How does auxin impact the way plant shoots grow toward the light?

One side of the tips are in more light than the other side. Auxin is in a greater concentration on the shaded side, causing the cells there to grow longer than the cells on the light side. Auxins have the opposite effect on root cells. This causes the root to bend away from the light.

The plant hormone auxin is responsible for a plant's ability to grow towards the light. Plants have developed a number of strategies to capture the maximum amount of sunlight through their leaves ... They do this by elongating the cells of the stem on the side that is farthest from the light.