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Stimulate Plant Growth Hormones

The role of Auxin

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Auxin refers to a class of hormones used by plants in the coordination of multiple essential growth and behavioral processes and are hence crucial in regulating and controlling plant growth. For example, auxin is responsible for phototropism in plants, which is the curving of plant stems towards light. While the term "auxin" is used for any organic or synthetic chemical that promotes cell elongation of the tip of plants, it most often refers to Indoleacetic acid (IAA), which was the result of purifying auxin in plants. The structure of IAA is shown on the right.

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Image 1: The full skeletal formula for Indoleacetic acid, the auxin that plants produce

Acid-Growth Hypothesis​

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Phototropism occurs when the darker parts of the plant contain more IAA than the parts of the plant which light is being shone upon, which causes the cells at the darker parts of the plant to elongate to a greater extent as compared to the cells at the lighter parts of the plant. IAA stimulates cell elongation over a concentration range of about 10    to 10   M. 

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According to the Acid-Growth Hypothesis, IAA stimulates proton pumps to pump H   ions into the cell. This increases the potential difference between the cell and the surroundings due to the increased difference of charge between the cell and the surroundings. This also lowers the pH of the cell, which is illustrated in the video below, where an increase in the concentration of H   ions by the proton pumps being activated is shown by the green line moving left and a corresponding decrease in the pH is shown by the red line moving downwards.

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©Lumination Incorporation 2023

Video 1:  This short clip shows the relation of the concentration of H+ ions and pH value, which causes the pH value to decrease from 7 to 6. IAA simulates the cell's proton pumps into the cell, decreasing its pH by increasing its concentration of H+ ions.

This acidification of the cell wall activates protein called expansins which increase the cell wall's plasticity by breaking the hydrogen bonds between components of the cell walls. In addition, the increase in the concentration of the H   ions in the cell, which causes osmotic uptake of water by the cell where the water travels down the water potential gradient from the surroundings into the cell. This exerts a turgor pressure on the cell wall, which causes the cell to expand and elongate.

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How wavelength determines seed germination

Light is a highly important factor in plant hormonal regulation. Various plant responses are determined by photoreceptors, which are also called pigments. Scientists have determined two major classes of photoreceptors, blue and red-light photoreceptors.

Blue Light

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Blue-light photoreceptors mediate some plant processes, which include the light-induced opening of stomata. These blue-light photoreceptors, also known as cryptochromes due to their elusive nature. In addition, phototropin, a protein kinase is also involved in phototropism in plants. The chemical structure of phototropin is shown on the right, while the effects on different wavelengths of light on the phototropic curvature of the plant caused by auxin is shown below.

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Image 2: The skeletal formula of phototropin, a protein kinase controlling phototropism in plants that is controlled by blue light.

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Image 3: An illustration showing how a plant stem would bend when exposed to different wavelengths of light. Since phototropism is mediated by phototropin, the plant stem would curve the greatest when exposed to blue light.

Red vs Far-red Light

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Plant possess pigments called phytochromes, which mostly absorb red and far-red light. These pigments are also known as red-light photoreceptors. These phytochromes have two forms, one that absorbs red light of around 660 nm (Pr) and the other absorbs far-red light of about 730 nm (Pfr). The image below shows the two forms of phytochrome, which have similar functional groups but different geometrical arrangements.

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Image 4: The chemical structures of an example phytochrome in its two versions.

As Pr absorbs red light, it is converted to Pfr. In addition, Pfr is also be converted back to Pr when it absorbs far red light. This reaction is photoreversible, meaning that a Pr phytochrome molecule can be converted to Pfr when it absorbs red light, and be reconverted back to Pr when it absorbs far red light.

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Pfr is the form of phytochrome that triggers seed germination in plants. Thus, seeds germinate only when exposed to red light as the red light converts Pr to Pfr. However, far red light negates the effect of the red light as it reconverts the Pfr back to Pr, hence preventing seed germination. An illustration of this reaction is shown below.

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Image 5: This shows how the two forms of phytochrome are synthesised when exposed to different wavelengths of red light. Hence, shining far red light on seeds prevents seed germination as the form of phytochrome required for seed germination is not synthesised by the plant.

Biological Clocks

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Many plant processes, like transpiration and enzyme synthesis, undergo a daily oscillation. These cyclic variations are synchronised to the day and night cycle, with its variations in temperature and light intensity. However, these cyclic variations still occur even when kept in constant light or darkness, and hence the term “circadian rhythms” has been given to them. A example of a circadian rhythm is shown below.

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Image 6: The following image shows an enzyme regulated circadian rhythm. In this page, we will study circadian rhythms with light as the determining factor for the oscillation, which would cause flowering in the plant.

The flowering in plants are controlled by the maximum amount of time the plant was exposed to continuous darkness. This critical night length is determined in a period of 24 hours, hence is a circadian rhythm. Short night plants flower only when the maximum amount of time the plant was exposed to continuous darkness is shorter than the critical night length, while long night plants flower only when the maximum amount of time the plant was exposed to continuous darkness is longer than the critical night length.

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Image 7: These diagrams display the effects of light on flowering in both short-night and long-night plants.

Red light is the most effective disruptor of the period of continuous of darkness as experienced by the plant. This is because the Pfr version of the phytochrome signals to the plant about the presence of light, effectively disrupting the period of continuous darkness. As with the germination of seeds, this is reversible when the plant is shone with far-red light. The image below shows shining red and far-red light affects flowering in both short-night and long-night plants.

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Image 8: These diagrams display the effects of red and far-red light on flowering in both short-night and long-night plants.

Impacts and Applications

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Plant growth hormones are essential in regulating plant growth, and it is shown that adequate light from all wavelengths would be required to facilitate plant growth, especially red light. From the above information, ideally, short night plants would be more ideal for indoor farming systems where the plant is constantly shown light throughout the whole day to maximise nutrient production. However, in order to induce flowering in long night plants, far red light could be shown at night time along with red light to have mutually cancelling effects on each other. Since the numbers of flowers produced by the plant will always be greater than the number of fruits produced, conditions ideal for flowering would be required in order to increase the number of fruits formed by the plant. The table below shows the some edible plants, along with whether are they short night or long night plants, and the part of the fruit consumed.

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Table 1: Plant Table showing the part of the plant consumed and whether the plant is a short-night or long-night plant.

In addition, light of similar intensity should be shone around the sides of the plant to prevent the unequal accumulation of auxin in the parts of the plant exposed to less light, allowing the plant to grow vertically and preventing the rejection of produce due to their bent "ugly" appearance.

Image References

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  1. https://phytotechlab.com/indole-3-acetic-acid-iaa.html (Image 1)

  2. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/phototropin (Image 2)

  3. Campbell, N. A., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Reece, J. B. (2020). Biology: A Global Approach, Global Edition. (pg 910) (Image 3)

  4. https://biologyreader.com/phytochrome-in-plants.html (Image 4)

  5. Campbell, N. A., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Reece, J. B. (2020). Biology: A Global Approach, Global Edition. (pg 911) (Image 5)

  6. https://www.frontiersin.org/articles/10.3389/fpls.2022.836244/full (Image 6)

  7. Campbell, N. A., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Reece, J. B. (2020). Biology: A Global Approach, Global Edition. (pg 913) (Image 7)

  8. Campbell, N. A., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Reece, J. B. (2020). Biology: A Global Approach, Global Edition. (pg 914) (Image 8)

  9. Table 1 and Video 1 was created by ourselves.

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