New research is now unravelling one of the most devious means plants use to deter attackers - and one whose effects are more insidious than a mouthful of bitterness or a punctured skin.
It is becoming clear that many plants, when set upon, are capable of summoning bodyguards in the form of predators or parasites that feed on the pest.
This 'cry for help', which is achieved by the plant releasing chemicals that are borne on the wind, is highly selective: only specific enemies of the pest respond to the call.
At Wageningen Agricultural University in the Netherlands, a team led by Dr Marcel Dicke has been studying how the lima bean plant, Phaseolus lunatus, responds when it is attacked by the tiny but highly destructive two-spotted spider mite, Tetranychus urticae. 'An individual spider mite does not cause a problem, but the population growth of these creatures is enormous, with the population doubling about every week,' said Dr Dicke.
When the two-spotted spider mite colony starts to feed on the leaves of the bean plant, the plant releases volatile chemicals, termed synomones, which drift in the air and alert a predator of T urticae to their presence.
This predator is another kind of mite, Phytoseiulus persimilis. The chemicals are released not only from the leaves which are being eaten, but from the whole plant. Furthermore, the chemical is given off only if the damage is caused by the two-spotted mite. When the scientists artificially damaged the leaves, the chemicals were not released.
Like an entomological version of the Bisto Kids, the predatory mites follow the attractant released by the plant to their meal. According to Dr Dicke, the predatory mites, like the spider mites, also have a very high population growth once they have found a rich food source. 'This has led us to the hypothesis that a plant that is infested by spider mites would be able to save its life by affecting the influx of predatory mites.'
It is now becoming apparent that this defence strategy is more widespread than at first suspected. 'When we started, we only showed it for the lima bean,' said Dr Dicke. 'But soon afterwards we discovered the phenomenon in many plants. In fact there is not a plant so far tested where this ability was not found, and in many cases similar chemicals are involved.'
Meanwhile at a US Department of Agriculture research station in Florida, Dr Ted Turlings has been working on a similar phenomenon. Dr Turlings's laboratory is concentrating on what happens when caterpillars attack corn seedlings. Dr Turlings has shown that, like the lima bean, the caterpillar-damaged seedlings release volatile chemicals. These attract to the plant a wasp which is a parasite of the caterpillar.
The researchers at both centres are now piecing together the events that result in this biochemical summoning of bodyguards.
Spider mites feed on plants by inserting a hollow feeding tube, the stylet, through the plant's outer cells and injecting saliva containing various digestive enzymes.
It is likely that the action of the enzymes results, directly or indirectly, in the formation of a specific chemical at the point of attack which is then transported through the plant, 'alerting' the other parts to the infestation and triggering the widespread production of the synomones. Internal chemical messengers of this sort are termed 'endogenous elicitors'.
In a recent series of experiments, Dr Dicke's team soaked infested leaves in water for seven days, removed them and then placed in the water leaves from an uninfested plant.
They found that following this treatment, the uninfested leaves became highly attractive to the predatory mites. This experiment demonstrated that an endogenous elicitor had indeed been produced in the infested leaves, had leached out into the water, and was capable of triggering the 'infestation response' in untouched leaves.
Dr Dicke believes that his team's research could lead to a number of strategies for pest control.
He has found that some strains, or cultivars, of the lima bean exhibit the 'bodyguard' response more strongly than others. This opens the prospect for selectively breeding those that produce higher concentrations of the chemical attractants when attacked.
In the US, Dr Turlings's team is now focusing its work on identifying the elicitor produced by corn seedlings in response to attack by caterpillars. If the elicitor can be identified, it may be possible to produce it synthetically. This, says Dr Turlings, 'may provide us with a tool to use in the possible manipulation of crops to directly and indirectly enhance control of pests'.Reuse content