how do auxins promote and inhibit plant growth?

Auxins are plant hormones that aid in the growth and development in a plant (and sometimes inhibits the growth). Before diferentiation of a cell auxins promote cell elongation. It does this by initiating the intake of water, increasing the elasticity of the cell to cope with the extra water, and it can also promote movement of the plant away from the sunlight (sunlight eradicates auxin).

Auxin can prevent growth through apical dominance which is the presence of auxins in the root and shoot tips.