Plant defense compounds: “The enemy of my enemy …”

Plants really have it rough; they struggle for sunlight, fight for resources with neighbors, and just have to make the best of things being at the bottom of the food chain. They are victims of circumstance, if they root in a sunny fertile meadow life might be smooth sailing, but if the neighborhood gets rough and predators are abundant, packing up and leaving town isn’t exactly an option. Just thinking about it makes me claustrophobic, but most plants are far from defenseless. Evolution has lead to some creative defensive adaptations that make herbivores think twice before munching away. The most noticeable herbivore deterrents include spines, thorns, toxins, and foul tasting compounds, but there is so much more going on in the world of plant protection than just these direct defenses.

For years it has been known that some plants emit volatile compounds when mechanically damaged. A chemical signal is released when a leaf or stem is broken, and traditionally it was understood that this chemical is relaying a message to other branches of the same tree, or nearby plant neighbors; acting as a SOS or call to man the molecular defenses1. A recent paper investigated this topic and found that, at least in one case, these volatile compounds are actually signaling to insects2.

In a two-year field study Schuman and colleagues tested the role of herbivory-induced plant volatiles (HIPV) in the wild tobacco plant (Nicotiana attenuata).   This research explored the HIPVs as part of a defense strategy against the tobacco hornworm, a natural and aggressive predator of the tobacco plant. The authors found that following the HIPV signal, higher order predators would flock to the besieged tobacco plant. In this case, the chemical signal isn’t really a SOS but perhaps it is a dinner bell. These higher order predators would make a meal of the tobacco hornworm and inadvertently defend the tobacco plant. Genetically engineered plants, unable to produce HIPVs, experienced much higher herbivore damage, highlighting the importance of this process.

It seems like cut-and-dry symbiosis, where everyone except the hornworm benefits from this arrangement of HIPV signaling. However, the tobacco plant leaves nothing to chance, and taking the role of dinner host seriously, it produces a secondary defense compound. An anti-digestive (trypsin protease inhibitor) is generated that makes the hornworms sluggish and easier prey, ensuring that more hornworms are removed from the plant. It is astonishing to realize how much time and energy plants have invested into defenses that are entirely external, relying on communication with other organisms for protection. Although not the first to suggest this, Schuman et al. provide a compelling argument for HIPV signaling as interspecies communication, and they remind us that nothing is ever as simple as it would seem, not even a plant.

  1. Heil, Martin, and Juan Carlos Silva Bueno. “Within-plant signaling by volatiles leads to induction and priming of an indirect plant defense in nature.” Proceedings of the National Academy of Sciences 104.13 (2007): 5467-5472.
  2. Schuman, Meredith C., Kathleen Barthel, and Ian T. Baldwin. “Herbivory-induced volatiles function as defenses increasing fitness of the native plant Nicotiana attenuata in nature.” Elife 1 (2012): e00007