Gravitropism is an adaptation that allows plants to direct their growth in response to gravity, ensuring shoots emerge upward and roots penetrate downward into the soil. This directional movement is regulated primarily by the plant hormone auxin, which acts as the signaling molecule. When a plant is placed horizontally, a cellular mechanism detects this change in orientation, leading to an asymmetrical distribution of auxin that causes the plant to bend. Understanding this process involves examining the specialized cells that sense gravity, the resulting hormone movement, and the different growth responses it produces.
Gravity Sensing by Plant Cells
The initial detection of gravity occurs within specialized cells called statocytes, located in the root cap (columella cells) and the stem’s endodermis. These cells contain dense, starch-filled organelles known as amyloplasts, which function as statoliths, or gravity sensors. In a vertically oriented plant, these statoliths settle uniformly at the bottom of the statocyte cell.
When the plant is tipped onto its side, the statoliths quickly sediment to the new lower side of the cell membrane within seconds. This physical repositioning of the statoliths is the first step in the signaling cascade. The interaction between the settled statoliths and internal cellular components triggers a signal communicating the plant’s new orientation.
This physical signal is then transduced into a biochemical signal that dictates the subsequent movement of the growth hormone. The way statoliths signal the cell remains a subject of ongoing research. It is theorized that their contact with the membrane or the cytoskeleton influences the activity of nearby proteins. This gravity-sensing process in the statocytes is localized and happens before any change in auxin concentration occurs in the elongation zone.
Auxin Redistribution and Lateral Transport
The signal generated by the sedimenting statoliths is translated into a change in the directional flow of auxin. Auxin is transported from the shoot tip downward and recycled within the root tip via polar auxin transport. This transport is regulated by specific proteins embedded in the cell membranes, particularly the PIN-FORMED (PIN) efflux carriers.
Upon a gravitational stimulus, the signal from the statoliths causes a rapid relocalization and asymmetrical distribution of PIN proteins, such as PIN3 and PIN7, within the statocytes. In a horizontal root, these PIN proteins quickly cluster on the lower side of the statocyte cell membrane, facing the gravity vector. This re-polarization creates a preferred route for auxin to exit and move laterally toward the lower flank of the organ.
The result is the establishment of a lateral auxin gradient across the horizontal organ within minutes. Auxin concentration becomes higher on the lower side of both the shoot and the root compared to the upper side. This asymmetrical flow of auxin, driven by the re-patterning of PIN efflux carriers, converts the perception of gravity into a growth-regulating signal.
Differential Growth and Curvature
The unequal distribution of auxin across the organ serves as the direct trigger for differential growth, which is the physical bending constituting the gravitropic response. The physiological outcome of the high auxin concentration differs between the shoot and the root, reflecting their opposite growth goals. This difference is due to the varying sensitivity of shoot cells and root cells to the hormone.
In the shoot, the higher concentration of auxin on the lower side promotes cell elongation. The cells on the lower flank of the stem grow faster than the cells on the upper flank, causing the shoot to curve upward (negative gravitropism). This response ensures the plant’s photosynthetic organs are directed toward the light.
Conversely, root cells are far more sensitive to auxin; the high concentration accumulating on the lower side is inhibitory to cell elongation. The cells on the upper side of the root, which have a lower concentration of auxin, elongate more rapidly. This differential growth forces the root to curve downward (positive gravitropism), anchoring the plant and allowing it to seek water and nutrients.
The bending continues until the organ is aligned with the gravity vector, at which point the statoliths return to their resting position at the bottom of the statocytes. This re-establishes a symmetrical PIN protein distribution and uniform auxin flow, stopping the differential growth and maintaining the new vertical orientation. The process illustrates how a single hormone, through concentration-dependent effects, coordinates the directional growth necessary for plant survival.