To test this assumption, the researchers started a series of experiments using Manduca sexta eggs and different blends of (Z)-3-hexenal and (E)-2-hexenal. The eggs were glued to tobacco plants growing in the field and different mixtures of odorant signals were applied to adjacently located cotton swabs. After 12 and 24 hours, the fate of eggs on the leaves was determined. The result: Plants that had been perfumed with the Z-lure had only 8% of their glued eggs attacked, whereas plants perfumed with the E-lure had lost 24% of the eggs. And the eggs had been eaten by Geocoris. The speed of plants' defense triggering was particularly impressive: it takes less than an hour to trigger the conversion of the Z-lure to the E-lure and less than 24 hours for the predation of the eggs. "Other indirect defense mechanisms of plants require the activation of new metabolic pathways for the release of odorant signals, and these responses are much slower," says Ian Baldwin. The release of (E)-2-hexenal after its rapid and simple conversion from (Z)-3-hexenal is so rapid that it provides the prey-searching predator with enough information to pinpoint the exact location of the feeding herbivore on a plant. And it is this information which makes the release of volatile "alarm calls" an effective defense strategy for plants.
But which substance is betraying the caterpillar? "The simplest assumption is that the larval oral secretions contain a special enzyme, probably an isomerase, which rearranges the Z:E ratio of the aldehyde in favor of (E)-2-hexenal," says Silke Allmann. To test this assumption, the oral secretion was briefly heated and subsequently applied on wounded leaves. (E)-2-hexenal was now no longer produc
|Contact: Ian T. Baldwin|
Max Planck Institute for Chemical Ecology