Defensive Plants?

Stop for a second and imagine that a mosquito is trying to suck the blood out of your arm. Or that you are walking down the street and suddenly you see a ferocious dog. How would you react to both situations? You would surely swat the mosquito away, and maybe run from the scary dog. Humans can react immediately to avoid dangerous situations and plants can do the same. It may not seem as obvious, but plants have various defenses to protect themselves from attacks, too. And this is important because plants are a critical part of life on Earth. They provide us with food, medicine, and wood. They also provide homes for other living things, produce the oxygen we breathe, keep soils healthy, regulate humidity, and contribute to climate stability.

Herbivores: The Enemies of Plants

Plants have many natural enemies, from bacteria to insects, fungi to mammals. These enemies are attracted to plants because they may have bright colors, nice smells, good-tasting leaves, or fruits. Herbivory is an important interaction between plants and animals because it is the process by which some animals feed on living plants. So, animals that eat plants are known as herbivores. Herbivores damage plant organs, such as leaves, stems, roots, and flowers, when they feed on plants. If a plant loses part of its living tissue, it can have trouble taking in water and nutrients, leaving the plant weaker. In some cases, herbivory decreases photosynthesis, growth, reproduction, and survival. This means that herbivory can control how abundant plants are in nature, and it can even determine if forests grow back after they are cut down or destroyed by wildfires.

Insects are the main plant herbivores, and they make up more than 50% of the known species in terrestrial ecosystems. There are at least 450,000 species of plants distributed throughout the world [1], so there are a lot of options on the menu for plant-eating insects! Unlike humans, plants cannot run away from dangerous predators—so how have plants managed to avoid being eaten by herbivores? The answer lies in the fascinating mechanisms plants have developed to protect themselves. But first, we need to explain how the environment affects how plants respond to threats.

The Environment Affects Plant Defenses

To survive and grow, humans need food and a healthy place to live in. Plants also need resources from the environment to grow and reproduce, including water, sunlight, good-quality soil, and access to nutrients. A scientific theory called Resource Availability Theory helps us understand how the environment, particularly the availability of resources, affects plant responses [2]. The theory says that plants living in ecosystems with high availability of natural resources will use their energy for growth and development. On the contrary, plants that do not get enough resources will put their energy toward resistance or tolerance mechanisms, which help them survive in the face of limited resources.

The availability of resources in an ecosystem depends on many environmental variables (Figure 1). The amount of rainfall, soil type, temperature, and wind intensity are examples of environmental variables that affect how plants respond. Throughout time, plants have grown in diverse environments, each with its specific levels of resources which has been important for the evolution of plants and their ability to adapt to many different conditions.

Differences in the natural abundance of a resource from area to area is called an environmental gradient. Environmental gradients can change the way plants and herbivores interact by affecting both herbivores and the physical and chemical traits of plants. For example, there are more plants in habitats with more resources so these patches are more attractive to herbivores increasing the chances of plants being eaten [3]. The traits of individual plants can also influence herbivory; for example, nutritious plants are more likely to be eaten if they don’t have defense mechanisms to keep herbivores away. These traits vary from plant to plant depending on environmental variables such as soil quality, amount of rainfall, and exposure to sunlight. In general, the environment in which plants live will influence their ability to protect themselves from external damage.

The Delicious Plant Leaves

You do not have to be a botanist to know that leaves are the part of a plant most eaten by herbivores—just go to your garden and observe. But what do leaves have to attract herbivores? Leaves are rich in nutrients that insects need, including carbohydrates, nitrogen, phosphorus, protein, and water. However, plants can develop chemicals that make their leaves less digestible, making it more difficult for herbivores to get nutrients from them, decreasing herbivory [4].

Similarly, the physical characteristics of leaves, such as their weight, thickness, and size, influence how attractive they are to herbivores (Figure 2). For example, larger leaves attract more attention from herbivores because bigger means more food. Likewise, thick, tough leaves attract less attention because they are more difficult to chew and digest.

Herbivores have managed to overcome some of these plant defenses thanks to their own adaptations. For example, some insects have developed mouth parts that penetrate thick, tough plant tissues. Others have developed the ability to break down toxic compounds using strong digestive proteins, making the leaves edible.

What About Roots, Stems, and Flowers?

Most herbivores eat leaves; however, some eat roots and stems and, therefore, also decrease the plant’s growth. The absorption of water and nutrients through the roots decreases, as does the amount of nutrient reserves that are stored underground. Root-eating herbivores are among the most damaging and dangerous pests. They reduce the ability of plants to reproduce. Like leaves, roots produce a variety of toxic and repellent compounds that scare off subterranean attackers. In addition, plants react to root attacks by redirecting roots into enemy-free space or storing nutrient reserves in undamaged root tissues [5]. Thanks to these nutrient reserves, damaged roots can sometimes be regenerated.

Herbivore attacks on stems are more dangerous than attacks on leaves, because stems transport vital nutrients along the plant. Stem-eating herbivores can eat the outer bark of the stem or suck out the sap inside. When the stem is damaged, the passage of nutrients from the root to the leaves is blocked, which can stop plant growth and even kill the plant [6]. Stems defend themselves by producing lignin, which makes the stem rigid and challenging to eat.

Flowers are delicious treats for herbivores. Flowers are crucial for the reproduction of plants. When herbivores eat flowers, less pollen is available for pollinators like bees, butterflies, hummingbirds, and bats. Further, if many flowers are removed, the plant becomes less attractive to pollinators, and they may stop visiting and fertilizing the plant. That is why flowers have also developed strategies to protect themselves from herbivore attacks. Flowers produce chemicals and toxins to repel herbivores; some can close their petals to protect their reproductive organs, and others release airborne compounds that attract the natural enemies of their herbivores [7].

Types of Resistance and Protection

Past studies have shown that plant defenses can be of two main types: constitutive or induced. Constitutive defenses are those that are always in the plant and constantly protect it. These include thorns and plant hairs called trichomes, as well as various chemical compounds. However, having these defenses around all the time, regardless of whether they are needed, can use a lot of energy. In contrast, induced defenses only develop or activate after an attack. Many plants only invest resources in these defenses once they know an attack is happening. This saves the plant energy.

Whether they are constitutive or induced, plant protection mechanisms can be classified into two main methods: mechanical and chemical [4]. Plants use mechanical protection, such as thorns, hairs or trichomes, as well as the firmness of their leaves, to resist wind, rain, and hail, for example. Mechanical protection is also used to protect against the attacks of some herbivores. Chemical protection involves the production of compounds that kill microbes and fungi, or that can poison an herbivore’s heart or cause it to hallucinate (Figure 3). These chemical defenses include terpenes, tannins, phenols, and oils [4]. The concentrations of these chemicals can vary according to things like the age of the leaves, soil quality, nutrient availability, sunlight, and water.

In Conclusion

This article has shown that plants are like a fortress, fighting to protect themselves and stay alive when they are under attack by herbivores. Now, it will be easier for you to understand how plants have dominated the world for millions of years and, despite adversity, are still a major component of our planet. In the end, plants may lose some battles, but they will rarely lose a war.

Glossary

Natural Enemies: ↑ Living organisms that feed on other organisms. They can kill, weaken, or reduce the reproductive potential of the species they feed on.

Herbivory: ↑ Biological interaction between two living organisms in which animals eat living plant tissues, like leaves, stems, roots, flowers, or fruits.

Herbivore: ↑ Animal that feeds mainly on plants.

Environmental Variables: ↑ Elements or aspects of the environment that can affect living organisms. Some of these include the amount and quality of water and soils, temperature, and the presence of other organisms.

Environmental Gradient: ↑ Differences in the environmental characteristics from one area to another.

Adaptations: ↑ Changes that organisms underwent over time according to what they need to survive in their natural environment.

Constitutive: ↑ Characteristic that is part of a whole.

Induced: ↑ Something that is produced thanks to an external stimulus.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors are grateful to María Fernanda Hernández Arroyo for her help in the design of the figures. Funding for the article was provided by IIES-UNAM through Presupuesto Operativo to AG.