July 23, 2021
In this IBDP Biology blog post, we will be learning about the hormone Auxin, an important plant hormone which functions during the day and night.
The Plant Hormone: Auxin
Auxin: Growth promoter. Used to control the growth at the shoot tip
Auxins increase flexibility of plant cell walls in young shoots
This results in cell elongation on the side of the shoot necessary to cause growth towards light
Auxin cannot diffuse through the plasma membrane - moved through membrane by Active Transport using a trans-membrane protein: Auxin Efflux pump
- Phototrophins (pigments) detect differences in the intensity of blue light
- This triggers the movement of Auxin by Active Transport via efflux pumps in the plasma membrane
- Efflux pumps pump Auxin from cytoplasm to cell wall
- Auxin efflux pumps move into the plasma membrane of shoot cells [where elongation is going to take place] on the shady side
- This sets up a concentration gradient of auxin (more in shade)
- More elongation (bend towards light) from the shady side which allows the shoot to grow towards the light
Concentration gradients of Auxin is necessary to control the direction of plant growth. Therefore auxin must be unevenly transported
Auxin
- Auxin is produced in the tip of the shoot in the apical meristem
- Auxin causes the cells to elongate, promoting formation of leaves, causing apical dominance
- Auxin accumulates on the shady side of the shoot
- Protein pumps (Auxin efflux pumps) in the cell membrane are positioned on the shady side of each cell in the shoot.
- They pump Auxin out of the cell on the shady side which moves the Auxin to the shady side of the shoot
- Auxin binds to auxin receptors in plant cells - transcription of specific genes are promoted
- Expression of specific genes causes secretion of hydrogen ions into the cell walls
- This loosens the connections between cellulose fibres allowing for cell expansion
Explain Auxin's role in Phototropism
- Auxin being produced causes transport of hydrogen ions from cytoplasm to the cell wall
- Hydrogen pumping breaks the bonds between the cell wall fibres
- This makes the cell walls flexible
- Auxin makes the cell walls grow
- Gene expression is also altered by Auxin to promote cell growth
- Positive phototropism is growth towards light
- Shoot tip senses the direction of the brightest light
That is all for this post!
References:
- https://image.shutterstock.com/image-vector/phototropism-vector-illustration-labeled-plants-260nw-1730182411.jpg
- https://www.google.com/url?sa=i&url=http%3A%2F%2Figcse-biology-2017.blogspot.com%2F2017%2F06%2F285-understand-role-of-auxin-in.html&psig=AOvVaw1I3fCOsHGohISQtdutE7Ja&ust=1627127955159000&source=images&cd=vfe&ved=0CAsQjRxqFwoTCIC4_sSS-fECFQAAAAAdAAAAABAD
Drafted by Venetia (Biology)