Cytokinin
Encyclopedia
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
, or cytokinesis
Cytokinesis
Cytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
, in plant roots and shoots. They are involved primarily in cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
growth and differentiation
Cellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
, but also affect apical dominance
Apical dominance
In plant physiology, apical dominance is the phenomenon whereby the main central stem of the plant is dominant over other side stems; on a branch the main stem of the branch is further dominant over its own side branchlets....
, axillary bud
Axillary bud
The axillary bud is an embryonic shoot which lies at the junction of the stem and petiole of a plant.As the apical meristem grows and forms leaves, a region of meristematic cells are left behind at the node between the stem and the leaf. These axillary buds are usually dormant, inibited by auxin...
growth, and leaf senescence
Senescence
Senescence or biological aging is the change in the biology of an organism as it ages after its maturity. Such changes range from those affecting its cells and their function to those affecting the whole organism...
. Their effects were first discovered through the use of coconut milk
Coconut milk
Coconut milk is the water that comes from the grated meat of a coconut. The colour and rich taste of the milk can be attributed to the high oil content. In many parts of the world, the term coconut milk is also used to refer to coconut water, the naturally occurring liquid found inside the hollow...
in the 1940s by a scientist at the University of Wisconsin–Madison
University of Wisconsin–Madison
The University of Wisconsin–Madison is a public research university located in Madison, Wisconsin, United States. Founded in 1848, UW–Madison is the flagship campus of the University of Wisconsin System. It became a land-grant institution in 1866...
named Folke Skoog.
There are two types of cytokinins: adenine-type cytokinins represented by kinetin
Kinetin
Kinetin is a kind of cytokinin, a class of plant hormone that promotes cell division. Kinetin was originally isolated by Miller and Skoog et al. as a compound from autoclaved herring sperm DNA that had cell division-promoting activity. It was given the name kinetin because of its ability to induce...
, zeatin
Zeatin
Zeatin is a plant hormone derived from the purine adenine. It is a member of the plant growth hormone family known as cytokinins. Zeatin was first discovered in immature corn kernels from the genus Zea...
, and 6-benzylaminopurine
6-Benzylaminopurine
6-Benzylaminopurine, benzyl adenine or BAP is a first-generation synthetic cytokinin that elicits plant growth and development responses, setting blossoms and stimulating fruit richness by stimulating cell division...
, and phenylurea-type cytokinins like diphenylurea and thidiazuron (TDZ). The majority of adenine-type cytokinins are synthesized in the roots. Cambium
Cambium (botany)
A cambium , in botany, is a tissue layer that provide undifferentiated cells for plant growth. It forms parallel rows of cells, which result in secondary tissues....
and other actively dividing tissues are also sites of cytokinin biosynthesis. There is no evidence that the phenylurea cytokinins occur naturally in plant tissues. Cytokinins are involved in both local and long distance signalling, the latter of which involves the same in planta transport mechanism as used for transport of purines and nucleosides. Typically, cytokinins are transported within the xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
.
Cytokinins act in concert with auxin
Auxin
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...
, another plant hormone stimulating cell expansion.
Mode of Action
The ratio of auxin to cytokinin plays an important role in the effect of cytokinin on plant growth. ParenchymaParenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...
tissue cultured with auxin and without cytokinin have cells that grow large but do not divide. When cytokinin is added along with auxin, the cells both expand and differentiate. However, when the plant cells are cultured with only cytokinin, there is no effect. When cytokinin and auxin are present in equal levels, the parenchyma cells form a callus
Callus (cell biology)
Plant callus is a mass of undifferentiated cells derived from plant tissue for use in biological research and biotechnology. In plant biology, callus cells are those cells that cover a plant wound. To induce callus formation, plant tissues are surface sterilized and then plated onto in vitro...
, or undifferentiated mass of cells. An increase in cytokinin will lead to the growth of shoot
Shoot
Shoots are new plant growth, they can include stems, flowering stems with flower buds, and leaves. The new growth from seed germination that grows upward is a shoot where leaves will develop...
buds, while an increase in auxin induces root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
formation.
Cytokinins are involved in many plant processes, including cell division and shoot and root morphogenesis. In particular, they are known to regulate axillary bud growth as well as affect apical dominance. These effects are a result of the cytokinin to auxin ratio, and termed the direct inhibition hypothesis. This theory states that the auxin, originating in the apical bud, travels down shoots to inhibit axillary bud growth. This promotes shoot growth, and restricts lateral branching. During this process, cytokinin moves from the roots and into the shoots, eventually signaling lateral bud growth. Simple experiments agree with this theory. When the apical bud—the major source of auxin—is removed, the axillary buds are liberated from inhibition. This allows the plant increased lateral growth, making the plant bushier. Applying auxin to the cleaved stem again inhibits lateral dominance.
While cytokinin action in vascular plants
Vascular plant
Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...
is described as pleiotropic, this class of plant hormones specifically induces the transition from apical growth to growth via a three-faced apical cell in moss
Moss
Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
protonema
Protonema
A protonema is a thread-like chain of cells that forms the earliest stage of a bryophyte life cycle...
. This
bud induction can be pinpointed to differentiation
Cellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
of a specific single cell, and thus is a very specific effect of cytokinin.
Cytokinins have also been known to slow the aging of plant organs. This process occurs by preventing protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
breakdown, activating protein synthesis, and assembling nutrients from nearby tissues. In a study that regulated leaf senescence in tobacco leaves, it was found that wild-type leaves showed yellowing of leaves, while the transgenic leaves remained mostly green. It was hypothesized that cytokinin may affect enzymes that regulate protein sythesis and degradation.
Biosynthesis
Adenosine phosphate-isopentenyltransferase (IPT) catalyses the first reaction in the biosynthesisBiosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...
of isoprene cytokinins. It may use ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
, ADP
Adenosine diphosphate
Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....
, or AMP
Adenosine monophosphate
Adenosine monophosphate , also known as 5'-adenylic acid, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid and the nucleoside adenosine. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine...
as substrate
Substrate (biochemistry)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...
s and may use dimethylallyl diphosphate (DMAPP) or hydroxymethylbutenyl diphosphate (HMBDP) as prenyl donors
Prenylation
Prenylation, or isoprenylation, or lipidation is the addition of hydrophobic molecules to a protein. It is usually assumed that prenyl groups facilitate attachment to cell membranes, similar to lipid anchor like the GPI anchor, though direct evidence is missing...
. This reaction is the rate-limiting step
Rate-determining step
The rate-determining step is a chemistry term for the slowest step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in. In...
in cytokinin biosynthesis. DMAPP and HMBDP used in cytokinin biosynthesis are produced by the methylerythritol phosphate pathway (MEP).
Cytokinins can also be produced by recycled tRNAs in plants and bacteria. tRNAs with anticodons that start with a uridine
Uridine
Uridine is a molecule that is formed when uracil is attached to a ribose ring via a β-N1-glycosidic bond.If uracil is attached to a deoxyribose ring, it is known as a deoxyuridine....
and carrying an already-prenylated adenosine adjacent to the anticodon release on degradation the adenosine as a cytokinin. The prenylation
Prenylation
Prenylation, or isoprenylation, or lipidation is the addition of hydrophobic molecules to a protein. It is usually assumed that prenyl groups facilitate attachment to cell membranes, similar to lipid anchor like the GPI anchor, though direct evidence is missing...
of these adenines is carried out by tRNA-isopentenyltransferase
TRNA isopentenyltransferase
In enzymology, a tRNA isopentenyltransferase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are isopentenyl diphosphate and tRNA, whereas its two products are diphosphate and tRNA containing 6-isopentenyladenosine....
.
Auxin is known to regulate the biosynthesis of cytokinin.