Question

The region behind the elongation zone has high pH in the surface. Briefly explain what is the underlying biochemical basis for this and how does this high pH impact on cell development in this region?

Answer 1

It can be seen from the experiment's case B and C that the root tends to grow towards gravity and this is because of more elongation of the upper portion compared to the lower one. The upper portion exhibiting more growth has low pH (as indicated in case B and C), which implies that there is more H+ concentration on the upper portion and we can say that protons are associated with cell growth.

The molecular mechanism behind this can be explained by Auxin transport. Auxin is also known as IAAH (Indole-3-acetic acid) is a growth hormone found in plants and is responsible for root growth. Auxin transport is polar and it moves from shoot to roots. The region where there is more auxin is associated with more growth or cell expansion.

Auxin enters in the form of IAAH in the cell, but to move on to the next cell, it needs to be effluxed by PIN protein efflux which can only move it in IAA- form. So to convert IAAH to IAA- it has to give up its H+. The H+ is given up by IAAH is pumped into the cell wall via H+ ATPases thus maintaining the cell wall at an acidic pH and the cytosol at neutral pH where IAA- predominates and thus it can be transported forward. Once it has left the cytosol, IAA- again combines with the H+ in the acidic cell wall and form IAAH for entry into the next cell. In this way, the growth will continue to take place because of auxin transport while protons are being extruded to the cell wall.

In this way, high pH is linked to auxin transport and thus impact cell development.