WordNet

an orienting response to gravity

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/02/14 04:09:13」(JST)

wiki en

Example of Gravitropism in the remains of a cellar of a Roman villa in the Archeologic Park in Baia, Italy

Gravitropism (also known as Geotropism) is a turning or growth movement by a plant or fungus in response to gravity. It is a general feature of all higher and many lower plants as well as other organisms. 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 behavior can be easily demonstrated with any potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing (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 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 versus changes in cell division, although a role for cell division in tropic growth has not been formally ruled out. 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 and continue to grow its roots in the same direction as gravity (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). This was described in the 1920s in the Cholodny-Went model. The model was independently proposed by the Russian scientist N. Cholodny of the University of Kiev in 1927 and by Frits Warmolt Went of the California Institute of Technology in 1928, both based on work they had done in 1926.^[1] Auxin exists in nearly every organ and tissue of a plant, but 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 shoots

Apex reorientation in Pinus pinaster during the first 24h after experimental inclination of the plant.

Gravitropism is very common and affects plant growth very much. The shoot, being negatively gravitropic, grows upwards. This also depends on the auxin concentration.

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".

Modulation by phytochrome

Apart being itself the tropic factor (phototropism), light may also suppress the gravitropic reaction. As changes introduced by red light can often be reversed by the far red light, this regulation is believed to be phytochrome - mediated.^[2]

Compensation

The compensation reaction of the bending Coprinus 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.

References

^ Janick, Jules (2010). Horticultural Reviews. John Wiley & Sons. p. 235. ISBN 0470650532. http://books.google.ca/books?id=JItGq_zGANAC&pg=PA235.
^ E. Liscum and R. P. Hangarter, 1993 Genetic Evidence That the Red-Absorbing Form of Phytochrome B Modulates Gravitropism in Arabidopsis thaliana. Plant Physiology, 103 no. 1 15-19

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.
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.

Wikimedia Commons has media related to: Gravitropism

English Journal

Gravitropism in Phycomyces: violation of the so-called resultant law - evidence for two response components.

Göttig M, Galland P.Author information Fachbereich Biologie, Philipps-Universität Marburg, Marburg, Germany.AbstractWe investigated gravitropic bending of sporangiophores of the zygomycete fungus Phycomyces blakesleeanus in response to centrifugal accelerations to test the so-called resultant law of gravitropism ('Resultantengesetz'; Jahrbuch der wissenschaftlichen Botanik, 71, 325, 1929; Der Geotropismus der Pflanzen, Gustav Fischer, Jena, Germany, 1932), which predicts that gravitropic organs orient in a centrifuge rotor parallel to the stimulus vector resulting from the centrifugal acceleration and gravity. Sporangiophores of wild-type strain C171 carAcarR and of several gravitropism mutants were subjected for 7 h to centrifugal accelerations in a dynamic range between 0.01 and 3 × g. The stimulus-response curves that were obtained for C171 carA carR, C2 carA geo and C148 carA geo madC were complex and displayed two response components: a low-acceleration component between 0.01 and 0.5 × g and a high-acceleration component above 0.5 × g. The low acceleration component is characterised by bending angles exceeding those predicted by the resultant law and kinetics faster than that of the second component; in contrast, the high-acceleration component is characterised by bending slightly below the predicted level and kinetics slower than that of the first component. Sporangiophores of the wild-type C171 centrifuged horizontally displayed the opposite behaviour, i.e. low accelerations diminished and high accelerations slightly enhanced bending. Further proof for the existence of the two response components was provided by the phenotype of gravitropism mutants that either lacked the first response component or which caused its overexpression. The tropism mutant C148 with defective madC gene, which codes for a RasGap protein (Fungal Genetics Reports, 60 (Suppl.), Abstract # 211, 2013), displayed hypergravitropism and concomitant deviations from the resultant law that were twice as high as in the wild-type C171. Gravitropism mutants with defects in the genes madF, madG and madJ lacked the low-response component below 0.5 × g. Our data are at variance with the so-called resultant law and imply that gravitropic orientation cannot depend exclusively on the classical sine stimulus (i.e. acting perpendicularly on the side walls); it rather must also be controlled by the cosine stimulus acting parallel to the longitudinal axis of the gravisensing organ. Our studies indicate that the threshold for the cosine response is the same as that of the sine response, and thus close to 0.01 × g.
Plant biology (Stuttgart, Germany).Plant Biol (Stuttg).2014 Jan;16 Suppl 1:158-66. doi: 10.1111/plb.12112. Epub 2013 Nov 6.
We investigated gravitropic bending of sporangiophores of the zygomycete fungus Phycomyces blakesleeanus in response to centrifugal accelerations to test the so-called resultant law of gravitropism ('Resultantengesetz'; Jahrbuch der wissenschaftlichen Botanik, 71, 325, 1929; Der Geotropismus der Pfl
PMID 24373014

Gravity-dependent differentiation and root coils in Arabidopsis thaliana wild type and phospholipase-A-I knockdown mutant grown on the International Space Station.

Scherer GF, Pietrzyk P.Author information Leibniz Universität Hannover, Institut für Zierpflanzenbau und Gehölzwissenschaften, Abt. Molekulare Ertragsphysiologie, Hannover, Germany.AbstractArabidopsis roots on 45° tilted agar in 1-g grow in wave-like figures. In addition to waves, formation of root coils is observed in several mutants compromised in gravitropism and/or auxin transport. The knockdown mutant ppla-I-1 of patatin-related phospholipase-A-I is delayed in root gravitropism and forms increased numbers of root coils. Three known factors contribute to waving: circumnutation, gravisensing and negative thigmotropism. In microgravity, deprivation of wild type (WT) and mutant roots of gravisensing and thigmotropism and circumnutation (known to slow down in microgravity, and could potentially lead to fewer waves or increased coiling in both WT and mutant). To resolve this, mutant ppla-I-1 and WT were grown in the BIOLAB facility in the International Space Station. In 1-g, roots of both types only showed waving. In the first experiment in microgravity, the mutant after 9 days formed far more coils than in 1-g but the WT also formed several coils. After 24 days in microgravity, in both types the coils were numerous with slightly more in the mutant. In the second experiment, after 9 days in microgravity only the mutant formed coils and the WT grew arcuated roots. Cell file rotation (CFR) on the mutant root surface in microgravity decreased in comparison to WT, and thus was not important for coiling. Several additional developmental responses (hypocotyl elongation, lateral root formation, cotyledon expansion) were found to be gravity-influenced. We tentatively discuss these in the context of disturbances in auxin transport, which are known to decrease through lack of gravity.
Plant biology (Stuttgart, Germany).Plant Biol (Stuttg).2014 Jan;16 Suppl 1:97-106. doi: 10.1111/plb.12123. Epub 2013 Nov 8.
Arabidopsis roots on 45° tilted agar in 1-g grow in wave-like figures. In addition to waves, formation of root coils is observed in several mutants compromised in gravitropism and/or auxin transport. The knockdown mutant ppla-I-1 of patatin-related phospholipase-A-I is delayed in root gravitropism
PMID 24373011

The sporangiophore of Phycomyces blakesleeanus: a tool to investigate fungal gravireception and graviresponses.

Galland P.Author information Fachbereich Biologie, Philipps-Universität Marburg, Marburg, Germany.AbstractThe giant sporangiophore of the single-celled fungus, Phycomyces blakesleeanus, utilises light, gravity and gases (water and ethylene) as environmental cues for spatial orientation. Even though gravitropism is ubiquitous in fungi (Naturwissenschaftliche Rundschau, 1996, 49, 174), the underlying mechanisms of gravireception are far less understood than those operating in plants. The amenability of Phycomyces to classical genetics and the availability of its genome sequence makes it essential to fill this knowledge gap and serve as a paradigm for fungal gravireception. The physiological phenomena describing the gravitropism of plants, foremost adherence to the so-called sine law, hold even for Phycomyces. Additional phenomena pertaining to gravireception, specifically adherence to the novel exponential law and non-adherence to the classical resultant law of gravitropism, were for the first time investigated for Phycomyces. Sporangiophores possess a novel type of gravisusceptor, i.e. lipid globules that act by buoyancy rather than sedimentation and that are associated with a network of actin cables (Plant Biology, 2013). Gravitropic bending is associated with ion currents generated by directed Ca(2+) and H(+) transport in the growing zone (Annals of the New York Academy of Sciences, 2005, 1048, 487; Planta, 2012, 236, 1817). A set of behavioural mutants with specific defects in gravi- and/or photoreception allowed dissection of the respective transduction chains. The complex phenotypes of these mutants led to abandoning the concept of simple linear transduction chains in favour of interacting networks with molecular modules of physically interacting proteins.
Plant biology (Stuttgart, Germany).Plant Biol (Stuttg).2014 Jan;16 Suppl 1:58-68. doi: 10.1111/plb.12108. Epub 2013 Nov 20.
The giant sporangiophore of the single-celled fungus, Phycomyces blakesleeanus, utilises light, gravity and gases (water and ethylene) as environmental cues for spatial orientation. Even though gravitropism is ubiquitous in fungi (Naturwissenschaftliche Rundschau, 1996, 49, 174), the underlying mech
PMID 24373010

Japanese Journal

Overexpression of MIZU-KUSSEI1 Enhances the Root Hydrotropic Response by Retaining Cell Viability Under Hydrostimulated Conditions in Arabidopsis thaliana

Miyazawa Yutaka,Moriwaki Teppei,Uchida Mayumi [他]
Plant and Cell Physiology 53(11), 1926-1933, 2012-11-00
NAID 40019500706

担子菌きのこの生産に関する基礎的研究 : 特に環境制御の観点から

鈴木彰
日本きのこ学会誌 : mushroom science and biotechnology 19(4), 155-166, 2012-01-31
担子菌きのこの生活史の各発育段階,特に子実体形成は,各きのこの菌株の遺伝的特性に加えて外部環境要因の影響を受ける.一般に,子実体形成が可能な温度域は栄養生長が可能な温度域よりも狭く,子実体形成の最適温度は,低温側に位置するものが多い.温度が低くなると,子実体形成速度が低下するとともに,柄が短くなり,傘が大きくなる傾向がみられる.湿度の低下も柄の短小化を招く.光照射は,多くのきのこで子実体形成過程の …
NAID 110009470735

13. 細胞内オーキシン分布の可視化に関する研究 : IAAのリアルタイムイメージングにむけて(口頭発表,植物化学調節学会第46回大会)

福永紫穂,青山卓史,野崎浩,林謙一郎
植物化学調節学会研究発表記録集 (46), 29, 2011-10-03
… Auxin is a master regulators of plant development including apical dominance, gravitropism, and phototropism. …
NAID 110008902422