Which plant hormone is responsible for cell elongation?

The correct response indicates that auxin is the plant hormone primarily responsible for cell elongation. Auxins promote the elongation of cells in the stem and help regulate various aspects of plant growth and development. By influencing the directionality of growth, auxins enable plants to respond to light, a phenomenon known as phototropism. For example, when light reaches one side of a plant, auxin accumulates on the shaded side, causing those cells to elongate more than those on the light-exposed side. This results in the plant bending toward the light source.

Gibberellin is another hormone involved in growth, particularly in seed germination and stem elongation, but its role is more specific and different compared to auxin's action across various tissues and responses. Cytokinins primarily promote cell division and are more involved in the growth of lateral shoots and roots rather than elongation. Ethylene is a gaseous hormone that primarily regulates fruit ripening and responses to stress, not directly associated with cell elongation. This understanding clarifies why auxin is recognized as the hormone responsible for cell elongation.