The Venus Flytrap, an Improbable Wonder that Baffled Darwin
Here in Seattle we've noticed Venus flytraps for sale in supermarkets -- some people swear by them for controlling fruit fly invasions like the particularly intense one we had this summer. They makes a good conversation starter, and an object lesson in intelligent design.
For the Venus flytrap does much more than just catch flies. A century and a half since this plant fascinated Darwin, biologists are still finding complex processes involved in its carnivorous lifestyle.
A paper in Current Biology by a dozen German biologists, while discussing new findings about an ammonium ion transporter, includes descriptions of the many actions that occur when the Venus flytrap snaps shut on an insect. More information is provided by Colin Brownlee in a Dispatch article in Current Biology about the research. Here's a summary of the steps involved:
- The traps open wide to the environment, exposing trigger hairs and attractive red leaves.
- Electrical action potentials are established for the trigger hairs on the inner leaf surface.
- The digestive glands remain quiescent till activated. Abscisic acid regulates their sensitivity, but is balanced by 12-oxo-phytodienoic acid (OPDA), which makes them more sensitive to touch.
- A trigger hair on the inner leaf is touched. If only one is touched, nothing happens.
- A second touch after a short delay, or touch of a second trigger hair, begins a cascade of events.
- Anion channels open. The action potential collapses, activating the motor center.
- Vascoelastic energy snaps the trap shut in a fraction of a second.
- If the triggering substance was not an animal, the trap re-opens after a short period.
- Escape movements by the trapped animal triggers synthesis of a touch hormone, and acidifies the trap.
- The trap edge hairs wrap more tightly around the edges, preventing escape.
- The trap seals hermetically around the prey like a "green stomach," exposing it to densely packed glands and chlorine ions.
- OPDA stimulates production of jasmonic acid, which triggers the glands to secrete an acidic cocktail with more than 20 ingredients, including chitinases to dissolve the saccharides of the exoskeleton, proteases to dissolve the proteins, nucleases to dissolve the nucleic acids, lipases to dissolve the fats, and phosphatases to isolate the phosphates. These only digest the prey, not the leaf. The proteins are hydrolyzed into their constituent amino acids.
- The amino acid glutamine is deaminated into ammonium, NH4+.
- Genes to make an ammonium transporter are activated, depending on the action of touch hormones and elicitors, so as to adapt to varying, prey-derived ammonium sources.
- The cell membrane becomes depolarized, ready to accept ammonium, even though it is not activated by pH. Only activation of the genes prepares the transporter for ammonium transport.
- The ammonium transporter increases uptake of NH4+ from the prey into the plant cells, satisfying the need for nitrogen in the nutrient-poor soils of the plant's habitat. It is described as "a voltage-dependent high-affinity NH4+ transporter optimised for NH4+ uptake at the membrane potential of gland cells." Counteracting the acidification of the trap, the transporter can "serve to counter the depolarising effects of electrogenic NH4+ uptake and help to maintain intracellular pH homeostasis."
- "At the same time, progressive acidification of the trap digestive fluid will allow optimal digestion of a wide range of protein and other substrates." If the pH drops below 3, additional digestive enzymes are synthesized to benefit from the additional NH4+ provided by the insect's haemolymph.
- Upon successful completion of the digestive cycle, the trap re-opens, and action potentials are set up for the next capture.
The authors attempt to draw evolutionary links between the ammonium transporter in the Venus flytrap and other known transporters from other plants and animals, but that's only one part of at least 18 systems listed above. Besides, where did the other transporters come from? Certainly this highly coordinated insect-digesting plant is unlike any other. Brownlee says:
The Venus flytrap digests and absorbs its prey, but how does it coordinate digestion and absorption to maximise the efficiency of this highly evolved mechanism? A new study that combines direct recordings from cells within the trap along with molecular characterization of nutrient transport reveals a complex and coordinated suite of mechanisms that underlie this elegant process.
The phrase about its being a "highly evolved mechanism" is used here as a Darwinian substitute for "well designed system." Observers are certainly justified in doubting the ability of blind, unguided natural processes to design optimized, efficient, coordinated, elegant systems as seen in the Venus flytrap.
Photo credit: NC Orchid/Flickr.