Auxin- influence many aspects of growth including elongation growth which determines the overall length of roots and shoots
Auxin is synthesised in the growing tips of shoots and roots, where the cells are dividing
It's transported back down the shoot, or up the root, by active transport from cell to cell.
Growth in plants occurs at meristems, such as those at shoot tips and root tips.
Growth occurs in 3 stages: Cell division by mitosis, cell elongation by absorption of water, and cell differentiation
Auxin stimulates cells to pump hydrogen ions into the cell wall. This acidifies the cell walls which leads to a loosening of the bonds between cellulose microfibrils and the matrix that surrounds them. The cells absorb water by osmosis and the pressure potential causes the wall to stretch so that these cells become longer (elongate)
Molecules of auxin bind to a receptor protein on the cell surface membrane. The binding of auxin stimulates ATPase proton pumps to move hydrogen ions across the membrane. In the cell walls are proteins known as expansins that are activated by the decrease in pH. The expansins loosen the linkages between cellulose microfibrils.