36.4:
Responses to Gravity and Touch
Plants typically have shoots that sprout from the ground and roots that grow downward into the soil. How do plants sense which way is up and which way is down?
Plants grow and move in response to gravity, a phenomenon called gravitropism. Growth in the direction of gravity, such as the downward growth of roots, is called positive gravitropism. Growth in the opposite direction of gravity, like the upward growth of shoots, is called negative gravitropism.
In roots, the root cap contains specialized gravity-sensing cells called statocytes. Within the statocytes, dense, starch-filled organelles called amyloplasts—or statoliths—settle downward in response to gravity.
In vertically-oriented roots, statoliths settle near the bottom of each cell. When roots are rotated to be horizontally-oriented, statoliths slide down and collect at the previous vertical wall that now forms the cell bottom.
The settling amyloplasts allow the plant to sense gravity, triggering an asymmetrical elongation pattern at the root tip; growth is stimulated at the upper side of the root and suppressed at the lower side. The rapid elongation of cells on the upper side causes the root to curve as it grows downward.
Plants also grow in response to touch. For example, the roots of many plants navigate around obstacles, while climbing plants have shoots that wrap around structures. Thigmotropism is the directional growth exhibited by plants in response to touch.
Roots generally grow away from objects they touch, allowing them to follow the path of least resistance through the soil; this is called negative thigmotropism.
In contrast, the threadlike tendrils of climbing plants grow toward objects they touch, sometimes rapidly coiling around them; this is positive thigmotropism.
The tendrils are highly touch-sensitive. Contact with a stimulus induces the contraction of cells at the contact side and the elongation of cells at the non-contact side. This differential growth eventually causes tendrils to twine around the object, thereby securing the plant.
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.
Suggested Reading
Braam, Janet. "In Touch: Plant Responses to Mechanical Stimuli." New Phytologist. 165 (2)2005: 373–89. [Source]
Jaffe, Mordecai J, A Carl Leopold, and Richard C Staples. "Thigmo Responses in Plants and Fungi." American Journal of Botany. 89 (3)2002: 375–82. [Source]
Su, Shih-Heng, Nicole M Gibbs, Amy L Jancewicz, and Patrick H Masson. "Molecular Mechanisms of Root Gravitropism." Current Biology. 27 (17)2017: PR964–R972. [Source]