Towering canopies, sprawling bushes and fruit-bearing vines all create an aura of peace and grace. Little do we realize that a silent war is constantly going on amidst the leaves we see blowing in the breeze. Plants of all shapes and sizes face a bombardment of pests and grazing animals. But while easy to pluck and disrupt, plants are far from defenseless. Plant defense mechanisms against herbivores — including beetles, worms, cattle, deer and other leaf eating animals — vary from physical, chemical and genetic tactics.
While commercial growers are currently relying on harmful pesticides to ward off invasive species and wayward grazers, research into plant defenses against herbivores could help us develop non-damaging methods of regulating pests and protecting our crops.
Physical Defense Mechanisms
Physical forms of protection are the only ones we can actually see. These defenses include thorns and thistles, cell wall thickness and surface wax on leaves and stems. Plant species have adapted these structural adaptations to act as protection against grazing animals and pests.
Cell wall thickness makes ingesting parts of the plant harder for small herbivores. Wax and small, hair-like thorns cover the leaves of several species of temperate shrubs to ward off feeding animals and protect against various weather patterns.
Thorny plants are particularly common and live in numerous ecosystems and climates. Holly and hawthorn shrubs are common examples, along with locust trees and brambles — which include raspberries and blackberries. Western landscapes sport cacti varieties with sharp thorns and spiky complexions, such as the agave plant and the prickly pear cactus.
Physical defenses were the first form of plant defenses against herbivores seen in ancient farming, with wooden spikes and great barriers to protect fields from wandering animals. Many modern farms still use physical security systems to protect their crops.
Chemical Defense Mechanisms
If a plant’s first defense doesn’t work, it relies on its second. There are a variety of chemical defenses, one of which being the release of toxins and enzymes. If an animal inflicts physical damage on the plant, it provides an entryway for pathogens that alert the plant’s system of the wound. This triggers the release of toxic metabolites, which can sometimes be lethal to animals that ingest them.
Not every variety of metabolite is lethal, however. Some plants give off a pungent odor instead, such as the volatile oils of mint and sage. Others incur a bitter taste.
In fact, the caffeine in your morning cup of joe is the result of the release of a metabolite that causes excessive stimulation! That’s why caffeine gives us an alert buzz, and multiple cups can cause hyperactivity. Other metabolites can cause lethargy upon ingestion. Eating large quantities of apple seeds can cause abdominal pain and lethargy — you can even enter a coma and die.
If the attacker is a microbe, Bactria or fungi, the plant must respond differently. It releases salicylic acid, which creates proteins to attack the cell walls of the invading microbe. These same proteins are also used for communication and can alert nearby plant cells of the attack to prepare them for defense.
The Colorado Beetle
Unfortunately, the Colorado beetle has figured out a way to trick the chemical defense mechanisms of the plants it attacks.
This tricky bug contains gut bacteria that tricks the plant’s system into a miscommunication. While the beetle munches away at the plant leaves of a tomato plant, the bacteria from its saliva tells the plant that instead of a physical wound, the plant is being invaded by a fungus or microbe. The plant then triggers the wrong defense mechanism, and the beetle continues eating, unharmed by the salicylic acid.
Researchers from a study group published in the research journal, “Nature,” in January 2017 are looking into the problem. They believe it could be possible to develop a solution that alters the way the bug’s digestive bacteria is released, and in turn naturally, induce the plant’s correct defense mechanism.
Perhaps the most fascinating form of plants’ defenses against herbivores is their ability to alter their genes. Through a process called “RNA interference,” plants can change their own genes to block protein translation, resulting in malnutrition for the insects that feed on them.
Genetic engineers are working on weaponizing plants’ natural abilities in hopes of developing specific RNA fragments that cause the bugs’ reproductive systems to be compromised and rendered sterile when ingested by invasive insects. This new technology could change the course of insecticide use and decrease the amount of damage done to the ecosystems surrounding large crop farms.
The system also allows researchers to get specific. Producing RNA fragments specifically made to target an individual species of worm or Beetle could minimize the impact on other insects that are beneficial for your garden.
Until new techniques come to light, the best way to protect your plants at home is crop rotation. Your plants have plenty of natural defense mechanisms against herbivores, and you might be surprised how well they can do their job! Let’s hear it for our beautiful, silent warriors.