A tropism is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus.
Tropisms (from Greek, tropos, to turn) are usually named for the stimulus involved (for example, a phototropism is a reaction to light) and may be either positive (towards the stimulus) or negative (away from the stimulus).
Viruses and other pathogens also affect what is called "host 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, or specific cell types within those species.
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).
Types of tropisms:
- Chemotropism, movement or growth in response to chemicals
- Gravitropism (or geotropism), movement or growth in response to gravity
- Heliotropism, movement or growth in response to sunlight
- Hydrotropism, movement or growth in response to water
- Phototropism, movement or growth in response to lights or colors of light
- Thermotropism, movement or growth in response to temperature
- Thigmotropism, movement or growth in response to touch or contact
- Host tropism or cell tropism, the host range of pathogens
HIV tropism refers to the cell type that the human immunodeficiency virus (HIV) infects and replicates in. HIV tropism of a patient's virus is measured by the Trofile assay.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)