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Bad Vibes

December 15, 2014
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D. plexippus eating a leaf

A larva feeding on a leaf sends vibrations throughout the plant, which will initiate a defensive response

I think that, mobile, intelligent creatures that we are, humans have an unconscious disdain for plants and their unique methods of protecting themselves from predators. I also think that this disdain stems from a certain ignorance of the way a plant will handle an herbivorous threat. Plants don’t respond to stimuli the same way humans do: If we are too hot in the sun, we can move to shade; if we are in the rain, we can move to shelter; if we feel threatened, we can move away from the threat. Plants cannot respond to stimuli through movement; nevertheless, a response does occur, no matter that it can’t be observed to the naked eye.

There are many types of plant defense mechanisms: physical, like thorns; location, like growth on the face of a cliff; timing, like variation of seasonal growth patterns; and chemical, to name a few. While defense is the focus of this article, first it must be understood how scientists study these mechanisms. One method is the use of model organisms. All model organisms have three things in common: they have been studied extensively and thoroughly, they grow or mature quickly, and are easy to maintain in a laboratory.There are hundreds of thousands of plant species that we know of, but a very select few are used in experimental studies as model organisms.  One model organism, Arabidopsis thaliana, is a small flowering plant that is commonly used in molecular biology, plant biology, and genetics experiments. This plant has one of the smallest genomes of all flowering plants, making it an ideal model organism for its simplicity. It is commonly found in Europe, Asia, and northwestern Africa, and is an annual plant.

Flowering Arabidopsis thaliana

Flowering Arabidopsis thaliana

 

In 2014 a study was conducted with Arabidopsis thaliana to examine the chemical response to vibrations caused by caterpillar feeding (Appel et al. 2014). Two scientists at the University of Missouri asked whether the acoustic energy generated by the feeding of herbivorous insects was detected by plants, and if so, what was their reaction. Two experiments were done. In the first, each plant had a module with a pre-recorded vibration of caterpillar feeding behavior placed on one leaf. Twenty four hours later, the leaves were harvested, freeze-dried, and ground into a powder to prepare for chemical extraction. The second experiment was similar to the first, but in addition to feeding vibrations, they also had wind and insect song vibrations placed on individual leaves, to see if the plants could distinguish herbivory from other, non-violent vibrations.

The leaves that had experienced feeding vibrations from the module released two chemicals. One, glucosinolate, is a pungent compound that is toxic in high amounts and is shown to act as a natural pesticide by altering the eating behavior of herbivores. This short-term defense is a generalized response to deter a large variety of herbivores after damage has occurred; it is a reactive mechanism.  The other chemical, anthocyanin, is believed to visually repel insects by indicating toxic properties through color. This chemical is responsible for the red, blue, and purple colors of many flowers, as well as the autumn foliage of many trees. This long-term defense is preventative. It is a specific, targeted response; a co-evolution of the plant and its common enemies who will recognize the threatening colors (Karageorgou et al. 2005). Interestingly, the researchers found that Arabidopsis thaliana could not only distinguish when their leaves were being eaten, but could also tell whether the vibrations were from wind or an insect song. Note that in this instance, the insect song the researchers used was for the purpose of mating or pollination, not herbivory.

This is only some of the many plant chemical defenses in nature. Some plants, when attacked by herbivores, will release salicylic acid, a plant hormone capable of causing chemical burns at high concentrations. Salicylic acid is also a crucial element of plant development and growth, as well as photosynthesis and the transportation of water throughout the plant. Other plants will release jasmonic acid when their leaves are eaten, another plant hormone that plays a role in regulating plant growth. Both jasmonic acid and salicylic acid prohibit growth and development in insect larvae (War et al. 2014).

Wind blowing through T. monococcum and M. chamomilla

Although wind will cause vibrations, plants recognize this as a non-hostile force

It is clear that plants are not intelligent as humans would understand it. They are not mobile, they cannot see, hear, or feel as humans do. What plants have is more subtle: millennia of evolution have provided a sensitivity to the very vibrations in the air. This sensitivity is so precise that plants can distinguish an herbivorous attack from an insect song or a breeze, and they have numerous chemical deterrents in response to threats. Plants may be sessile, but they are not helpless. This research sheds new light on plants and I believe it also lends them a certain amount of respect for the methods developed to protect them from harm.

Resources:

      1. War A, Sharma H (2014) Effect of Jasmonic Acid and Salicylic Acid Induced Resistance in Groundnut on Helicoverpa Armigera. Physio Entomol 39:136-142. doi: 10.1111/phen.12057
      2. Appel H, Cocroft R (2014) Plants Respond to Leaf Vibrations Caused by Insect Herbivore Chewing. Oecologia 175:1257-266. doi: 10.1007/s00442-014-2995-6
      3. Karageorgou P, Manetas Y (2005) The importance of being red when young: anthocyanins and the protection of young leaves of Quercus coccifera from insect herbivory and excess light. Tree Physiol 26:613-621. doi: 10.1093/treephys/26.5.613
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