Author: Ellie Wang
Editors: Sophia Chen and Rachel Chen
Artist: Felicia Chen
Compared to animals, plants seem to be utterly defenseless. However, that could not be further from the truth. As sessile organisms, plants have developed their own unique responses to threats, utilizing both physical and chemical defenses.
The first and simplest line of defense for many plants is their anatomy. Some physical traits can help prevent plants from being eaten. For example, roses have thorns and hawthorn trees have spikes, which make it painful for animals to consume them. Other plants have more subtle methods of warding off herbivores. Grasses, such as corn, rice, and wheat, absorb silicon from the soil they grow in, turning the leaves abrasive. Although this does not directly prevent animals or insects from consuming them, it does dull their teeth or mandibles. There are also specific defense mechanisms for protecting against insects. Some leaves have trichomes on their surfaces, creating a fuzzy outer layer that insects have a difficult time penetrating.
Besides physical deterrents, plants also utilize chemical substances to fend off herbivores. Plants have much larger genomes than animals in part due to the number of enzymes required to create their many chemical compounds. Plants in the same families usually have similar compounds, but convergent evolution sometimes causes very disparate plants to produce similar effective chemicals. To ensure that the toxin only harms the herbivore instead of the plant itself, the toxin is stored in an inactive form; the reactive portion of the compound binds to larger molecules, unbinding and activating only when the toxin is ingested. In the case of maize, benzoxazinoids are stored in its vacuoles, which spill into the cell cytoplasm and activate as the herbivore takes a bite or when it is digested, protecting the plant from further damage.
A notable type of chemical defense consists of proteins. Enzymes called protease inhibitors disable the function of a protein called protease, which are essential for digestion. Nitrogen, a necessary element for survival, is often found in the amino acids that make up proteins, but with nonfunctioning proteases, herbivores cannot break down and absorb plant proteins, leading to slowed development and potentially fatal consequences. Other protein-disabling enzymes include threonine deaminase, which breaks down the amino acid threonine. Once a herbivore eats a tomato or potato plant, this enzyme deprives them of the essential amino acid, which once again limits the herbivore’s growth.
But why aren’t humans affected by this vast arsenal of defenses? The answer is simple: food processing and cooking. Properly preparing and cooking plants will get rid of the toxins that incapacitate other animals. For example, cassava, which is full of hydrogen cyanide, can be neutralized by simply soaking its roots in water, physically removing the compound from the crop. Cooking eliminates anything that cannot be easily removed, since enzymes deform under temperatures that the protein structure cannot stay stable in. Enzymes deform and lose their function in heat, becoming perfectly safe for consumption.
Although humans do not need to worry about plant defenses that other species do, it is still beneficial to study them to develop the best defenses for our crops. With the genetic engineering available to us, we can develop crops that can better withstand herbivores, eliminating the use of harmful pesticides and reducing the costs for farmers, consumers, and the environment alike.
Citations:
How Plants Make, Store and Use Toxins.
Plant Defenses. learn.genetics.utah.edu/content/herbivores/defenses.
War, Abdul Rashid, et al. “Mechanisms of Plant Defense Against Insect Herbivores.” Plant
Signaling & Behavior, vol. 7, no. 10, Oct. 2012, pp. 1306–20.