How is the process by which plants turn towards a light source called and what is the role of auxin (IAA) in it?

The process is called phototropism (‘photo’-light, ‘tropism’ – ‘tendency of an animal or plant to turn or move in response to a stimulus’). IAA (Indole Acetic Acid) is a plant hormone belonging to a class called auxins. It is produced by the meristem cells and is involved in the process of phototropism by stimulating cell elongation. When a plant is exposed to a unilateral source of light (in other words illuminated from one side), the auxin concentration in the shaded side increases. In turn, this auxin stimulates proton pumps in the membranes of the nearby cells, which causes them to pump protons in their surrounding space. The drop in pH creates optimal conditions for the proteins expansins, which act to loosen the cell wall, thus promoting cell elongation. As a result of this uneven growth the tip of the plant turns toward the light source.