Plants have an awkward predicament. Since they’re unable to simply get up and walk, or even shout when in danger, we often think of plants as passive receivers of whatever the environment throws at them. As one group of authors noted, “Plants are dumb and deaf, and plant communication runs counter to human common sense.”1 However, plants are far from passive. In fact, plants are highly active and communicative. Plant communication comes in many forms and is found across many plant families. But how do scientists explain how and why plants developed this complex system?
First demonstrated in trees in 1983, plant communication is still shrouded in a bit of mystery.2 What has been discovered indicates that plants communicate in a number of ways. This article will focus on above-ground communication, but plants also communicate below ground. When communicating above ground, plants use special chemicals called volatiles. These volatiles are emitted into the air and received by plants nearby. Over twenty volatile compounds have been identified. 3
Plants of the same species have been shown to “help out” their nearby relatives. One study involved researchers allowing a pest herbivore to feed freely on one set of lima beans. They then checked the nearby lima bean plants which had not been eaten and found they showed enhanced resistance to herbivore attack.4 Another example involved lodgepole pine trees, which were attacked by a herbivorous beetle. Attacked pines sent out a volatile signal of monoterpenes so nearby related pines were able to prepare to be attacked.5 If plants can communicate, it would be expected that they would communicate with members of their own species in an evolutionist worldview, using the same chemical compounds and receptors. To the creationist, this communication also makes perfect sense given that members of the same species are, by default, members of the same kind.
Other studies are less easy to reconcile with an evolutionary worldview. Researchers have found that plant volatile emission does not just affect other plants for herbivore defense; it also may attract the natural predators of the herbivore to come feed on the pest. In one study, bean plants infested with aphids were planted in the same pot as plants without aphids. The non-infected plants were then shown to release the same compounds that made them attractive to a parasitic wasp that preys on aphids as infested plants. This also applied when uninfested plants were grown in a hydroponic solution which had previously contained infested plants.6 This would appear to indicate that the volatiles were passed from plant to plant by means of the soil, as this occurred even when the roots did not touch or did not share the same container simultaneously.
The cress plant, a common model organism in botany, releases volatiles in response to predation by the caterpillars of cabbage white butterflies. As the caterpillars chew on the cress, it releases volatiles that attract a parasitic wasp. The parasitic wasps responded more to plants damaged specifically by cabbage white caterpillars, though they also were attracted to plants which had been damaged manually and those with generic herbivore damage.7 This result could indicate that the cress was able to recognize the herbivore feeding on it and tune its volatile release to attract the right predator to deal with it.
Another particularly clever experiment was done with lima beans, corn, and tobacco. Rather than assume that the uninfested plants that sensed volatiles had a better defense against herbivores, these researchers tested the idea by placing non-infested tobacco plants in a wind tunnel and placing lima bean plants down wind. Then, the lima beans were deliberately infested with herbivorous mites and the tobacco plants removed. New, uninfected lima bean plants were then introduced. After a period of days, the herbivorous mites were introduced to the uninfected plants. The same process was applied to the corn plants, except with a different herbivore-predator combination. It was found that, in a lab environment with air flow, uninfected plants were more resistant to their herbivores, while in a greenhouse environment with no air flow, there was no difference.8 This is expected as plant volatiles are carried by wind above ground and normally have a range of a few centimeters even in the best conditions.
A study done on the common backyard weed goldenrod revealed some interesting discoveries in a series of five experiments. When exposed to a mobile predator, such as the beetle used in the study, goldenrods were able to increase their volatile production both to warn other nearby goldenrods and to encourage the beetle larva to move to a different, nearby plant.9 Goldenrods damaged by beetles and those undamaged but in close proximity to the damaged plants exhibited very similar responses, though this did vary with time. This result demonstrates that plant volatiles are not always just a warning for other plants. The researchers refer to this preparatory ability as priming. Priming allows plants to respond to a potential herbivore attack by producing chemicals that make them less attractive to a nearby herbivore, either by tasting bad, producing toxins, or making themselves appear unattractive.10
Not all priming takes place between members of the same species or even the same kind.
Not all priming takes place between members of the same species or even the same kind. Tobacco plants have demonstrated the ability to receive signals from sagebrush plants which prepare the tobacco for the potential onset of herbivores.11 Cotton plants have demonstrated the ability to prime the defenses of both clover and alfalfa. In this particular experiment, cotton plants damaged by a worm emitted volatiles which warned nearby clover and alfalfa of the coming danger in both the lab and field studies. However, the reverse is not true.12 Neither clover nor alfalfa prime the defenses of cotton when they are the ones damaged by an herbivore. This means that volatiles are not a universal plant language. Some plants can “understand” members of other species, but the converse is not always true.
Interestingly, it appears the priming can be remembered by a plant, at least for a short time. Tests on corn plants using a common infestation worm determined that the priming lasts a minimum of five days and perhaps longer.13 These changes are mediated epigenetically. Since researchers have demonstrated that epigenetic changes can be inherited for a few generations, it is possible that this priming could be passed to any seeds produced while the plant is primed.14 This idea has been demonstrated in the rock cress and tomatoes. Both were found to be able to pass resistance to herbivores to at least the next generation of plants. 15
While priming is not all that surprising to either the evolutionist or the creationist, the fact that unrelated species can be primed by the same signal that primes the members of the same species is surprising. There is no evolutionary reason for a sagebrush to prime such a distantly “related” tobacco plant. It confers no evolutionary advantage. If anything, it would be to the sagebrush’s advantage to avoid priming tobacco defenses so that it diverts herbivore attack from other members of the sagebrush species. Thus cross-species priming should have been selected against in the evolutionary paradigm. However, if viewed through a creation lens, cross-species priming makes much more sense. In the perfect pre-fall world, plants still would have been eaten by herbivores. However, since the curse had not been enacted, it is possible the plants were designed to cooperate with one another and share information about herbivore activity by the sharing of volatiles.
Most plant-communication studies have been done under laboratory conditions, leading some researchers to ask whether the communication between plants or between plants and insects can occur in nature. A review of the topic suggested that plant communication would be the same “in the field” but warned against making sweeping statements with little available evidence.16 From a creation perspective, it is expected plants should be able to communicate under natural conditions as well as laboratory conditions if God built this ability into them.
For some plants, their entire chemical suite of volatiles has been analyzed and published. The rock cress is one such example. Over one hundred such compounds were identified.17 Another example is the sagebrush, which was found to have two very different “chemotypes” within the same species that were heritable.18 Most of these volatiles are termed lipidophilic, or attracted to natural fats.19 Because of this, and the fact that cell membranes are made of a phospholipid bilayer, volatiles are able to quickly escape from the plant cells into the atmosphere when the plant is damaged.20 The method by which the plant receiving the volatiles “understands” them is, however, still largely unknown.21 There has, however, been a few recent hints that protein-binding is involved in receiving volatiles.22
There are four common types of chemical compounds in plant volatiles.23 The first are the terpenoid compounds, which can take on a number of roles. Over 22,000 terpenoids have been identified.24 Terpenoids are synthesized from isopentenyl pyrophosphate to form building blocks. These building blocks serve as substrates for special enzymes, called terpene synthase enzymes, which then catalyze the production of the terpenoid volatiles.25 One example of a terpenoid involved in plant communication is (E,E)-α-farnesene, which is found in cucumbers.26
The second class of volatiles are derivatives of fatty acids. Depending on the fatty acid involved, the biosynthetic pathway is different. However, in general, it involves oxidizing the fatty acids before they are released as volatiles.27 Probably the most common fatty acid derivatives are the jasmonates. One such chemical, methyl jasmonate, has been shown to help initiate plant defense in multiple plants when received from the air, even when those plants are not the same species.28
The third type of plant volatiles are aromatic compounds. In chemistry, these are compounds which include a benzene ring (C6H6) molecule. In plants, these compounds are often derived from the amino acid L-phenylalanine.29 Aromatics are not as common as terpenoids, but they do appear in plant volatiles with regularity. Benzyl alcohol is one example that was found in the rock cress.30
Amino acid derivatives are the fourth type of plant volatiles, excluding the aforementioned L-phenylalanine derivatives discussed above. Different amino acids will be broken down to form different plant volatiles, and some will form more than one volatile. For example, methionine can be broken down into dimethyl disulfide and various thioesters.31
Explaining the origin of plant communication has provided a challenge for evolutionists. Very few articles have attempted to address the topic. One paper, in the prestigious Proceedings of the Royal Society B, argued that plants respond better to signals of close relatives. This response corresponds with the recent research on lodgepole pines mentioned above.32 The researchers claim in their conclusion that kin recognition “makes such evolution more likely.”33 However, while kin recognition may explain why some plants understand their relative’s volatile emissions, it does not explain why the emitters produce volatiles or why plants can respond to volatiles of different species.34 The researchers admit that the emitters did not “know” that they were emitting volatiles for relatives, so evolution is not explained.35
The inability to empirically explain how and why plants communicate has left many evolutionists frustrated.
The inability to empirically explain how and why plants communicate has left many evolutionists frustrated. One paper acknowledged, “In summary, there are theories at hand that could explain the evolution of emitting airborne signals but there is a lack of empirical data to test them. It is known empirically that plants can perceive VOCs (volatile organic compounds) but there are no theoretical models to understand the evolutionary origin of this capacity, neither is it completely understood how volatiles are perceived and translated into signals.”36
Plant communication is a marvel of God’s design. In the pre-fall world, plants were perhaps able to exchange information freely across the kind boundary to alert their neighbors to an incoming herbivore. Perhaps the ability to communicate with insects that prey on herbivores is a post-fall adaptation, either due to unlocking of genes at the fall, or due to speciation and genetic drift. The chemical complexity of these volatiles and the fact that we still do not know how exactly plants translate the signals they receive speak to the complexity of design God built into His perfect creation. And for a creation scientist, what a joy to be able to continue learning more about the Creator through the intricate workings of his creations.