Dutch botanist Frits Went first isolated the hormone auxin, which stimulates growth in plant shoots and allows them to respond to light and gravity. But how exactly does auxin enable plant growth? Plant cells are surrounded by a rigid cell wall, so this cell wall must somehow be relaxed before the cells can begin to expand.
Plasticity refers to the ability of a plant cell wall to stretch irreversibly. Auxin increases wall plasticity, and thereby enables shoot elongation and such phenomena as phototropism, the shoot's tendency to grow in the direction of a light source. For the plant to grow, its cells must expand, and for this to happen the rigid structural elements of the cell wall must relax. The cell wall can stretch when its cellulose fibers are allowed to slide past one another.
Auxin loosens the meshwork of cellulose fibers and matrix polysaccharides by stimulating the transport of H+ ions into the cell wall. The resulting decrease in pH activates cell wall proteins called expansins, which alter the hydrogen bonding between the matrix polysaccharides that link the cellulose fibers together. Scientists still don't know exactly how expansins loosen the meshwork, but one suggestion is that expansins allow the cellulose fibers and matrix polysaccharides to slide relative to one another, so that the wall stretches and the cell expands.
Textbook Reference: Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action