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Upside-Down Plants
What's going on with our tomatoes?
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Contents
Articles
Gravitropism
1
Phototropism
3
References
Article Sources and Contributors
5
Image Sources, Licenses and Contributors
6
Article Licenses
License
7
Gravitropism
1
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 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
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.
Example of Gravitropism in the remaints of a
cellar of a roman villa in the Archeologic Park in
Baia, Italy
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 and
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 is
called "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.
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).
Gravitropism
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
The compensation reaction of the bending Coprinus
multi-parameter function, depending on time, the current
stem. C - the compensating part of the stem.
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. 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.
2
Gravitropism
See also
• Clinostat - a device used to negate the effects of gravitational pull.
• Amyloplast - vesicle involved in 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.
mhmmmm
Phototropism
Phototropism is directional growth in which the direction of growth is
determined by the direction of the light source. In other words, it is the
growth and response to a light stimulus. Phototropism is most often
observed in plants, but can also occur in other organisms such as fungi.
The cells on the plant that are farthest from the light have a chemical
called auxin that reacts when phototropism occurs. This causes the
plant to have elongated cells on the farthest side from the light.
Phototropism is one of the many plant tropisms or movements which
respond to external stimuli. Growth towards a light source is a positive
phototropism, while growth away from light is called negative
phototropism (or Skototropism). Most plant shoots exhibit positive
phototropism, while roots usually exhibit negative phototropism,
although gravitropism may play a larger role in root behavior and
growth. Some vine shoot tips exhibit negative phototropism, which
allows them to grow towards dark, solid objects and climb them.
The Thale Cress (Arabidopsis thaliana) is
Phototropism in plants such as Arabidopsis thaliana is directed by blue
[1]
[2]
regulated by blue to UV light (plantphys.net )
light receptors called phototropins. Other photosensitive receptors in
plants include phytochromes that sense red light[3] and
cryptochromes that sense blue light[4] . Different organs of the plant may exhibit different phototropic reactions to
different wavelengths of light. Stem tips exhibit positive phototropic reactions to blue light, while root tips exhibit
negative phototropic reactions to blue light. Both root tips and most stem tips exhibit positive phototropism to red
light.
3
Phototropism
Phototropism is enabled by auxins. Auxins are plant hormones that
have many functions. In this respect, auxins are responsible for
expelling protons (by activating proton pumps) which decreases pH in
the cells on the dark side of the plant. This acidification of the cell wall
region activates enzymes known as expansins which break bonds in the
cell wall structure, making the cell walls less rigid. In addition, the
acidic environment causes disruption of hydrogen bonds in the
cellulose that makes up the cell wall. The decrease in cell wall strength
causes cells to swell, exerting the mechanical pressure that drives
phototropic movement.
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Phycomyces, a fungus, also exhibit phototropism
Other light responses
• Etiolation is the response of a plant when light is nearly (or completely) absent.
• Heliotropism is the diurnal motion of plant parts (flowers or leaves) in response to the direction of the sun. It is
not a phototropism since it does not involve growth.
• Photonasty involves the movement of plant parts that does not involve growth but is triggered by light. The plant
movement is not determined by the direction of light so it is not a phototropism. Photonasty in prayer plant
(Maranta leuconeura) involves the downward movement of leaves when they receive light in the morning.
• Phototaxis is movement of an entire organism in which the direction of movement is determined by the direction
of light. It occurs in some motile microbes such as Euglena and algae. It is not a phototropism because growth is
not required.
• Photo-orientation occurs within a plant cell when chloroplasts change their positions depending upon light
intensity. This was discovered in 1987 by Chelsea Polevy and Kelsey Joyce when experimenting in their
laboratory. When the light intensity is high, chloroplasts move to the edge of the cell to reduce photobleaching
(destruction of chlorophyll).[5] In low light, chloroplasts tend to spread out within the protoplasm to maximize
their capture of light energy. Photo-orientation is also not a phototropism.
See also
• Scotobiology
External links
• Time lapse films [6], Plants-In-Motion
References
[1]
[2]
[3]
[4]
[5]
http:/ / www. plantphys. net/ article. php?id=266
http:/ / abstracts. aspb. org/ pb2004/ public/ S01/ 9179. html American Society of Plant Biologists
http:/ / plantphys. info/ plant_physiology/ phytochrome. shtml
http:/ / www. plantcell. org/ cgi/ content/ full/ 15/ 5/ 1051
Takagi, Shingo 23 December 2002, Actin-based photo-orientation movement of chloroplasts in plant cells (http:/ / jeb. biologists. org/ cgi/
content/ full/ 206/ 12/ 1963), Journal of Experimental Biology 206: 1963-1969
[6] http:/ / plantsinmotion. bio. indiana. edu/
Article Sources and Contributors
Article Sources and Contributors
Gravitropism Source: http://en.wikipedia.org/w/index.php?oldid=354194401 Contributors: 5 albert square, 7, Againme, Ahoerstemeier, Anetode, Ardric47, Arkon, Artarro, Atif.t2, AtticusX,
Audriusa, Bartledan, Beano, Bit Lordy, Bobo192, Brainyiscool, Brickbeard, Burzmali, CanadianLinuxUser, Capricorn42, Cometstyles, DRosenbach, Da monster under your bed, Daf,
Danlewis1979, David D., Daycd, DieYuppieScum, Dina, Discospinster, Dominic, Drunken Pirate, Duncharris, EEMIV, Epbr123, Frosted14, HalfShadow, HappyCamper, II MusLiM HyBRiD II,
Ian Pitchford, Inderonline1988, J.delanoy, JForget, JRSP, Jachym.czech, Jeff G., Jusdafax, Krich, Ld100, Lx Rogue, MPF, Mackeriv, Marek69, Marshman, Mgiganteus1, Momhoff, Moreschi,
N0d3, Nazlfrag, Neutrality, NightFalcon90909, Oxymoron83, Photon03, PigFlu Oink, Pixeltoo, Possum, Razorflame, Retama, RoyBoy, Saric, SeventyThree, Shenme, Sintaku, Snigbrook,
Snowolf, Sup trix, Tarheel95, The Thing That Should Not Be, Ttiotsw, Ulric1313, Uncle G, Vcu123, Vojtech.dostal, Whpq, Yt95, 232 anonymous edits
Phototropism Source: http://en.wikipedia.org/w/index.php?oldid=358259897 Contributors: Alan Liefting, Alansohn, Antandrus, Boing! said Zebedee, Bradjamesbrown, Burn, Cantor, CarinaT,
Ceinturion, Chris G, Cobaltbluetony, Crazedgiggles, Dabrssmnky, DarkFalls, David D., Daycd, Donarreiskoffer, Drunken Pirate, E Wing, Empco, EncycloPetey, EoGuy, Eog1916, Epbr123,
Escape Orbit, Everyking, Fritzpoll, GeneralCheese, Hoof Hearted, Ipatrol, JaadesA, Jake swallow, Jimmyt2009-10, Jnb, Kilva, Korg, Lexor, Mandarax, Matthudson, Maxis ftw, MikeLeeds,
MisterSheik, Nancy, NawlinWiki, Neutrality, NightFalcon90909, Nummer29, Numsgil, Perdika92, PeteShanosky, Plantguy, Prari, R Lee E, Redvers, Rich257, Richard001, Riptide3568,
Rpyle731, SJFriedl, Sadi Carnot, Spezdispenser, Spiff, Stardust8212, Stemonitis, Steven Zhang, TerriersFan, The High Fin Sperm Whale, TheAlphaWolf, Typhoonchaser, Vacuum, Wavelength,
Wireless Keyboard, 172 anonymous edits
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Image Sources, Licenses and Contributors
Image Sources, Licenses and Contributors
File:Upsidedown-tree.JPG Source: http://en.wikipedia.org/w/index.php?title=File:Upsidedown-tree.JPG License: Creative Commons Attribution 2.5 Contributors: Kleuske
Image:Compensation mushroom.png Source: http://en.wikipedia.org/w/index.php?title=File:Compensation_mushroom.png License: GNU Free Documentation License Contributors:
Audriusa, Ies
Image:Arabidopsis thaliana.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Arabidopsis_thaliana.jpg License: GNU Free Documentation License Contributors: User:Roepers
Image:Phycomyces3.JPG Source: http://en.wikipedia.org/w/index.php?title=File:Phycomyces3.JPG License: GNU Free Documentation License Contributors: TheAlphaWolf
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License
License
Creative Commons Attribution-Share Alike 3.0 Unported
http:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/
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