Thigmotropism In Tendrils
Published on May 10, 2017
Thigmotropism is the directional response of a plant organ to touch or physical contact with solid object. This directional response is generally caused by induction of some pattern of differential growth. This phenomenon is clearly illustrated by the climbing tendrils of some plants, such as the sweet pea. The tendrils actually “feel” the solid object, which results in the coiling response.
So plants actually have a sense of touch?
Yes. In fact, some plants are actually much more sensitive to touch than human beings! For example, human skin can minimally detect a thread weighing 0.002mg being drawn across it. However, a feeding tentacle of the insectivorous sundew plant responds to a thread of 0.0008 mg, and a climbing tendril of Sicyos actually responds to a thread weighing just 0.00025mg! Therefore, some plants have a sense of touch which is nearly 10 times as sensitive as human skin!
What Parts of the Plant Can Respond to Touch?
The clearest example of thigmotropism is the coiling that occurs in some tendrils. However, roots also depend on touch sensitivity to navigate their way through the soil. The general touch response in roots is negative. That is, when a root "feels" an object, the root grows away from the object. In comparison, most tendrils grow toward the touch stimulus, allowing for the tendril to wrap around the object which it is in contact with.
Therefore, roots are said to be "negatively thigmotropic". This allows the roots to follow the line of least resistance through the soil. In addition to thigmotropic responses, roots (as well as other organs) are known to grow in response to gravity. This "gravitopism" allows the roots to grow in the direction of gravity, which is down into the earth. Interestingly, thigmotropism seems capable of overriding the strong graviptropic responses of even primary roots. Gravitropism overrides thigmotropism in horizontally oriented roots. This interaction, or "cross-talk" between thigmotropism and gravitropism likely regulates the path finding of roots, but significant studies on the nature of this interaction have yet to be performed.
How Do Tendrils Actually Curve?
In general, tendrils are able to curve by employing a process known as "differential growth". This process involves the stimulation of growth in particular regions of the tendril. In positive thigmotropism, for example, the side of the tendril which is opposite to the side of contact will grow at a faster rate than the contact side. In some cases, the cells on the contact side will actually compress, which enhances the curving response. Therefore, the non-contact side begins to elongate faster than the rest of the tendril, while the contact side actually compresses. This causes the tendril to curve toward the site of contact.
In addition to differential growth, some tendrils exhibit a type of coiling response which is referred to as "rapid contact-coiling". This type of response is, as the name suggests, very rapid. It is caused by changes in cell turgor which alter the shape of the tendril, causing it to curve. The cells on the non-contact side of the tendril expand, while the cells on the contact side contract, similar to the differential growth patterns in the animation above. Therefore, the rapid contact-coiling response is a rapid initial response, while differential growth is a somewhat slower, but more "permanent" response.
The objective of this plant biology project is to investigate the response of a plant's tendrils to touch.
Materials and Equipment
• Small pots
• Potting soil
• Permanent marker
• Masking tape
• Pencils for smaller plants
• Lab notebook
Growing the Plants
1. Plant two morning glory seeds in a small pot. First put about 3 inches of potting soil into the pot. Form a hole and place the seeds in the hole. Then cover them with about 1–2 more inches of potting soil.
2. Water the seeds regularly and keep the pots in a warm area, out of the direct sunlight.
3. Record the date, the time, the common and scientific name of the plant.
4. It will take up to three weeks for the plant to grow and form tendrils.
Observing Tendril Response to a Solid Support
1. When the plants have produced several tendrils, insert a pencil into the soil near one of the tendrils.
a. Place the pencil in the pot so that it is touching the tendril.
b. Arrange the tendril and support so that the middle of the tendril is in direct contact with the support.
c. Gently secure the stem of the plant to the pencil with a little piece of tape so that the tendrils are in contact with the pencil.
2. Repeat step 1 for two more tendrils.
3. Observe the tendrils for the next 24 hours, as each first touches the pencil, and then as it curls around the pencil.
a. Keep a record of your observations, including times, in the lab notebook.
Experimenting with Stimuli
Now that we have healthy plant with tendrils, experiment with the stimuli that result in the tendrils starting to curl. Specifically, investigate how the frequency of physical contact affects curling. To stimulate curling in the tendrils, first mark a region on tendrils with a permanent marker. Then touch them at different times, with a pencil, and record their response to touch.
1. Mark one spot on few tendrils with a permanent marker, as follows.
a. Mark each tendril in approximately the same region; for example, near the
middle of the tendril.
b. The mark should be about 1 cm long.
2. Keep track of each of the tendrils.
3. Pick three tendrils that will not be marked with the permanent marker. These tendrils will be the negative controls. These tendrils should not touch the pencil or any other solid object.
4. Touch (always with the pencil and always timing with the stopwatch) tendrils 1– 12 at different times of the day, as follows:
a. Tendrils 1, 2, and 3: One time per day for 30 seconds in the morning.
b. Tendrils 4, 5, and 6: Three times per day, for 30 seconds each time, in the morning, afternoon, and at night.
c. Tendrils 7, 8, and 9: Six times per day for 30 seconds. Six times over the course of the day.
d. Tendrils 10, 11, 12: These tendrils should be in constant contact with a support, such as a pencil, as they were in the previous section.
e. Tendrils 13, 14, and 15: These tendrils receive no stimulus.
5. Record the time it takes for the tendrils to start curling.
6. Record the degree of curl—such as 90 degrees, one full rotation, two full rotations, etc.—in the lab notebook.
Thigmotropism is very complex! However, consider that the initial signal must be generated by an action potential. This action potential leads to the establishment of an ionic gradient, which results in increased turgidity in the non-contact side cells, and decreased turgidity in the contact-side cells. This process allows for the initial, rapid bending of the tendril. This rapid bending is then followed by a slower process of differential growth. Jasmonate production may then be increased, which would promote growth in the non-contact side cells. In addition, up regulation of the TCH genes may act in concert with the jasmonates to induce cell growth. Although there are "missing links" in this mechanism which we have not yet uncovered, further research may elucidate the entire mechanism. Studies which focus on the regulation of the TCH genes may prove invaluable in determining how certain areas of the tendril grow at a faster rate than others in response to touch.