Let’s start with plant defense in general. To follow this blog, here are some key things to know:
Plants can’t run away from their attackers. Instead, over evolutionary time, plants have developed secondary metabolism (aka, not photosynthesis- that is primary metabolism) to make toxic, tough, unpalatable, or otherwise unpleasant experiences for the bugs that try to eat the plants (I use the term bug here loosely to include all herbivorous arthropods, fully knowing that technically a true bug belongs to the phylogenetic order Hemiptera). Plant defense is when plants resist being eaten by bugs.
Plant defense has many flavors:
Chemical defense: the toxic and bitter stuff. We like to consume many of these things that plants designed in order to kill their opponents. (Small apology- I will only minimally anthropomorphize plants throughout my blog.)
Mechanical defense: the tough stuff. Usually this comes in the form of thicker and rougher leaves, or leaves covered in trichomes (fancy word for hair, and very effective: picture walking on velcro as an aphid… difficult right?).
Direct defense: the plant makes a toxic or tough compound that deters bugs.
Indirect defense: the plant relies on predators to provide the defensive service. (This is my personal favorite!) Here is how it works: plants under attack send out a cry for help either as a sugar reward or a signal in the air to attract predator bugs (ants, wasps, or spiders) that fill a hit-man role and kill or evict the herbivore from the plant. Badass! Can you attract ants to be your bodyguards? Didn’t think so.
Inducible defense: This is the on/off switch for plant defense. Usually it behooves the plant to use energy defending themselves when herbivores are around, and to save that energy when they are not under attack.
Constitutive defense: Leaving the secondary metabolite lights on all the time, even when there is nobody home (aka no herbivore attack)
That’s all for vocabulary today. I know, plant biology can get pretty crazy exciting. Get ready for the underlying evolutionary tradeoffs and hypotheses explaining why, how, and when plants defend themselves.