Mechanisms of Trap Closure in Dionaea muscipula

Laurent Legendre

University of Reims • France

Leaf movement is one of the most intriguing feature of plant sciences. A large body of research has now tried to unveil the biochemical mechanisms behind the rapid closure of the Venus fly trap. Some of these advances will be described and illustrated with experiments conducted on live plants. The evolutionary relationship between this mechanism of leaf movement and the one of other members of the carnivorous family, Droseraceae, will then be explained.

Touching one of the 6 hairs present on the upper trap surface sensitizes all of them to initiate the rapid closure of the trap when touched a second time. Even though this double touch mechanism is required to prevent artificial closure by rain drops or plant debris pushed by the wind, the nature of the sensitization signal that rapidly travels through the whole trap is still speculative. Surprisingly, the Venus fly trap seems to have a very short memory of the first touch so that many insects could get by if they knew it. An increasing number of touches is indeed required to obtain full trap closure when the time between the two touches is increased.

Hurting a trigger hair generates a heavy mechanical stress at its base, a hinge region that is slender and more flexible than the rest of the hair. Under this mechanical stress, the cells at this hinge region liberate a chemical signal that will travel through the trap at high speed (10 cm/s) without losing intensity. When reaching the cells at the outer surface of the trap it will force them to grow rapidly. As the size of the cells on the inner surface do not increase their size, the trap will close, the curvature of the trap being greater at its edge. Even though the full nature of the traveling signal is not known, it has been suggested that ions such as chloride and calcium are involved. As these are charged particles, their movement across cellular membranes generates a depolarization signal which, when moving from one part of the trap to the next, resembles an electrical current. Interestingly, the cells at the hinge region that initiates this signal contain cellular structures which are unique to the plant kingdom but are found in animal muscle cells. In spite of this convergent evolution feature, plant and animal structures work on opposite ways as one responds mechanically to an electrical nerve signal while the other one produces an electrical signal under mechanical stimulation

The differential growth of the outer cell layers is obtained via an acidification outside their cells (acid growth theory). This acidification loosens their cell wall fibers so that the cells will elongate due to their internal turgor pressure. This phenomenon is irreversible and the reopening of the trap occurs via the elongation of the cell layer on the inner side of the trap to equal the one of the external surface. Thus, a trap that has already closed and reopened is larger. Moreover, the elongation of the cell wall fibers cannot repeat forever (until they are parallel) and a trap can only work 3 times in its life. The fast closure movement of the trap is followed by a slow movement induced by the degradation products that leach from the early digestion of the prey. This second closure mechanism will allow the trap edges to come into contact with each other and seal the trap to facilitate digestion. Remarkably, the pressure exerted by the two trap lobes on each other varies according to a circadian rhythm and is maximum each day (starting two days after prey capture) early in the afternoon.