polar transport of auxin

[MrMistery]

Ok, this has been bugging me for quite some time, and i can not seem to find the answer anywhere. Auxin only moves from top to base of a plant, at a speed of 1 cm/hour. The current model is that this polar transport is due to an assimetry in auxin transport proteins, that they are only located at the bottom of parenchyma xylem cells(through which auxin moves).
However, if auxin transport proteins are only located at the bottom of cells, how does the auxin get into the cell beneath it?
My hypothesis is that these auxin transport proteins actively pump the auxin out of the cell, and the auxin then diffuses into the next cell through another transport protein(passive transport). Can anyone confirm this?

[asquiths]

Hi andrew

Wikipedia covers auxin polar transport, just type it in!

cheers

[MrMistery]

looked there. no specific answer to my question, it just says that the transport proteins are at the bottom..

[Volvox]

According to the text that is used for the bio course I TA, auxin will enter cells via a cotransport protein and two protons, which will set-up the gradient required for the auxin to enter the cell. Once in the cell, auxin will disperse to the bottom of the cell and will be transported to the cell below it by carrier proteins. This leads to an asymmetrical distribution of auxin, and leads to other phenomena with plant growth, like gravitropism.

I hope this helps. If you would like to know where I found this information, just let me know.

[Linn]

However, if auxin transport proteins are only located at the bottom of cells, how does the auxin get into the cell beneath it?

I dont really know, but do you think gravitrophic response is involved?

[MrMistery]

@volvox
thank you SO MUCH. It's ok, i believe you. so the transport into the cell is active and the one out of the cell is passive.. I got it..

@Linn
No, polar transport of auxin has nothing to do with gravity. If you put a plant upside down, experiments show it travels the same way(which i now understand thanks to volvox)

[Volvox]

No problem MrMistery. Just thought I'd help you out.

[SororSaudade]

doesn't it have to do with changes between basic and acidic forms of auxins? I can't remember that well what I learned in college but I think it had something to do with this, which allowed a continuous flow of auxins (since there was always a gradient of the two forms).
Forgive me if I'm wrong or if I'm not making myself clear

[SororSaudade]

maybe this can help:

http://4e.plantphys.net/article.php?ch=19&id=264

[MrMistery]

thanks... it took me a while to understand all of that... but thank you... that covers any question i would ever have on the subject...