Tropism

This article is about physical movement in organisms. For infection specificity of pathogens, see Host tropism and Tissue tropism.

Daisies (Bellis perennis) facing the sun after opening in the morning showing heliotropism

Phycomyces, a fungus, exhibiting phototropism

A tropism (from Greek τρόπος, tropos, "a turning") 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 opposed to nastic movements which are non-directional responses). Viruses and other pathogens also affect what is called "host tropism", "tissue tropism", or "cell tropism"; in which case tropism refers to the way in which different viruses/pathogens have evolved to preferentially target specific host species, specific tissue, or specific cell types within those species. Tropisms are usually named for the stimulus involved (for example, a phototropism is a reaction to sunlight) and may be either positive (towards the stimulus) or negative (away from the stimulus).

Tropisms occur in three sequential steps. First, there is a sensation to a stimulus, which is usually beneficial to the plant. Next, signal transduction occurs. And finally, the directional growth response occurs.

Tropisms are typically associated with plants (although not necessarily restricted to them). Where an organism is capable of directed physical movement (motility), movement or activity in response to a specific stimulus is more likely to be regarded by behaviorists as a taxis (directional response) or a kinesis (non-directional response).

In English, the word tropism is used to indicate an action done without cognitive thought: However, "tropism" in this sense has a proper, although non-scientific, meaning as an innate tendency, natural inclination, or propensity to act in a certain manner towards a certain stimulus.

In botany, the Cholodny–Went model, proposed in 1927, is an early model describing tropism in emerging shoots of monocotyledons, including the tendencies for the stalk to grow towards light (phototropism) and the roots to grow downward (gravitropism). In both cases the directional growth is considered to be due to asymmetrical distribution of auxin, a plant growth hormone.

Types

in plants (and bacteria)

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

• Aerotropism, growth of plants towards or away from a source of oxygen
• Chemotropism, movement or growth in response to chemicals
• Electrotropism, movement or growth in response to an electric field
• Exotropism, continuation of growth "outward," i.e. in the previously established direction
• Geotropism (or gravitropism), movement or growth in response to gravity
  • Apogeotropism, negative geotropism
• Heliotropism, diurnal motion or seasonal motion of plant parts in response to the direction of the sun, (e.g. the sunflower)
  • Apheliotropism, negative heliotropism
• Hydrotropism, movement or growth in response to water; in plants, the root cap senses differences in water moisture in the soil, and signals cellular changes that causes the root to curve towards the area of higher moisture
  • Prohydrotropism, positive hydrotropism
• Hygrotropism, movement or growth in response to moisture or humidity
• Magnetotropism, movement or growth in response to magnetic fields
• Orthotropism, movement or growth in the same line of action as the stimulus
• Plagiotropism, movement or growth at an angle to a line of stimulus such as gravity or light
• Phototropism, movement or growth in response to lights or colors of light
  • Aphototropism, negative phototropism
  • Skototropism, negative phototropism of vines
• Selenotropism, motion of plant parts in response to the direction of the moon
• Thermotropism, movement or growth in response to temperature
• Thigmotropism, movement or growth in response to touch or contact

in viruses

• Amphotropism, wide host range (e.g. infects many species or cell types)
• Ecotropism, limited host range (e.g. infects only one species or cell type)
• HIV tropism, the means of entry into cells used by a given strain of HIV
• Neurotropism, a virus that preferentially infects the host's nervous system

See also

• Chemotaxis
• Rapid plant movement
• Tropisms, French novel