- Green Plants Fight Back - Heeran Rathod
Picture this: A group of creatures in the forest feels the burn of an insect bite and calls out to others of their kind nearby to brace for attack. A desert dweller, under attack by a predator, defends itself by squirting poisonous spittle at its enemy. A tropical carnivore, sensing its last meal has been digested, opens its “mouth” and waits for something tasty to come by.
These would be unremarkable behaviors in animals but each of these examples is taken from a plant. Furthermore, they represent a mere fraction of the astonishing behavioral repertoire that botanists have begun to uncover in the verdant realm of plants.
Plant Physiology Mimics Animal Physiology
As they are eaten by insects, chemical reactions almost identical to the neurohormonal reactions that regulate pain and injury repair in animals surge through plants. When leaves get nibbled, they release a hormone resembling the endorphins made by animals after injury. So similar are these chemical responses to injury that a sprinkling of aspirin or ibuprofen on plant tissue will squelch the reaction, as it does in people.
Recently botanists at the University of Clermont in France intentionally punched holes in the young leaflets of a bur – marigold plant (Bidens laevis). Then, they defoliated the plant altogether. Soon, the marigold sprouted new leaves, but not in the symmetrical pattern typical of the species. Instead, the plant seemingly “remembered” that some of its leaves had been altered, and it grew its new foliage in precisely the same unnatural, damaged pattern that the botanists had created. Apparently, even though they have no nervous systems, some plants do have the ability to store and transmit information, a “memory,” so to speak.
Can Plants Communicate With Each Other?
Plants also seem to have the ability to communicate with one another. Some trees may actually send warning signals to nearby trees when they are attacked by insects. For example, when either alder or willow trees are infested by tent caterpillars, they change their leaf chemistry to repel the invaders. At the same time, neighboring trees of the same species, though not yet infested, also change their chemistry.
Does some sort of wound hormone from attacked plants waft through the air and stimulate the defenses of nearby plants that have not yet come under siege? To test this possibility, a potted sugar maple tree, surrounded by an airtight enclosure, was purposefully infested with insects. As long as the enclosure remained in place, nearby sugar maples did not react. However, when the enclosure was removed from the tree under attack, the nearby maples began to change their leaf chemistry.
Clearly, some warning signal must be passing through the air. Recent research also suggests that some chemical messages may also be passed underground from plant to plant through the intertwining of their root systems.
Plants Wage Chemical Warfare
Given the number of insects and the sheer size of the grazing mammals on earth and considering the ravenous appetites of both groups, plants should have been eaten into extinction by now. The green survivors are dramatic proof that plants are far from defenseless and passive victims. On the contrary, many plants wield a potent arsenal of chemical weapons to ward of voracious insects, nibbling mammals, disease, and even other plants.
When attacked, some plants alter the chemistry in their leaves to produce chemicals (such as tannin) which makes the leaves bitter tasting to mammals and which makes them indigestible or poisonous to insects. Other plants produce chemicals that permanently gum up the mouth parts of insects with a rubbery goo so effective that the insect eventually starves to death. Still others produce insect hormone – mimicking chemicals that disrupt the molting process of insects, resulting in gross deformities and death of the developing infant insects.
Some types border on the aggressive. In a bold act of biological comeuppance, leaf-chomping beetles are sent fleeing by a toxic spray squirted by a Mexican shrub of the genus Bursera. Bursera plants are laced with canals filled with irritating resins. When an insect bites into one of these canals – or when a goat browses on the plant – the result is like a break in a high-pressure underground pipe filled with toxic wastes. The noxious squirt from these plants can reach 5 feet.
Traditionally, plants have been viewed as passive life forms that endure whatever insults the environment, herbivores, and humans dish out to them. Now, however, it is apparent that green plants are far feistier than first believed. Clearly, a new image of plant life is emerging. As we learn ever more of the secret life of plants, we can only stand in awe in the face of the complexity and chemical ingenuity of the citizens of GreenWorld.
Dennis Holley - Looking back, it's clear that I have been a science nerd my entire life. A simple, almost toy-quality microscope my poor parents scraped ...
