What is Tropism?
Tropism is an involuntary response of an organism (plant or lower animal) or one of its parts to a source of environmental stimulation. The organism responds either by movement (curving or turning) or by differential growth to the stimulus that acts with greater intensity from one direction than another. In tropisms, the organism’s response is dependent on the direction of the stimulus. Nevertheless, tropism could be a positive or negative response and may be achieved by structural alteration or by active movement. An organism will move towards the stimulus in a positive reaction whereas an organism will move away from the stimulus in a negative reaction.
There are various types of tropism in response to several stimuli. The various forms of tropism include a response to light, gravity, substances, water, mechanical stimulation, wound lesion, and electric current. The various types of tropisms are normally named for the stimulus involved. For instance, chemotropism is a reaction to chemical substances and phototropism is a reaction to sunlight.
Furthermore, a movement caused by a tropism is referred to as a taxis. Most tropic movements are orthotopic which are directed towards the source of the stimulation. However, some tropic movements could be Diatropic (right angle to the direction of the stimulus) or Plagiotropic (oblique to the direction of stimulus).
Positive tropism is a movement or growth of an organism, typically a plant, towards a stimulus.
Examples of positive tropism
- The growth of plants shoots towards the light.
- Some single-celled organisms move towards certain chemical substances.
- Some organisms show positive thermotropism as they move towards specific temperatures.
- When tendrils and twining plants touch a hard surface, they grow or move towards the stimulus.
Negative tropism is the growth or movement of an organism away from a stimulus. A common example that can be used to explain negative tropism is the shoot of the plants that grow against gravity.
Examples of negative tropism
- The shoot of the plant grows against gravity showing negative gravitropism.
- Beetles seek darkness for their safety showing negative phototropism.
- A growing root underneath the soil can show negative thigmotropism when it is in contact with an object such as rock and grows away from it.
- Some varieties of fish move away from hazardous chemicals showing negative chemotropism.
In nastic movement, the organism responds to a stimulus but not in the direction of the stimulus. Nastic movements are non-directional responses to stimulus whereas in tropisms the response to the stimulus is solely dependent on the direction of the stimulus.
Tropisms are different from nastic movements because the direction of a nastic movement is independent of where the signal comes from. Additionally, nastic movements are temporary. Moreso, nastic movements are more particular in distribution and function than tropisms. Some insectivorous plants, for instance, capture their prey by moving trap organs together.
- Plant exhibit thigmotropism, a response to touch. The roots of plants may turn when they touch a hard surface, like a rock.
- Some animals may be drawn to certain magnetic poles and use magnetic fields as a source of direction. Many animals and plants exhibit magnetotropism.
- Chemotropism is seen in fishes where they are both drawn to and avoid a variety of different chemicals.
- Salmon show chemotrophic ability when they are able to travel far in the ocean and still return to the same stream in which they were spawned. Salmon are drawn to the unique chemical signatures that the stream has.
- When a seed is planted, the roots always establish themselves toward gravity. This is geotropism.
- Plant exhibit hydrotropism as their roots are drawn to water and will move and grow towards the most water.
- Some echinoderms like brittle stars show negative phototropism. They live on hard substrates in water where little or no light penetrates.
- Sharks exhibit positive chemotropism for blood as they tend to move towards blood.
- Galvanotropism or Electrotropism
Phototropism is the response of the organism in response to light stimulus. It involves growth or movement in response to lights or colors of light. The response could be towards the source of light or away from it. The growth towards a light source is positive phototropism. Aphototropism is negative phototropism that involves the response of the organism away from the source of the light stimulus.
This type of tropism is often observed in plants and can also occur in fungi. The plant cells that are furthest from the source of light have a chemical called auxin that reacts when there is phototropism. This makes the plant have elongated cells on the part of the plant that is furthest from the light.
Many shoots of the plant show positive phototropism as they rearrange their chloroplasts in the leaves to promote growth and maximize photosynthetic energy. However, some vine shoot tips show negative phototropism as they grow towards dark objects and climb them. The negative phototropism of vines is called skototropism. Additionally, the combination of gravitropism and phototropism allows plants to grow in the correct direction.
Geotropism is a type of tropism that involves movement or growth in response to gravity. This occurs typically in plants and fungi. According to Charles Darwin, roots show positive geotropism whereas stems show negative geotropism. Apogeotropism is negative geotropism.
Geotropism is also caused by an uneven distribution of auxin, like in phototropism. When a root is placed horizontally, the bottom side will contain more auxin and will grow less. Hence, causing the root to grow in the direction of the force of gravity. However, the reverse happens in a stem. If a stem is placed horizontally, the bottom side will contain more auxin and will grow more. Hence, causing the stem to grow upwards against the force of gravity.
Chemotropism is the growth of organisms navigated by external chemical stimuli. This type of tropism has been observed in plants, fungi, and bacteria. Positive chemotropism is characterized by growing towards a chemical stimulus whereas negative chemotropism is growing away from the chemical stimulus. Chemotropism is quite different from Chemotaxis being that chemotaxis is related to locomotion (movement) whereas chemotropism is related to growth.
A major example of chemotropism can be seen in plant fertilization and pollen tube elongation of flowering plants. Since plants cannot move, they need a suitable mechanism for sexual reproduction. Hence pollens that contain the male gametophyte are transferred to another plant by wind or insects. Then, the pollen will germinate and begin to grow if compatible. The ovary, however, releases chemicals that trigger a positive chemotropic response from the developing pollen tube. Then, the pollen tube in response develops a defined tip growth area. The tip growth area enhances directional growth and elongation of the pollen tube as a result of a calcium gradient that is localized in the tip. This calcium gradient is crucial for growth to take place. However, the male sperm remains in the apical region as the pollen tube continues to grow towards the ovules. The male sperm is then transported to the female ovule.
Also, a typical example of positive and negative chemotropism is observed in plant roots. The roots of plants will grow towards essential minerals showing positive chemotropism. on the other hand, the root grows away from harmful substances showing negative chemotropism.
The growth of individual neuronal cell axons in response to extracellular signals is an example of chemotropism in more complex organisms. Also, chemotropism is seen in yeast (a fungus). In order to attract mates, yeast releases chemical pheromones.
Hydrotropism is a growth response in which the direction of growth or movement is determined by a stimulus in water concentration. A typical example is a plant root growing in humid air and bending toward a level of higher relative humidity. Prohydrotropism is positive hydrotropism and Hygrotropism is the movement or growth of the organism in response to humidity or moisture.
The hydrotropism process starts with the root cap sensing water. After sensing the difference in water moisture in the soil, it sends a signal to the elongating part of the root. This causes the root to curve towards the area of higher moisture in the soil. Plants recognize water in their habitat in order to absorb it for metabolic purposes. Hence the direction and growth rate of the root go towards the higher water concentration in the soil.
Thigmotropism is growth or movement in response to contact or touch. This type of tropism is a directional growth movement that happens as a mechanosensory response to a touch stimulus. Thigmotropism is also referred to as haptotropism and can be in the form of the plant coiling around the surface, closing or opening parts of the plant such as the petals or leaves, and other ways. This response or reaction can be positive or negative. For example, a positive thigmotropism is a stem climbing an object and growing upwards towards the sun to get light for photosynthesis. A negative thigmotropism example could be a root growing away from an object in the soil.
Usually, thigmotropism is found in tendrils and twining plants. This form of tropism has also been found in fungi and flowering plants by botanists. Basically, this behavior happens as a result of unilateral growth inhibition. This means the growth rate on the side of the stem that is being touched is slower than the side opposite the touch. Hence, the resultant growth pattern is to attach or sometimes roll around the object that is touching the plant. Moreso, flowering plants like Portulaca grandiflora, have been studied to grow or move their sex organs toward the pollinator that lands on the flower.
Galvanotropism or Electrotropism
Galvanotropism or electrotropism is the directional growth of an organism, most especially a plant in response to an electrical stimulus. This form of tropism is the process by which a cell alters its shape to grow directionally in response to an electric field. Galvanotropism is seen in a broad variety of eukaryotic cells.
An example of electrotropism is growing axons in the nervous system that are directed by an extracellular electric field. Galvanotropism plays an important role in the initial orientation of axons that extend over long distances and the extension of developing axons in the embryonic brain.
Magnetotropism is the growth or movement of plants in response to the stimulus provided by the magnetic field in plants, especially agricultural plants.
The variations of magnetic field level cause many biological effects, including activation of cryptochrome, germination rate, photosynthesis, flowering time, biomass accumulation, and shoot growth.
Aerotropism is the growth or response in plants towards the presence of air. In negative aerotropism, the plant grows away from where oxygen is present.
Selenotropism is growth in response to moonlight. This type of tropism involves the tendency of the organism to turn towards the rays of the moon. There have been some visible movements of plant parts in response to the direction of the moon.
Thermotropism is the growth or movement of an organism or cell to changes in temperature or heat. This form of tropism may be positive or negative. Thermotropism is positive when the movement or growth of an organism is towards the source of heat. Whereas, thermotropism is negative when the organism grows or moves away from the heat source.
However, the curling of Rhododendron leaves during cold temperatures is an example of thermotropism. Also, the collapsing of leaf petioles which leads to the folding of leaflets of Mimosa pudica in cold temperature is another example.
Tropism in Microbiology
Tropism in microbiology refers to the way in which pathogens or viruses have evolved to target specific cell types, specific tissues, or specific host species. Pathogens and viruses affect what is referred to as host tropism, cell tropism, or tissue tropism. In relation to microbiology, there are various forms of tropism.
Types of tropism in microbiology
- Viral tropism
- Host tropism
- Tissue tropism
- HIV tropism
The viral tropism is the ability of a virus to effectively infect a particular host, cell, or tissue. It involves different viral strains infecting different tissues or cell types and inducing syncytia formation or chronic infectious virus production due to the infection.
For viruses to survive, being parasites, they have to compromise with a number of negative and positive factors present in target cells. If they don’t interact appropriately with cells, they cannot replicate. Therefore, viral tropism can be determined at each replication step. Hence beginning with entrance into cells and ending with the progeny production from cells.
However, viral tropism is of two major types. This includes receptor-dependent tropism and receptor-independent tropism. In the receptor-dependent viral entry step, restriction of viral replication occurs on the surface of the cell. Then, restriction of viral replication occurs intracellularly in receptor-independent post-entry replication steps.
Host tropism is the specification of infection of certain pathogens to specific host tissues and host. This sort of tropism justifies why most pathogens are only able to infect a limited range of host organisms.
Pathogens can be classified based on their host tropism. For example, pathogens that are capable of infecting a wide range of tissues and hosts are said to be amphotropic. Then, pathogens that are only able to infect a narrow range of host tissues and host are said to be ecotropic pathogens.
However, the knowledge of the host tropism of a pathogen allows professionals in medical and research industries to study the pathogen and develop medication, vaccines, and preventive measures to fight against the infection.
A pathogen will show tropism for a particular host if it can interact with the cells of the host in such a way that supports pathogenic infection and growth. There are various factors that affect the ability of a pathogen to infect a particular cell. Such factors are:
- The presence of chemical or physical barriers within the cells and around the surrounding tissue.
- Structure of the cell’s surface receptors. These cellular receptors are the proteins seen on a viral or cell surface, allowing the viral cell to attach itself or fuse with a cell.
- The ability of the tissue and cells to support bacterial or viral replication.
- Availability of transcription factors that can identify pathogenic RNA or DNA.
Tissue tropism is the tissues and cells of a host that support the growth of specific bacteria or viruses. Some viruses and bacteria having a broad tissue tropism can infect many tissues and cell types. Whereas other viruses may infect mainly a single tissue. Rabies virus, for instance, affects mainly neuronal tissue. However, there are factors that influence viral tissue tropism. Such factors are:
- The presence of cellular receptors. These cellular receptors permit viral entry.
- Availability of transcription factors that are involved in viral replication.
- The viral tropogen molecular nature
These are the following stages at which tissue tropism develops:
- A virus with GPX enters the body. GP – glycoprotein, and X is the numeric value given to the glycoprotein.
- Cell with GPX receptors is targeted by the viral cell.
- The viral cell eventually fuses with the cell of the host.
- Viral cell then inserts its contents into the host cell
- Reverse transcription then occurs.
- The Viral DNA is incorporated with host DNA by the viral enzyme.
- There is a production of RNA and viral protein.
- Viral particle is then assembled.
- The viral particle then buds out of the cell.
- Taking a chunk of the cell membrane with it, the viral particle acquires a new tissue with all the receptors it needs to continue tissue tropism.
HIV Tropism involves the cell type that a give+n strain of virus (HIV) infects and replicates in. It’s the means of entry used by the Human immunodeficiency virus into cells. However, the Monogram Trofile assay is used to measure the HIV tropism of a patient’s virus. Read more on the HIV Replication cycle.
Amphotropism shows that a pathogen, virus, or bacterium can infect many cell types or species and have a wide host range.
Neurotropism indicates that a virus preferentially infects the nervous system of the host. A neurotropic virus is a virus that infects nerve cells.
Ecotropism shows that a pathogen, virus, or bacterium has a limited host range and can infect only one cell type or species.
Tropism in plants
Tropisms are typically associated with plants and occur in 3 sequential steps. Firstly, there is a sensation to a stimulus that is normally beneficial to the plant. Then, signal transduction occurs and lastly, the directional growth reaction occurs. However, the directional growth in plants is considered to be due to the asymmetrical distribution of auxin. Auxin is a plant growth hormone.
Since plants cannot move like animals to more favorable locations. They survive by adjusting their growth to their immediate environment. The main way this is done is by them sensing the directions of environmental signals like gravity and light. They then use this sensory information to orient the direction of their growth either toward or away from the stimulus. This process is what is called tropism in plants. Only part of the plant body grows in the direction of a stimulus since they are not motile.
By this process, the shoots of the plants grow up from the ground into the light. This eventually increases the amount of sunlight absorbed by chlorophyll and enhances biomass and photosynthesis. Also, the raised stature of the plant enhances seed dispersal and pollination. Hence, increasing plant competitiveness.
However, the growth of the plant organ towards the stimulus is positive tropism and the growth of the plant organ away from the stimulus is negative tropism. Gravitropism and phototropism are the most widespread and important tropisms in plants. Although, other physical stimuli like water, touch, and temperature can orient growth in some plants and organs.
Tropism allows plants to adjust their growth direction when their environment changes. However, only the plant’s growing regions are capable of tropisms in most cases. In a plant, there are many such regions. For instance, the tips of roots and stems contain meristems. Meristems are regions where new growth occurs and cell divisions in these meristems contribute to the elongation of roots and stems. They also form new stem branches and each stem branch is capable of gravitropism and phototropism. Therefore, their collective responses help in determining the overall shape of the shoot.
Tropism in plants examples
- Houseplants on windows will show positive phototropism by growing toward the light.
- A growing root underneath the soil can show negative thigmotropism when it is in contact with an object such as a rock and grows away from it.
- Plant stems that emerge from seeds sowed in the soil will grow upward away from gravity, hence exhibiting negative gravitropism.
- When tendrils and twining plants touch a hard surface, they grow or move towards the stimulus.
- Modified stems such as rhizomes grow at a right angle to gravity along the surface of the soil as in iris plants.
Tropism in animals
Tropism in animals is the innate ability of the organism to turn or move in response to a stimulus. These reactions are genetically programmed and a stimulus can be any signal from the environment. In a positive tropism, the animal will move toward the stimulus whereas the animal will move away from the stimulus in a negative tropism. Certain stimuli are always beneficial or detrimental to an organism. However, chemotropism is a widely spread tropism in the animal kingdom. This is the ability of an organism to turn or move towards or away from a specific chemical substance.
For instance, a chemical emitted may indicate the presence of a mate and another chemical may signify danger to an organism. Hence, an organism will either move towards the stimuli or away from it in the direction that their ancestors found to be most evolutionary rewarding. For example, sharks exhibit a positive chemotropism for blood hence they navigate towards it. Though a shark would usually test or examine a meal before eating it, showing that other natural processes can overshadow a tropism.