Relaxing in a summer meadow, listening to the buzz of bees, watching a caterpillar leisurely inch its way up a stalk, aphids obligingly allowing ants to rub them with their antennae and milk them – a peaceful scene on a gentle slope above. But what an illusion!

These are dramas playing out in meadows, forests and hedges – dramas of death and downfall, hunting and enticement, attack and defense. Just because humans lack the appropriate senses, we fail to notice the daily fight for survival that continuously goes on between plants and their adversaries: insects, fungi, bacteria and viruses.

Alluring calls, warning calls, calls for help – we rarely perceive them because they are not acoustically or optically encoded. We know them only as the seductive fragrance of a spray of flowers, or as the heavy bouquet of a freshly mown field. Fragrant chemicals, emitted in extremely low concentrations, are what plants use to communicate. If we could see them, a meadow or a section of forest would be awash in trails of molecules and softly billowing clouds of fragrance. If we could hear them, the screams of battle would destroy any summer idyll. But people have no natural sense or comprehension of the molecular language of plants.

While primary metabolites, such as amino acids and sugars, perform fundamental survival functions, secondary metabolites are rather an indirect benefit to survival: Plants use them, for instance, as dyes and scents in flowers and fruits to attract pollinators or animals that disperse the seeds. But they also make up a large part of the plants’ defense forces, either as messenger substances, like the plant hormones jasmonic acid and salicylic acid, or as defense substances, like the mustard oil glycosides, the terpenes or the tannins.

It takes some time before a plant activates its defenses: genes for their production are adjusted up and down, enzymes are produced, and defensive compounds are synthesized.  The feeding attack of a caterpillar changes the electrical potential of the affected cells and their neighbors in such a way that the resulting depolarization wave propagates alarmingly, within just minutes, across the entire leaf.

Initial analyses using Arabidopsis thaliana show that the saliva does, in fact, cause numerous completely different genes in the plant to be adjusted up or down differently than with pure wounding. “And not only in the wounded leaf, but also in far distant regions of the plant,” says Boland. This is evidence that there are signal substances in the saliva that are responsible for transmitting the stimuli from the damaged location to other parts of the plant.

Bean, rice and potato plants, form new tissue or produce toxic substances to effectively repel the pests’ eggs. Elms, pines and even brussels sprouts use fragrances to call parasitoid wasps to their aid. These destroy the leaf parasite by using its eggs as living incubators for their offspring.

The lima bean from Latin America,  uses a “sweet call for help” to attract virtual defense troops when attacked by aphids or beetles. Nibbling on their leaves triggers nectar production in the plant: Tiny shimmering beads of nectar are then secreted by small extrafloral nectaries. Their sweet fragrance attracts ants that take on the attackers and suck up the high-calorie treat as their reward.  But the lima bean not only attracts foreign aid, it also uses fragrances to warn neighboring plants that can thus prevent an attack by likewise producing drops of nectar.

"Can you smell what I'm trying to say?"

Source:  The Max Planck Institute of Chemical Ecology, Marcus Anhäuser