Welcome back to my channel, in this video we are going to go over polar auxin transport.
Polar auxin transport refers to how auxin primarily moves in a unidirectional way
through the plant from the apex to the base. Auxin is primarily produced in the apical
meristems of the shoots and this auxin travels from here down through
the plant cells via polar auxin transport. It also moves through the plant via the phloem
as well as produced on site in the roots. In this video we are going to focus on the
movement of auxin that occurs through the process of polar auxin transport through the cells.
The chemiosmotic model is a model used to describe how polar auxin transport is thought to work.
First we’ll briefly go over a few key features of a plant cells.
The first feature is the outer most structure of a plant cell, the cell wall which is
represented by the outer green area here. Then there is the cell membrane in from this.
Inside of the cell membrane we have the cytosol and the large vacuole.
These plant cells have several membrane proteins that are important for the steps involved in polar
auxin transport between cells. One type of membrane protein
important in this model are PIN proteins. PIN proteins function as auxin efflux carriers.
These PIN protein efflux carriers are located primarily on the basal end of each cell.
Another type of membrane protein important in this model are the AUX1 influx carrier proteins.
A third type of membrane protein, hydrogen ATPase (H+-ATPase), is another important
membrane protein in this model. They function in pumping
hydrogen across the cell membrane. Auxin can be found in the cell in two forms;
undissociated (IAAH) and dissociated (IAA-). These different forms have different
methods of moving between cells. There are 4 general steps that occur in
the process of auxin transport through cells. The first step is auxin influx into the cell.
This step involves auxin from the more apical cell entering the more basal cell.
This auxin influx occurs in one of two ways. Auxin in the undissociated form (IAAH) is
able to move by passive diffusion across the cell membrane into the cytosol of the cell.
Auxin in the dissociated form (IAA-) is negatively charged and
unable to move freely across the membrane. Auxin in this form is able to move into the
cell through secondary active transport via the AUX 1 influx carrier proteins.
These are symporter proteins that move the dissociated auxin across whilst carrying 2
hydrogen molecules across the membrane. The next step going on is cell
wall acidity maintenance. The hydrogen ATPase membrane
proteins are invovled in pumping hydrogen out across the cell membrane into the cell wall.
This keeps the cell wall at an acidic pH, whilst keeping the cytosol at a neutral pH.
The third step is the acidity regulated concentration gradient.
The high acidity in the apoplast and neutral pH in the cytosol create a pH concentration gradient.
The high acidity of the apoplast (which is the space between the
cell membrane and cell wall) means the undissociated form (IAAH) dominates here.
The neutral pH of the cytosol means the dissociated form (IAA-) is the dominant form here.
The undissociated form moves through diffusion as
already discussed into the cell down its concentration gradient.
Once in the cell it converts to the dissociated form.
The dissociated form that hasn’t converted into the undissociated form in the apoplast moves
along with hydrogen molecules through a symporter protein down the hydrogen concentration gradient.
This combined movement allows for greater accummulation of auxin in the cytosol than
what would occur through just diffusion alone. The next and final step in polar auxin
transport is auxin efflux. During this step auxin leaves
the cell in the dissociated form. The dissociated form of auxin is not
able to diffuse across the cell membrane and so it accumulates in the cytosol.
They require specialised membrane proteins to be able to leave the cell.
They leave via PIN proteins located in larger numbers on the basal end of the cell.
These PIN proteins act as auxin efflux carriers for auxin to leave out this basal end of the cell.
Once out of the cytosol a lot of the auxin in the dissociated
form is converted into the undissociated form and diffuses into the cell below it.
The remaining dissociated form can go through the AUX1 influx cotransporter carriers.
And so the transport of auxin from cell to cell in this polar manner continues.
The auxin efflux protiens being primarliy at the basal end of the cell causes the auxin to move in
this apical to basal pattern throughout the plant. Because the auxin leaves through the basal portion
of the cell it is then taken up into the next cell in the apical end, then is again primarily
able to leave through the basal end. This creates a directional movement
downwards in the plant apically from the shoot to basally in the root tip.
This unidirectional movement is called polar transport.
So when it pertains to the unidirectional movement of auxin it is called polar auxin transport.
And that concludes this video on polar auxin transport.