Introduction

There is a certain competition between plants for the two: Gravitropism and phototropism. Gravitropism, as the name suggests is defined to be the differential plant growth in response to gravity. To make it a bit simpler, let us divide the word into two. Gravity means in response to gravity whereas tropism refers growth of plants towards a certain direction in response to external stimulus. The gravitropism in general terms is also known as ‘Geotropism’.

Light and gravity are factors that account for proper plant growth. To manage this process plants direct their photosynthesizing organs towards the face of the sun while roots have gravitated to turn out to be a better absorbent system for the plant. There are several gravity-sensing cells in the plants’ organs which are having a few amyloplasts in them. Amyloplasts are the organelles that produce and store starch within the plant’s cell. The plants have several phytohormones that regulate the physiological processes within the plant cells.

Likewise, the auxin is one of the plants’ growth regulators which allows lateral gradient within the plant parts which grows against gravity. This works by promoting cell elongation in the shoot of the plants.

Scientific Invention of Gravitropism

It was no other than Charles Darwin, a famous biologist, and geologist who contributed much to the science of evolution. He discovered that roots show positive gravitropism. This means roots grow towards the direction of gravity whereas shoots that grow away shows negative gravitropism. He demonstrated this principle through a small plant pot which shows such changes throughout their elementary growth days.

The very earliest tropism depiction was done through the ‘Cholodny-Went Model”. This model explained both gravitropism and phototropism under the influence of the growth hormone auxin. Habeverns are the non-woody plants used for such demonstrations earlier.

Positive and negative gravitropism

• Positive gravitropism can be defined as the growth of the root (downwards) in the direction of gravity.
• Negative gravitropism can be defined as the stem’s growth against gravity upwards.

The growth is monitored by unequal auxin distribution within the shoots and the roots of the plant. 

Gravitropism in the roots

In roots, the growth is all the way managed through prior cell division. The cells are known to divide asymmetrically on the tip of the root entering the elongation phase. The cells are totipotent and pluripotent in origin driven from the root meristem. Gravity plays an evident role at this point where it is censored by gravity sensing receptors in the root. The account for basic management of cell expansion in contrast to cell division.

This is known to be under the greatest influence by auxin as stated earlier. Although auxin is known to be found in every organ of the plant it is known to reorient itself in the plants’ gravity field causing differential growth in the roots. The auxins in a different high and low concentrations account for symmetrical growth of the roots. The auxin undergoes a tipping point mechanism where it runs towards the stimulus causing its effect.

Gravitropism in the shoots

This process not just allows proper root orientation but also accounts for a proper shoot position about gravity and light. Though auxin is found in both shoots and stem the difference in response to gravitropism is accountable via differential sensitivity. The sensitivity of what? The auxin. The master hormone is responsible to explain Darwin’s theory.

 Likewise, in the root’s auxins get accumulated to the lower side of the shoot organs letting two things happen. One it allows cell elongation and another curving up of the stem against gravity. This also causes less cell growth on the top cells of the shoot letting auxin gravitate down the shoot towards the root.

Advantages of gravitropism in roots

• It allows the root to anchor strongly into the soil. This allows the plant to stay erect and bear environmental pressure. This allows other tropisms like phototropism to interact evidently with the plant growth.
• This also allows the plant to search for water. This will cause hydrotropism allowing water to pass semi-permeable root cell membranes into the plant down the concentration gradient. This process is called ‘Osmosis’. The water is then dragged to other parts of the plant via the xylem vessels.
• This ensures that the radicle of the seedlings grows downwards changing into roots. It is independent of the position of the seed in the soil. This is positive gravitropism.

How roots find their way during the process of Root Growth?

The roots are buried deep down into the soil. The soil is all the way dark. The scientists explain the root growth fact extensively via root propelling actions. The root growth is monitored well by the root hairs encoded in the furry coat. These hairs tend to grow when there is not an obstacle in their way. The roots undergo a self-reinforcing chemical cycle. There is a well-known protein at the root hair tips known as RHD2. The self-reinforcing cycle stops at the point when the root gets to an obstacle further preventing Calcium intake. This accounts for path discovery by the roots.

Knight’s experiment to show gravitropism

In about 1806, the British physiologist Knight devised an experiment to see how to plant’s growth depends on gravity. What he does was very simple. He took a stand and devised several seedlings vertically into it. These seedlings used to revolve on the stand all incorporated into a small millwheel bringing about the task. The centrifugal force brings about rotation. As the rotation increase, the root shows a growth away from the centre.

Centrifugal force is a phenomenon, which you might have studied in physics. It states that in circular motion gravity tends to act into the centre of the circle from all dimensions. So forth, this phenomenon devises a study argument which later discovered to be a truth. Summing up the theory B Frank, another scientist introduced us to the term of geotropism about sixty-two years later. He came up with three types of geotropism which are:

• Positive geotropism
• Negative geotropism
• Transverse geotropism

Already discussed negative and positive geotropism in terms of the orthotropic (vertical) orientation of plant on its longitudinal axis. Transverse geotropism is termed to be a directional growth regarding the main perpendicular root axis. This defines the existence of lateral or transverse auxiliary plant parts. These parts include stems, leaves, side roots, and lateral shoots.

J.V. Sachs introduced clinostat in 1879 regarding complementary bending about centrifugal forces. He seconds the theory given by Knight and B Frank in the paragraph above.

Read more about Tropism

Practical investigation of auxin

The auxin and phototropism both have a profound effect on the seedling of the plant. The steps to carry out the practice are as follow:

• At very first, you must record the initial length of the seeds.
• For this investigation, mustard or cress seeds be considered an ideal seed type.
• We will look for germination which is defined to be the transformation of seedlings into seeds and then eventually into a plant.
• The dependent variables are the directions in which the seedlings will grow recording the curvature. The independent variable is the suitable light exposure influencing auxin concentration in the seedlings.
• Take cotton wool into the petri dish soaked into water.
• Add ten same types of seeds into the dish. Make sure they are evenly spaced.
• Keep the petri dish into the warm place.
• Manage light orientation at a certain angle to investigate the effect of light and gravity.
• Do measure angle regularly continuously for a week and record the readings.
• To calculate average growth.

Mechanisms of gravity sensation by the plants

If you remember we have already discussed these organelles in the introductory part of the article. These as in a broader term are known as statocytes, where stato means ‘balance’ and cyte means ‘cell’. These cells are found on the tip of the root cells. These statocytes are also having starch grains named as statoliths. The biological gravity-sensing device contains certain receptors which can transform physical information into electrical signals which arise through deformation or displacement of certain things.

In shoots of the plant, amyloplasts are found in vascular tissues whereas it is in columella of the cap in the roots. They sense the external stimulus in the form of gravity and reorient them by expanding both longitudinally and radially. Arabidopsis plant clearly shows several amyloplasts while stained with potassium iodide. This test is a standard test for checking starch.

The statocytes are polarized cells. This means that they have a centrally positioned nucleus with peripheral endoplasmic reticulum and amyloplasts are ‘sediment’ to the bottom. The amyloplasts are known to bring about signal transduction leading to the curvature of the tip.

Role of Phytochromes in gravity sensation

Phytochromes are vital in plant development to a certain extent. These are extensively acting hormones inhibiting negative tropism. They disrupt the formation of starch-filled prototypes endodermal amyloplasts causing them to change their morphology into photosynthesizing organelles chloroplasts. The activity is following the phototropism effect at all times.

Amyloplast signal transduction to a physiological response

The amyloplast sedimentation is the main factor which brings about gravitropism. It is known that amyloplasts are having short actin microfilaments which are in turn attached to the statocytes cortex. This does not allow amyloplast to get itself accumulated into the periphery of the statocytes.

The sedimentation of amyloplasts generates a pull. This eventually. Produce stretch in the plasma membrane of the endoplasmic reticulum. This causes activation of certain channels to find on to the membrane. These channels increase the intracellular flow of calcium ions initiating signal transduction. The Calcium ions act as a second messenger system, in this case, inhibiting calmodulin complexes.

Recent studies suggest that the phosphoinositide pathway influences gravitropism. The phosphatidylinositol-4-phosphate-5-kinase is found to increase in concentration after gravistimulation. This is also co-supported by calcium ions triggering effect. 

Auxin transport from gravity stimulant site to the site of curvature

The auxin is known to be transported from the shoots to the roots. This gets itself distributed into periphery but then its sediments down into the basal regions. The basal regions auxin plays a vital role in cell division and elongation. This will eventually cause root formation. Within cells, auxin transport via utilization of ATP that accommodates well through cellular influx and efflux carriers. The basal polarity is responsible for efflux basally. The greater the accumulation the better the differential accommodation of the root.

Gravitropic mutants

Arabidopsis thaliana is a small flowering plant that is majorly used in several cases to study gravitropism. They showed an odd activity in response to gravity so were purposely isolated from the population. Mutants have found to affect variable gravitropic responses by eliminating growth in response to gravity. The variation in the genetic makeup of the PGM1 gene in these species. The PGM! Gene encodes the enzyme phosphoglucomutase causing statoliths to be less dense. The mutation affects auxin transfer to the basal counterparts inducing further regression in plant growth. The mutation is also seen in Zea mays and few varieties of rice, tomatoes, barley, and maize.

Summary

• The amyloplasts serve to be the primary gravitropic receptors that carry out signal transduction.
• Auxin plays a vital role in such a type of tropism. These activations are transduced into physiological actions. The other hormones which are involved in this response are ethylene and cytokinin which play a valid role in root and shoot gravitropism.
• They do so by transporting auxin to the basal sides. In 1995 Masson discovered that other factors like touch, temperature, humidity, and oxygen concentration affect tropisms in certain ways.
• To study such elements new tools based on reverse genetics have been discovered. These imply a great deal of extensive research in the fields of physiology, biochemistry, structural biology, and functional genomics. To give you an idea just, clinostat is a device that measures the amount of gravitational pull.

Frequently Asked Questions

What is gravitropism?

Gravitropism is a response of a plant’s roots towards gravity. Because of gravitropism, the roots of plants grow downwards under the influence of the earth’s gravity.

How does gravitropism work?

Gravitropism works by the redistribution of auxins in the developing root. Auxins are plant hormones responsible for root initiation and development. These auxins accumulate in the lower part of roots and cause them to bend downwards, instead of causing elongation in the longitudinal direction.

What is negative gravitropism?

The shoots of plants grow upwards, opposite to the gravity of the earth. It is called negative gravitropism.

What is the benefit of gravitropism?

Gravitropism causes roots to grow deep into the soil so that the plant can get water, minerals and other nutrients from the ground. It also allows roots to tightly anchor into the soil. The radicle of the seedling grows downwards into the soil due to gravitropism.

References

1. Darwin, Charles; Darwin, Francisc (1881). The power of movement in plants. New York: D. Appleton and Company. Retrieved 24 April2018.
2. Hangarter, R.P. (1997). “Gravity, light, and plant form”. Plant, Cell & Environment. 20 (6): 796–800. doi:10.1046/j.1365-3040.1997.d01-124.x.
3. Chauvet, Hugo; Pouliquen, Olivier; Forterre, Yoël; Legué, Valérie; Moulia, Bruno (14 October 2016). “Inclination not force is sensed by plants during shoot gravitropism”. Scientific Reports. 6 (1): 35431. doi:10.1038/srep35431. PMC 5064399. PMID 27739470.
4. “Gravitropism Lesson”. herbarium.desu.edu. Retrieved 2018-07-08.
5. Chen, Rujin; Rosen, Elizabeth; Masson, Patrick H. (1 June 1999). “Gravitropism in Higher Plants”. Plant Physiology. 120 (2): 343–350. doi:10.1104/pp.120.2.343. PMC 1539215. PMID 11541950.