Spring has sprung! Plants are sprouting and popping up out of the ground. Buds are turning green and flowers are blooming. But how do plants know which way to grow? The roots always go down, the stems grow up, and sometimes the leaves can even turn to follow the sun! The magic behind this is that plants grow in the direction that a molecule called auxin tells them to. With spring coming, this molecule is changing the way our world looks to make it greener.

Quick note – there’s actually a whole family of molecules we refer to as ‘auxins’. As ever with biology, things are not as simple as they might first appear!

As I write this article there are sprouting plants following the plant pied piper’s call – auxin tells the sprouts which way to go.

What kind of molecule is auxin?

Auxin is quite a small molecule, especially compared with some of the proteins we commonly talk about in biology! Remember the size of the proteins we talked about last halloween with blood and cobwebs?

The ‘typical’ auxin molecule is shown below. It has a 6 membered carbon ring attached to a 5 membered ring that has a nitrogen atom in it. When there is an element in one of these rings that is not carbon we call it a heteroatom. Heteroatoms in these kinds of rings can do some interesting chemistry, so they’re useful! Keep an eye out for them in drug molecules and neurotransmitters.

This particular type of heteroatom ring structure is called indole. Auxin has a small carbon chain with a carboxylic acid attached to the 5 membered ring of indole. The carboxylic acid group is another chemically interesting group – so this molecule will be able to interact with a lot of different targets.

The structure of the molcule auxin. An indole ring structure comprising a 6 membered carbon ring with a 5 membered ring with a nitrogen heteroatom is seen, with a 2 carbon chain terminated by a carboxylic acid on the carbon opposite the heteroatom in the 5 membered ring.
The typical auxin structure.

What effect does auxin have on plants?

Plants need light to grow. One adaptation that helps them thrive is to grow towards light. Plants also need water, pulled through the roots. Since water is in the ground below, the roots need to grow downwards. Without a brain or any capacity to ‘decide’, plants rely on chemical signals to make these decisions. Leaves towards light, up. Roots towards water, down. Auxin is one of those chemical signals.

Getting bigger

So how do plants grow?


If we were building a structure with traditional materials and we wanted to make it bigger we would add more blocks. Imagine playing with LEGO – you just add blocks until the structure is as tall as you want it. That is one option in biology – encourage cells to replicate to get a certain organ to grow.


But biology is clever and almost always has more than one option to play with. You could also make your LEGO structure taller by making each of the blocks stretch up and get longer and taller.

Think of filling up a water balloon. If you just pumped water in, the balloon gets bigger. It will get big and fat and round. The same thing would happen if biology just pumped water into plant cells to make them bigger. The walls all the way around the cell would get stretched and turn the cell into a big sphere.

a plant cell on the left is rectangular and short. As more water is pumped in the cell gets fat and round.
A plant cell filling up with water – without help and structural support it will just get fat and round!

What auxin can do is control where the cell can grow. This is more like placing a water balloon inside a tube – think of the cardboard tube from kitchen roll – and then filling it up with water. By controlling which sides of the balloon can grow you’ll have a really long thin water balloon! Right up until you take the tube away, anyway. Auxin molecules can encourage the long sides of the cell to be more elastic – more stretchy – and the cell can get longer and taller.

With support from auxin the cell walls can elongate selectively – making the cell longer and taller instead!

Reach for the sky

This makes the parts of the plant with lots of auxin grow taller. Areas of the plant that are pointing straight at the light will grow a lot – to reach out for more sun! Areas of the plant that are hit by light on one side will grow more on the dark side of the plant than the light side – this pushes and turns the plant towards the light! Auxin is produced in the right spots to make all this happen.

Spring has sprung

No matter how dark the winter, the green shoots of spring will always appear again. Beautiful yellow sunflowers will rise against a glorious blue sky once again. Remember that whenever things seem dark and hopeless – with a little help from wonderful molecules like auxin – we can all grow and bloom.

Read more

BBC Bitesize – Auxin and Phototropism



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