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Cytokinin
 
 
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Cytokinins (CK) are a class of plant growth substances (plant
Plant

Plants are Life organisms belonging to the Kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae....
 hormone
Hormone

Hormones are chemicals released by cells that affect cells in other parts of the body. Only a small amount of hormone is required to alter cell metabolism....
s) that promote cell division
Cell division

Cell division is a process by which a cell , called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle....
. They are primarily involved in cell
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
 growth, 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 single zygote to a complex system of Tissue and cell types....
, and other physiological
Physiology

Physiology is the study of the mechanical, physical, and biochemical functions of living organisms. Physiology has traditionally been divided between plant physiology and animal and all living things physiology but the principles of physiology are universal, no matter what particular organism is being studied....
 processes. Their effects were first discovered through the use of coconut milk
Coconut milk

Coconut milk is a sweet, milky white cooking base derived from the meat of a mature coconut. The color and rich taste of the milk can be attributed to the high coconut oil content and sugars....
 in the 1940s by Adicott at the University of Wisconsin-Madison 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 Carlos Miller and Folke K....
, 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 which elicits plant growth and development responses, setting blossoms and stimulating fruit richness by stimulating cell division....
, as well as phenylurea-type cytokinins like diphenylurea or thidiazuron (TDZ).






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Cytokinins (CK) are a class of plant growth substances (plant
Plant

Plants are Life organisms belonging to the Kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae....
 hormone
Hormone

Hormones are chemicals released by cells that affect cells in other parts of the body. Only a small amount of hormone is required to alter cell metabolism....
s) that promote cell division
Cell division

Cell division is a process by which a cell , called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle....
. They are primarily involved in cell
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
 growth, 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 single zygote to a complex system of Tissue and cell types....
, and other physiological
Physiology

Physiology is the study of the mechanical, physical, and biochemical functions of living organisms. Physiology has traditionally been divided between plant physiology and animal and all living things physiology but the principles of physiology are universal, no matter what particular organism is being studied....
 processes. Their effects were first discovered through the use of coconut milk
Coconut milk

Coconut milk is a sweet, milky white cooking base derived from the meat of a mature coconut. The color and rich taste of the milk can be attributed to the high coconut oil content and sugars....
 in the 1940s by Adicott at the University of Wisconsin-Madison 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 Carlos Miller and Folke K....
, 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 which elicits plant growth and development responses, setting blossoms and stimulating fruit richness by stimulating cell division....
, as well as phenylurea-type cytokinins like diphenylurea or thidiazuron (TDZ). The adenine-type cytokinins are synthesised in stems, leaves and roots, which is the major site. Cambium
Cambium

In botany the cambium is a layer or layers of tissue, also known as meristems, that are the source of cells for secondary growth. There are two types of cambium...
 and possibly 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.

Mode of Action


Cytokinins are involved in many plant processes, including cell division, shoot and root morphogenesis, chloroplast
Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryote organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve Thermodynamic free energy in the form of Adenosine triphosphate and reduce NADP to NADPH through a complex set of processes called photosynthesis....
 maturation, cell enlargement, auxiliary bud release and senescence
Senescence

Senescence encompasses all of the biological processes of a living organism's approaching an advanced age . The word senescence is derived from the Latin word senex, meaning "old man" or "old age" or "advanced in age"....
. The ratio of auxin
Auxin

Auxins are a class of plant growth substance . Auxins play an essential role in coordination of many growth and behavioral processes in the plant life cycle, they and the behavior they played in plant growth was first revealed by a Dutch scientist named Fritz Went ....
 to cytokinin is crucial during cell division and the differentiation of plant tissues.

While cytokinin action in vascular plants
Vascular plant

Vascular plants are those plants that have lignin tissue for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the ferns, clubmosses, flowering plants, conifers and other gymnosperms....
 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....
 protonema
Protonema

A protonema is a thread-like chain of cells that forms the earliest stage of a bryophyte life cycle. When a moss or Marchantiophyta first grows from the spore, it grows as a protonema which develops into a leafy gametophore....
. 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 single zygote to a complex system of Tissue and cell types....
 of a specific single cell, and thus is a very specific effect of cytokinin.

Biosynthesis


Adenosine phosphate-isopentenyltransferase (IPT) catalyses the first reaction in the biosynthesis
Biosynthesis

Biosynthesis is a phenomenon wherein chemical compounds are produced from simpler reagents. Biosynthesis, unlike chemosynthesis, takes place within living organisms and is generally catalyst by enzymes....
 of isoprene cytokinins. It may use ATP
Adenosine triphosphate

This article is about the chemical used by cells as an energy carrier. For other uses, see ATP .Adenosine-5'-triphosphate is a multifunctional nucleotide, and plays an important role in cell biology as a coenzyme that is the "molecule unit of currency" of intracellular energy transfer....
, ADP
Adenosine diphosphate

Adenosine diphosphate, abbreviated ADP, is a nucleotide. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate Functional 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 found in RNA. It is an ester of phosphoric acid and the nucleoside adenosine....
 as substrate
Substrate (biochemistry)

In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalysis chemical reactions involving the substrate. The substrate binds with the enzyme active site, and an enzyme-substrate complex is formed....
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 anchored protein 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 reaction 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 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 ?-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 of these adenines is carried out by tRNA-isopentenyltransferase.

Auxin is known to regulate the biosynthesis of cytokinin.

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