Chemotropism is defined as the growth of organisms navigated by chemical

stimulus A stimulus is something that causes a physiological response. It may refer to: *Stimulation **Stimulus (physiology), something external that influences an activity **Stimulus (psychology), a concept in behaviorism and perception *Stimulus (economi ...

from outside of the organism. It has been observed in

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

s and

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

. A chemical gradient can influence the growth of the organism in a positive or negative way. Positive growth is characterized by growing towards a stimulus and negative growth is growing away from the stimulus. Chemotropism is slightly different from

Chemotaxis Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemica ...

, the major difference being that chemotropism is related to growth, while chemotaxis is related to locomotion. A chemotropic process may have an underlying chemotactic component, as is the case with mating yeast.

Chemotropism in plants

PSM V77 D352 The course of the pollen tube in a rock rose

One prime example of chemotropism is seen in plant

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

and

pollen tube A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells fro ...

elongation of

angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...

, flowering plants. Unlike animals, plants cannot move, and therefore need a delivery mechanism for sexual reproduction.

Pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

, which contains the male

gametophyte A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...

is transferred to another plant via insects or wind. If the pollen is compatible it will germinate and begin to grow. The ovary releases chemicals that stimulates a positive chemotropic response from the developing pollen tube. In response the tube develops a defined tip growth area that promotes directional growth and elongation of the pollen tube due to a calcium gradient. The steep calcium gradient is localized in the tip and promotes elongation and orientation of the growth. This calcium gradient is essential for the growth to occur; it has been shown that inhibiting the formation of the gradient results in no growth. As the pollen tube continues to grow towards the ovules, the male sperm remains in the apical region and is transported to the female ovule. The pollen tube elongates at a rate comparable to neurite development An example of positive and negative chemotropism is shown by a plant's roots; the roots grow towards useful

minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

displaying positive chemotropism, and grow away from harmful

acids In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

displaying negative chemotropism.

Chemotropism in animals

In more complex organisms an example of chemotropic movement includes the growth of individual neuronal cell

axons An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, th ...

in response to extracellular signals. Secreted protein can either repel or attract specific neurons. Some signal proteins such as

netrin Netrins are a class of proteins involved in axon guidance. They are named after the Sanskrit word "netr", which means "one who guides". Netrins are genetically conserved across nematode worms, fruit flies, frogs, mice, and humans. Structurally, ...

semaphorin Semaphorins are a class of secreted and membrane proteins that were originally identified as axonal growth cone guidance molecules. They primarily act as short-range inhibitory signals and signal through multimeric receptor complexes. Semaphorins ...

neurotrophins Neurotrophins are a family of proteins that induce the survival, development, and function of neurons. They belong to a class of growth factors, secreted proteins that can signal particular cells to survive, differentiate, or grow. Growth factor ...

and

fibroblast growth factor Fibroblast growth factors (FGF) are a family of cell signalling proteins produced by macrophages; they are involved in a wide variety of processes, most notably as crucial elements for normal development in animal cells. Any irregularities in the ...

s have been identified in aiding neuronal growth. These signals guide the developing axon to

innervate A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

the correct target tissue. The neuronal growth cones are guided by gradients of

chemoattractant Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemical ...

molecules released from their intermediate or final targets. There is evidence that the axons of peripheral neurons are guided by chemotropism and the directed growth of some central axons is also a chemotropic response, it remains to be determined whether chemotropism also operates in the central nervous system. Evidence has also been noted in neuronal regeneration, where chemotropic substances guide the ganglionic neurites towards the degenerated neuronal stump.

Chemotropism in Fungi

Fungal chemotropism was first reported over 100 years ago by Anton de Bary. One example of fungi using chemotropism is seen in Yeast.

Yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...

release chemical

pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...

s in order to attract mates. Each haploid yeast cells express specific haploid genes; haploid α-cells express α-genes and haploid a-cells express a-genes. Each cell type releases a unique pheromone: a- or α-factor. By secreting these factors a chemical gradient is formed that attracts the other type of yeast cell during mating. In order for the yeast to sense the gradient they have to have proper receptors that bind a- or α-factor: Ste3 and Ste2 respectively. The receptors to detect the pheromones are seven transmembrane G-protein coupled receptor (

GPCR G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...

). Once activated, there is a signaling cascade that results in the activation of transcription factors for mating-specific genes such as those involved in cell cycle arrest, directional polarization towards the chemical gradient, and sexual hyphae formation. When fungal hyphae touch, the cells merge to form a diploid zygote. Mating pairs of budding yeast cells may polarize away from each other, but are able to adjust the location of polarity to enable successful alignment and fusion.

References

{{Tropism Tropism Pollination

Chemotropism in plants

Chemotropism in animals

Chemotropism in Fungi

See also

References