Plant Hormones

Plant Hormones

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Plant hormones are special chemicals that plants use to grow and change in response to their surroundings. Unlike animals, plants don't have nervous systems to help them react to changes. Instead, they use these hormones to control how they grow and respond to their environment. These hormones work like little messengers, telling specific cells what to do. Even though they're small, they can have a big impact! The plant makes these hormones in one part of its body and sends them to other parts where they can take action. They can also be made in a lab and added to plants to help them grow better. These hormones are pretty amazing! And they help plants do some pretty amazing things. One of the coolest things is how they can grow towards the light, even if the light source is far away.

Functions of Plant Growth Hormones

Plants have a clever way of sensing and responding to their surroundings, even though they can't move like animals can. They do this through a system of reactions to different things in their environment, called stimuli. These stimuli can be things like light, touch, gravity, water, and changes in seasons. When a plant responds to a stimulus, it's called a tropism. Tropisms help plants grow towards things like light and water so they can do important processes like photosynthesis. There are two types of tropisms: positive, where the plant grows towards the stimulus, and negative, where it grows 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. 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.


Plants need auxins, a type of hormone, to grow properly. Auxins are found in growing stems and roots, and they travel to other parts of the plant to help with growth. They're really important - in fact, without auxins, a plant would die! One thing auxins do is help plants grow towards light. This is called positive phototropism. The side of the stem that's in the shade has more auxins, which makes those cells grow faster and causes the plant to bend towards the light. If there are a lot of auxins at the tip of the stem, the main stem will grow more than the side branches. This is called apical dominance. It was Charles Darwin who first noticed the effects of auxins on plants way back in 1880. But it took until the 1920s for scientists to figure out that auxins are what cause phototropism.

Gibberellins and Ethene

Gibberellins are a group of hormones that help plants grow and develop. They're involved in things like seed germination, signaling to seeds that it's time to start growing. They also help promote growth by making plant stems longer. It's similar to what auxins do, but gibberellins are a different type of hormone. Ethene is another plant hormone, but it's unusual because it's a gas. It can move through the air and affect nearby fruit, like how bananas make other fruit ripen faster. Ethene is also involved in something called abscission, which is when parts of the plant, like ripe fruit or dead flowers, naturally fall off. So even though they're different types of hormones, gibberellins and ethene both help plants grow and change in different ways.


The ripe red tomato is releasing ethene to start the ripening process in the other tomatoes
The ripe red tomato is releasing ethene to start the ripening process in the other tomatoes

Cytokinin and ABA

Cytokinin is a hormone that acts like an anti-ageing cream for plants. It helps to delay the natural ageing process, called senescence, by promoting cell division. This is why florists sometimes use it to keep cut flowers from wilting too quickly. Cytokinins travel passively, without using energy, from the root to the stem by riding on water molecules. ABA is another hormone that helps plants deal with stress, like when there's not enough water. It does this by encouraging the stomata, which are tiny pores on the surface of leaves, to close up and prevent further water loss. So, cytokinin keeps plants young, while ABA helps them conserve water during times of stress.

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

Plant hormones are essential for controlling and coordinating plant growth and responses to the environment. Plants respond to stimuli like light, gravity, water, and touch through tropisms. The five main types of plant hormones are Auxins, Gibberellins, Ethene, Cytokinins, and ABA. These hormones are used in agriculture and horticulture for various purposes, such as promoting growth in tissue culture, speeding up germination, promoting year-round flowering, increasing fruit size and yield, and controlling the ripening of fruit. Understanding and using plant hormones can help farmers and horticulturists to achieve better plant growth and yields.


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

Plant Hormones

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.

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.

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