The Web of Life (79-3) Plant Self-Protection


Plants are winning mostly. Insects are always trying to catch up. ~ American entomologist Mark Mesche

They are not called bugs for nothing. Insects pester plants more than any other animal. A plant often outsmarts its 6-legged opponent, sometimes with considerable cunning. Goldenrods deterring egg-laying on their leaves by gallflies, their nemesis, through specialized toxins, is one of many such examples.

 Inciting Cannibalism

At the end of the day, somebody gets eaten. ~ John Orrock

When the supply of their favorite vegetable is running low, herbivorous insects start eating their competition. Plants know this and produce metabolites that engender pest-on-pest predation.

From the plant’s perspective, this is a pretty sweet outcome; turning herbivores on each other. ~ John Orrock

When being unrelentingly attacked by caterpillars, tomato plants produce a compound that induces the pests to lay off their leaves and turn into cannibals. The cost to the plant of mounting this defense is considerable, but it is much better than being eaten alive.

Plants strike a balance and decide if the attack is serious enough to activate the defenses. ~ John Orrock

 Beetle Juice

Symbionts can influence plant–insect interactions. ~ Korean American entomologist Seung Ho Chung et al

A plant can tell what is eating at it. The defense against gnawing insects proceeds along a pathway that creates chemicals to stop the digestion and growth of the pest.

This pathway is mediated by jasmonic acid. If attacked by a microbe, a different, mutually exclusive pathway is employed which is mediated by salicylic acid.

Potato bug larvae have friends that get them past the plant defense system. Before chowing down on a potato or tomato plant, bug gut bacteria are brought up to fool the plant as to what is going on.

These beetles don’t have salivary glands. They regurgitate some stomach contents onto leaves to begin digestion. These secretions have the bacterial tricksters.

The bacteria make the plant think it is being attacked by microbes instead of an insect. While some symbionts are sacrificed in the deception, the beetle dines and keeps the microbiome within well-fed.


Plants have strategies to preemptively protect themselves, and to do so efficiently. Several of these strategies are contemplated decisions to wisely use resources and protect against predators.

Plants tightly control the onset and amplitude of potent immune responses to pathogens for optimal growth and development. ~ American botanist Walter Gassmann et al

To avoid predators on the ground, the Australian brushtail possum lives in the trees. Eucalyptus leaves are a mainstay of the possum’s diet.

Once a tree realizes that its leaves are being munched, it quickly pumps eucalyptol, a possum preventative, into its leaves, to get the possum to forage somewhere else. Eucalyptol also acts as an insecticide. A eucalyptus tree controls its production and application of eucalyptol to meet its needs.

Many plants have subtle mechanisms that promote production of protective secondary metabolites only when needed. They know what time it is, and the time of day that common pests rouse themselves to bring ruin. In anticipation, plants ramp their defenses in likely locations of assault.

New leaves begin to grow only when stimulated by light: the prospect of profit. The setup involves a subtle subterfuge.

The region at the top of the plant where new leaves emerge (the shoot apical meristem) is sheltered by existing leaves. The covering hides the new shoots from predation by herbivores, as this new growth has yet to develop its own chemical protection. Plant development is a timed investment, intelligently made in concert with proper conditions.

Once they appear, the chemically unprotected shoots and leaves of early spring are highly prized by herbivores. Besides hiding behind veteran leaves, some plants color new growth in ways that disguise its digestibility. Plants somehow understand the inner workings of the vision system of herbivores.

Roots, particularly those subject to subterranean grazing by parasitic fungi and bacteria, commonly contain the highest concentrations of metabolites designed to ward off microbial predation. It is no coincidence that ancient Greek herbalists, who knew where the metabolite medicines were stored, were called “root diggers.”

Variations in the distribution of secondary compounds extend beyond anatomy and season. Plants are aware of predators.

After gypsy moths have defoliated oak trees, new leaves emerge with a higher tannin concentration. Tannin molecules bind with plant tissue proteins, rendering otherwise tasty bits indigestible to plant-eating insects. Such leaves are also tougher, with lower water content. The combination retards development of gypsy moth caterpillars and the growth of adults.

Besides inciting local defense responses, wounding provokes plant-wide resistance to further herbivore attack. A plant increases production of protease inhibitors that resist insect proteases, and so thwart predator digestion. Proteases are enzymes used by all organisms to facilitate digestion, particularly breaking apart the peptide bonds that hold proteins together.

Potato plants damaged by insects ramp the making of protease inhibitors and other chemicals that disrupt predator digestion and stunts its growth. The first damaged leaf signals others to crank up protection production.

Plants determine who is attacking them. They’re even capable of distinguishing between a native and an exotic herbivore. ~ Dutch botanist Nicole van Dam