Tropism is the movement that a plant develops in response to an external stimulus.
The notion of tropism is used in biology to name the movement that plants or certain organs carry out to respond to a stimulus that comes from the outside.
There are different kinds of tropisms according to the nature of the stimulus. When the plant 's reaction is caused by the force of gravity and its acceleration, we speak of gravitropism .
The roots, in this case, grow towards the ground while the stems develop upward until they come to the surface. It is said that the tropism of the roots is positive (accompanies the force of gravity), while the tropism of the stems is negative (opposite to the aforementioned force).
Various types of tropisms
Due to phototropism , on the other hand, plants react to light , allowing photosynthesis to develop. Unlike what happens with gravitropism, in this case the roots exhibit negative phototropism and the stems have positive phototropism. Within phototropisms, heliotropism stands out: the tropism that implies a movement of the plant according to the orientation of the sun .
Other classes of tropisms are those recognized as part of chemotropisms , which are linked to the responses of plants to chemical elements. When these elements are beneficial to the plant organism, aerotropism (the stems are oriented to the aerated regions of the surface) or hydrotropism (they are directed to the water) can occur.
When coming into contact with a solid , plants can also develop thigmotropism , a reaction that produces a change in their growth. This type of tropism is what climbing plants perform.
The idea of tissue tropism is linked to the affinity that a virus has with one or more tissues of its host.
The concept in virology
In virology, the branch of microbiology that is dedicated to studying viruses , we speak of tissue tropism to refer to the affinity that a given virus has with one or more tissues of its host (also called host). Some of the factors that influence its distribution are the following: how susceptible the host is; that there are receptors for the virus in question in the host organism; the rate of reproduction of the virus (this cycle is also known as viral replication ).
Tissue tropism can also occur or increase thanks to the presence of soluble proteins or cytokines (also known as cytokines , they are proteins that are in charge of regulating the functioning of certain cells and their communication, among other tasks) that counteract the action of the host. It is worth mentioning that this phenomenon can also favor the development of bacteria, and that the affinity can be to a particular tissue or to a large number of different tissues and cells.
A clear example of tissue tropism occurs in porcine adenovirus type 3, which appears to be related to columnar epithelial cells. Similarly, bovine adenovirus is attracted to capillary endothelial-type cells. The rabies virus, for its part, has an almost exclusive affinity for neuronal tissue .
Stages of tissue tropism
The proteins found in cells are known as cellular receptors and these have the power to allow a virus to fuse with the cell. Through the process of tissue tropism, the shape that these proteins acquire is comparable to that shown during an infection.
Some of the stages that are identified throughout tissue tropism are the following:
* the virus enters the body ;
* its cells fuse with others;
* A process known as reverse transcription takes place, which consists of generating double-stranded DNA from single-stranded RNA;
* Virus particles sprout and acquire the tissue necessary to continue tropism.