The Plant Armory Part 1 introduced plants as predominantly sessile organisms: they tend to live out their lives in one spot. Being sessile is a significant disadvantage when you are at the bottom of the food web. When it comes to flight or fight against the herbaceous predators prowling about, flight is not an option, so plants have become masters of putting up a fight.

In Part 1, the physical traits used for defense were described. Plants, however, also have toxic chemical defenses. Plant toxins can be present in all parts of a plant, such as the leaves, stems, buds, fruits, and roots. These toxins are Plant Secondary Metabolites (PSMs), primary metabolites being the carbohydrates and proteins a plant needs to live.

Plant toxin. photo by Sarah Lorse

Many of the plant toxins most familiar to people, such as nicotine, caffeine, opioids, and capsaicin (the compound that gives peppers their kick), are found in plants growing in warmer climates. However, that does not mean plants in regions such as the PNW are lacking in toxins of interest to people. Kinnikinnick (Arctostaphylos uva-ursi) is a plant native to the Salish Sea region that has hallucinogenic properties when smoked. Common foxglove (Digitalis purpurea) is a non-native plant that has been naturalized in our area. Foxglove is highly toxic to most animals, including humans, when the plant is ingested, but that same toxin is used to make a heart medicine. Many of the beneficial properties of native plants are only known to indigenous people and those studying ethnobotany. The discovery of a beneficial plant toxin can lead to industrial growing of the plant species or exploitation of the wild population.

Kinnikinnick. photo by John F. Williams

Skunk cabbage. photo by John F. Williams

Some VOCs are produced regularly by plants, while others are only produced as a response to an attack. Jasmonic acid is a plant hormone responsible for the production of VOCs. When a pest, such as a parasitic microorganism, decides to make a plant its host, the hormone triggers the expression of genes that produce the compounds used to battle the microorganism. The smell of fresh cut grass in the summer brings up fond memories for many of us. That recognizable scent is due to VOCs. One theory for why grass is so fragrant when mowed is that when a blade of grass is cut, like a cut to an animal’s skin, it creates an opening into the internal structure making the plant vulnerable to bacterial infection. As the blade of grass seals up the wound, VOCs are released.

VOCs are also important in plant-to-plant interactions. While animals use many methods to communicate with others of their species, for most of recorded history it was assumed that plants were not much more interactive with each other than a rock is to a neighboring rock. Research has shown that is grossly incorrect. Plants do communicate with other plants, both with individuals of their own species and with other species. One method of communication is through VOCs. In a study it was shown that when potato plants were exposed to VOCs of onion plants, there was a decrease in how many aphids were found in the potato field compared to the fields without exposure to onions.

Some plants have developed relationships with animals where the animals help defend the plant. Syrphid fly (hover fly) larva are voracious predators, seeking out aphids. The adult flies are attracted to flowers and will lay their eggs on flowering plants with an aphid population. The plant provides a source of food for the syrphid fly larvae, and in return the flies keep the plant from being overrun by aphids. A study from Portland, Oregon, indicated that English Laurel (Prunus laurocerasus), which is invasive across the PNW, produces chemicals that incite local ants to protect the laurel when under attack from insect herbivores, such as weevils.

Both physical and chemical defenses require energy to be produced, the same energy a plant uses to grow. Some constitutive defenses, such as bark, are constantly being produced. But others are inducible, meaning they are only produced as needed. A plant that is growing in ideal conditions is usually able to fend off pathogens and herbivores, but when conditions are not ideal a plant’s defenses are weakened, and they may not survive an attack.

With so many different species of plants, it is not surprising that there are many different lines of defense against a diverse horde of attackers. A plant species’ defenses evolve to protect them from other inhabitants in their ecosystem, from large mammals to microscopic bacteria. However, nothing in nature is static, and new threats can put a plant’s defenses to the true test.

Citrus longhorn beetle. Photo by 哲聖 林 via Pexels

Humans are part of the natural world, and our actions can be beneficial or harmful to other species. The biggest way humans can assist in the defense of the plant kingdom is by decreasing the effects of climate change through personal changes and by influencing corporations to make climate-friendly changes. One easy personal change is through how we garden. The principle of “right plant, right place” helps ensure the plants we add to our yards are growing in their preferred conditions and allowed to grow true to their form instead of pruning them to what we visualize as ideal.

Planting a variety of plants is also important. Biodiversity helps to reduce the spread of diseases and pests, providing habitat to beneficial animal species, and it helps in other ways we have yet to discover. Emerging research shows that ecosystems with high biodiversity reduce pathogen transmission in humans, wildlife, and plants. By having more diversity, the concentration of susceptible species decreases in the ecosystem, diminishing the number of individuals likely to become infected. Emerging research is strongly indicating that plants, animals, and fungi communicate with each other and, although some species prey on others, there are species that come to the defense of others. An ecosystem, whether an entire region or your backyard, benefits from having biodiversity. The more diversity there is, the more likely species will help each other survive.

Plant Defense Mechanisms – Against Herbivores in LibreTexts, Biology

Plant Defenses in the Genetic Science Learning Center

An Overview of Plant Defenses against Pathogens and Herbivores by Brian C. Freeman and Gwyn A. Beattie

Volatile-mediated plant–plant interactions: volatile organic compounds as modulators of receiver plant defence, growth, and reproduction by Agnès Brosset, James D Blande in Journal of Experimental Botany

Spines, Prickles, and Thorns by Sara Gage, Washington Native Plant Society

Poisonous Plants of Washington State, Washington State Department of Transportation

Plant Toxins by Ogori Akama Friday in The Americal Journal of Biomedical Science and Research

Mycorrhizal Fungi, National Park Service

Study shows pest attack-order changes plant defenses by Sara Zaske, WSU Insider

Biodiversity loss and the ecology of infectious disease by Richard S. Ostfeld in The Lancet

The Remarkable Benefits of Biodiversity by Francine Vito, earth.org

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