What role does auxin play in the processes of phototropism?

Auxin is a plant hormone that is often made in the growing tips of roots and stems, the hormone can then diffuse to other parts of the plant. It controls the growth of plants by promoting cell division and cell elongation. The general rule is that, under high concentrations of auxin, stems grow more and roots grow less. However, it is important to remember that auxin doesn't make the root shrink! In plant stems, the shaded side contains more auxin. This means that the cells on that side of the stem grow at a faster rate than those on the illuminated side. The shaded cells become elongated, which causes the plant to grow towards the light. It is important to note the plant doesn't bend towards the light! Auxin has the opposite effect on roots, the shaded side contains less auxin and so grows less, thus roots grow away from the light.