cytokinins
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Cytokinins (CK) are a class 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 that promote
cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
, or
cytokinesis Cytokinesis () is the part of the cell division process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and meios ...
, in plant roots and shoots. They are involved primarily in
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
growth and differentiation, but also affect apical dominance,
axillary bud The axillary bud (or lateral bud) is an embryonic or organogenic shoot located in the axil of a leaf. Each bud has the potential to form shoots, and may be specialized in producing either vegetative shoots (stems and branches) or reproductive sh ...
growth, and leaf
senescence Senescence () or biological aging is the gradual deterioration of functional characteristics in living organisms. The word ''senescence'' can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence inv ...
. There are two types of cytokinins: adenine-type cytokinins represented by
kinetin Kinetin (/'kaɪnɪtɪn/) is a type of cytokinin, a class of plant hormone that promotes cell division. Kinetin was originally isolated by Carlos Miller and Skoog ''et al.'' as a compound from autoclaved herring sperm DNA that had cell division- ...
,
zeatin Zeatin is a cytokinin derived from adenine, which occurs in the form of a ''cis''- and a ''trans''- isomer and conjugates. Zeatin was discovered in immature corn kernels from the genus '' Zea''. It promotes growth of lateral buds and when sprayed ...
, and 6-benzylaminopurine, and phenylurea-type cytokinins like
diphenylurea 1,3-Diphenylurea is a phenylurea-type compound with the formula (PhNH)2CO (Ph = C6H5). It is a colorless solid that is prepared by transamidation of urea with aniline. DPU is a cytokinin Cytokinins (CK) are a class of plant hormones that pr ...
and thidiazuron (TDZ). Most adenine-type cytokinins are synthesized in roots.
Cambium A cambium (plural cambia or cambiums), in plants, is a tissue layer that provides partially undifferentiated cells for plant growth. It is found in the area between xylem and phloem. A cambium can also be defined as a cellular plant tissue from w ...
and other actively dividing tissues also synthesize cytokinins. No phenylurea cytokinins have been found in plants. Cytokinins participate in local and long-distance signalling, with the same transport mechanism as purines and nucleosides. Typically, cytokinins are transported in the
xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
. Cytokinins act in concert with
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 essenti ...
, another plant growth hormone. The two are complementary, having generally opposite effects.


History

The idea of specific substances required for cell division to occur in plants actually dates back to the German physiologist J. Wiesner, who, in 1892, proposed that initiation of cell division is evoked by endogenous factors, indeed a proper balance among endogenous factors. Somewhat later, the Austrian plant physiologist, G. Haberlandt, reported in 1913 that an unknown substance diffuses from the phloem tissue which can induce cell division in the parenchymatic tissue of potato tubers. In 1941,
Johannes Van Overbeek Johannes is a Medieval Latin form of the personal name that usually appears as "John" in English language contexts. It is a variant of the Greek and Classical Latin variants (Ιωάννης, '' Ioannes''), itself derived from the Hebrew name '' Ye ...
found that the milky
endosperm The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the embryo and ...
of immature
coconut The coconut tree (''Cocos nucifera'') is a member of the palm tree family ( Arecaceae) and the only living species of the genus ''Cocos''. The term "coconut" (or the archaic "cocoanut") can refer to the whole coconut palm, the seed, or the ...
also had this factor, which stimulated cell division and differentiation in very young
Datura ''Datura'' is a genus of nine species of highly poisonous, vespertine-flowering plants belonging to the nightshade family Solanaceae. They are commonly known as thornapples or jimsonweeds, but are also known as devil's trumpets (not to be conf ...
embryos. Jablonski and Skoog (1954) extended the work of Haberlandt and reported that a substance present in the vascular tissue was responsible for causing cell division in the pith cells.
Miller A miller is a person who operates a Gristmill, mill, a machine to grind a grain (for example corn or wheat) to make flour. Mill (grinding), Milling is among the oldest of human occupations. "Miller", "Milne" and other variants are common surname ...
and his co-workers (1954) isolated and purified the cell division substance in crystallised form from autoclaved herring fish sperm DNA. This active compound was named as
Kinetin Kinetin (/'kaɪnɪtɪn/) is a type of cytokinin, a class of plant hormone that promotes cell division. Kinetin was originally isolated by Carlos Miller and Skoog ''et al.'' as a compound from autoclaved herring sperm DNA that had cell division- ...
because of its ability to promote cell division and was the first cytokinin to be named. Kinetin was later identified to be 6-furfuryl-amino purine. Later on, the generic name kinin was suggested to include kinetin and other substances having similar properties. The first naturally occurring cytokinin was isolated and crystallised simultaneously by Miller and D.S. Lethum (1963-65) from the milky endosperm of
corn 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 ...
(''Zea mays'') and named Zeatin. Lethem ((1963) proposed the term Cytokinins for such substances.


Function

Cytokinins are involved in many plant processes, including cell division and shoot and root morphogenesis. They are known to regulate axillary bud growth and apical dominance. According to the "direct inhibition hypothesis", these effects result from the ratio of cytokinin to auxin. This theory states that auxin from apical buds travels down shoots to inhibit axillary bud growth. This promotes shoot growth, and restricts lateral branching. Cytokinin moves from the roots into the shoots, eventually signaling lateral bud growth. Simple experiments support this theory. When the apical bud is removed, the axillary buds are uninhibited, lateral growth increases, and plants become bushier. Applying auxin to the cut stem again inhibits lateral dominance. Moreover, it has been shown that cytokinin alone has no effect on
parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word π ...
cells. When cultured with auxin but no cytokinin, they grow large but do not divide. When cytokinin and auxin are both added together, the cells expand and differentiate. When cytokinin and auxin are present in equal levels, the parenchyma cells form an undifferentiated
callus A callus is an area of thickened and sometimes hardened skin that forms as a response to repeated friction, pressure, or other irritation. Since repeated contact is required, calluses are most often found on the feet and hands, but they may o ...
. A higher ratio of cytokinin induces growth of
shoot In botany, a plant shoot consists of any plant stem together with its appendages, leaves and lateral buds, flowering stems, and flower buds. The new growth from seed germination that grows upward is a shoot where leaves will develop. In the spri ...
buds, while a higher ratio of auxin induces
root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...
formation. Cytokinins have been shown to slow aging of plant organs by preventing
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
breakdown, activating protein synthesis, and assembling
nutrients 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 ...
from nearby tissues. A study that regulated leaf senescence in tobacco leaves found that wild-type leaves yellowed while
transgenic A transgene is a gene that has been transferred naturally, or by any of a number of genetic engineering techniques, from one organism to another. The introduction of a transgene, in a process known as transgenesis, has the potential to change the ...
leaves remained mostly green. It was hypothesized that cytokinin may affect enzymes that regulate protein synthesis and degradation. Cytokinins have recently been found to play a role in plant pathogenesis. For example, cytokinins have been described to induce resistance against ''
Pseudomonas syringae ''Pseudomonas syringae'' is a rod-shaped, Gram-negative bacterium with polar flagella. As a plant pathogen, it can infect a wide range of species, and exists as over 50 different pathovars, all of which are available to researchers from internat ...
'' in ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...
'' and ''
Nicotiana tabacum ''Nicotiana tabacum'', or cultivated tobacco, is an annually grown herbaceous plant of the ''Nicotiana'' genus. The plant is tropical in origin, is commonly grown throughout the world, and is often found in cultivation. It grows to heights bet ...
''. Also in context of
biological control Biological control or biocontrol is a method of controlling pests, such as insects, mites, weeds, and plant diseases, using other organisms. It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also invo ...
of plant diseases cytokinins seem to have potential functions. Production of cytokinins by ''
Pseudomonas fluorescens ''Pseudomonas fluorescens'' is a common Gram-negative, rod-shaped bacterium. It belongs to the ''Pseudomonas'' genus; 16S rRNA analysis as well as phylogenomic analysis has placed ''P. fluorescens'' in the ''P. fluorescens'' group within the genu ...
'' G20-18 has been identified as a key determinant to efficiently control the infection of ''A. thaliana'' with ''P. syringae.''. While cytokinin action in
vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They al ...
s is described as
pleiotropic Pleiotropy (from Greek , 'more', and , 'way') occurs when one gene influences two or more seemingly unrelated phenotypic traits. Such a gene that exhibits multiple phenotypic expression is called a pleiotropic gene. Mutation in a pleiotropic ge ...
, this class of plant hormones specifically induces the transition from apical growth to growth via a three-faced apical cell in
moss Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) '' sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hor ...
protonema A protonema (plural: protonemata) is a thread-like chain of cells that forms the earliest stage of development of the gametophyte (the haploid phase) in the life cycle of mosses. When a moss first grows from a spore, it starts as a ''germ tube'', ...
. This bud induction can be pinpointed to differentiation of a specific single cell, and thus is a very specific effect of cytokinin.


Mode of action

Cytokinin signaling in plants is mediated by a two-component phosphorelay. This pathway is initiated by cytokinin binding to a
histidine kinase Histidine kinases (HK) are multifunctional, and in non-animal kingdoms, typically transmembrane, proteins of the transferase class of enzymes that play a role in signal transduction across the cellular membrane. The vast majority of HKs are homod ...
receptor in the
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
membrane. This results in the
autophosphorylation Autophosphorylation is a type of post-translational modification of proteins. It is generally defined as the phosphorylation of the kinase by itself. In eukaryotes, this process occurs by the addition of a phosphate group to serine, threoni ...
of the receptor, with the phosphate then being transferred to a phosphotransfer protein. The phosphotransfer proteins can then phosphorylate the type-B response regulators (RR) which are a family of transcriptions factors. The phosphorylated, and thus activated, type-B RRs regulate the transcription of numerous genes, including the type-A ''RR''s. The type-A RRs negatively regulate the pathway.


Biosynthesis

Adenosine phosphate-isopentenyltransferase (IPT) catalyses the first reaction in the
biosynthesis Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules. ...
of
isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. Isoprene is an unsaturated hydrocarbon. It is produced by many plants and animals ...
cytokinins. It may use ATP, ADP, or
AMP #REDIRECT Amp {{Redirect category shell, {{R from other capitalisation{{R from ambiguous page ...
as substrates and may use
dimethylallyl pyrophosphate Dimethylallyl pyrophosphate (DMAPP; or alternatively, dimethylallyl diphosphate (DMADP); also isoprenyl pyrophosphate) is an isoprenoid precursor. It is a product of both the mevalonate pathway and the MEP pathway of isoprenoid precursor biosynt ...
(DMAPP) or hydroxymethylbutenyl pyrophosphate (HMBPP) as prenyl donors. This reaction is the
rate-limiting step In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate-determining step (RDS or RD-step or r/d step) or rate-limiting step. For a given reaction mechanism, the prediction of the ...
in cytokinin
biosynthesis Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules. ...
. DMADP and HMBDP used in cytokinin biosynthesis are produced by the methylerythritol phosphate pathway (MEP). Cytokinins can also be produced by recycled
tRNA Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes), that serves as the physical link between the mRNA and the amino ac ...
s in plants and
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 ...
. tRNAs with
anticodon Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes), that serves as the physical link between the mRNA and the amino ac ...
s that start with a
uridine Uridine (symbol U or Urd) is a glycosylated pyrimidine analog containing uracil attached to a ribose ring (or more specifically, a ribofuranose) via a β-N1-glycosidic bond. The analog is one of the five standard nucleosides which make up nuclei ...
and carrying an already-prenylated adenosine adjacent to the anticodon release on degradation the adenosine as a cytokinin. The
prenylation Prenylation (also known as isoprenylation or lipidation) is the addition of hydrophobic molecules to a protein or a biomolecule. It is usually assumed that prenyl groups (3-methylbut-2-en-1-yl) facilitate attachment to cell membranes, similar to ...
of these adenines is carried out by tRNA-isopentenyltransferase.
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 essenti ...
is known to regulate the
biosynthesis Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules. ...
of cytokinin.


Uses

Because cytokinins promote plant
cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
and growth, they have been studied since the 1970s as potential
agrochemical An agrochemical or agrichemical, a contraction of ''agricultural chemical'', is a chemical product used in industrial agriculture. Agrichemical refers to biocides ( pesticides including insecticides, herbicides, fungicides and nematicides) an ...
s, however they have yet to be widely adopted, probably due to the complex nature of their effects. One study found that applying cytokinin to
cotton Cotton is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus ''Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose, and can contain minor perce ...
seedling A seedling is a young sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle (embryonic root), the hypocotyl (embry ...
s led to a 5–10% increase in yield under drought conditions. Some cytokinins are utilized in
tissue culture Tissue culture is the growth of tissues or cells in an artificial medium separate from the parent organism. This technique is also called micropropagation. This is typically facilitated via use of a liquid, semi-solid, or solid growth medium, su ...
of plants and can also be used to promote the
germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...
of
seed A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
s.


References


External links


Agrares Fertilizer with cytokinins
*


Regulation of Leaf Senescence by Cytokinin, Sugar, and Light
{{Plant_hormones Plant hormones Aging-related substances in plants