This is something that’s been puzzling me for a while. I know that several plants will react with movement when stimulated - often by touch. Venus flytraps and those plants that fold up all their leaves when touched come to mind. With fly traps, for example, I know how the movement is triggered (special hairlike structures being brushed), but I’ve never been able to figure out how this stimulus was translated to a reaction. What exactly happens when one of those hairs are triggered?
It’s hard to relate to this process, because every reaction - to the best of my knowledge - in mammals utilizes nerve cells and a central nervous system. Plants definitely have neither, so what do they do?
Any hardcore plant biologists able to answer this? I’m looking for all the nitty gritty details.
Dirx
Well, Dirx, I’m not a plant biologist . . . but under my belt I have a degree in microbiology, a significant amount of work in plant genetics, and a lot of textbooks.
Movement in plants is generally divided into two categories, both with sub-categories: tropisms (which include phototropism, gravitropism, and thigmatropism; growth in response to light, root/shoot orientation in response to gravity, and growth in response to touch, respectively), and turgor movements (rapid leaf movements, to which you’re referring, and sleep movements).
Rapid leaf movements result from a rapid loss of turgor by cells within motor organs at the base of the leaf (called pulvini); this loss of rigidity is caused when water leaves the cells by osmosis as a result of potassium loss.
A large factor in signal transmission is electrical impulses, similar to nerve impulses in animals; however, the analagous impulses in plants are MUCH slower.
Yeah, I remember being told about turgor movement some time last year, and my lab professor vaguely mentioned electric action potentials today when I asked him the same question, but I still don’t understand how such a potential is triggered, or how it initiates the loss of turgor.
One of my text books (AP Biology, written between '95 and '98, I think) briefly stated that the exact process was so far unknown. Of course, text books often do make mistakes (and this book was certainly no exception). I was just really curious.
So, if I’m with you, RM, while the resultant movement is hydraulically driven (when the flytrap closes), the stimulus to do so is electronically transmitted. How? Some charge differential is passed from cell to cell?
I’ve got one paragraph in my general bio textbook that I will copy here… (come and get me Benjamin/Cummings Publishing;))
[blah blah blah]… “this response, which takes only a second or two, results from a rapid loss of turgor by cells within pulvini, specialized motor organs located at the joints of the leaf. The motor cells suddenly become flaccid after stimulation because they loose pottassium, which causes water to leave the cells by osmosis. It takes about 10 minutes for the cells to regain their turgor and restore the natural form of the leaf”. [blah blah blah]… “A remarkable feature of rapid leaf movements is the transmission of the stimulus through the plant. If one leaf on a sensitive plant is touched with a hot needle, first that leaf collapses, then the next leaf along the stem until all the plants leaves are drooping. From the point of stimulation, the message that produces this response travels wavelike through the plant at a speed of about 1cm per second. Chemical messengers probly have a role in this transmission, but an electrical impulse can also be detected by attaching electrodes to the plant. These impulses, called action potentials, resemble nervous messages in animals, though the action potentials of plants are thousands of times slower than those of animals. Action potentials, which have been discovered in many species of algae and plants, may be widely used as a form of internal communication”.
So that doesn’t really add much here, but it’s all my book says. You’d have to consult a more serious plant book or person to get the response you’re looking for.
It looks as though Campbell’s Biology text is quite widely used.
The signals are passed directly from ordinary cell to ordinary cell via channels called plasmodesmata, unlike the specialized nervous system in animals . . . this explains why the process is so slow. Apparently transduction can occur via phloem, also (vascular tissue that is primarily responsible for the transport of nutrients throughout the plant).
Just like Mike (cough) I’m not exactly an expert, but I’m not so sure that the “nitty gritty details” about how the process is initiated/carried out are completely known just yet; I’ve searched a bit on the net, and everything on the subject seems to be speculative to a certain point. My biochemistry text contains nothing on the subject.
Anyone out there with an advanced plant physiology text?
Alrighty, with Reinhold Messner’s mention of plasmodesmata and phloem (we just learned about phloem yesterday!), and mmmiiikkkeee’s cite about the loss of potassium in specialized motor cells, we’re really getting somewhere. This is actually more information that I had expected to find out.
Now, if we just knew the details of how the the action potential is started, and how it triggers the loss of potassium in the motor cells…
But thanks a bunch you guys, the information you came up with has been most helpful!
To the best of my knowledge, the only work that has been done on the initiation of mechanosensory impulses has been done in nematodes (microscopic worms). Probably other animal species have similar mechanisms, plants may or may not share them also.
So FWIW, this is how worms work it: there are pores which cross the cell membrane which are normally blocked. When the sensory hair is touched, it physically levers the block out of the pore temporarily. This allows charged ions to flow in, and changes the charge potential across the cell membrane. This change in charge is then propagated via voltage-gated channels, which open pores in response to the change in membrane charge. This can be passed directly between cells that have channels between them for the ion flux to disperse through, or through chemical signalling between cells, like neurons. One of the principle ions involved here is potassium, although in animals calcium plays an equally important role.
I think we’ve got a winner! mischievous, I’ll bet dollars to doughnuts that the process you describe is almost exactly what we’d find in the plants. So it’s not too unlike a nervous system, afterall. Just without the neurotransmitters deal. This is really cool…
Do you think this ion flow business is descendent all the way back to a common ancestor, or possibly evolved independently? Think it’s common enough to evolve independently often?
Whatever the case, it seems as if my questions have all been answered, and I thank you people tremendously for your information.
Again, I don’t have the answer to your question, but I do have some information to speculate with:
I don’t know if maintaining a charge potential across the membrane is universal, but I do know that it is done in plant, animal, and bacterial species. Flagellar bacteria use the membrane potential to power the rotation of their flagella, so they can swim. I can’t think of any examples of bacteria using membrane potential for cellular signalling, so that may be unique to multicellular organisms. I have a vague recollection of slime molds using a similar mechanism to trigger the release of spores from the fruiting bodies. Slime molds are old indeed, they spend most of their time as single-celled organisms and only become multicellular when starved. So, if my recollection is correct (but please, can someone check?), then charge signaling evolved very early in the process of multicellularization.
