The plant hormone that promotes cell elongation and helps shoots bend toward light is auxin.
Auxin is a class of plant hormones, with indole-3-acetic acid (IAA) being the principal naturally occurring auxin in many plants. It influences cell elongation, root formation, vascular development, apical dominance, and responses to gravity and light.
In phototropism, light from one side of a shoot causes auxin to redistribute toward the shaded side. Cells there elongate more than cells on the illuminated side, making the shoot curve toward the light. This classic explanation is why auxin is strongly associated with phototropism.
Auxin is not simply a “growth chemical” that always increases growth. Its effect depends on concentration, tissue, and developmental stage: low concentrations can stimulate root formation, while higher concentrations may inhibit roots. Gibberellins, cytokinins, and abscisic acid are different hormone groups with different primary roles.