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Gravitropism

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Gravitropism

Have you ever tipped a potted plant by accident? Or encountered a plant uprooted after a spell of bad weather? Did you reorient the plant back upright? What do you think would have happened if you had left it that way?

Given enough time, its roots will eventually find their way downwards by bending. Likewise, its stems and leaves will find their way upwards. This is due to a plant response called gravitropism.

In this article, we will discuss what gravitropism is, its various manifestations, its mechanisms, and why it is important to the growth and development of plants.

Definition of gravitropism

Plants use hormones (chemical messengers) and other sophisticated biological mechanisms to respond to environmental stimuli in which they develop their stems, roots, or leaves toward or away from the stimulus. Such behaviors are called tropisms.

Tropisms in plants include phototropism (response to light), thigmotropism (response to touch), and gravitropism (response to gravity).

Gravitropism in plants is the development or growth of plant organs in a specific direction in response to gravity. It is what causes roots to grow downward or shoots to grow upward even in the absence of light. Gravitropism is also sometimes referred to as geotropism because the growth can be described as moving towards the earth (“geo”).

Types of gravitropism

Various plant organs respond to gravity in different ways. Here we will discuss orthogravitropism, diagravitropism, and plagiogravitropism. We will also distinguish between two types of orthogravitropism: positive and negative gravitropism.

Orthogravitropism

Gravitropism in plants can be orthogravitropic, meaning the direction of plant growth is parallel to the pull of gravity. Orthogravitropism can be further subdivided into positive and negative gravitropism.

Positive gravitropism

Positive gravitropism refers to the growth in the same direction as the pull of gravity, that is, downward (or toward the center of the earth). Such growth is typically displayed by primary roots to take up sufficient water and nutrients as well as to ensure secure anchorage in the soil.

Negative gravitropism

Negative gravitropism refers to upward growth, typically displayed by shoots that grow upwards even in the absence of light. Such growth is called negative gravitropism because its direction is opposite of the pull of gravity. This upward growth ensures that the leaves are positioned properly for efficient photosynthesis and respiration.

There are some cases, however, where roots also exhibit negative gravitropism. For instance, some species of mangrove trees produce specialized aerial roots called pneumatophores. Pneumatophores grow upward from deep lateral roots to help mangroves absorb oxygen in waterlogged soil.

Other gravitropic responses

Besides orthrogravitropic, plant responses to gravity can also be diagravitropic or plagiogravitropic.

Diagravitropic means that the growth of the plant organ is perpendicular to the pull of gravity. This can be observed in rhizomes (such as ginger) and stolons (such as strawberries) that grow horizontally.

On the other hand, plagiogravitropic means that the growth of the plant organ is at an angle in relation to the pull of gravity. This can be observed in lateral roots. The lateral outgrowth of roots at the preferred angle from the primary root (which typically grows vertically) provides the plant with a more efficient means of exploring the environment to acquire local resources.

Gravitropism example

If a plant’s orientation is changed, its roots will start a gravitropic reaction to reorient to its original downward growth orientation. Let’s say a corn seedling was flipped to its side. In about 20 to 30 minutes, cells along the higher side of the root close to the root tip will begin to elongate quicker than cells along the lower side, causing the root to bend downward. As time passes, the root will grow longer and curve until it is reoriented such that the root tip faces down, as it was before the plant was flipped on its side.

This example of positive gravitropic behavior in a corn seedling is visualized in Figure 1 below.

On the other hand, plant shoots tend to grow upward. Let’s say a Coleus plant is turned on its side. After an hour, gravitropic curvature begins to form at the tip of the shoot. The leaves around the plant's base realign and settle into a more horizontal posture, just as they were before the plant was flipped on its side. Note that gravitropism is not confined to stems and roots since other organs on a plant, such as the leaves, perceive and respond to gravity as well.

This example of positive gravitropic behavior in a Coleus plant is visualized in Figure 2 below.

How about lateral roots? As mentioned earlier, gravitropic response can also be observed in lateral roots. In Arabidopsis–a small weed that belongs to the mustard family– lateral roots grow at a progressively oblique angle.

After emergence, their lateral roots tend to curve toward an initial preferred angle (which is usually quite shallow), then they straighten up and grow at this angle for a while. They may begin to steeply curve until they reach an angle that is nearly vertically downward which could result in an axial root system, or they may maintain a shallow angle for a long time which could result in a radially expanded root system.

This example of plagiogravitropic behavior in Arabidopsis is visualized in Figure 3 below.

How does gravitropism occur in plants?

Gravitropism is a complex process that can be summed up in four basic phases: perception, intracellular transduction, translocation, and reaction.

We will go through each of these phases briefly, but keep in mind that there are varying explanations to gravitropism and its mechanisms continue to be the subject of investigation by many scientists.

Perception phase

The perception phase is where the cell perceives the direction of gravity due to the displacement of a cell component. Perception generally occurs in the root cap which covers the root apical meristem. Such displacement could be in the form of:

  1. The protoplast pressing against the cell wall.

  2. A heavy starch-filled organelle such as the amyloplast or statolith, allowing the plant to sense gravity by accumulating and weighing down the cell.

The protoplast is basically the whole plant cell without the cell wall.

The amyloplast is a membrane-bound plastid that store and produce starch, while statolith is a general term used for starch-rich organelles that play a role in gravitropism. The two terms are sometimes used interchangeably.

Intracellular transduction phase is where the perception of the direction of gravity produces asymmetry in the cell via its structure and/or biochemistry. While it is uncertain how this step in the process occurs, scientists think that such asymmetry could take the form of:

  1. Asymmetry in calcium in the cells.

  2. The localized activation of certain ion channels in the plasma membrane.

  3. A change in structure and distribution of elements of the cytoskeleton.

  4. An alteration in the plasmodesmata.

Plasmodesmata are microscopic channels that link the cytoplasm of one plant cell to adjacent plant cells.

During the translocation phase a shift occurs in the perceiving cells, resulting in the movement of one or more signals to the site of the reaction phase on both sides of the organ. One of these signals is auxin, a plant hormone that promotes cell elongation.

Cell elongation is the growth or enlargement of cells that have already undergone differentiation.

The reaction phase is where the accumulation of signals like auxin promotes a differential rate of cell elongation:

  1. In roots, a high concentration of auxin slows cell elongation, causing cells to grow slowly on the lower side while growing normally on the upper side, causing the root to bend down toward the region where auxin is highly concentrated.

  2. In shoots, a high concentration of auxin stimulates cell elongation, leading the shoot to bend upward, away from the location with the highest concentration of auxin.

Importance of gravitropism in plants

Gravitropism is important in the growth and development of plants. It is a strategy that enables plants to compete for resources in their immediate surroundings and to resume vertical growth after being toppled down by wind, rain, or other environmental factors.

In addition, negative gravitropism enables plants to protect their seeds from moisture and pathogens that may be present in the soil.

Gravitropism - Key takeaways

  • Gravitropism in plants is the development or growth of plant organs in a specific direction in response to gravity.
  • Orthogravitropism is where the growth is parallel to the pull of gravity.
  • Positive gravitropism is where growth is in the same direction as the pull of gravity, while negative gravitropism is where growth is in the direction opposite of the pull of gravity.
  • Diagravitropism means that the growth of the plant organ is perpendicular to the pull of gravity. This can be observed in rhizomes (such as ginger) and stolons.
  • Plagiogravitropism means that the growth of the plant organ is at an angle in relation to the pull of gravity. This can be observed in lateral roots.

References

  1. “Behavior.” Exploring Our Fluid Earth, University of Hawai‘i, https://manoa.hawaii.edu/exploringourfluidearth/biological/aquatic-plants-and-algae/behavior.
  2. National Academies of Sciences, Engineering, and Medicine. 1998. A Strategy for Research in Space Biology and Medicine in the New Century. Washington, DC: The National Academies Press. https://doi.org/10.17226/6282.
  3. Hangarter, Roger P. Plants in Motion. Indiana University, 2000, https://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html.
  4. Sato, Ethel Mendocilla, et al. “New Insights into Root Gravitropic Signalling.” Journal of Experimental Botany, Oxford University Press, Apr. 2015, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4986716/.
  5. Chen, Rujin, et al. “Gravitropism in Higher Plants.” Plant Physiology, vol. 120, June 1999, pp. 343–350.
  6. Su, Shih-Heng, et al. “Molecular Mechanisms of Root Gravitropism.” Current Biology, vol. 27, 11 Sept. 2017, pp. R964–R972.

Frequently Asked Questions about Gravitropism

Gravitropism in plants is the development or growth of plant organs in a specific direction in response to gravity.  

Gravitropism is an important strategy because it enables plants to compete for resources in their immediate surroundings and to resume vertical growth after being toppled down.

Tropism is the development of plant organs toward or away from stimuli that it is responding to. Phototropism is the response to light while gravitropism is the response to gravity.

Root gravitropism can be caused by the protoplast (the whole plant cell minus the cell wall) pressing against the cell wall; and/or a heavy starch-filled organelle such as the amyloplast or statolith, allowing the plant to sense gravity by accumulating and weighing down the cell. 

Some examples of gravitropism include the tendency of roots to grow downward and of shoots to grow upward even in the absence of light.

Final Gravitropism Quiz

Question

What is gravitropism?

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Answer

Gravitropism in plants is the development or growth of plant organs toward a certain direction in response to gravity.

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Question

This type of gravitropism is where the growth of plant organs is parallel to the pull of gravity.

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Answer

Orthogravitropism

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Question

This type of gravitropism is where the growth of plant organs is in the same direction as the pull of gravity.

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Answer

Positive gravitropism

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Question

This type of gravitropism is where the growth of plant organs is in the direction that is opposite of the pull of gravity.

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Answer

Negative gravitropism

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Question

This type of gravitropism can be observed in stolons and rhizomes due to the horizontal growth of their roots or stems.

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Answer

Diagravitropism

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Question

Explain how positive gravitropism helps in plant growth.

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Answer

By growing vertically downwards, positive gravitropism enables primary roots to efficiently take up water and nutrients as well as to secure anchorage in the soil.

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Question

Explain how negative gravitropism helps in plant growth.

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Answer

Negative gravitropism manifests in the upward growth of shoots. By growing upward, leaves are positioned properly for efficient photosynthesis and respiration. 

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Question

Summarize the process of gravitropism in plants.

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Answer

The plant perceives the direction of gravity, which leads to an asymmetry in cell structure or biochemistry. A signal is sent to the site in the plant organ causing cells to elongate at a differential rate.

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Question

Explain the role of amyloplasts in gravity perception.

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Answer

Amyloplasts are heavy starch-filled organelles that can weigh down the cell due to the pull of gravity. Its weight can cause asymmetry in the cell structure or biochemistry.

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Question

What happens during the reaction phase?

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Answer

In roots, a high concentration of auxin slows cell elongation, causing the root to bend down toward the region where auxin is highly concentrated.

Show question

Question

Why is gravitropism important in plants?

Show answer

Answer

Gravitropism is important in the growth and development of plants. It is a strategy that enables plants to compete for resources in their immediate surroundings and to resume vertical growth after being toppled down by wind, rain, or other environmental factors. It also enables plants to protect their seeds from moisture and pathogens that may be present in the soil.

Show question

Question

A corn seedling was flipped to its side. In about 20 to 30 minutes, cells along the higher side of the root close to the root tip will begin to elongate quicker than cells along the lower side, causing the root to bend downward. What type of gravitropism does this illustrate?

Show answer

Answer

Positive gravitropism

Show question

Question

A Coleus plant is turned on its side. After an hour, gravitropic curvature begins to form at the tip of the shoot. The leaves around the plant's base realign and settle into a more horizontal posture, just as they were before the plant was flipped on its side. What type of gravitropism does this example demonstrate?

Show answer

Answer

Negative gravitropism

Show question

Question

Explain how lateral roots exhibit gravitropism.

Show answer

Answer

Gravitropic response can also be observed in lateral roots which tend to grow at a progressively oblique angle. After emergence, lateral roots would curve toward an initial preferred angle in relation to the pull of gravity (which is usually quite shallow), then straighten up and grow at this angle for a while. They may begin to steeply curve until they reach an angle that is nearly vertically downward which could result in an axial root system, or they may maintain a shallow angle for a long time which could result in a radially expanded root system. 

Show question

Question

Which statement best describes the translocation phase?

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Answer

One or more signals move to the site of the reaction phase on both sides of the organ

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