thigmotropism
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Thigmotropism is a directional growth movement which occurs as a mechanosensory response to a touch stimulus. Thigmotropism is typically found in twining plants and tendrils, however plant biologists have also found thigmotropic responses in flowering plants and fungi. This behavior occurs due to unilateral growth inhibition. That is, the growth rate on the side of the stem which is being touched is slower than on the side opposite the touch. The resultant growth pattern is to attach and sometimes curl around the object which is touching the plant. However, flowering plants have also been observed to move or grow their sex organs toward a pollinator that lands on the flower, as in ''
Portulaca grandiflora ''Portulaca grandiflora'' is a succulent flowering plant in the family Portulacaceae, native to southern Brazil, Argentina, and Uruguay and often cultivated in gardens.Huxley, A., ed. (1992). ''New RHS Dictionary of Gardening''. Macmillan . It h ...
''.


Physiological factors

Since growth is a complex developmental procedure, there are indeed many requirements (both biotic and abiotic) that are needed for both touch perception and a thigmotropic response to occur. One of these is calcium. In a series of experiments in 1995 using the tendril ''
Bryonia dioica ''Bryonia dioica'', known by the common names red bryony and white bryony, also English mandrake or ladies' seal, is a perennial climbing vine indigenous to Central and Southern Europe. It is a flowering plant in the cucumber family Cucurbitac ...
'', touch-sensing
calcium channels A calcium channel is an ion channel which shows selective permeability to calcium ions. It is sometimes synonymous with voltage-gated calcium channel, although there are also ligand-gated calcium channels. Comparison tables The following tables ex ...
were blocked using various antagonists. Responses to touch in treatment plants which received calcium channel inhibitors were diminished compared to control plants, indicating that calcium may be required for thigmotropism. Later in 2001, a membrane depolarization pathway was proposed in which calcium was involved: when a touch occurs, calcium channels open and calcium flows into the cell, shifting the electrochemical potential across the membrane. This triggers voltage-gated chloride and potassium channels to open and leads to an action potential that signals the perception of touch. The plant growth hormone
auxin Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essenti ...
has also been observed to be involved in thigmotropic behavior in plants, but its role is not well understood. Instead of asymmetric auxin distribution influencing other tropisms, it has been shown that a unidirectional thigmotropic response can occur even with a symmetric distribution of auxin. It has been proposed that the action potential arising from a touch stimulus leads to an increase of auxin in the cell, which causes the production of an contractile protein on the side of the touch that allows the plant to grip onto an object. Further, it has been shown that when auxin (typically leading to growth ''away'' from the side of its localization) and a touch stimulus (causing a tropic response ''toward'' its localization) were applied on the same side of a cucumber hypocotyl, the stem will curve towards the touch.
Ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...
, another plant hormone, has also been shown to be an important regulator to the thigmotropic response in ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...
'' roots. Under normal circumstances, high ethylene concentrations in the roots promote straight growth. When the root encounters a rigid object, the thigmotropic response is activated and ethylene production is down-regulated, leading to the root to bend while growing rather than growing straight. Like phototropism, a thigmotropic response in stems requires light. Plant biologist Mark Jaffe performed a simple preliminary experiment using pea plants that led to this conclusion. He found that when he snipped a tendril off of a pea plant and placed it in the light, then repeatedly touched one side of it, the tendril would begin to curl. However, when performing this same experiment in the dark, the tendril would not curl.


In roots

Root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...
s also rely on touch to navigate their way through the soil. Generally, roots have a negative touch response, meaning when they feel an object, they would grow away from the object. This allows the roots to go through the soil with minimum resistance. Because of this behavior, roots are said to be negatively thigmotropic. Thigmotropism seems to be able to override the strong gravitropic response of even primary roots.
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
performed experiments where he found that in a vertical bean root, a contact stimulus could divert the root away from the vertical.


Misconception

''
Mimosa pudica ''Mimosa pudica'' (from la, pudica "shy, bashful or shrinking"; also called sensitive plant, sleepy plant, action plant, touch-me-not, shameplant) is a creeping annual or perennial flowering plant of the pea/legume family Fabaceae. It is often g ...
'' is well known for its
rapid plant movement Rapid plant movement encompasses plant movements, movement in plant structures occurring over a very short period, usually under one second. For example, the Venus flytrap closes its trap in about 100 milliseconds. The traps of Utricularia are muc ...
. The leaves close up and droop when touched. However, this is not a form of
tropism A tropism is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the stimulus (as oppose ...
, but a
nastic movement Nastic movements are non-directional responses to stimuli (e.g. temperature, humidity, light irradiance), and are usually associated with plants. The movement can be due to changes in turgor. Decrease in turgor pressure causes shrinkage while ...
, a similar phenomenon.
Nastic movements Nastic movements are non-directional responses to stimuli (e.g. temperature, humidity, light irradiance), and are usually associated with plants. The movement can be due to changes in turgor. Decrease in turgor pressure causes shrinkage while ...
are non-directional responses to stimuli (e.g. temperature, humidity, light irradiance), and are usually associated with plants.


References


External links


Thigmotropism in Tendrils
{{Tropism Tropism Plant physiology Auxin action