Editing Gravitropism

{{Short description|Plant growth in reaction to gravity and bending of leaves and roots}}
[[File:Example of gravitropism found in a tree from Central Minnesota.jpg|alt=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.|thumb|300x300px|Example of gravitropism in a tree from central Minnesota. This tree has fallen over and due to gravitropism exhibits this arched growth.]]
[[File:Gravitropism tree.jpg|thumb|300px|right| 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.]]

'''Gravitropism''' (also known as '''geotropism''') is a coordinated process of differential growth by a [[plant]] in response to [[gravity]] pulling on it. It also occurs in [[fungi]]. Gravity can be either "artificial gravity"{{clarify|reason=What does this mean?|date=November 2024}} or natural gravity. It is a general feature of all [[higher plants|higher]] and many [[lower plants|lower]] plants as well as other organisms. [[Charles Darwin]] was one of the first to scientifically document that [[root]]s show ''positive gravitropism'' and stems show ''negative gravitropism''.<ref>{{cite book|last1=Darwin|first1=Charles|last2=Darwin|first2=Francisc|title=The power of movement in plants|date=1881|publisher=D. Appleton and Company|location=New York|url=https://archive.org/details/powermovementin06darwgoog|access-date=24 April 2018}}</ref> That is, [[root]]s grow in the direction of gravitational pull (i.e., downward) and [[Plant stem|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 [[Plant stem|stem]] begin to display negative gravitropism, growing (biologists say, turning; see [[tropism]]) upwards. Herbaceous (non-woody) stems are capable of a degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside.{{clarify|reason=Outside what?|date=November 2024}} The mechanism is based on the [[Cholodny–Went model]] which was proposed in 1927, and has since been modified.<ref>{{cite journal
 |title=The Rice Coleoptile Phototropisim1 Gene Encoding an Ortholog of Arabidopsis NPH3 Is Required for Phototropism of Coleoptiles and Lateral Translocation of Auxin(W)
 |journal=Plant Cell |volume=17
 |issue=1
 |pages=103–15
 |date=January 1, 2005 |doi=10.1105/tpc.104.028357 |pmid=15598797
 |pmc=544493
 |last1=Haga |first1=Ken |last2=Takano |first2=Makoto |last3=Neumann |first3=Ralf |last4=Iino |first4=Moritoshi}}</ref> Although the model has been criticized and continues to be refined, it has largely stood the test of time.{{cn|date=August 2022}}

== In roots ==

[[File:Gravitropism in Roots.gif|thumb|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.<ref name=":5" />|alt=[this image is incorrect! the high auxin is always on the opposite side of the tropic movement!]]Root growth occurs by  division of stem cells in the root [[Meristem#Indeterminate growth of meristems|meristem]] located in the tip of the root, and the subsequent asymmetric expansion of cells in a shoot-ward region 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 effective growth responses to changes in orientation to and continue to grow its roots in the same direction as gravity.<ref name=":0">{{Cite journal|last1=Perrin|first1=Robyn M.|last2=Young |first2=Li-Sen|last3=Narayana Murthy|first3=U.M.|last4=Harrison |first4=Benjamin R.|last5=Wang|first5=Yan|last6=WILL|first6=Jessica L.|last7=Masson |first7=Patrick H.|date=2017-04-21|title=Gravity Signal Transduction in Primary Roots|journal=Annals of Botany|volume=96|issue=5|pages=737–743|doi=10.1093/aob/mci227|issn=0305-7364|pmc=4247041|pmid=16033778}}</ref>

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]].<ref>{{Cite journal|last1=Swarup|first1=Ranjan|last2=Kramer|first2=Eric M.|last3=Perry|first3=Paula|last4=Knox|first4=Kirsten|last5=Leyser|first5=H. M. Ottoline|last6=Haseloff|first6=Jim|last7=Beemster|first7=Gerrit T. S.|last8=Bhalerao|first8=Rishikesh|last9=Bennett|first9=Malcolm J.|date=2005-11-01|title=Root gravitropism requires lateral root cap and epidermal cells for transport and response to a mobile auxin signal|journal=Nature Cell Biology|volume=7|issue=11|pages=1057–1065|doi=10.1038/ncb1316|issn=1465-7392|pmid=16244669|s2cid=22337800}}</ref>  This was described in the 1920s in the [[Cholodny-Went model]]. 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.<ref>{{cite book |url=https://books.google.com/books?id=JItGq_zGANAC&pg=PA235 |page=235
 |title=Horticultural Reviews
 |first=Jules |last=Janick
 |publisher=John Wiley & Sons |year=2010 |isbn=978-0-470-65053-0}}</ref> 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.

Experiments show that auxin distribution is characterized by a fast movement of auxin to the lower side of the root in response to a gravity stimulus at a 90° degree angle or more. However, once the root tip reaches a 40° angle to the horizontal of the stimulus, auxin distribution quickly shifts to a more symmetrical arrangement. This behavior is described as a "tipping point" mechanism for auxin transport in response to a gravitational stimulus.<ref name=":5">{{cite journal|last1=Band|first1=L. R.|last2=Wells|first2=D. M.|last3=Larrieu|first3=A.|last4=Sun|first4=J.|last5=Middleton|first5=A. M.|last6=French|first6=A. P.|last7=Brunoud|first7=G.|last8=Sato|first8=E. M.|last9=Wilson|first9=M. H.|last10=Peret|first10=B.|last11=Oliva|first11=M.|last12=Swarup|first12=R.|last13=Sairanen|first13=I.|last14=Parry|first14=G.|last15=Ljung|first15=K.|last16=Beeckman|first16=T.|last17=Garibaldi|first17=J. M.|last18=Estelle|first18=M.|last19=Owen|first19=M. R.|last20=Vissenberg|first20=K.|last21=Hodgman|first21=T. C.|last22=Pridmore|first22=T. P.|last23=King|first23=J. R.|last24=Vernoux|first24=T.|last25=Bennett|first25=M. J.|title=Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism|journal=Proceedings of the National Academy of Sciences|date=5 March 2012|volume=109|issue=12|pages=4668–4673|doi=10.1073/pnas.1201498109|pmid=22393022|pmc=3311388|bibcode=2012PNAS..109.4668B|doi-access=free}}</ref>

== In shoots ==
[[File:Gravitropism on flat and sloped ground.svg|thumb| ]]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. Growth due to gravitropism is mediated by changes in concentration of the plant hormone auxin within plant cells.

[[File:Gravitropism in Shoots.gif|thumb|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.<ref name=":4">{{Cite web |title=Gravitropism Lesson |url=http://herbarium.desu.edu/pfk/page8/page9/page9.html |access-date=2018-07-08 |website=herbarium.desu.edu}}</ref>]]

As plants mature, gravitropism continues to guide growth and development along with phototropism. While amyloplasts continue to guide plants in the right direction, plant organs and function rely on phototropic responses to ensure that the leaves are receiving enough light to perform basic functions such as photosynthesis. 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.<ref>{{Cite journal|last=Hangarter|first=R.P.|date=1997|title=Gravity, light, and plant form|journal=Plant, Cell & Environment|volume=20|issue=6|pages=796–800|doi=10.1046/j.1365-3040.1997.d01-124.x|pmid=11541207|doi-access=}}</ref> 

Differential sensitivity to auxin helps explain Darwin's original observation that stems and roots respond in the opposite way to the forces of gravity. 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).<ref name=":0" /><ref>{{Cite journal|last=Hou, G., Kramer, V. L., Wang, Y.-S., Chen, R., Perbal, G., Gilroy, S. and Blancaflor, E. B.|date=2004|title=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|journal=The Plant Journal|volume=39|issue=1|pages=113–125|doi=10.1111/j.1365-313x.2004.02114.x|pmid=15200646|doi-access=free}}</ref>  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).<ref name=":2">{{Cite journal|last1=Masson|first1=Patrick H.|last2=Tasaka|first2=Masao|last3=Morita|first3=Miyo T.|last4=Guan|first4=Changhui|last5=Chen|first5=Rujin|last6=Boonsirichai|first6=Kanokporn|date=2002-01-01|title=Arabidopsis thaliana: A Model for the Study of Root and Shoot Gravitropism|journal=The Arabidopsis Book|volume=1|pages=e0043|doi=10.1199/tab.0043|pmc=3243349|pmid=22303208}}</ref>

A recent study showed that for gravitropism to occur in shoots, a lot of an inclination, instead of a weak gravitational force, is necessary. This finding sets aside gravity sensing mechanisms that would rely on detecting the pressure of the weight of statoliths.<ref>{{cite journal|last1=Chauvet|first1=Hugo|last2=Pouliquen|first2=Olivier|last3=Forterre|first3=Yoël|last4=Legué|first4=Valérie|last5=Moulia|first5=Bruno|title=Inclination not force is sensed by plants during shoot gravitropism|journal=Scientific Reports|date=14 October 2016|volume=6|issue=1|pages=35431|doi=10.1038/srep35431|pmid=27739470|pmc=5064399|bibcode=2016NatSR...635431C}}</ref>

== In fruit ==

A few species of fruit exhibit negative geotropism. Bananas are one well-known example.<ref>{{Cite journal |last1=Hu |first1=Wei |last2=Zuo |first2=Jiao |last3=Hou |first3=Xiaowan |last4=Yan |first4=Yan |last5=Wei |first5=Yunxie |last6=Liu |first6=Juhua |last7=Li |first7=Meiying |last8=Xu |first8=Biyu |last9=Jin |first9=Zhiqiang |date=2015-09-15 |title=The auxin response factor gene family in banana: genome-wide identification and expression analyses during development, ripening, and abiotic stress |journal=Frontiers in Plant Science |volume=6 |pages=742 |doi=10.3389/fpls.2015.00742 |issn=1664-462X |pmc=4569978 |pmid=26442055|doi-access=free }}</ref> Once the canopy that covers the fruit dries, the bananas will begin to curve upwards, towards sunlight, in what is known as [[phototropism]]. The specific chemical that initiates the upward curvature is a phytohormone in the banana called [[auxin]]. When the banana is first exposed to sunlight after the leaf canopy dries, one face of the fruit is shaded. On exposure to sunlight, auxin in the banana migrates from the sunlight side to the shaded side. Since auxin is a powerful plant growth hormone, the increased concentration promotes cell division and causes the plant cells on the shaded side to grow.<ref name="Strohm 358–361">{{Citation |last1=Strohm |first1=A. |title=Gravitropism in Arabidopsis thaliana |date=2013|url=https://www.sciencedirect.com/science/article/pii/B9780123749840006628 |encyclopedia=Brenner's Encyclopedia of Genetics |edition=2nd |pages=358–361 |editor-last=Maloy |editor-first=Stanley |place=San Diego |publisher=Academic Press |language=en |isbn=978-0-08-096156-9 |access-date=2022-04-06 |last2=Baldwin |first2=K. |last3=Masson |first3=P. H. |editor2-last=Hughes |editor2-first=Kelly}}</ref> This asymmetrical distribution of auxin is responsible for the upward curvature of the banana.<ref name="Strohm 358–361"/><ref>{{Cite journal |last1=Liscum |first1=Emmanuel |last2=Askinosie |first2=Scott K. |last3=Leuchtman |first3=Daniel L. |last4=Morrow |first4=Johanna |last5=Willenburg |first5=Kyle T. |last6=Coats |first6=Diana Roberts |date=January 2014 |title=Phototropism: Growing towards an Understanding of Plant Movement[OPEN] |journal=The Plant Cell |volume=26 |issue=1 |pages=38–55 |doi=10.1105/tpc.113.119727 |issn=1040-4651 |pmc=3963583 |pmid=24481074}}</ref>

== Gravity-sensing mechanisms ==

=== Statoliths ===
[[File:Banana Bunch At Chinawal.jpg|thumb|Banana fruit exhibiting negative geotropism.]]
Plants possess the ability to sense gravity in several ways, one of which is through statoliths. 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.<ref>{{cite journal|last1=Chen|first1=Rujin|last2=Rosen|first2=Elizabeth|last3=Masson|first3=Patrick H.|title=Gravitropism in Higher Plants|journal=Plant Physiology|date=1 June 1999|volume=120|issue=2|pages=343–350|doi=10.1104/pp.120.2.343|pmid=11541950|pmc=1539215}}</ref> These specialized amyloplasts are denser than the cytoplasm and can sediment according to the gravity vector. The statoliths are enmeshed in a web of actin and it is thought that their sedimentation transmits the gravitropic signal by activating mechanosensitive channels.<ref name=":0" /> The gravitropic signal then leads to the reorientation of [[auxin]] efflux carriers and subsequent redistribution of auxin streams in the root cap and root as a whole.<ref>{{Cite journal|last1=Sato|first1=Ethel Mendocilla|last2=Hijazi|first2=Hussein|last3=Bennett|first3=Malcolm J.|last4=Vissenberg|first4=Kris|last5=Swarup|first5=Ranjan|date=2015-04-01|title=New insights into root gravitropic signalling|url= |journal=Journal of Experimental Botany|volume=66|issue=8|pages=2155–2165|doi=10.1093/jxb/eru515|pmid=25547917|issn=0022-0957|pmc=4986716}}</ref> Auxin moves toward higher concentrations on the bottom side of the root and suppresses elongation. The asymmetric distribution of auxin leads to differential growth of the root tissues, causing the root to curve and follow the gravity stimuli. Statoliths are also found in the endodermic layer of the [[hypocotyl]], stem, and inflorescence stock. 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.

=== Modulation by phytochrome ===
[[Phytochrome]]s are red and far-red photoreceptors that help induce changes in certain aspects of plant development. Apart being itself the tropic factor ([[phototropism]]), light may also suppress the gravitropic reaction.<ref name=":3">{{Cite journal|last1=Kim|first1=Keunhwa|last2=Shin|first2=Jieun|last3=Lee|first3=Sang-Hee|last4=Kweon|first4=Hee-Seok|last5=Maloof|first5=Julin N.|last6=Choi|first6=Giltsu|date=2011-01-25|title=Phytochromes inhibit hypocotyl negative gravitropism by regulating the development of endodermal amyloplasts through phytochrome-interacting factors|journal=Proceedings of the National Academy of Sciences|language=en|volume=108|issue=4|pages=1729–1734|doi=10.1073/pnas.1011066108|issn=0027-8424|pmc=3029704|pmid=21220341|bibcode=2011PNAS..108.1729K|doi-access=free}}</ref> In seedlings, red and far-red light both inhibit negative gravitropism in seedling hypocotyls (the shoot area below the [[cotyledon]]s) causing growth in random directions. However, the hypocotyls readily orient towards blue light. 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.<ref name=":3" />

==Compensation==<!-- This section is linked from [[Compensation]] -->
[[File:Compensation mushroom.png|thumb|200px|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.<ref name=":1">{{Cite journal|last=Meškauskas, A., Novak Frazer, L. N., & Moore, D.|date=1999|title=Mathematical modelling of morphogenesis in fungi: a key role for curvature compensation ('autotropism') in the local curvature distribution model|journal=New Phytologist|volume=143|issue=2|pages=387–399|doi=10.1046/j.1469-8137.1999.00458.x|pmid=11542911|doi-access=free}}</ref> This effect is called ''compensation'' (or sometimes, ''autotropism''). The exact reason of such behavior is unclear, and at least two hypotheses 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).<ref name=":1" />
* 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, straightening resulting.<ref>{{Cite journal|last=Meškauskas A., Jurkoniene S., Moore D.|date=1999|title=Spatial organization of the gravitropic response in plants: applicability of the revised local curvature distribution model to Triticum aestivum coleoptiles|journal=New Phytologist|volume=143|issue=2|pages=401–407|doi=10.1046/j.1469-8137.1999.00459.x|pmid=11542912|doi-access=free}}</ref>

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 stimulate less 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.<ref name=":2" />  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. In the same way that gravity has an effect on winding and circumnutating, thus aspects of morphogenesis have defects on the mutant. 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 affect starch accumulation, such as those affecting the ''PGM1'' (which encodes the enzyme [[phosphoglucomutase]]) gene in ''Arabidopsis'', causing plastids – the presumptive statoliths – to be less dense and, in support of the starch-statolith hypothesis, less sensitive to gravity.<ref>{{Cite journal|last1=Wolverton|first1=Chris|last2=Paya|first2=Alex M.|last3=Toska|first3=Jonida|date=2011-04-01|title=Root cap angle and gravitropic response rate are uncoupled in the Arabidopsis pgm-1 mutant|journal=Physiologia Plantarum|language=en|volume=141|issue=4|pages=373–382|doi=10.1111/j.1399-3054.2010.01439.x|pmid=21143486|issn=1399-3054}}</ref>  Other examples of gravitropic mutants include those affecting the transport or response to the hormone auxin.<ref name=":2" />  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.

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 [[Plant habit|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 ==
* [[Amyloplast]] – starch organelle involved in sensing gravitropism
* [[Astrobotany]] – the field of science concerned with plants in a spaceflight environment
* [[Clinostat]] – a device used to  the effects of gravitational pull
* [[Random positioning machine]] – a device used to negate the effects of gravitational pull
* [[Free fall machine]] – a device used to negate the effects of gravitational pull
* [[Large diameter centrifuge]] – a device used to create a hyper-gravity pull
* [[Prolonged sine]] – reaction of plants to turning from their usual vertical orientation
* [[Phototropism]] – growth of a plant in response to a light stimulus

== References ==
{{Reflist}}

{{Tropism}}
{{Commons category|Gravitropism}}

[[Category:Tropism]]
[[Category:Auxin action]]
[[Category:Articles containing video clips]]