Photoreceptors in plants detect light, but the actual response of growth or movement towards light is mediated by hormones, particularly auxin, also known as indoleacetic acid. Auxin is crucial for plant growth, especially in seedlings, where it is produced at the tips of coleoptiles—the protective coverings of cotyledons in monocots. The Colin D. Wendt hypothesis explains that auxin moves from the light side of the plant to the shaded side, creating an asymmetric distribution. This concentration of auxin on the shaded side stimulates those cells to elongate more than those on the light side, resulting in the plant bending towards the light source.

The mechanism behind this elongation is described by the acid growth hypothesis. Proton pumps in the plant cell membranes actively transport protons out of the cell, leading to a high concentration of protons in the cell wall. This increase in protons activates expansins, proteins that loosen the hydrogen bonds in cellulose, allowing water to enter the cell wall. As water enters, potassium ions also move into the cell due to an electrochemical gradient, causing further water influx through aquaporins. This process results in cell swelling and elongation, enabling the plant to grow towards light.

Auxin is not only involved in phototropism (growth towards light) but also plays a significant role in various plant functions. It is transported in a polar manner from the shoots to the roots, maintaining this direction regardless of gravity. This unidirectional transport is essential for several processes, including pattern formation in developing plants, phyllotaxy (the arrangement of leaves on a stem), and abscission (the shedding of leaves and fruits). Additionally, auxin is key in apical dominance, where the central stem inhibits the growth of lateral stems, ensuring that the plant grows efficiently towards light.

In summary, auxin is a vital hormone that orchestrates plant growth and behavior in response to light, influencing everything from leaf arrangement to overall plant structure and health.