Gravitropism is an important plant growth response to the environment that directs shoots upward and roots downward.
when a plant shoot is inclined horizontally, the primary inflorescence stem grows upwards against gravity, referred to as negative gravitropism. In contrast, plant roots exhibit downward growth in accordance with gravity upon reorientation, which is referred to as positive gravitropism.
Gravity sensing occurs mainly in specialized cells, named “statocytes,” through the sedimentation of starch-filled plastids (amyloplasts).
This movement triggers a pathway that leads to lateral auxin transport across the stimulated organ, responsible for curvature at the elongation zone.
Gravity sensing and gravity signalling progress sequentially in statocytes in plant gravitropism.
During plant reorientation, a plant is rotated relative to the gravity vector.
This results in the sedimentation of dense amyloplasts within the statocytes.
In roots the statocytes are the columella cells, whereas in stems they are the endodermal cells.
Each endodermal cell contains a large vacuole, and the amyloplasts must traverse it by tunneling through transvacuolar strands in order to reach the new lower side of the cell.
This requires proper vacuole structure, which the SGR proteins mediate.
Amyloplast sedimentation is then thought to activate signal transduction through second messengers, possibly calcium ions or protons.
Another second messenger is InsP3, which is produced by cleavage of the phospholipid, PIP2.
In a process that is not completely understood, the second messengers activate the relocalization of auxin transporters, such as PIN3 and PIN7 in the columella cells.
The new polarized distribution of these auxin efflux carriers changes the flow of auxin throughout the plant.
This differential auxin transport affects cell elongation rates, thereby resulting in organ curvature as the plant grows.