How do plants defend themselves? Defense mechanisms

One of the main characteristics of plants , which they share with fungi, is that they are sessile organisms. That is, adult individuals lack the ability to move. This determines that they are unable to flee from unfavorable conditions or attack by predators, which is why they have developed a series of adaptive responses to cope with the environment in which they find themselves. In this article we will review the main defense mechanisms of plants against herbivorous animals.

Plant defense mechanisms

Physical barriers against herbivores

Depending on the diet, herbivores can feed on different parts of plants: some eat the leaves, others the seeds, or pierce stems or roots to feed on the sap.

Different plant species have developed common responses that are aimed at minimizing the impact of these attacks, causing the animal in question to look for another plant to feed on. They are the defense mechanisms of plants based on physical barriers.

Reduced palatability

Thus, a widespread response is the reduction in the palatability of soft tissues, through the accumulation of molecules that make chewing difficult and are not easily digestible by animals. Three molecules widely produced by plants for this purpose are cellulose, hemicellulose, and lignin. 

The cellulose is a polymer formed by glucose monomers. The enzyme necessary to degrade cellulose is called cellulase and it is generally accepted that no animal species can produce it, only fungi and bacteria possess it, so those animals that eat cellulose (such as ruminants) digest it thanks to the ratio of symbiosis with cellulase-producing bacteria.

As for hemicellulose , it is a polymer composed of different types of monosaccharides. As in the previous case, the degradation of this biomolecule is carried out by enzymes of bacterial origin.

Finally, of the three biomolecules mentioned previously, lignin is the most complex and resistant. It is a polyphenolic network, which contains numerous aromatic rings and has a heterogeneous composition. Only a few fungi are capable of degrading lignin by enzymes, particularly those that grow on the bark of trees.

Plants accumulate cellulose and hemicellulose in their cell walls, while lignin accumulates in the trunk of trees, as well as in some roots and around the seeds of various species. An extreme case of the latter is the accumulation of lignin in coconuts.

These biomolecules are not the only physical barrier against herbivores. The grasses accumulate silicon in their leaves , wearing animal teeth and jaws of insects. Herbivorous animals such as ruminants have teeth that grow continuously and endowed with large crowns to counteract this wear. In some cases, such as the Pampas grass,  Cortaderia Selloana , the accumulation of silicon in the leaves reaches the point where they are not edible by any herbivore. In fact, the name of the plant comes from the peculiarity that its leaves can cut the skin if you run your hand through them.

Another silicon-based plant defense mechanism is the spines on the leaves of nettles . These spines are very sharp but fragile, so they pierce the skin of the animal at the same time that they break, spilling their content in the wound, mainly formic acid, which causes itching.

As for those herbivores that pierce the stem to feed on the sap, plants respond to this type of attack by producing sticky and irritating resins or latex .

Other physical barriers

There are also plants that have lignified thorns on the stem, such as brambles or acacias. This is an effective barrier against large herbivores.

The leaves can also have thorns (like those of holly), or undergo a modification whereby they lose all soft tissue and completely transform into thorns (as is the case of cacti), preventing the plant from being eaten by an animal.

Another additional barrier is the production of waxes on the surface of leaves and fruits , which makes them waterproof and reduces vulnerability to pathogens and certain insects.

Putting all of the above together, an example of a well-protected leaf would be that of holly , since it is a hardened tissue, covered with wax and whose edges are protected by thorns.

Another example of complex defense would be the seeds of the chestnut tree, which are protected by a hard covering (which forms the skin of the chestnut) which in turn is surrounded by a protective layer of thorns.

Chemical defenses of plants

In addition to all the physical barriers mentioned above, plants have a very complete arsenal of chemical molecules that are harmful to herbivores. They are the defense mechanisms of plants based on chemical defenses.

While the physical defenses are oriented to deter herbivores, being able to injure them, some chemical compounds are capable of killing them . Generally, chemical defense molecules fall into two categories: quantitative toxins and qualitative toxins.

Qualitative toxins

Qualitative toxins are the most potent , a small amount being enough to cause a great effect. This is beneficial for the plant, as it thus needs to produce less toxins and can allocate more resources for growth.

However, specialist herbivores (like some insects) develop immunity against qualitative toxins , even going so far as to “sequester” them. That is, the herbivore incorporates the toxin into its body and uses it to defend itself against predators.  

Qualitative toxins include alkaloids, some of which are very toxic to humans, such as strychnine, ricin, scopolamine or digitalis, among many others.

On the other hand, there are also alkaloids that despite being toxic are not lethal to humans, and have certain commercial or medical uses, such as caffeine , nicotine, cocaine or morphine. Another type of qualitative toxin that is present in the seeds of some plants is hydrogen cyanide.

Quantitative toxins

On the other hand, quantitative toxins are less toxic than qualitative ones , they affect both specialist and general herbivores, and the plant produces them in greater quantity (hence its name).

The two main groups of quantitative toxins are terpenoids and phenolic compounds. Some terpenoids are aromatic compounds, such as citronella, menthol or camphor and many essential oils that, as we see, are defense mechanisms of plants.

Known phenolic compounds are tannins, flavonoids, the aforementioned lignin, and cannabinoids. Of this group , tannins are interesting , which bind to the digestive enzymes of insects, causing their death . It is the molecule that gives red wine its characteristic color and astringency to some fruits such as persimmon. 

As we have seen, plants have a wide range of mechanisms to defend themselves against herbivores. However, they also have resources to attract animals that perform useful functions, such as brightly colored flowers to attract pollinating insects or fruits with an edible part that favor the dispersal of the seeds by animals (especially birds).

An unusual example of mutualistic defense is that of acacias and ants. Trees have hollow thorns on their branches, which serve as protection against large herbivores, and which in turn serve as a refuge for ants, which defend the plant from other insects with less friendly intentions.