Plants move, even though they don’t have muscles. They can respond to touch even though they don’t have nerves, and to differences in light and dark without having eyes.
The mechanisms that make this possible are known as tropisms, growth patterns in plants in response to environmental signals. Guided by plant hormones, tropisms allow plants the ability to “move” toward favorable conditions or away from unfavorable ones, which compensates in some ways for plants’ inability to actually move. Tropisms are directional — one example is a seedling growing toward the light — and the word is derived from the Greek word trope, which means “turn.”
Guided By The Light
Phototropism, the response to light, is the most familiar to people. We’ve all seen plants grow toward a bright window. The first plant hormone ever identified, auxin, (also known as indole-3-acetic acid or IAA) is responsible for this response.
Auxin is involved in a number of plant processes, but one of its main functions is in cell elongation. It’s produced in the apical meristem, or growing tip of the plant, and moves down through the stem in decreasing concentration. When light is directly overhead, auxin concentrations remain equal on all sides of the plant, but when more light comes from one direction, auxin migrates to the shaded side, causing cells on that side to lengthen, which causes the stem to turn toward the light. In some plants, auxin has this effect on the leaf petioles as well, causing individual leaves to turn towards the light and maximizing photosynthetic activity.
Auxin works by causing the release of hydrogen ions from cells, which changes the pH of the cell wall, which in turn causes the cell wall to soften. This allows the relatively rapid expansion and elongation of the cell.
Why do shoots grow up and roots grow down? You could reasonably guess it’s because shoots grow toward the light and roots grow toward the dark. But it’s actually a response to gravity called gravitropism — either negative or positive — that guides this. Once again, auxin is involved, although other plant hormones may be as well. Studies are ongoing — even in space — to learn more about how plant cells perceive and respond to gravity.
A simple explanation however, is that similar to phototropism, the directional response is caused by cell elongation on one side of the stem or root, causing uneven growth. If a potted plant falls on its side, auxin accumulates on the lower side of the stem, causing elongation on that side, which turns the stem back upwards. This is a negative gravitrophic response, meaning it grows away from the gravitational pull.
Roots do the opposite, of course, growing toward the gravitational pull, a positive gravitrophic response. Interestingly, if roots encounter an obstacle, the gravitrophic response is temporarily overruled so the root can grow upwards if needed. Once the obstacle is surmounted, the root grows downward again.
In roots, the role of auxin is less clear, but it is still thought to be involved. The cells that are gravity sensitive are in the root tips, but the elongating cells are further away. Researchers are trying to determine how the information between the two is transmitted (possibly an electric signal) and what other plant hormones may be contributing to the process.
A Touching Response
Both roots and shoots exhibit thigmotropism, the response to touch. In roots, a negative thigmotropic response, growing away from the touch, allows roots to navigate up over and around obstacles, in concert with the gravitropic response. In leaves and stems, the response can be either positive or negative. In twining plants such as clematis or morning glory, tendrils coil around the object, sometimes in as little as 3 to 10 minutes.
When plants are capable of this quick response, it is due to changes in cell turgor pressure (water content). The tips of most tendrils contain tiny hairs that, when touched, cause changes in the cell’s turgor pressure that cause the cell to shrink or expand. The side of the tendril away from the object expands rapidly while the side next to the object stays the same or in some cases, shrinks. This causes a curvature in the tendril. Later on, as plant hormones have time to respond, auxin and another plant hormone, ethylene, play key roles in causing differential growth in the cells, making the coiling more permanent.
Researchers believe that different species of plants likely have unique responses to touch — that it’s not the same for every plant. Studies have shown that some plants are more sensitive to touch than humans, in one case nearly 10 times as sensitive.
It’s not just vining plants that have thigmotropic responses. Plants have to tolerate and survive wind, insect and herbivore feeding, and other potentially damaging physical mechanisms. These longer-term responses are called thigmomorphogenesis (thigma (touch) and morphogenesis meaning development of form) Using a complex and varied array of signalling molecules and plant hormones, plants respond in a variety of ways, from strengthening or relaxing cell walls, slowing growth, changing flowering time or chlorophyll content, dropping leaves or closing leaf stomata to reduce water loss. Studies in ____ showed significant differences in height between Arabidopsis plants touched twice per day and those that were not. Not only were the touched plants shorter and more compact, plants nearby that were not touched also were shorter, showing the existince of a long-distance signaling mechanism.
A number of genes are induced in response to touch, whether constant or incremental, aiding in the plant’s response. The specific chemical pathways that are responsible for triggering the responses are not well understood and scientists continue to look for answers.
Vartanian, Steffan “Thigmotropism in Tendrils.” Kenyon College Web. 6 May 2016.
Gao, Q., Kachroo, A. and Kachroo, P. (2013) “Chemical Inducers of Systemic Immunity in Plants.” Journal of Advanced Botany. Web. 6 May 2016.
Métraux, J., Nawrath, C. and T. Genoud. (2002) “Systemic Acquired Resistance.” Euphytica. Web. 6 May 2016.
Raven, P. and G. Johnson. 2001. Biology. Chapter 9: How Plants Grow in Response to Their Environment. McGraw-Hill Higher Education: Columbus, OH.
Choudhary, D., Prakash, A. and B. N. Johr. (2007) “Induced Systemic Resistance (ISR) in Plants: Mechanism of Action.” Indian Journal of Microbiology. Web. 6 May 2016.
Vallad, G., and R. Goodman. (2004) “Systemic Acquired Resistance and Induced Systemic Resistance in Conventional Agriculture.” Crop Science. Web. 6 May 2016.