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.