemicals that make them and their neighbours unpalatable to the mites, Junji Takabayashi of Kyoto University and colleagues now report¹. In the laboratory at least, these airborne alarm signals activate genes in neighbouring plants, prompting them to produce the same repellents to ward off the mites.

But when the experimenters chopped the leaves with scissors, the plants switched on genes for "green leaf volatiles," Takabayashi explains, "that are antiseptic and act locally to sterilise wounds." Just as the human body quickly sets about closing an open wound, a trampled or cut plant seals off the area to protect itself from microbes and to prevent opportunistic insects from gaining a free dinner.

Intriguingly, neighbouring plants largely ignore these similar yet distinct chemical distress signals. They seem to be 'aware' that something is happening (certain genes are activated). But they do not issue defence chemicals. Which makes sense. To do so is energetically expensive and is unlikely to deter a rampaging cow or combine harvester. Also, some defence compounds are very toxic and can damage the plant.

So how can plants tell the difference between an experimenter armed with scissors and an aphid sporting gnashing mandibles? Compounds in the insect's saliva probably give its presence away.

Takabayashi's team are not the first to spot plant defence. Some plants produce weapons tailored to their attackers. One South American potato releases a volatile that mimics the alarm call of infesting aphids, making them flee in panic. And several plants, including lima beans, emit substances to attract other insects that prey on or parasitize the attacking insect.

"[Plant defence] has potential evolutionary value for both plant and predator," comments John Pickett of the Institute of Arable Crops Research, Rothamsted, UK. "Or it could be that predators have simply learnt to associate volatiles with particular insects." He adds, "it is much easier for the predator to find an insect if the whole plant acts as a beacon".

Takabayashi's group is now looking to see if plants downwind from an insect-damaged plant produce volatiles to recruit predators as well as switching on their internal defences in preparation for being attacked.

They hope that improved understanding of these airborne communications systems among plants will suggest new, more environmentally friendly methods of insect control, as they explains in Nature1. "We may be able to produce synthetic volatiles that could activate defence genes," says Takabayashi, "[or] genetically manipulate the distress responses in plants.

Alternatively, you could avoid genetic engineering Pickett suggests, "by planting mixed crops where one plant is particularly susceptible to insect attack and it switches on the defences in the harvest crop."