Thigmotropism in Tendrils
Steffan "Plant Wizard" Vartanian, '97
What is Thigmotropism?
Thigmotropism is the directional response of a plant organ to touch or physical contact with a solid object. This directional response is generally caused by the 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. An example of such a response is illustrated below:
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 0.0008mg, and a climbing tendril of Sicyos actually repsonds 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. Darwin himself performed experiments which compared these two responses. He found that in a veritcal bean root, a contact stimulus could divert the root away from the vertical, i.e. thigmotropism overrides gravitropism. However, in a horizontally oriented root, downward curvature always resulted, even against a contact stimulus. Therefore gravitropism overrides thigmotropism in horizontally oriented roots. This interaction, or "cross-talk" between thigmotropism and gravitropism likely regulates the pathfinding of roots, but significant studies on the nature of this interaction have yet to be performed.
How Do Tendrils Actually Curve?
Good question! 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, as shown below.
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.
How Do the Cells on the Opposite Side of the Tendril Sense the Stimulus?
The question of how the touch stimulus is transduced from the site of contact to the sites of differential growth is quite complicated. First of all, it is believed that different species of plants transduce this signal in different ways. Secondly, numerous possible mechanisms exist by which this signal could be transduced. However, we know that the signal must orignate from the epidermal cells, because these are the cells which are in direct contact with the touch stimulus. The epidermal cells of most tendril tips have a very dense distribution of "hairs", or tactile papillae, as well as plasmodesmata. When these hairs are touched, the membrane of the cell is temporarily deformed. This membrane deformation causes a change in the cell's ionic permeability, which results in a regulatory action potential. This action potential causes turgor movements which drive the initial curving response. In addition, the high concentration of plasmodesmata on the epidermal cells of tendrils suggests that the whole surface tissue acts as physically unified system. The relationship between these turgor movements and the initiation of differential growth is not well-understood. However, we will consider both mechanisms, and examine their possible relationships.
Putting it all Together....
We have discovered that 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, upregulation 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 reserach 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.
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