What are the five major plant hormones and their actions?

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

What is plant rooting hormone?

Auxin is necessary plant hormone for establishing roots.

What is the role of hormones in plants?

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

Where are plant hormones produced?

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

Plant Hormones: Definition, Examples & Types

Q: What are 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.

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.

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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

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

Plant Hormones

StudySmarter Editorial Team

Functions of Plant Growth Hormones