A tobacco plant may look like a harmless, tempting feast for herbivores, but it is capable of both direct and indirect chemical warfare. When attacked by hungry insects, the plant releases a complex mix of volatile chemicals. Some of these chemicals directly assault herbivore senses to make feeding more difficult, but a new study indicates that GLVs (Green Leaf Volatiles) go for an indirect attack by attracting natural predators that will feast on either the herbivores or their eggs.
A number of laboratory studies have shown that GLVs attract carnivores, but not many observations of this phenomenon have occurred in nature, and nobody was really sure how GLVs manage to draw them in. Silke Allmann from the Max Planck Institute of Chemical Biology in Germany and the Swammerdam Institute for Life Sciences in the Netherlands and Ian T. Baldwin from the Max Planck Institute worked together to show that certain herbivores undermine their own survival, as plants can use a component of the insect's spit to create their defenses. Their work appears in today's issue of Science.
One of Allmann and Baldwin’s key observations is that there is a difference between the GLVs released by mechanically damaged and herbivore ravaged tobacco plants (Nicotiana attenuata plants). The difference lies in isomerization, specifically in how chemical groups are arranged around a double bond.
Mechanically injured plants release mostly (Z)-GLVs, isomers where the main chemical groups lie on the same side of a double bond. On the other hand, plants damaged by larvae of the tobacco hornworm (Manduca sexta), release a roughly equal mixture of (Z)-GLVs and (E)-GLVs, isomers where the main chemical groups lie on different sides of a double bond.
There is an energy barrier between the (Z)- and (E)-isomers, so some sort of catalyst is required for the chemical transformation to occur. Thus, something unique to the bite of the tobacco hornworm induces a rearrangement around the double bond of GLVs.
Allmann and Baldwin confirmed that oral secretions are responsible for the isomer rearrangement by isolating hornworm saliva and testing its effectiveness at converting a control (Z)-isomer. They mixed saliva with (Z)-hex-2-enal, an extremely common aroma product of green leaves like cabbage, and found that over 50 percent was converted into (E)-hex-2-enal.
Allmann and Baldwin attempted to determine the ingredient in the oral secretion that is directly responsible for the isomerization by running isomer conversion reactions using known components of Manduca sexta saliva. They also tried experiments at an elevated temperature of 90°C and in a basic buffer with a pH of 9 (Manduca sexta saliva is alkaline).
In the end, they were unsuccessful in figuring out what caused isomer conversion, but they did eliminate several Manduca sexta components (fatty acid-amino acid conjugates, β-glucosidase, and glucose oxidase) from the list of possibilities. In addition, they determined that the isomerization reaction is sensitive to heat, as the isomer conversion will not occur at 90°C. Notably, they also found that the source of saliva is important. Manduca sexta saliva is far more active than the oral secretions of the two other herbivores they tested (Spodoptera exigua and S. littoralis).
Ultimately, Allmann and Baldwin wanted to demonstrate the importance of (E)-GLVs to plant defenses in a field experiment. They conducted their work on native tobacco plants of the same size and developmental stage in the Great Basin desert of southwest Utah. They swabbed the plants with either a solution of 1:1 (Z)/(E)-GLVs ratio to mimic herbivore wounded plants, or 9:1 ratio to mimic mechanically damaged plants.
They then glued three herbivore eggs to each plant. They counted the eggs at 12 and 24 hours after the application of GLV mixtures to see how many survived predators. Eggs glued to plants treated with a solution of 1:1 (Z)/(E)-GLVs ratio were significantly more likely to be eaten.
Allmann and Baldwin’s findings enhance our understanding of plant defenses. An obvious direction for future work involves determining how common this defense mechanism is. Out of the three herbivores they tested, only one produces significantly active oral secretions. They also tested only a single type of plant (Nicotiana attenuata); it is important to extend these studies to include other herbivore and plant combinations. Finding the exact saliva enzyme or combination of components that causes the isomer transformation may be critical for practical applications, as well.
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