Gravitropism

Gravitropism [or geotropism] is a turning or growth movement by a plant or fungus in response to gravity. Charles Darwin was one of the first Europeans to document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull (i.e., downward) and stems grow in the opposite direction (i.e., upwards). This behaviour can be easily demonstrated with a potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, bending (biologists say, turning; see tropism) upwards. Herbaceous (non-woody) stems are capable of a small degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth in a new direction.

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Gravitropism in the root

If the root cap is removed, root growth ceases to respond to gravity. The root cap is vital for gravitropism since it contains cells with sensors called statoliths, which are amyloplasts packed with starch. Amyloplasts are a type of plastid similar to chloroplasts. Statoliths are dense organelles that settle to the lowest part of the root cap cells in response to a change in the gravity vector. This initiates differential cell expansion in the root elongation zone causing a reorientation of the root growth (see below). The location of the elongation zone is many cells above the root cap, so intercellular signal transduction must occur from the site of gravity perception, in the root cap, to the growth response in the elongation zone. As of 2002, the nature of this signal is an active area of research in plant biology.

Roots bend in response to gravity due to a regulated movement of the plant hormone auxin known as polar auxin transport. In roots, an increase in the concentration of auxin will inhibit cell expansion, therefore, the redistribution of auxin in the root can initiate differential growth in the elongation zone resulting in root curvature.

Gravitropism in the stem

A similar mechanism is known to occur in plant stems except that the shoot cells have a different dose response curve with respect to auxin. In shoots, increasing the local concentration of auxin promotes cell expansion; this is the opposite of root cells.

The differential sensitivity to auxin helps explain Darwin's original observation that stems and roots respond in the opposite way to the gravity vector. In both roots and stems auxin accumulates towards the gravity vector on the lower side. In roots, this results in the inhibition of cell expansion on the lower side and the concomitant curvature of the roots towards gravity (positive gravitropism). 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).

Hypothetical models of gravitropism

As the mechanism of gravitropism is still up for question, several hypothetical models have been asserted.

Starch-statolith hypothesis

Sedimentation of the amyloplasts play the role of the statoliths.

Protoplast pressure hypothesis

The weight of the entire protoplast changes the gravity perception of the plant. Some sort of sensing mechanism detects the pulling and/or pushing forces of the protoplast on the cell walls and adjusts growth accordingly.

Tensegrity model

The word tensegrity is a contraction of tensional integrity. This model postulates that the interaction of falling amyloplasts with the structural integrity of the cell is responsible for gravitropism. Actin filaments form a structural meshwork anchored to the plasma membrane. The amyloplasts create tension which leads to disruption of the actin meshwork. Because actin tension affects calcium channels on the plasma membrane, we expect a transient increase in cytosolic Ca²⁺ level. Presumably, the Ca²⁺ activates tryptophan transcription factors that synthesize auxin. Alternativley localized changes in calcuim could alter membrane dynamics, alter auxin transport activity, or act in other pathways as a second messenger in gravity signaling. The involvement of actin in such a model must be considered carefully in light of experiments showing that treatment of Arabidopsis thaliana with Latrunculin B (an actin de-polymerizing drug) at levels sufficient to disrupt fine actin structures in the root cap actually increases gravity sensitivity as well as increasing the magnitude of several downstream gravity signaling events (Hou et al., 2004). These results suggest that fine actin structures play a role in dampening gravity sensing or functions to down-regulate gravity signaling events.

Experiments using Arabidopsis thaliana question the role of cytosolic Ca²⁺ in gravity signaling. Some experiments indicate that cytosolic Ca²⁺ levels increase in root gravity sensing cells following mechanical stimulation (such as the root hitting a barrier).^{[citation needed]} Such increases appear to dampen the gravitropic response, and may instead be associated with activity of the touch-response (thigmotropism) pathway.

Compensation

  Bending mushroom stems follow some regularities that are not common in plants. After turning into horizontal position the mushroom stem starts bending near the cap, as some signal about the improper orientation is sensed there and then gradually transmitted towards the base. But very soon, well before reaching the normal vertical orientation the apical part (region C in the figure below) starts to straighten. Finally this part gets straight again, and the curvature concentrates near the base of the mushroom. This effect is called compensation (or, sometimes, autotropism). The exact reason of such behavior is unclear, and at least two hypothesis exist.

• The hypothesis of plagiogravitropic reaction supposes some mechanism that sets the optimal orientation angle other than 90 degrees (vertical). The actual optimal angle is a multi-parameter function, depending on time, the current reorientation angle and from the distance to the base of the fungi. The mathematical model, written following this suggestion, can simulate bending from the horizontal into vertical position but fails to imitate realistic behavior when bending from the arbitrary reorientation angle (with unchanged model parameters).
• The alternative model supposes some “straightening signal”, proportional to the local curvature. When the tip angle approaches 30° this signal overcomes the bending signal, caused by reorientation, resulting straightening.

Both models fitted the initial data well, but the latter was also able to predict bending from various reorientation angles. Compensation is less obvious in plants, but in some cases it can be observed combining exact measurements with mathematical models.

Gravitropic mutants

There are cultivars known that are mutants for mechanisms thought to be required for gravitropism. Usually these are trees that have a weeping or pendulate growth habit. The branches still respond to gravity, but with a positive response, rather than the normal negative response. Some agravitropic mutants have also been isolated in Arabidopsis thaliana (one of the genetic model systems for plant research) and their roots have a weak response to gravity and grow in random directions. One agravitropic mutant cannot produce starch causing the statoliths to be less dense and thus reducing their ability to function as sensors. Another example of an agravitropic mutant lacks the proposed auxin transporter responsible for transducing the lateral auxin asymmetry from the site of gravitropic perception (root cap) to the site of action (elongation zone).

References

Hou G, Kramer VL, Wang YS, Chen R, Perbal G, Gilroy S, Blancaflor EB (2004). The promotion of gravitropism in Arabidopsis roots upon actin disruption is coupled with the extended alkalinization of the columella cytoplasm and a persistent lateral auxin gradient. Plant J. 39(1):113-25.

• Meškauskas A., Moore D., Novak Frazier L. (1999). Mathematical modelling of morphogenesis in fungi. 2. A key role for curvature compensation ('autotropism') in the local curvature distribution model. New Phytologist, 143, 387-399.
• Meškauskas A., Jurkoniene S., Moore D. (1999). Spatial organization of the gravitropic response in plants: applicability of the revised local curvature distribution model to Triticum aestivum coleoptiles. New Phytologist 143, 401-407.