What is the primary role of auxin in plant development?

The primary role of auxin in plant development is promoting cell elongation. Auxin is a plant hormone that is crucial in the regulation of various growth processes. It influences the direction of growth by causing cells to elongate on one side of the plant, which leads to bending towards light—a phenomenon known as phototropism. This differential growth allows plants to optimize their exposure to light, which is essential for photosynthesis. Auxin also plays a significant role in other developmental processes such as root and bud growth, but its central function revolves around facilitating cell elongation and enhancing vertical growth in plants.

The other options, while they may relate to broader aspects of plant responses, do not directly encapsulate the main function of auxin itself. For instance, while auxin may indirectly influence flowering through its involvement in other growth processes, its most critical effect is on cell elongation. Similarly, auxin does not play a direct role in deterring herbivory or affecting soil nutrient uptake, which are governed by different plant mechanisms and hormones.