Auxin is a plant hormone which controls stem elongation, synthesised in meristems of shoots and roots. A principle auxin is IAA.
Binds with receptors which stimulate ATPase. This pumps hydrogen ions into cell wall, acidifying it.
The decrease in pH activates expansins which loosen linkages of microfibrils, so it can stretch and therefore elongate. The loosening is only temporary so the overall strength of the plant is not compromised.