A nervous system in plants?
During the time I worked for the educational TV channel “NDR Das Dritte” and produced a film on carnivorous plants with the title “Pflanzen, die von Tieren leben” (Plants that live off animals), I was of course aware of Charles Darwin’s inspiring and careful studies, published in 1875, on what was then called “insectivorous plants”. I also knew of the experiments by one of the scientific giants of India, namely Sir Jagadish Chandra Bose, who in 1926 had unequivocally shown that plants not only generate action potentials similar to those known from the nervous systems of animals, but that these plants transmitted the electrical impulse over considerable distances via their vascular bundles (of which as we now know, the phloem plays the part of a “nerve”). However, a great deal more information especially on how those sensitive plant species like the Venus flytrap Dionaea muscipula, sundews of the genus Drosera, or Mimosa spp., etc. react physiologically to mechanical or thermal stimulation is now available than it was at the time when I produced my film.
Plant cells, like animal cells, possess a resting potential of around -120 mV. Sensitive plants have sensory hairs as in case of the Venus flytrap with its 3 hairs on either side of its bilobed leaves which have become modified to trap prey or they have numerous sticky and glistening tentacles on the leaf’s surface as in the sundews to ensnare prey. If in the Venus Flytrap a small invertebrate touches one of the sensory hairs twice within not more than maximally 40 sec, ion channels in the cells of the sensor allow calcium to enter and K+ and Cl- to leave that results in a graded non-propagated transmembrane voltage change: a depolarization. If this depolarization is strong enough to reach a threshold, an action potential (AP) of around 100 mV amplitude is generated. In sundews and other sensitive plants these initial steps are more or less identical, but while the AP in the Venus flytrap rises quickly, lasts a few seconds before repolarization occurs and is then conducted at a speed of 50-100 mm/sec, the action potential in the sundew rises more slowly, lasts much longer (at least 4 minutes in which it generates additional APs) and is transmitted at a the much lower speed of 3-5 mm/sec. The difference is mirrored by the different speeds of the movements that the two plants employ. Mimosa, incidentally, reaches conduction velocities of its action potential of 20-30 mm/sec and responds not just to mechanical, but also thermal (cold and hot) stimuli. Action potential conduction velocities may not seem terribly fast compared with those of mammals that can reach 100 m/sec, but they are actually no different from signal transmission velocities in, for example, some slow invertebrates like mussels and bivalves.
What I found interesting when producing that film on carnivorous plants is that bending of the trigger hairs in the Venus flytrap by strong wind or raindrops does not cause the trap to close and therefore does not apparently generate an action potential. On the other hand, two ever so slight touches of one of the six hairs with a little stick or a brush will lead to the closure of a trap. That there is a smaller, but rather a rare version of the Venus flytrap in the form of the aquatic Aldrovanda vesiculosa, a species that occurs in some freshwater lakes and ponds of Eurasia and Australia and catches waterfleas, is not well known, but that plant also featured in my film. The two Japanese plant physiologists T. Iijima and T. Sibaoka recorded resting potentials from Aldrovanda vesiculosa’s trigger hairs (of which there are many more than just the 6 on the leaf blades of the Venus flytrap) of -110 mV and an action potential that lasted 1 sec, had an amplitude of 130 mV and was transmitted at a speed of 80 mm/sec.
Despite these wonderful and exciting findings, many questions remain. For example, how exactly do the action potentials cause the movements of the plants’ structures involved in trapping the prey? Animal neurons work with Na+, K+ and Cl-, why not the plants as well? How come that the Venus flytrap requires two stimuli before initiating a response? There has to be a ‘memory’, but how does memory work in plants that do not, after all, have a brain? And where does the stimulus come from to re-open a trap in case the prey escaped (which does happen!). Perhaps we’ll need another J. Chandra Bose.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2019.
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