Statolith

From Freepedia

Statoliths are a specialised form of amyloplasts involved in gravity perception by plants.

This specialised amyloplasts are denser than the cytoplasm and can sediment according to the gravity vector. They are found in a special subset of cells of the root cap (a tissue at the tip of the root) called statocytes. Statolith are enmeshed in a web of actin and it is thought that their sedimentation transmit the gravitropic signal by activating mechanosensing channels. The gravitropic signal than leads to reorientation of auxin efflux carriers and subseqent redistribution of auxin stream in root cap and root as a whole. The changed relations in concentration of auxin leads to differerential growth of the root tissues. Taken together, the root is then turning, following the gravity stimuli.