  Directional growth caused by gravitropism and corresponding bending of plant
cells has been explored since 19th century, however, many aspects of mechanisms
underlying the perception of gravity at the molecular level are still not well
known. Perception of gravity in root and shoot gravitropisms is usually
attributed to gravisensitive cells, called statocytes, which exploit
sedimentation of macroscopic and heavy organelles, amyloplasts, to sense the
direction of gravity. Gravity stimulus is then transduced into distal
elongation zone, which is several mm far from statocytes, where it causes
stretching. It is suggested that gravity stimulus is conveyed by gradients in
auxin flux. We propose a theoretical model that may explain how concentration
gradients and/or stretching may indirectly affect the global orientation of
cortical microtubules, attached to the cell membrane and induce their dynamic
reorientation perpendicular to the gradients. In turn, oriented microtubules
arrays direct the growth and orientation of cellulose microfibrils, forming
part of the cell external skeleton and determine the shape of the cell.
Reorientation of microtubules is also observed in reaction to light in
phototropism and mechanical bending, thus suggesting universality of the
proposed mechanism.
