hinge region between the lobes (Fagerberg and Allain 1991), rapid turgor loss in motor
cells in this region (Hill and Findlay 1981), and an inherent tendency of the lobes to
“snap” shut due to the elasticity and geometry of the trap itself (Forterre et al. 2005). The
sensor that triggers this closure is a series of modified hairs on the inner trap surface.
Activating the trap requires multiple stimulations on these hairs as the insect climbs
across the leaf. The requirement for several signals is likely a safeguard to prevent clo-
sure of the trap by random mechanical stimuli, such as the impact of a raindrop, which
tend to be solitary events. The multiple stimulations then trigger action potentials that are
transmitted to the hinge region to effect closure (Jacobsen 1965; Simons 1981). The con-
tinued struggling of the insect, likely supplemented with some chemical sensory events,
then closes the trap even tighter, forming a chamber in which the insect is digested
(Fagerberg and Allain 1991).
Similar mechanosensory triggers are seen in other carnivorous plants, such as the suc-
tion trap of the bladderworts (Utricularia) and the leafrolling sticky trap of the sundews
(Drosera) (Darwin 1893; Lloyd 1942). In this latter case the mechanosensor is respon-
sive to microgram stimuli, yet is able to ignore the presumably much larger mechanical
signals from the impact of rain or wind (Darwin and Darwin 1880; Darwin 1893).
Although these plants have received much attention due to their dramatic carnivorous
responses, we are largely ignorant of how their mechanosensors operate. The rapidity of
triggering the response seen in DionaeaandUtricularia(which responds in the millisec-
ond range) strongly suggests the involvement of a mechanosensory ion channel. Indeed,
the inherent speed and signal amplification of channels makes them the top candidates
for most rapid mechanosensory responses (Gillespie and Walker 2001, and see below).
However, at present the molecular identity of plant mechanosensors remains unknown.
Identification of this initial signaling system will be an important step toward answering
the many perplexing questions raised by the touch response in these carnivorous plants.
For example, how does Dionaeasuppress its response until multiple touch signals have
been received, and how can Droseratell the difference between an insect and a raindrop
impacting on the leaf?
A similarly highly specialized touch response is seen in the thigmonastic leaf move-
ments of the sensitive plant (Mimosa pudica) and its relatives. In these plants touch stim-
ulation leads to rapid folding of the stimulated leaflet, possibly as an anti-herbivory/
defense response (Simons 1981). This collapse of the leaflets propagates along the leaf
and, depending on the strength of stimulation, may lead to folding of the petiole and even
induce a response in adjacent leaves. Similar to the carnivorous plants, the mechanisms
driving the movements have been described but the nature of the mechanosensor or early
signaling events remain undetermined. Thus, electrical signals, either a propagating action
potential or a slow wave potential (Simons 1981; Fromm and Eschrich 1988; Fleurat-
Lessard et al. 1997), perhaps coupled to changes in hydraulic pressure in the vasculature
(Malone 1994) and a chemical messenger (Schildknecht and Meier-Augenstein 1990;
Varin et al. 1997), likely trigger the local and systemic movements. These signals then
elicit ion efflux and loss of turgor in motor organs (pulvini) located at the base of each
leaflet and the petiole (Simons 1981; Fromm and Eschrich 1988). The possibility that
plants measure xylem pressure as a systemic signal remains an intriguing idea potentially
allowing rapid communication throughout the plant via mechanosensors located around
frankie
(Frankie)
#1