What is the hormone that controls the growth of plant roots?

The hormone that primarily controls the growth of plant roots is auxin. Auxins are a group of plant hormones that play a crucial role in coordinating various growth and behavioral processes in the plant's life cycle. Specifically, they promote the elongation of cells in roots, influencing their growth direction and allowing them to respond to gravitational pull (a phenomenon known as gravitropism). This directional growth is essential for the establishment and stability of the plant as it seeks water and nutrients from the soil.

Auxins also contribute to the differentiation of root cells and can regulate processes such as lateral root formation and the development of root hairs, enhancing the plant's ability to absorb water and nutrients. Their concentration often dictates the growth patterns in plants, helping roots to grow more deeply and establish a more robust support system beneath the soil surface.

In contrast, gibberellins are more closely associated with stem elongation and seed germination, cytokinins primarily stimulate cell division and shoot growth, and glucose is not a hormone but rather an energy source for plants. Each of these substances has important roles in various aspects of plant growth, but auxin is specifically integral to root development.