36.4: Responses to Gravity and Touch
Gravitropism: Plant Responses to Gravity
Higher plants sense gravity using statocytes, cells found near the vascular tissue in shoots, and in the root cap columella in roots. Statocytes contain starch-filled organelles called statoliths. The statoliths settle, or sediment, at the bottom of the statocyte in the direction of gravity.
Statolith sedimentation triggers a signaling cascade, resulting in the asymmetrical distribution of the plant hormone auxin across root and shoot tips. This process generates a lateral auxin gradient, in which auxin levels are higher on the lower sides of roots and shoots.
In roots, the higher auxin concentration on the lower side inhibits cell expansion. Cells will, therefore, expand more rapidly on the upper side, causing the root to bend downward. In contrast, the higher auxin concentration on the lower side of shoots promotes cell expansion. Cells expand more rapidly on the lower side, causing shoots to bend upward.
Thigmotropism: Plant Responses to Touch
Climbing plants have tendrils - modified shoots that coil around objects. The tips of such tendrils have touch-sensitive sensory epidermal cells that trigger differential growth. Here, cells on the side of the tendril that touches the object grow more slowly than those on the side opposite the point of contact, allowing the tendrils to curve toward objects they touch.
Interactions between Thigmotropism and Gravitropism
When a vertically oriented root encounters an obstacle during downward growth, it grows away from the point-of-contact. Thus, for vertically oriented roots, thigmotropism overrides gravitropism. Horizontally-oriented roots, however, grow downward even in the presence of obstacles. The path that roots take during their growth is, therefore, likely regulated by interactions between these responses.