The plant hormone primarily involved in directional growth responses like the bending of shoots towards light (phototropism) is auxin. Auxin is produced in the tip of the shoot and redistributed unevenly when light shines predominantly from one side. It accumulates on the shaded side of the shoot, causing cells there to elongate more than those on the lighted side, resulting in the shoot bending toward the light. This process involves auxin transporters and is regulated by photoreceptors called phototropins that sense light direction and modulate auxin distribution.

How Auxin Controls Phototropism

• Auxin is synthesized mainly in the shoot tip and moves downward.
• When light comes from one side, auxin accumulates on the shaded side.
• This promotes cell elongation on the shaded side, causing the shoot to bend toward light.
• Phototropin proteins detect light and trigger auxin transporter relocalization, establishing the auxin gradient.

Thus, auxin is the key hormone controlling phototropic bending in plant shoots.