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Plant callus (plural ''calluses'' or ''calli'') is a growing mass of unorganized plant
parenchyma upright=1.6, Lung parenchyma showing damage due to large subpleural bullae. Parenchyma () is the bulk of functional substance in an animal organ such as the brain or lungs, or a structure such as a tumour. In zoology, it is the tissue that ...
cells. In living plants, callus cells are those cells that cover a plant wound. In biological research and biotechnology callus formation is induced from plant tissue samples (explants) after surface sterilization and plating onto tissue culture medium
in vitro ''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...
(in a closed culture vessel such as a
Petri dish A Petri dish (alternatively known as a Petri plate or cell-culture dish) is a shallow transparent lidded dish that biologists use to hold growth medium in which cells can be cultured,R. C. Dubey (2014): ''A Textbook Of Biotechnology For Class- ...
). The culture medium is supplemented with plant growth regulators, such as
auxin Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essent ...
,
cytokinin Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in Cell (biology), cell growth and cellular differentiation, differentiation, but also affect apical ...
, and
gibberellin Gibberellins (GAs) are plant hormones that regulate various Biological process, developmental processes, including Plant stem, stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. They are one of th ...
, to initiate callus formation or
somatic embryogenesis Somatic embryogenesis is an artificial process in which a plant or embryo is derived from a single somatic cell. Somatic embryos are formed from plant cells that are not normally involved in the development of embryos, i.e. ordinary plant tissue. N ...
. Callus initiation has been described for all major groups of land plants.


Callus induction and tissue culture

Plant species representing all major land plant groups have been shown to be capable of producing callus in tissue culture. A callus cell culture is usually sustained on gel medium. Callus induction medium consists of agar and a mixture of
macronutrient A nutrient is a substance used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
s and
micronutrient Micronutrients are essential chemicals required by organisms in small quantities to perform various biogeochemical processes and regulate physiological functions of cells and organs. By enabling these processes, micronutrients support the heal ...
s for the given cell type. There are several types of basal salt mixtures used in plant tissue culture, but most notably modified Murashige and Skoog medium, White's medium, and woody plant medium. Vitamins, such as Gamborg B5 vitamins, are also provided to enhance growth. For plant cells, enrichment with
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
,
phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
, and
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
is especially important. Plant callus is usually derived from
somatic Somatic may refer to: * Somatic (biology), referring to the cells of the body in contrast to the germ line cells ** Somatic cell, a non-gametic cell in a multicellular organism * Somatic nervous system, the portion of the vertebrate nervous syst ...
tissues. The tissues used to initiate callus formation depends on the plant species and which tissues are available for
explant culture In biology, explant culture is a technique to organotypically culture cells from a piece or pieces of tissue or organ removed from a plant or animal. The term ''explant'' can be applied to samples obtained from any part of the organism. The extract ...
. The cells that give rise to callus and somatic embryos usually undergo rapid division or are partially undifferentiated such as
meristem In cell biology, the meristem is a structure composed of specialized tissue found in plants, consisting of stem cells, known as meristematic cells, which are undifferentiated cells capable of continuous cellular division. These meristematic c ...
atic tissue. In alfalfa ('' Medicago truncatula''), however, callus and somatic embryos are derived from mesophyll cells that undergo
dedifferentiation Dedifferentiation (pronounced dē-ˌdi-fə-ˌren-chē-ˈā-shən) is a transient process by which cells become less specialized and return to an earlier cell state within the same lineage. This suggests an increase in cell potency, meaning that, f ...
.
Plant hormones Plant hormones (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis, the regulation of Organ (anat ...
are used to initiate callus growth. After the callus has formed, the concentration of hormones in the medium may be altered to shift the development from callus to root formation, shoot growth, or somatic embryogenesis. The callus tissue then undergoes further cell growth and differentiation, forming the respective organ primordia. The fully developed organs can then be used for the regeneration of new mature plants.


Morphology

Specific
auxin Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essent ...
-to-
cytokinin Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in Cell (biology), cell growth and cellular differentiation, differentiation, but also affect apical ...
ratios in plant tissue culture medium give rise to an unorganized growing and dividing mass of callus cells. Callus cultures are often broadly classified as being either compact or friable. Compact calli are typically green and sturdy, while friable calli are white to creamy yellow in color, fall apart easily, and can be used to generate cell suspension cultures and somatic embryos. In
maize Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout grass that produces cereal grain. It was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. Native American ...
, these two callus types are designated as type I (compact) and type II (friable). Callus can directly undergo direct
organogenesis Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation (the ectoderm, endoderm, and mesoderm) form the internal org ...
and/or
embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...
where the cells will form an entirely new plant.


Callus cell death

Callus can brown and die during culture, mainly due to the oxidation of phenolic compounds. In '' Jatropha curcas'' callus cells, small organized callus cells became disorganized and varied in size after browning occurred. Browning has also been associated with oxidation and phenolic compounds in both explant tissues and explant secretions. In rice, presumably, a condition which is favorable for scutellar callus induction also induces necrosis.


Uses

Callus cells are not necessarily genetically homogeneous because a callus is often made from structural tissue, not individual cells. Nevertheless, callus cells are often considered similar enough for standard scientific analysis to be performed as if on a single subject. For example, an experiment may have half a callus undergo a treatment as the experimental group, while the other half undergoes a similar but non-active treatment as the
control group In the design of experiments, hypotheses are applied to experimental units in a treatment group. In comparative experiments, members of a control group receive a standard treatment, a placebo, or no treatment at all. There may be more than one tr ...
. Plant calluses derived from many different cell types can differentiate into a whole plant, a process called regeneration, through addition of plant hormones to the culture medium. This ability is known as
totipotency Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potential within a cell, which like a continuum ...
. A classical experiment by Folke Skoog and Carlos O. Miller on tobacco pith used as the starting explant shows that the supplementation of culture media by different ratios of auxin to cytokinin concentration can induce the formation of either shoots or roots – with higher auxin to cytokinin ratio, the rooting (rhizogenesis) is induced, applying equal amounts of both hormones stimulates further callus growth and changing the auxin to cytokinin ratio in favor of the cytokinin leads to the development of shoots. Regeneration of a whole plant from a single cell allows transgenics researchers to obtain whole plants which have a copy of the transgene in every cell. Regeneration of a whole plant that has some genetically transformed cells and some untransformed cells yields a
chimera Chimera, Chimaera, or Chimaira (Greek for " she-goat") originally referred to: * Chimera (mythology), a fire-breathing monster of ancient Lycia said to combine parts from multiple animals * Mount Chimaera, a fire-spewing region of Lycia or Cilicia ...
. In general, chimeras are not useful for genetic research or agricultural applications. Genes can be inserted into callus cells using biolistic bombardment, also known as a
gene gun In genetic engineering, a gene gun or biolistic particle delivery system is a device used to deliver exogenous DNA (transgenes), RNA, or protein to cells. By coating particles of a heavy metal with a gene of interest and firing these micro-projec ...
, or ''
Agrobacterium tumefaciens ''Agrobacterium tumefaciens'' (also known as ''Rhizobium radiobacter'') is the causal agent of crown gall disease (the formation of tumours) in over 140 species of eudicots. It is a rod-shaped, Gram-negative soil bacterium. Symptoms are cause ...
''. Cells that receive the gene of interest can then be recovered into whole plants using a combination of
plant hormone Plant hormones (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis, the regulation of Organ (anat ...
s. The whole plants that are recovered can be used to experimentally determine gene function(s), or to enhance crop plant traits for modern agriculture. Callus is of particular use in
micropropagation Micropropagation or tissue culture is the practice of rapidly multiplying plant stock material to produce many progeny plants, using modern plant tissue culture methods. Micropropagation is used to multiply a wide variety of plants, such as ...
where it can be used to grow genetically identical copies of plants with desirable characteristics. To increase the yield, efficiency and explant survivability of micropropagation, a thorough care is taken for the optimization of the micropropagation protocol. For example, using explants composed of low totipotency cells may prolong the time necessary to obtain callus of sufficient size, increasing the total length of the experiment. Similarly, various plant species and explant types require specific plant hormones for callus induction and subsequent organogenesis or embryogenesis – for the formation and growth of maize calluses, auxin
2,4-Dichlorophenoxyacetic acid 2,4-Dichlorophenoxyacetic acid is an organic compound with the chemical formula . It is usually referred to by its ISO common name 2,4-D. It is a systemic herbicide that kills most broadleaf weeds by causing uncontrolled growth, but most gra ...
(2,4-D) was superior to
1-Naphthaleneacetic acid 1-Naphthaleneacetic acid (NAA) is an organic compound with the formula C10H7CH2CO2H. This colorless solid is soluble in organic solvents. It features a carboxylmethyl group (CH2CO2H) linked to the "1-position" of naphthalene. Use and regulation N ...
(NAA) or
Indole-3-acetic acid Indole-3-acetic acid (IAA, 3-IAA) is the most common naturally occurring plant hormone of the auxin class. It is the best known of the auxins, and has been the subject of extensive studies by plant physiologists. IAA is a derivative of indole, con ...
(IAA), while the development of callus was hindered in prune explants after applying auxin
Indole-3-butyric acid Indole-3-butyric acid (1''H''-indole-3-butanoic acid, IBA) is a white to light-yellow crystalline solid, with the molecular formula C12H13NO2. It melts at 125°C in atmospheric pressure and decomposes before boiling. IBA is a plant hormone in th ...
(IBA) but not IAA.


History

Henri-Louis Duhamel du Monceau Henri-Louis Duhamel du Monceau (; 20 July 170013 August 1782) was a French physician, naval engineer and botanist. Biography Henri-Louis Duhamel du Monceau was born in Paris in 1700, the son of Alexandre Duhamel, lord of Denainvilliers. I ...
investigated wound-healing responses in elm trees, and was the first to report formation of callus on live plants. In 1908, E. F. Simon was able to induce callus from poplar stems that also produced roots and buds. The first reports of callus induction ''in vitro'' came from three independent researchers in 1939. P. White induced callus derived from tumor-developing procambial tissues of hybrid ''
Nicotiana glauca ''Nicotiana glauca'' is a species of flowering plant in the tobacco genus Nicotiana of the nightshade family Solanaceae. It is known by the common name tree tobacco. Its leaves are attached to the stalk by petioles (many other ''Nicotiana'' spec ...
'' that did not require hormone supplementation. Gautheret and Nobecourt were able to maintain callus cultures of carrot using auxin hormone additions.


See also

* Embryo rescue *
Somatic embryogenesis Somatic embryogenesis is an artificial process in which a plant or embryo is derived from a single somatic cell. Somatic embryos are formed from plant cells that are not normally involved in the development of embryos, i.e. ordinary plant tissue. N ...
*
Chimera (genetics) A genetic chimerism or chimera ( or ) is a single organism composed of cells of different genotype, genotypes. Animal chimeras can be produced by the fusion of two (or more) embryos. In plants and some animal chimeras, Mosaic (genetics), mos ...
* Hyperhydricity


References

{{DEFAULTSORT:Callus (Cell Biology) Cell culture Biotechnology Cell biology