How plants 'shout attack!'

The mites themselves could soon become the meal

By BBC News Online's Jonathan Amos

Plants know the difference between being eaten and being trampled on, according to Japanese and German researchers.

The Kyoto University-led team has found that lima beans under attack from spider mites will release chemicals that not only attract the insects' natural predators but also warn nearby plants that there is danger about.

However, if the lima beans are simply damaged - by an agricultural tool or a clumsy cow - the subtly different chemical distress signals sent out by the plants are ignored by their neighbours.

The research team say their knowledge could lead to new ways to protect plants from insect pests.

Details of their work are published in a paper that appears in the current edition of the journal Nature.

Plant aroma

Dr Junji Takabayashi, an associate professor at Kyoto, and colleagues, monitored the chemical interactions of lima bean ( Phaseolus lunatus) leaves in a glass container over several days. Some of the leaves were infested with female spider mites (Tetranychus urticae), others were not.

The team found that the plant tissue under direct attack activated genes, some of which contributed to the release of volatile compounds known as terpenoids. These are a well-known class of chemicals that influence plant aroma.

Lima beans are cultivated for their edible seeds

"Very probably, the spider mites inject some saliva into the leaves and this acts as an elicitor," Dr Takabayashi told BBC News Online. "When the plant tissue receives the elicitor, the leaves would start responding by producing volatiles."

These organic compounds make the lima bean leaves more attractive to the carnivorous natural enemies of the mites. But the researchers were also able to show that the terpenoids could be "smelt" by the neighbouring leaves with no infestation, prompting them to roll out their own defences.

"We found that a large number of genes are activated, not only in the infested leaves but also in the receiver leaves," Dr Takabayashi said.

Modified plant

However, when the plant tissue was "artificially wounded" by having holes punched through it by the researchers, fewer genes were activated and subtly different chemicals - green-leaf volatiles - were released.

This "distress call" did not trigger the defence response in nearby plants, the team were able to show.

"Neighbouring plants can recognise such specificity and differences," the scientists write in their Nature paper.

They hope the knowledge could prove useful in developing new ways to protect plants from insect pests.

"Maybe we can enhance the ability to respond to the volatiles so that clean plants standing next to infested plants react quicker and more vigorously than usual when the terpenoids are picked up," Dr Takabayashi said. "This would be a modification in a transgenic plant."