by Alice Ren
Venus flytraps, also called dionaea muscipula, are arguably the most well known type of carnivorous plant. With appearances in games such as Plants vs Zombies and many films, these plants have become famous for snapping their jaw-like leaves shut, and capturing and digesting insects. Like other plants, venus flytraps obtain nutrients from the air and soil, but due to often growing in soil that lacks the needed minerals, they benefit more from getting their resources from insects and other small animals. However, despite venus flytraps attracting scientific attention since as early as the time of Darwin, few people know about why exactly they are able to capture their meals in this way.
First, let’s outline the basic sequence of events in how the venus flytrap obtains its food. The insect / fly lands inside the two leaves that make up the “mouth”. In the picture above, you’ll notice that there are a few small hair-like projections coming out of both leaves; these are known as trigger hairs (or sensor hairs), each of which being no longer than 0.5cm. When the prey lands onto the leaves, they also exert pressure on these hairs, causing them to bend. This bending action then forces the leaves to snap shut in around 100 milliseconds, which is 1/10 of one second. The leaves do not close completely for a few seconds in order to allow smaller insects that would not provide sufficient food, to escape. When they do close fully, the cilia (the spiky hairs coming out of the edges of the leaves) on both leaves lace together, trapping the larger insects inside. Within minutes, the trap becomes sealed air-tight, in order to keep digestive fluids inside and bacteria outside. In a few days, the trap will have finished digestion of its prey, and open up once more to await the arrival of its next meal.
The snapping shut of the trap is still not completely understood, but it has been theorised that electrical stimulation and osmoregulation are part of the reason why flytraps are able to do this. The trigger hairs mentioned earlier act as mechanosensors, which are able to detect when mechanical stimulus has been applied onto them and cause the plant to respond to this change. When the prey touches these mechanosensors, an electrical signal is generated. This signal acts as action potential, which can be described as a sudden change in the membrane potential above a certain threshold value, caused by increased membrane permeability. The increased permeability means that the ion carriers in the membrane allow certain ions into and out of the cells. It is this action potential that activates the motor cells (plant cells that act as a hinge at joints to allow movement of plant parts); the ions entering the motor cells causes water to also enter via osmosis, making the cells turgid. This turgor pressure in the motor cells either side of the “hinge” (the connecting part of the two leaves that make up a single flytrap) changes the shape of the cells in such a way that they push, and open up the trap. Meanwhile, the action potential also means that ions can osmose out of the cell, causing the motor cells either side of the hinge to become flaccid, which causes the closing motion of the trap. Neuromotoric components have also been found in venus flytraps, including neurotransmitters, the messenger protein calmodulin, and sensors for light, touch, gravity and temperature- these components form a system that is similar to what we know as the nervous system, allowing different parts of the venus flytrap to communicate with each other efficiently over long distances. However, although it is true that electrical signalling and loss of turgor pressure have been found to be involved in the snapping shut of the flytrap’s leaves, the mechanism is still poorly understood, as osmosis is a relatively slow process while the closing of the leaves is rapid.
Sources:
https://www.tandfonline.com/doi/full/10.4161/psb.2.3.4217
https://www.nature.com/articles/nature03185
http://www.plantphysiol.org/content/149/4/1661.short
https://www.sciencedirect.com/science/article/pii/S002192582036066X
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