Experiment: Sensitive Mimosa Electrophysiology
Using the Venus Fly Trap, we previously introduced you to plant electrophysiology, showing that plants can generate electrical impulses too! We now move to a second exquisite "rapid movement plant" - the sensitive mimosa.
What will you learn?
You will learn more details about rapid plant movement, how water is moved rapidly through cells, plant morphology, and the ion channels used by plants.
- Venus Fly Trap Electrophysiology - Our Venus Fly trap experiment is easier and serves as a good start to plant electrophysiology before moving to the more challenging mimosa preparation.
Note: As you cannot normally buy sensitive mimosa plant seedlings, you have to buy seeds and grow them. Mimosas are famously tricky to germinate, and see our note at the bottom of this page for recommendations on mimosa growth.
With its lovely purple flowers and the hypnotic ways the leaves fold when touched, the sensitive mimosa (Mimosa pudica) has enraptured home gardeners and plant physiologists alike for its beauty and its unique behavior.
In a healthy mimosa plant, you can observe two "rapid movement" responses to touch. With a light touch brushed along the leaves (called pinnules), the leaves fold together at points (pulvinules) along the rib (rachis). With a strong touch, the leaves will fold and the branch will drop along the point (pulvinus) where the main branch (petiole) joins the stem.
How does such dramatic movement occur? How does the plant even detect the touch to begin with? Hard working scientists have hypothesized that small red mechanoreceptor cells on the underside of the leaves respond to mechanical disturbance. This initiates an electrical impulse (action potential) propagation along the rachis that results in the plant movement behavior we observe.
With a strong touch, the action potential travels along the rachis, down the petiole, and all the way to the main joint (pulvinus) via "phloem tubes." The exact nature of this signal propagation is still actively being investigated.
But how then do the plants actually move? Since plants do not have muscles like we do, plant movement occurs through hydraulic forces (the flow of water). Plant cells have special large organs called "vacuoles" which are filled with water and can make up 70-80% of the cell volume. Plants thus have developed ways to rapidly move water in and out of the the vacuoles through special transport channels in their cell walls called "aquaporins." These are like ion channels, but instead of permitting ions to flow across membranes, they permit the rapid flow of water. As such, plants capable of rapid movement quickly flush water out of select cells. Such efflux of water shrinks the cell, and the shrinking of multiple cells at once, depending on location in the plant, can cause a mechanical strain that results in rapid movement.
What initiates the water movement to begin with? Why, the action potential itself! The movement of ions across the cell membrane, which causes the action potential we observe, also creates the osmotic imbalance that results in water movement
The illustration below depicts this process. The action potential begins with an increase in the intracellular calcium levels, which, being positively charged, makes the voltage on inside of the cell more positive. This increase in voltage then opens the voltage sensitive chloride channels, causing chloride to flow out of the cell, making the inside of the plant cell yet even more positive. In response to this chloride efflux, potassium channels then open to permit potassium to also flow out of the plant cell. Potassium being positive, this restores the resting potential and balances the chloride charges.
Buuuuuttttt...we now have an ionic situation the plant cell can exploit: an excess of chloride and potassium ion are now outside the cells, or, we have an osmotic imbalance. Through the aquaporins, Water will then "chase" the potassium and chlorine ions, causing the plant cells to lose water rapidly, shrink, and, ultimately, result in the rapid movement of plant structures.
After the cells have shrunk, they can be refilled with water again by moving the chloride and potassium ions back into the cells, but this requires energy expenditure, and is a slower process. In mimosas, this takes ~10 minutes, in Venus Fly Traps: ~1-2 days.
We will now observe and measure this sensitive mimosa action potential. Join us as we continually expand our plant electrophysiology knowledge!
Before you begin, make sure you have the Backyard Brains Spike Recorder and Arduino Programs installed on your computer. The Arduino "Sketch" is what you install on your Arduino circuit board using the Arduino laptop software (your board comes preinstalled if you bought the Arduino from us), and the Backyard Brains Spike Recorder program allows you to visualize and save the data on your computer when doing experiments. We made a software video for you to explain this!
Tutorial Video of Experiment
In this experiment, we are going to measure the action potentials generated at the stem/petiole joint of mimosa plants.
- You will use our Plant SpikerShield Bundle which has all the materials you need (sans plant) to do the experiment.
- Grow a sensitive mimosa. You will need to start about 3-4 months before you do your experiments to have plants large enough for recording. Some planning is necessary. Our mimosas were grown in pots exposed to ambient light and air and not in a green house. Thus, the research for these experiments was always done during Spring/Summer. Life. Plants. Seasons. Cycles.
- Place our plant electrode in the manipulator, and position your manipulator such that an inch of free silver wire is close to a stem/petiole joint.
- Now, carefully wrap the silver wire around the union of the joint. Note that the plant branch will droop (move) as you do this due to the mechanical disturbance of wrapping the wire around the branch.
- Put the ground wire pin in the...wait for it...ground of the plant pot.
- Wait about 10 minutes for the plant to recover from its droopping movement. As said before, an advantage of the sensitive mimosa is that, unlike the Venus Fly Trap, it only takes 10 minutes to recover instead of 1-2 days.
- Place some conductive gel along the silver spiral wrapped around the petiole. Note: we have noticed that placement of excessive conductive gel (perhaps due to ionic shunting?) prevents movement of the branch. Place only a small a small amount of conductive gel along the silver spiral.
- To get a clean signal we do what we like to call "Plant time", a time where we are devoted to only the Mimosa pudica. How do we do this? Not singing or praying to the plant, but turning off all the lights and unplugging every single power outlet that's in the room, including the internet router if it's near the experiment setup. This will reduce the electromagnetic noise that can interfere with your recording. We recommend to give heads up to people near you by yelling "Plant time!" before unplugging everything.
- Open our SpikeRecorder software, and in the settings window (click on the gear shaped symbol in the upper left hand of the screen to get there), connect to your USB port by clicking on the plug button.
- Now, the line on your screen should become flat, and we are likely recording from the plant if we have a proper interface. Press the "Record" button (red button on top right of screen) to begin saving your data as a .wav file.
- With a plastic probe, tap in one smart motion the leaves of the branch you are recording from. The plant branch should move in a dramatic fashion, and, in the SpikeRecorder software, you should notice a long deflection! Congratulations! you have just recorded an action potential in the mimosa! Such a universal signal that keeps us all functioning.
- if your action potential is too big, resulting in "flat tops," your gain is too high and you need to reduce it on the SpikerShield. The SpikerShield has gain wheel that can appear counter-intuitive, as counterclockwise movement increases gain but clockwise movement decreases gain. We have found one quarter gain works well.
- To analyze the data, such as the duration and amplitude of the plant action potential, you can open your .wav files by clicking the "open button" (looks like three vertical lines) next to the "record button."
- Now go investigate further the electrophysiology of the Mimosa and make new discoveries!
Discussion / Further Work
Notes on Growing Mimosas
We have been growing mimosa plants in various attempts for the last three years, and below we summarize our efforts to ensure generation. A surprise to us was our observation that the mimosas have very deep roots, notable while the seedlings are still small. We present our growing recommendations, and let us know if you have any insider tips.
- The seeds are fairly easy to obtain, you can purchase them on Amazon, but the germination rate is not high. We have only observed that only 10-20% of the seeds will germinate, and of, these, only 1/3 will survive the seedling phase.
- With a pair of pliers, lightly compress a seed to cause the hard outer shell to "crack." This will allow water to diffuse into the seed more readily and cause germination.
- Place about 50 "cracked" seeds between two layers of soaked paper towels, to make a "moist paper towel - mimosa seed - moist paper towel" sandwich.
- Keep the towels moist everyday, and everyday, check the seeds to see if you observe small white "rootlets" coming out of the seeds. This is a sign of germination. This can take from 3 days to 3 weeks to occur (seriously).
- Place any seeds with rootlets in medium-sized pots. Keep the soil moist. We usually put three to a pot, as typically only one will survive anyway.
- Once the mimosas reach 2-4 cm (~1-1.5 inches) in height, with three-four small branches, you have survived the most difficult part. Now care for the mimosa as you would any other plant, giving it sunlight and water, and transplanting it to a large pot, is it has very deep roots.
- After summer flowering and the arrival of fall, the leaves will yellow and whither away, leaving a plant structure consisting of nothing more than stems and sticks. We thought ours plant were annuals and thus had died... but when we cut the stem, the inside was still vibrant green! And indeed, when spring arrived, the plants began growing new branches and leaves! We have two plants that are now in their second year, appearing thus to be perrenials. It is worth maintaining healthy mimosas indefinitely once you have them, as we have said before, the hardest part of cultivation is the germination of the seeds.
- Edwards, Florencia (writer of Backyard Brains). 2015. Organized Notes I
- Edwards, Florencia (writer of Backyard Brains). 2015. Organized Notes II
- Fleurat-Lessard P, Frangne N, Maeshima M, Ratajczak R, Bonnemain JL, Martinoia E. Increased Expression of Vacuolar Aquaporin and H+-ATPase Related to Motor Cell Function in Mimosa pudica. L. Plant Physiol. 1997 Jul;114(3):827-834.
- Fromm J, Lautner S. 2007. Electrical signals and their physiological significance in plants. Plant Cell Environ. 2007 Mar;30(3):249-57.
- Pavlovic, Andrej. Effect of Electrical Signals on Photosynthesis and Respiration. in Volkov, Alexander (Ed.). 2012. Plant Electrophysiology, Volumes I (Signaling and Responses) / II (Methods and Cell Electrophysiology). Springer Press.
- Song K, Yeom E, Lee SJ. 2014. Real-time imaging of pulvinus bending in Mimosa pudica. Sci Rep. 2014 Sep 25;4:6466. 25253083.
- Visnovitz T, Világi I, Varró P, Kristóf Z. Mechanoreceptor Cells on the Tertiary Pulvini of Mimosa pudica L. Plant Signal Behav. 2007 Nov;2(6):462-6.