Gravitropism
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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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[edit] Gravitropism in the root
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.
A "tropism" is a plant movement triggered by stimuli. The term "geotropic" refers to a plant whose roots grow down into the soil as a response to gravity. Plants commonly exist in a state of "anisotropic growth," where roots grow downward and shoots grow upward. Anisotropic growth will continue even as a plant is turned sideways or upside down. In other words, no matter what you do to a plant within Earth's atmosphere, it will still grow roots down, stem up. The reason for this comes from the nature of a plant, and its general response to gravity.
Upward growth of plant parts, against gravity, is called negative geotropism, and downward growth of roots, positive geotropism.
Various external factors, often acting together with hormones, are also important in plant growth and development. One important class of responses to external stimuli is that of the tropisms—responses that cause a change in the direction of a plant's growth. Examples are phototropism, the bending of a stem toward light, and geotropism, the response of a stem or root to gravity. Stems are negatively geotropic, growing away from gravity, whereas roots are positively geotropic. Photoperiodism, the response to 24-hour cycles of dark and light, is particularly important in the initiation of flowering. Some plants are short-day, flowering only when periods of light are less than a certain length (see Biological Clocks). Other variables—both internal, such as the age of the plant, and external, such as temperature—are also involved with the complex beginnings of flowering.
[edit] 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).
[edit] Compensation
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 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.
[edit] 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.
[edit] Gravity, Structures and life
Life is strongly affected by gravity. Organic chemistry, molecular biology, cell biology and studies of longevity must include gravity-induced effects on organelles and multi-cell structures such as gravity-induced orientation-adaptation of organisms. These occur when organisms orient themselves, say, by gravity pointers and simple gradients of specific gravity density differences, or gravity-induced ‘hydrostatic pressure’
Plants and animals use build-in "gravity receptors", "g-perception", "bio accelerometers", "biological clocks", etc.
A considerable empirical evidence has already been accumulated on these subjects by biophysicists, biologists, plant Physiologists, botanists, zoologists, neuro-Physiologists, etc. [THERO For Anyone, SCRIBD]
• However, the accumulated evidence is not yet systematically studied, even though a voluminous literature has been published on these disciplinary-controlled investigations. But what we already know justifies the central role we expect gravity to play in medical sciences, biology and biological adaptive systems.
• The connection between gravity and the origin of life, on the one hand, and between gravity and form, adaptive structure, growth rate, growth direction, adaptive behavior, navigation, and adaptive space perception, on the other, must also be unified and investigated on the basis of a single Central Theme.
• Acceleration, which via general relativity is equivalent to gravity, also causes stresses to be set-up in cell membranes or in the organ as a whole, which, in turn, generates a 'proper' response vis-à-vis the gravity vector.
• Nevertheless, small organisms, like bacteria, may have no direct means to sense gravity. But that does not mean that they have not been affected by it from their very origin, say, via the origin of DNA and other gravity-induced biological effects.
A response of most living organisms to gravity is, inter alia, initiated by changes in the distribution of gravity-induced ‘hydrostatic pressure’ on sensitive locations, exerted either by and on the entire cell content, or inside it by particles (statoliths) ‘heavier’ or ‘lighter’ than the surrounding cytoplasm.
A more familiar response of living organisms to gravity is, inter alia, initiated by changes in the distribution of gravity-induced ‘hydrostatic pressure’ on sensitive locations, exerted either by or on the entire cell content, or inside it by particles (statoliths) ‘heavier’ or ‘lighter’ than the surrounding cytoplasm.
“Gravity perception” is often ignored or hidden. It is often prompted or caused by movements, or reorientation of the aforementioned gravity-pointers -- and their associated driving mechanisms -- to restore the original plant orientation in line with the invariant and common gravity vector. This also applies, with modifications, to animals and humans in various forms of life. [THERO For Anyone, SCRIBD]
If some plants are displaced only briefly and then restored to their original orientation well before their growth response sets in, they may still cause a gravity-induced response to that displacement.
Some organelles and nuclei are ‘heavier’ than the rest of the biological cell. Referred to as ‘gravity pointers’ they control plants to grow "vertically upward", irrespective of surrounding surface orientation.
Conclusion: Gravity controls the direction of growth in roots, leaves, branches, etc. It affects the movement of all animals in the sense of giving a common reference for orientation, 'balancing', ‘coordination’, 'walking' and space perception. An example is vertigo.
[edit] See also
- Clinostat - a device used to negate the effects of gravitational pull.
[edit] 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.
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