How do plants protect themselves from the bugs that chew on their leaves? In the case of the wild tobacco Nicotiana attenuata, when tobacco hornworm (manduca sexta) caterpillars feed on the leaves a collection of molecules called Green Leaf Volatiles (GLV's) is released by the plant. GLV's are released any time a leaf is damaged, but the interesting thing is that when the damage is done by chewing caterpillars, a different form of the GLV's are produced which attracts Big-Eyed Bugs (Geocoris spp) - a predator for the caterpillars.
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Plants emit two main types of volatile molecules: terpenoids and Green Leaf Volatiles. The terpenoids are emitted from the whole plant and usually after a delay - maybe as much as a day after the damage. The green leaf volatiles are more specific - they are emitted from the damaged leaf itself and it looks like they are produced at the same time as the damage.
Green Leaf Volatiles are typically 6-carbon alcohols, aldehydes or esters. In the case of Nicotiana Attenuata they seem to mostly consist of hexenal, hexenol and simple esters of hexenol. The interesting bit is the alkene portion of these molecules. Alkenes can have one of two basic geometries around the double bond: the Z (or cis) isomer is locked into a u-turn shape and the E (or trans) isomer is locked into a zigzag-like orientation.
Normally, Nicotiana attenuata produces mostly the Z isomer of these molecules and a relatively small amount of the E isomer. However something unusual happens when the damage is caused by caterpillars chewing on the leaves: in this case the plant produces roughly equal amounts of the Z isomer and the E isomer. You and I would probably not notice a difference in the smell of the leaves, but apparently there are bugs that can. When more E isomer is produced, more Big-Eyed Bugs are attracted to the plants. And the big-eyed bug eats caterpillars and their eggs. The E isomer GLV's are a plant distress call and the big-eyed bugs are the cavalry.
How exactly does the plant "decide" which GLV isomers to make? After testing a variety of possible candidates, it looks as though there is an enzyme in the caterpillars' saliva that causes the Z isomers to isomerize to the corresponding E isomers. It is the caterpillar spit that produces the distress call.
If you look closely at the Z molecules and the E molecules you will notice that there are actually two changes that take place. First, the geometry around the alkene switches. In general, the E isomer is more spread-out than the Z isomer and as a result it is lower in energy. Given a choice the alkene will usually adopt the E geometry. If there is a catalyst available, this change is pretty easy to understand.
The second thing that changes is the location of the alkene, the alkene moves closer to the oxygen end of the molecule. Enzymes are very efficient molecules and they are very sensitive to shape. My guess is that the "real" target for the isomerase in the caterpillar saliva is the aldehyde. The aldehyde has a carbonyl group as well as the alkene and the most stable arrangement for these two functional groups is the one in hex-2-enal. When the two double bonds are separated by only one single bond their orbitals are able to interact and form a conjugated system. The conjugated version is more stable than the one where the two double bonds are farther apart and unable to interact with one another.
If improved conjugation in the product is the reason that the alkene moves from the 3-position to the 2-position, why does the alkene move in the alcohol and ester molecules too? The alcohol has only one double bond since there is no C=O, so conjugation is not possible in this molecule. And while the ester does have a C=O, it is too far away to interact with the 2-alkene to form a conjugated system. What gives?
Enzymes can be very selective about the molecules that they react with, but they can also be forgiving if the structure is not exactly correct. A lot of drugs affect specific enzymes in the body - the drug isn't exactly the correct shape, but it's close enough to bind to the enzyme. In the case of the GLV's, the alcohol and ester molecules are close enough to the right shape to bind to the enzyme and react. In the aldehyde the enzyme causes the alkene to migrate as well as change shape because it forms conjugated molecule. Even though the alcohol and ester don't benefit from forming a product molecule that has conjugation, the enzyme treats them the same way it treats the aldehyde and the alkene migrates to the 2-position.
The other curious thing about this is the isomerase enzyme in the caterpillar saliva. I would bet the reason the caterpillars make this enzyme has nothing to do with attracting big-eyed bugs to come eat the caterpillars, that would be counter productive. The plants probably evolved their GLV's to take advantage of this enzyme that the caterpillars make anyway. So what is the isomerase "supposed" to do that benefits the caterpillars?
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Allmann S, & Baldwin IT (2010). Insects betray themselves in nature to predators by rapid isomerization of green leaf volatiles. Science (New York, N.Y.), 329 (5995), 1075-8 PMID: 20798319