Explain Auxin Role in Cell Elongation & Acid Growth Hypothesis

[siewwen168]

Can anyone here explain to me the role of auxin in cell's elongation?

I think it's related to the acid growth hypothesis,but i need the explanation on how this hypothesis works.

thanks a lot for anyone who help

 

[Moonbear]

I only know that auxin has a role. I'm just giving this one a bump to the top in the hopes that we do have someone lurking around here who has sufficient plant biology knowledge to answer your question.

 

[Ouabache]

I have dabbled a bit with plant physiology, perhaps I can provide a simple explanation:
Auxin's are plant growth hormones (one of the most familiar is IAA indole-acetic acid).
Living plant cells are under a high turgor pressure from electrolytes present in their cytoplasm. The auxin IAA, increases the elasticity of the cell walls and because they are under high turgor pressure, they elongate.

One explanation is the so called Acid-Growth Hypothesis.
Here is how it works. IAA activates a proton (H+) pump in the cell membrane, causing H+ to move into the cell wall, increasing its acidity. (An increase in hydrogen ion concentration decreases the pH and therefore increases acidity). This low pH activates the enzyme expansin (appropriate name ), which breaks bonds within the cellulose fibers. With these bonds broken, the cell wall becomes more elastic and expands under the turgor pressure.

You may have noticed, when you put a plant near a window, after some time, the leaves point in direction of the bright light. If you then turn the plant around 180 deg, after some time, the leaves bend upright and continue to bend once again towards the bright light. This is a the phototropic effect. One explanation of this, is that auxin is being released in the cells on the dark side of the plant and by the "acid growth hypothesis", those cells elongate causing the leaves to bend towards the light.

For a more scientific description, see ref especially sections 3,4 & 5

 

[Moonbear]

Thanks Ouabache! Now I'll have to tuck that explanation away in my mind somewhere safe in case I ever have to teach general biology again. One more question I'll be able to answer about plants if it gets asked.

 

[Ouabache]

Plants are somewhat easier to understand in terms of physiology compared to their animal cousins. With hormones, there are presently only 5 groupings: gibberelllins, auxins, cytokinins, abscisic acid and ethylene.

If feeling studious, here is a little quiz to test our understanding of plant hormones. quiz