Gravitropism

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). The definition of gravitropism is shown below.

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.

Intracellular transduction phase

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.

Translocation phase

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.

Reaction phase

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.