Gravitropism

in the remains of a cellar of a roman villa in the Archeologic Park in Baia, Italy]] Gravitropism is a turning or growth movement by a plant or fungus in response to gravity. Charles Darwin was one of the first to scientifically 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.

Gravitropism in the root

Root growth occurs by the division of stem cells in the root meristem located in the tip of the root, and the subsequent expansion of cells in a region just proximal to the tip known as the elongation zone. Differential growth during tropisms mainly involves changes in cell expansion verses changes in cell division, although a role for cell division in tropic growth has not been formally ruled out. Interestingly gravity is sensed in the root tip and this information must then be relayed to the elongation zone so as to maintain growth direction and mount an effective growth responses to changes in orientation (Perrin et al., 2005).

Abundant evidence demonstrates that roots bend in response to gravity due to a regulated movement of the plant hormone auxin known as polar auxin transport (Swarup et al., 2005). Auxin exists in nearly every organ and tissue of a plant, however its concentration in an organ/tissue is regulated by the auxin transport, synthesis and conjugation. In roots, an increase in auxin concentration generally inhibits cell expansion. Therefore the redistribution of auxin toward the lower flank of a root, that has been reoriented in the gravity field, can initiate differential growth 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 (statolithic gravitropism).

Upward growth of plant parts, against gravity, is called "negative gravitropism", and downward growth of roots is called "positive gravitropism".

==Compensation== stem. C - the compensating part of the stem.]] Bending mushroom stems follow some regularities that are not common in plants. After turning into horizontal 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 fit 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. The more-sensitive roots are stimulated by lower levels of auxin...higher levels of auxin in lower halves result in less-stimulated growth...resulting in downward curvature (positive gravitropism).

Gravitropic mutants

Mutants with altered responses to gravity have been isolated in several plant species including Arabidopsis thaliana (one of the genetic model systems used for plant research). These mutants have alterations in either negative gravitropism in hypocotyls and/or shoots, or positive gravitropism in roots, or both. Mutants have been identified with varying effects on the gravitropic responses in each organ, including mutants which nearly eliminate gravitropic growth, and those whose effects are weak or conditional. Once a mutant has been identified, it can be studied to determine the nature of the defect (the particular difference(s) it has compared to the non-mutant 'wildtype'). This can provide information about the function of the altered gene, and often about the process under study. In addition the mutated gene can be identified, and thus something about its function inferred from the mutant phenotype.

Gravitropic mutants have been identified that effect starch accumulation, such as those affecting the PGM1 gene in Arabidopsis, causing plastids - the presumptive statoliths - to be less dense and, in support of the starch-statolith hypothesis, less sensitive to gravity. Other examples of gravitropic mutants include those affecting the transport or response to the hormone auxin. In addition to the information about gravitropsim 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.

There are also several cultivated plants that display altered gravitropism compared to other species or to other varieties within their own species. Some 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. Others are the lazy (i.e. ageotropic or agravitropic) varieties of corn (Zea mays) and varieties of rice, barley and tomatoes, whose shoots grow along the ground.

See also

Clinostat - a device used to negate the effects of gravitational pull.

Amyloplast - starch organelle involved in sensing gravitropism

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.

Benjamin Gal-Or, “Cosmology, Physics and Philosophy”, Springer Verlag, 1981, 1983, 1987, ISBN 0-387-90581-2, ISBN 0387965262. Gravity, the cell, biological structuring and biological clocks.

Perrin RM, Young LS, Murthy U M N, Harrison BR, Wang Y, Will JL, Masson PH (2005). Gravity signal transduction in primary roots. Ann Bot. 2005 Oct;96(5):737-43.

Swarup R, Kramer EM, Perry P, Knox K, Leyser HM, Haseloff J, Beemster GT, Bhalerao R, Bennett MJ (2005). Root gravitropism requires lateral root cap and epidermal cells for transport and response to a mobile auxin signal. Nat Cell Biol. Nov;7(11):1057-65.

Category:Tropism