Callus (cell biology)
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Plant callus (plural ''calluses'' or ''calli'') is a growing
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
of unorganized plant parenchyma 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'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology ...
(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 Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pa ...
, such as auxin,
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 growth and differentiation, but also affect apical dominance, axillary bud growth, and le ...
, and
gibberellin Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. GAs are one of the longest-known classes of p ...
, 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 excre ...
s and micronutrients 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 Murashige and Skoog medium (or ''MSO ''or '' MS0'' ''(MS-zero)'') is a plant growth medium used in the laboratories for cultivation of plant cell culture. MS0 was invented by plant scientists Toshio Murashige and Folke K. Skoog in 1962 during Mura ...
, White's medium, and woody plant medium. Vitamins are also provided to enhance growth such as Gamborg B5 vitamins. For plant cells, enrichment with
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
,
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
, and
potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...
is especially important. Plant callus is usually derived from somatic tissues. The tissues used to initiate callus formation depends on 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 extracti ...
. The cells that give rise to callus and somatic embryos usually undergo rapid division or are partially undifferentiated such as
meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
atic tissue. In alfalfa, ''
Medicago truncatula ''Medicago truncatula'', the barrelclover, strong-spined medick, barrel medic, or barrel medick, is a small annual legume native to the Mediterranean region that is used in genomic research. It is a low-growing, clover-like plant tall with trifol ...
'', however callus and somatic embryos are derived from
mesophyll A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, s ...
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 a cell potency, meaning that a ...
. Plant hormones 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 tissues then undergo further cell growth and differentiation, forming the respective organ primordia. The fully developed organs can then be used for the regeneration of the new mature plants.


Morphology

Specific auxin 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 growth and differentiation, but also affect apical dominance, axillary bud growth, and le ...
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 calluses are typically green and sturdy, while friable calluses 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'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...
, 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 where the cells will form an entirely new plant.


Callus cell death

Callus can brown and die during culture, mainly due to oxidation of phenolic compounds. In ''
Jatropha curcas ''Jatropha curcas'' is a species of flowering plant in the spurge family, Euphorbiaceae, that is native to the American tropics, most likely Mexico and Central America. It is originally native to the tropical areas of the Americas from Mexico t ...
'' callus cells, small organized callus cells became disorganized and varied in size after browning occurred. Browning has also been associated with oxidation and
phenolic Phenolic is an adjective and a substantive (noun) that may apply to : * Phenol (or carbolic acid), a colorless crystalline solid and aromatic compound * Phenols, a class of chemical compounds that include phenol * Phenolic content in wine * Phenol ...
compounds in both explant tissues and explant secretions. In rice, presumably, a condition which is favorable for scutellar callus induction induces necrosis too.


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 An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs when ...
, while the other half undergoes a similar but non-active treatment as the control group. 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 Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta. According to Latin terms, Pluripotentia means the ability for many thin ...
. A classical experiment by
Folke Skoog Folke Karl Skoog (July 15, 1908 – February 15, 2001) was a Swedish-born American plant physiologist who was a pioneer in the field of plant growth regulators, particularly cytokinins. Skoog was a recipient of the National Medal of Science 19 ...
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 induces the formation of roots – with higher auxin to cytokinin ratio, the rooting (rhizogenesis) is induced, applying equal amounts of both hormones stimulates further callus growth and increasing 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 A genetically modified organism (GMO) is any organism whose genetic material has been altered using genetic engineering techniques. The exact definition of a genetically modified organism and what constitutes genetic engineering varies, with ...
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 Cilici ...
. In general, chimeras are not useful for genetic research or agricultural applications. Genes can be inserted into callus cells using
biolistic bombardment 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-proje ...
, 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-proje ...
, or '' Agrobacterium tumefaciens''. Cells that receive the gene of interest can then be recovered into whole plants using a combination of
plant hormone Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pa ...
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 th ...
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-D) was superior to 1-Naphthaleneacetic acid (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 ...
(IBA) but not IAA.


History

Henri-Louis Duhamel du Monceau Henri-Louis Duhamel du Monceau (20 July 1700, Paris13 August 1782, Paris), 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 Denai ...
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 Embryo rescue is one of the earliest and successful forms of in-vitro culture techniques that is used to assist in the development of plant embryos that might not survive to become viable plants. Embryo rescue plays an important role in modern plan ...
*
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) *
Hyperhydricity Hyperhydricity (previously known as vitrification) is a physiological malformation that results in excessive hydration, low lignification, impaired stomatal function and reduced mechanical strength of tissue culture-generated plants. The conseque ...


References

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