Venus Flytrap De-Darwinized
Darwin had a fascination for the Venus flytrap, but is it appropriate to conjure up his ghost when talking about it? The carnivorous plant still defies evolutionary explanations, especially now, when a recent paper drew attention to more amazing design features from macro to micro. For some reason, writers still feel a compulsion to mention Darwin’s name when talking about a plant that defies his evolutionary ideas.
Even though the Venus flytrap (Dionaea muscipula) grows only in America (particularly, in the Carolinas), it has long fascinated botanists from around the world. Recently, a team of plant biophysicists from Germany and Saudi Arabia went hunting for the clever plant’s secrets. Publishing in PNAS,1 they concentrated on plant hormones involved in the stimulation of the traps, the fast closure followed by slow constricting closure, and the formation of an “external stomach” on the trap surfaces that digests the prey. They found that signals for trap closure follow different signaling pathways than those for digestion. In sum, “These findings demonstrate that prey-catching Dionaea combines plant-specific signaling pathways… with a rapidly acting trigger, which uses ion channels, action potentials, and Ca2+ [calcium ion] signals.” That’s a lot of cooperation between multiple parts, each exquisitely adapted to their role in the goal of catching bugs.
Students who have played with Venus flytraps in school know that it takes two strokes of the trigger hairs separated by a short pause, or two strokes of separate hairs, to make the trap shut. This prevents needless trap resets from falling leaves or other non-prey objects. But they probably didn’t realize they have just tripped an electrical switch leading to a series of mechanical events: “Insects touching these mechanosenory organs protruding from the upper leaf epidermis of the Venus flytrap activate mechanosensitive ion channels and generate receptor potentials, which induce an action potential.” Prior to closure, the trap has been set by storing elastic energy, allowing it to close within 100 milliseconds (1/10 second). But even when sprung, that’s not all. A whole sequence of coordinated events is set into action:
However, the trap is not completely closed at this moment. To hermetically seal the trap, it requires ongoing activation of the mechanosensitive hairs by the trapped prey. Unless the prey is able to escape, it will further stimulate the inner surface of the lobes, thereby triggering further APs [action potentials]. This forces the edges of the lobes together, sealing the trap hermetically (prey-dependent slow closure) to form an external “stomach” in which prey digestion occurs. The second phase of trap closure is accompanied by secretion of lytic enzymes from the glands covering the inner surface of the bilobed leaf trap. Thus, digestive glands do not secrete until stimulated by natural or artificial prey. Additionally, prey-derived compounds stimulate digestive glands leading to acidification of the external stomach and production of lytic enzymes.
The authors seem fascinated by this process, as any observer would be. How can a plant act like a meat-eating animal? “Many similarities between fast processes involved in carnivory and signals of the nervous system suggest similarity in mechanisms,” they said. And sure enough, “ion channels and chemical factors are at the basis of triggering mechanisms in both systems.” Both tigers and Venus flytraps employ calcium signals, ion channels, exocytosis, and secreted substances. In particular, the researchers discovered an interplay between two plant hormones, jasmonic acid and abscisic acid: “Whereas the former systematically alerts neighboring traps to the presence of prey and elicits secretion, the latter regulates trap sensitivity, protecting the carnivore from untimely prey catching during periods of drought.” This little plant thought of everything (so to speak).
The analogy between carnivory in two completely different kingdoms of life is so uncanny, the authors elaborated on it in their concluding discussion:
Our results imply that phosphorylation/dephosphorylation reactions are important modulators of plant carnivore excitability. This reveals another analogy to the nervous system, where both electrical excitability and synaptic transmission is strongly modulated by such posttranslational modifications. Dionaea assembles an extensive signaling network that relies partly on plant-specific components (which it uses for its own specific purposes) and partly on mechanisms resembling those of higher animals. However, in contrast to nerve cells in animals, terrestrial plants lack fast voltage-dependent Na+ [sodium] channels, possibly due to the fact that Na+ gradients in such plants are minor. Thus, Na+ currents would not be very efficient to transiently depolarize the membrane potential. Instead, plants possess a rapid (R-type) anion channel current component in addition to the slow (S-type) anion channels (SLAC1). This anion channel exhibits voltage-dependent features of neuronal calcium and sodium channels. Upon depolarization, this channel type activates with fast kinetics, whereas hyperpolarization causes deactivation. Hence, this type of plant anion channel has all of the properties to substitute for Na+ channels and to drive the Dionaea AP [action potential].
Of all the nerve – a plant that is vastly unrelated to the animal kingdom has analogous components for hunting prey found in mammals with central nervous systems. Why, then, did these authors call it the “Darwin plant”? No common ancestor of plants and animals would have had all these components, and no sequence of transitional forms shows carnivory from the most primitive plants to this exotic angiosperm – a flowering plant, by the way, that does not need bug meat to survive, since it has all the photosynthetic gear to get by. What’s Darwin got to do with it?
Nevertheless, the ghost of Darwin lurked in the shadows from beginning to end:
Venus flytrap’s leaves can catch an insect in a fraction of a second. Since the time of Charles Darwin, scientists have struggled to understand the sensory biology and biomechanics of this plant, Dionaea muscipula.
Although this type of plant carnivory has been known since Darwin’s time, insights about the hapto-electrochemical coupling associated with the trapping behavior of Dionaea remain rather limited.
Urea had already been shown by Darwin to induce secretion in Dionaea.2
Sequencing the genome of the Venus flytrap and identifying the genes encoding key elements in mechanoelectric trap contraction will allow us to further understand the action of the Darwin plant and to characterize both similarities and differences between analogous processes in the two kingdoms. Furthermore, with the ion channel genes identified and functionally expressed, it will be possible to reconstitute the Dionaea AP and secretion process known since Darwin’s time.
This repetition of Darwin’s name is especially puzzling since the authors made no attempt to explain how this amazing plant evolved (they didn’t even invoke the phrase convergent evolution). Darwin wrote a book on insectivorous plants in 1875, in which he described experiments he performed on the Venus flytrap, but he did not provide a theory for how the plant might have evolved, nor did he mention it in his best-known work, On the Origin of Species by Natural Selection. To the contrary, the quotations above almost emphasize that Darwin did not understand how this wonder of nature originated.
Nor was Darwin the first to study the plant. It had been discovered in 1763, almost a century before the Origin. A description and illustration of it was sent by British naturalist John Ellis to Linnaeus in 1769, with the comment, “My dear Friend, I know that every discovery in nature is a treat to you; but in this you will have a feast” (source: Hunt Institute). William Paley gave it a short description in his 1804 book Natural Theology, or, Evidences of the Existence and Attributes of the Deity Collected from the Appearances of Nature.3 So by all rights, he could have been honored instead of Darwin in the current paper, having it called “the Paley plant.”
1. Escalante-Perez et al., “A special pair of phytohormones controls excitability, slow closure, and external stomach formation in the Venus flytrap,” PNAS, published online before print September 6, 2011, doi: 10.1073/pnas.1112535108.
2. Footnote in paper refers to Charles Darwin, Insectivorous Plants (John Murray, 1875), available at Darwin-Online.UK.
3. Paley, Natural Theology, p. 367, published at Darwin-Online.UK.
If you stand for fairness and historical accuracy, snatch that well-designed plant out of Charlie’s gnarly hands and let’s set the record straight. This plant is more irreducibly complex than Behe’s man-made mousetrap. (The scientific name, by the way, means “Dione’s daughter’s mousetrap”). It’s even more exquisite than Ellis, Linnaeus or Paley could have imagined. Darwin would have croaked if he had been told what these scientists found. Since the Venus flytrap clearly bears the hallmarks of intelligent design, let’s call it “the Paley plant, known since the time of the famous Biblical creationist, Linnaeus.”