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Plant Hormones

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Plant Hormones

Plants do not have nervous systems that help them respond to changes happening around them. Instead, they use chemical molecules, known as plant hormones, to control and coordinate growth and responses to their environment. They are regulatory messengers produced by the plant (or sometimes synthetically added) to stimulate an action or behaviour in specific cells. A little goes a long way! These hormones are made in one part of the plant and are transported to others parts, where they have their effect.

Functions of Plant Growth Hormones

Plants are quite sensitive but since they aren't able to move, they have developed a system to help them respond to the various stimuli in their environment. These stimuli include gravity, light, touch, water and changes in seasons.

Stimuli (plural) or stimulus (singular) is something that triggers a specific reaction in an organism.

These responses by the plant to the stimuli in the environment are known as tropisms. These help plants to grow towards a light and water source so that processes such as photosynthesis can take place more efficiently. There are two categories of tropisms:

  • Positive tropism: the growth or movement of the plant towards the stimulus.

  • Negative tropism: the growth or movement of the plant away from the stimulus.

Types of Plant Growth Hormones

There are five main types of plant hormones that coordinate the growth and development of a plant. These plant hormones all serve different purposes and sometimes have more than one function (see table below). They can work independently or together to help affect plant growth.

Plant Hormone

Function

Auxins

Affects tropisms and apical dominance

Gibberellins

Initiates seed germination

Ethene

Controls cell division and ripening of fruits

Cytokinins

Promotes cell division and delays the natural ageing process

Abscisic Acid (ABA)

Closes the stomata in times of stress

Apical dominance – the phenomenon by which the main stem of the plants grows more than the side stems.

Stomata – little openings (pores) in the leaves or stem of a plant, which allow substances such as gases to leave and enter.

Auxins

Auxins are a family of hormones that are very important for the growth of plants and can be found in growing stems and roots where they diffuse to other parts of the plant. They are involved in a few of the growth processes and it is likely that you have seen their effects.

Auxins are so important that if a plant doesn't produce auxin naturally, then it will die!

Plants tend to grow (NOT bend) towards the direction of a light source – this is known as positive phototropism. This is due to an unequal distribution of auxins, causing unequal growth rates in the stem. The side that is in the shade contains a higher concentration of auxins, leading to cell elongation and the plant growing towards the light.

When a higher concentration of auxins is found at the tip of the stem, it leads to apical dominance. This is when the main stem of a plant grows more strongly than the side branches. If the auxin-containing tips are cut off, then phototropism will not take place.

Auxins were the first of the 5 main plant hormones that were discovered and studied. The effects of the auxins were noted in a book written by Charles Darwin and published in 1880, that depicts the movements of plants. But it wasn't until the 1920s that auxins were discovered to be the cause of phototropism.

Gibberellins and Ethene

Gibberellins are a group of plant hormones that are involved in several different development phases, such as initiating seed germination. They signal to seeds that it is time to sprout, putting a pause on the resting time (dormancy) of the seed or bud. Despite being a completely different hormone, gibberellins, similarly to auxins, also help to promote plant growth by cell elongation. They essentially help to make the stems of plants grow longer.

Ever wonder why when you buy bananas, they ripen so quickly and cause all of the other fruit in the bowl to ripen too? The reason for this is due to ethene. This is an unusual plant hormone, as it is the only plant hormone to exist as a hydrocarbon gas. This means that it can travel through the air, and have an effect on any nearby fruit! Ethene also is involved in cell division, where it controls the process of abscission.

Abscission – the natural dropping of parts of the plant, such as ripe fruit or dead flowers.

Cytokinin and ABA

Cytokinin is like an anti-ageing cream for plants! It helps to delay the natural ageing process, known as senescence, by encouraging cells to divide. It is sometimes used by florists to delay wilting of cut flowers.

Cytokinins are passive: they do not require energy to travel from the root (where they are produced) up the stem (where they have their effect). They travel by hitching a ride on water molecules!

ABA helps to prevent further water loss. It does this by encouraging the stomata to close in times of stress, such as when there is a lack of water.

Plant Rooting Hormone

Auxin, just like with stem growth, is a necessary plant hormone for rooting. It helps plants to establish roots! As discussed above, auxins are involved in phototropism. The growth of the stem goes towards the stimulus of light. However, when it comes to the roots, the unequal distribution of auxins leads to the growth of the roots towards the direction of gravity – known as positive geotropism (or gravitropism) and the stem to grow away from gravity (negative geotropism).

Uses of Plant Hormones

These plant hormones are used in agriculture and horticulture for various reasons to help control plant growth in some way.

Auxins are plant hormones that are used:

  • as rooting powder

  • for promoting growth in tissue culture (along with cytokinin)

  • as weedkiller

Gibberellins are used:

  • to speed up germination by ending seed dormancy
  • to promote year-round flowering
  • to increase fruit size and yield

Ethene is used:

  • to control the ripening of fruit

Plant Hormones - Key takeaways

  • Plant hormones help plants to control and coordinate growth and responses to their environment.
  • Environmental stimuli for plants include light, gravity, water, changes in seasons and touch.
  • Responses by the plant to environmental stimuli are known as tropisms.
  • The five main types of plant hormones are Auxins, Gibberellins, Ethene, Cytokinins and ABA.
  • These plant hormones are used for various reasons in agriculture and horticulture to control plant growth.

References

  1. Lincoln Taiz et al., Plant Physiology and Development, 2018
  2. René Benjamins and Ben Scheres, Auxin: The Looping Star in Plant Development, Annual Review of Plant Biology, 2008
  3. Shinichiro Komaki and Keiko Sugimoto, Control of the Plant Cell Cycle by Developmental and Environmental Cues, Plant and Cell Physiology, 2012

Frequently Asked Questions about Plant Hormones

Plant hormones are regulatory messengers produced by the plant (or sometimes synthetically added) to stimulate an action of behaviour in specific plant cells.

  • Auxins - affects tropisms and apical dominance
  • Gibberellins - initiates seed germination
  • Ethene - controls cell division and delays the natural ageing process
  • Cytokinins - promotes cell division and delays the natural ageing process
  • Abscisic Acid (ABA) - closes the stomata in times of stress


Auxin is necessary plant hormone for establishing roots.

Plant hormones help plants to control and coordinate growth and responses to their environment.

They are made in one part of the plant and transported to others, where they have their effect.

Final Plant Hormones Quiz

Question

Name 5 different environmental stimuli that plants respond to.

Show answer

Answer

Light, gravity, water, changes in seasons and touch.

Show question

Question

Briefly describe auxins and its functions?

Show answer

Answer

Auxins are a family of plant hormones that are involved in plant growth and establishing roots. Auxins cause cell elongation, where the unequal distribution of auxins in the stem leads to phototropism and in the roots, geotropism. A high concentration in the tip of the stem leads to apical dominance.

Show question

Question

What are gibberellins?

Show answer

Answer

A group of plant hormones involved in initiating germination by pausing seed dormancy and helping stems to grow longer via cell elongation.

Show question

Question

What is ethene?

Show answer

Answer

Ethene is a plant hormone that is a hydrocarbon gas. It controls fruit ripening and cell division.

Show question

Question

What happens when the tips of the plant stems are cut off? Why?

Show answer

Answer

Phototropism does not take place, because a higher concentration of auxin is found at the tips of stems (leading to apical dominance). When this is removed, auxin can no longer trigger the response of growth towards the stimulus of light.

Show question

Question

When a plant grows towards the stimulus of light, then it is described as __________?

Show answer

Answer

Positive phototropism

Show question

Question

What controls a plant's response to environmental stimuli? How do plants respond to these stimuli?

Show answer

Answer

Plant hormones, by controlling and coordinating growth.

Show question

Question

Define positive tropisms.

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Answer

The growth or movement of the plant towards the stimulus.

Show question

Question

Define negative tropisms.


Show answer

Answer

The growth or movement of the plant away from the stimulus.

Show question

Question

When the stem of a plant grows away from the stimulus of gravity, then it is described as _________?

Show answer

Answer

Negative geotropism

Show question

Question

What is senescence? Name the plant hormone responsible for delaying this process?

Show answer

Answer

It is the natural ageing process, cytokinins help to delay senescence.

Show question

Question

Abscisic acid (ABA) _______ the stomata (pores in plants) in times of _______.

Show answer

Answer

closes, stress

Show question

Question

What plant hormone is commonly sprayed on fruits, like bananas, to help them ripen?

Show answer

Answer

Ethene

Show question

Question

Gibberellins can be used to...

(select all that apply)

Show answer

Answer

End seed dormancy

Show question

Question

What plant hormone is used as a weedkiller?

Show answer

Answer

Auxins

Show question

Question

What are plant hormones?

Show answer

Answer

Simple, organic, chemical messengers that help regulate plant growth and development

Show question

Question

What are phytohormones?

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Answer

plant hormones

Show question

Question

What are auxins?

Show answer

Answer

Plant hormones that control cell elongation.

Show question

Question

What are gibberellins? 

Show answer

Answer

A plant hormone involved in germination, flowering and fruit formation.

Show question

Question

What is ethene/ ethylene?

Show answer

Answer

Plant hormone that controls fruit ripening.

Show question

Question

Why is phototropism so important?


Show answer

Answer

Plants need to absorb light for photosynthesis so grow towards the light.

Show question

Question

What is a tropism?

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Answer

Growth or turning movement of a plant, in response to an environmental stimulus.

Show question

Question

What is positive tropism?

Show answer

Answer

Growth towards a stimulus.

Show question

Question

What is negative tropism?


Show answer

Answer

Growth away from a stimulus.

Show question

Question


What are auxins?

Show answer

Answer

Auxins are plant hormones that are made in the tips of growing roots and stems. They can diffuse from the apical meristem to other regions of the plant where they can have an effect and cause cells to grow more or grow less.

Show question

Question

What effect do auxins have in the root?


Show answer

Answer

They cause the cells to slow down growth.

Show question

Question

What effect do auxins have in the shoot?


Show answer

Answer

They cause the cells to elongate. 

Show question

Question

______ distributions of auxin cause unequal growth rates in plant roots and shoots.


Show answer

Answer

unequal

Show question

Question

What happens when a plant is exposed to light on one side only?


Show answer

Answer

  1. Auxin moves to the shaded side of shoot.
  2. Auxin stimulates cells to grow more on the shaded side.
  3. The shoot bends towards the light.
  4. The plant receives more light, speeding up photosynthesis.

Show question

Question

How do weed killers that use auxin work?

Show answer

Answer

Auxin causes the cells to grow too rapidly and the weed dies.


Show question

Question

How do rooting powders work?


Show answer

Answer

Rooting powder speeds up the growth of plants. The plant will grow at a quicker rate.


Show question

Question

How do auxins promote growth in tissue cultures?

Show answer

Answer

The auxins cause the cells to form shoots and roots at a quicker rate.

Show question

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