For details on the perception of the signal, click here.
For details on differential growth responses, click here.

THE CHOLODNY-WENT HYPOTHESIS:
According to this widely-accepted model, auxin is redistributed toward the lower side of roots, stems, and coleoptiles in response to gravity, accounting for the resulting asymmetric growth.
Although all aspects of this hypothesis have been questioned, auxin is widely believed to be the primary effector of gravitropism (Philosoph-Hadas et al, 1996).
In addition to the distribution of auxin, differential sensitivity to auxin (perhaps time- and tissue-despendent), the presence of calcium, the presence of other growth regulators such as ethylene and gibberellic acid, and the action of microfilament bundles may play crucial roles in graviresponsiveness.

Auxin Distribution:
In the roots, auxin is directed toward the lower side of a horizontal root by the root cap. If the root cap is removed and auxin is added to one side, growth is toward that side. The addition of auxin inhibitors overcomes the gravitropic response, but also reduces growth (Salisbury and Ross, 1992).
In stems and coleoptiles, low levels of auxin preclude a gravitropic response, and, as with roots, auxin inhibitors halt both graviresponsiveness and growth (Salisbury and Ross, 1992).
The Cholodny-Went suggestion that these asymmetries in auxin distribution preceed and directly lead to the gravitropic growth response has been questioned on the basis of the timing and extent of the asymmetric distribution. These questions are partially answered by the evidence that "auxin has become laterally asymmetric within 5 to 10 minutes of initiation of gravitropic stimulation....as early as a macroscopic curvature response is seen in most dicot seedling stems" (Harrison and Pickard, 1989). Additionally, the relationship between auxin asymmetry and gravitropic curavature is roughly linear (Harrison and Pickard, 1989).

Auxin Sensitivity:
Despite the evidence that auxin distribution plays a significant role in the gravitropic growth response, the kinetics of curvature and growth rate suggest that "time dependent changes in sensitivity of the responding organ to the gravitropic effector or to the gravity stimulus itself" (Evans, 1991) are also crucial. In fact, "the two halves of horizontal soybean hypocotyls reacted differently to the same exogenous auxin stimulus over a range of concentrations. The upper half was consistently less responsive to added auxin than the lower half" as analyzed by Michaelis-Menten kinetics (Rorabaugh and Salisbury, 1989). Rorabaugh and Salisbury go on to suggest that the mechanisms of changing auxin sensitivity may be closely tied to the presence of calcium and the settling to amyloplasts.

Calcium:
There is evidence that calcium plays a role in all steps of the signal-transduction pathway, acting as a second messenger to mediate auxin redistribution or action. When the levels of cytosolic calcium are limited by the addition of calcium chelators or calcium-channel antagonists, graviresponsiveness is significantly limited. In contrast, calcium-channel agonists stimulate gravitropic bending ( Philosoph-Hadas, et al, 1996).
The changing electrical field which accompanies calcium binding to settling amyloplasts in a gravistimulated plant could alter auxin sensitivity by influencing auxin receptors (Rorabaugh and Salisbury, 1989). Calcium is also linked to other factors which affect the gravitropic growth response, and its action is "manifested as increased ethylene production" ( Philosoph-Hadas, et al, 1996).
Click here for a closer look at the Philosoph-Hadas study.

Ethylene:
The role of ethylene in gravitropism has not been widely understood, but there is evidence that gravitropic bending is inhibited by the reduction of functional ethylene ( Philosoph-Hadas, et al, 1996). The amount of functional ethylene can be controlled experiementally via inhibitors, but also varies naturally, with more ethylene produced closer to the apex on the lower side of the stem. It appears that auxin may stimulate ethylene production adn that "high levels of ethylene may be required for the elongation response of the lower stem half, leading to bending" ( Philosoph-Hadas, et al, 1996).
Click here for a closer look at the Philosoph-Hadas study.

Gibberellic Acid:
Gibberellic acid is present in higher concentrations on the lower side of a gravistimulated stem. This gradient is not always observed until after the gravitropic response has occured (Salisbury and Ross, 1992).

Microfilament Bundles:
In the roots, amyloplasts are linked to bundles of microfilaments. It has been proposed that these bundles link settling statoliths, and trigger the gravitropic response (Salisbury and Ross, 1992).

Return