Gravitropism

The model was independently proposed by the Ukrainian scientist N. Cholodny of the University of Kyiv in 1927 and by Frits Went of the California Institute of Technology in 1928, both based on work they had done in 1926.

[6] Auxin exists in nearly every organ and tissue of a plant, but it has been reoriented in the gravity field, can initiate differential growth resulting in root curvature.

However, once the root tip reaches a 40° angle to the horizontal of the stimulus, auxin distribution quickly shifts to a more symmetrical arrangement.

[3] Gravitropism is an integral part of plant growth, orienting its position to maximize contact with sunlight, as well as ensuring that the roots are growing in the correct direction.

In complete darkness, mature plants have little to no sense of gravity, unlike seedlings that can still orient themselves to have the shoots grow upward until light is reached when development can begin.

[8] Differential sensitivity to auxin helps explain Darwin's original observation that stems and roots respond in the opposite way to the forces of gravity.

[4][9] In stems, the auxin also accumulates on the lower side, however in this tissue it increases cell expansion and results in the shoot curving up (negative gravitropism).

[10] A recent study showed that for gravitropism to occur in shoots, a lot of an inclination, instead of a weak gravitational force, is necessary.

Statoliths are dense amyloplasts, organelles that synthesize and store starch involved in the perception of gravity by the plant (gravitropism), that collect in specialized cells called statocytes.

Statocytes are located in the starch parenchyma cells near vascular tissues in the shoots and in the columella in the caps of the roots.

The statoliths are enmeshed in a web of actin and it is thought that their sedimentation transmits the gravitropic signal by activating mechanosensitive channels.

The redistribution of auxin causes increased growth on the lower side of the shoot so that it orients in a direction opposite that of the gravity stimuli.

This process may be caused by phytochrome disrupting the formation of starch-filled endodermal amyloplasts and stimulating their conversion to other plastid types, such as chloroplasts or etiolaplasts.

After turning into horizontal the normal vertical orientation the apical part (region C in the figure below) starts to straighten.

[10] In addition to the information about gravitropism which such auxin-transport or auxin-response mutants provide, they have been instrumental in identifying the mechanisms governing the transport and cellular action of auxin as well as its effects on growth.

This is an image taken of a tree from Central Minnesota. The tree was on the face of a hill and had blown over in a storm or fell over due to erosion in the soil surrounding it. The tree continues to grow however, and because it was horizontal, its growth exhibits gravitropism which can be seen in its arched growth.
Example of gravitropism in a tree from central Minnesota. This tree has fallen over and due to gravitropism exhibits this arched growth.
Gravitropism maintains vertical orientation of these trees. These trees, typical of those in steep subalpine environments, are covered by deep snow in winter. As small saplings, they are overwhelmed by the snow and bent nearly flat to the ground. During spring growth, and more so as larger trees, gravitropism allows them to orient vertically over years of subsequent growth.
[this image is incorrect! the high auxin is always on the opposite side of the tropic movement!
In the process of plant roots growing in the direction of gravity by gravitropism, high concentrations of auxin move towards the cells on the bottom side of the root. This suppresses growth on this side, while allowing cell elongation on the top of the root. As a consequence of this, curved growth occurs and the root is directed downwards. [ 3 ]
In the process of plant shoots growing opposite the direction of gravity by gravitropism, high concentration of auxin moves towards the bottom side of the shoot to initiate cell growth of the bottom cells, while suppressing cell growth on the top of the shoot. This allows the bottom cells of the shoot to continue a curved growth and elongate its cells upward, away from the pull of gravity as the auxin move towards the bottom of the shoot. [ 7 ]
Banana fruit exhibiting negative geotropism.
The compensation reaction of the bending Coprinus stem. C – the compensating part of the stem.