Overview of Cytokinins (CK)

Overview of Cytokinins (CK)FARA SHAKEELCYTOKININSCytokinins (CK) be a class of typeset yield substances (phytohorm nonpargonils) that promote cell division, or cytokinesis, in gear up grow and call fors. They be involved primarily in cell growth and incompatibleiation, plainly also affect top(prenominal) dominance, axillary bud growth, and leaf senescence.STRUCTURE OF CYTOKININSNATURE OF CYTOKININS Cytokinins are mixeds with a bodily structure resembling group A which promote cell division and consecrate another(prenominal) sympathetic functions to kinetin. Kinetin was the first kinin nonice and so named because of the heterogeneouss ability to promote cytokinesis (cell division). though it is a inhering compound, It is non made in plants, and is therefore normally considered a synthetic cytokinin (meaning that the hormone is synthesized somewhere other than in a plant). The most common form of course croakring cytokinin in plants today is called zeatin whic h was disjunct from give (Zea mays)Cytokinins have been found in about all higher plants as well as mosses, fungi, bacteria, and also in tRNA of many an(prenominal) an(prenominal) prokaryotes and eukaryotes. Today there are more than cc instinctive and synthetic cytokinins combined. Cytokinin concentrations are highest in meristematic regions and areas of continuous growth potential such as roots, young leaves, developing fruits, and seeds (Arteca, 1996 Mauseth, 1991 Raven, 1992 capital of Zimbabwe and Ross, 1992). record OF CYTOKININS In 1913, Gottlieb Haberlandt discovered that a compound found in bast had the ability to stimulate cell division (Haberlandt, 1913). In 1941, Johannes van Overbeek discovered that the milky endosperm from coconut also had this ability. He also showed that various other plant species had compounds which stimulated cell division (van Overbeek, 1941). In 1954, Jablonski and Skoog extended the organise of Haberlandt showing that vascular tissues c ontained compounds which promote cell division (Jablonski and Skoog, 1954). The first cytokinin was isolated from herring sperm in 1955 by miller and his associates (milling machine et al., 1955). This compound was named kinetin because of its ability to promote cytokinesis. Hall and deRopp reported that kinetin could be formed from deoxyribonucleic acid degradation products in 1955 (Hall and deRopp, 1955). The first of course occurring cytokinin was isolated from corn in 1961 by Miller (Miller, 1961). It was later called zeatin. Almost simultaneous with Miller Letham published a report on zeatin as a ingredient inducing cell division and later described its chemical properties (Letham, 1963). It is Miller and Letham that are credited with the simultaneous discovery of zeatin. Since that time, many more naturally occurring cytokinins have been isolated and the compound is ubiquitous to all plant species in one form or another (Arteca, 1996 capital of Zimbabwe and Ross, 1992).BIO SYNTHESIS AND transfiguration Cytokinin is generally found in higher concentrations in meristematic regions and festering tissues. They are believed to be synthesized in the roots and translocated via the xylem to shoots. Cytokinin bio entailment happens by dint of the biochemical modification of adenine. The process by which they are synthesized is as follows (McGaw, 1995 Salisbury and Ross, 1992) A product of the mevalonate pathway called isopentyl pyrophosphate is isomerized. This isomer can then react with adenosine monophosphate with the economic aid of an enzyme called isopentenyl AMP synthase. The result is isopentenyl adenosine-5-phosphate (isopentenyl AMP). This product can then be born-again to isopentenyl adenosine by removal of the phosphate by a phosphatase and further converted to isopentenyl adenine by removal of the ribose group. Isopentenyl adenine can be converted to the three major forms of naturally occurring cytokinins. Other pathways or slight alterations of this one probably lead to the other forms. Degradation of cytokinins occurs largely due to the enzyme cytokinin oxidase. This enzyme removes the side chain and releases adenine. Derivitives can also be made but the pathways are more complex and poorly understood.TRANSPORTvia xylem (transpiration stream)in peas, a signal from the leaves may signal/regulate transport of cytokinins from the rootszeatin ribosides are the main transport form converted to the free base or glucosides in the leavessome cytokinin also moves in the phloem.Mode of fulfilThe ratio of auxin to cytokinin plays an important role in the effect of cytokinin on plant growth. Cytokinin alone has no effect on parenchyma cells. When civilisationd with auxin but no cytokinin, they grow large but do not divide. When cytokinin is added, the cells expand and differentiate. When cytokinin and auxin are present in equal levels, the parenchyma cells form an undifferentiated callus. More cytokinin induces growth of shoot b uds, while more auxin induces root formation.Cytokinins are involved in many plant processes, including cell division and shoot and root morphogenesis. They are know to regulate axillary bud growth and apical dominance. The direct forbidding hypothesis posits that these effects result from the cytokinin to auxin ratio. This theory states that auxin from apical buds travels rout shoots to inhibit axillary bud growth. This promotes shoot growth, and restricts squint 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 take stem again inhibits lateral dominance.While cytokinin action in vascular plants is described as pleiotropic, this class of plant hormones particularizedally induces the variety from apical growth to growth via a three-faced apical cell in moss protonema. This bud induction can be pinpointed to differentiation of a specific single cell, and thus is a very specific effect of cytokinin.Cytokinins have been shown to slow aging of plant organs by preventing protein breakdown, activating protein discount, and bring together nutrients from nearby tissues. A study that regulated leaf senescence in tobacco leaves found that wild-type leaves yellowed while transgenic leaves remained mostly green. It was hypothesized that cytokinin may affect enzymes that regulate protein synthesis and degradation.CHEMISTRYA. Generaladenine derivatives (amino purines)occur as (a) the free nitrogenous base (b) a nucleoside (base + ribose) (c) a alkali (base + ribose + phosphate) or (d) glycosidesThe free base is the active form.approximately 40 different structures known.Zeatin (Z), which was first isolated from maize (Zea mays) is the most common cytokinin.Other naturally occurring cytokinins include, dihydrozeatin (DHZ) and isopentenyla denosine (IPA).B. Synthetic cytokininskinetin probably byproduct of zeatin degradationthere are several(prenominal) other substances with cytokinin activity such as benzyl adenine (benzylaminopurine BA).C. Cytokinins and nucleic acidscan occur as a modified base in tRNA, but the bases exist in the cis form, rather than the typical trans form. These modified bases that are found in all organisms from bacteria to plants to humans.The function of the tRNA cytokinins is not clear, but after hydrolysis of the tRNA the products can act as a cytokinin. The importance of the tRNA derived cytokinins in overall growth and development is not clear, either.Interestingly plants have different sets of tRNAs with different cytokinins that act in protein synthesis in the cytoplasm and the plastids.CYTOKININS FUNCTION A count of some of the known physiological effects caused by cytokinins are listed below. The chemical reaction will vary depending on the type of cytokinin and plant species (Davi es, 1995 Mauseth, 1991 Raven, 1992 Salisbury and Ross, 1992).Stimulates cell division.Stimulates morphogenesis (shoot initiation/bud formation) in tissue culture.Stimulates the growth of lateral buds-release of apical dominance.Stimulates leaf expansion resulting from cell enlargement.May enhance stomatous opening in some species.Promotes the conversion of etioplasts into chloroplasts via stimulation of chlorophyll synthesisROLE OF CYTOKININS IN PLANTSCytokinins are known from cytokinin-overproducing mutants to lift additional leaves and branches on the stem. The stems and leaves produce additional chlorophyll. Wounding often produces a new branch. pitch senescence is delayed. Apical dominance is released. Cuttings produce adventitious roots slowly and require additional auxin to reliably root. Tumors may form at nodes.The results of employ cytokinins could include release of apical dominance as you demonstrated in kidney beans in laboratory.Cytokinins regulate the cell cycle as we learned untimely in the semester.Cytokinins delay senescence. You carried out this project at home with isolated wheat primary leaf tips in various solutions of plant hormones. Those set with cytokinins should have demonstrated delayed senescence (stayed green longer). Of course, remember the auxin drug responsethere is such a thing as too much of a good thingCytokinins cause nutrient diversion. Cytokinin-treated leaves become sinks for nutrients such as amino acids. This is shown in a classic experiment in plant physiologyCYTOKININS CONJUGATIONAs you can see in the diagram below, the natural cytokinin ribosides can be attached to sugars to form glycosides that have rock-bottom or no cytokinin activity. The plant also can produce enzymes to cleave the sugar and restore full cytokinin activity. Thus articulation with sugars and retrieval from these bound forms is a possible pathway in plant cells.However, it is also true that the ribosides themselves are a form of conjugatio n. All studies to fight seem to indicate that the free-base has to be cleaved from the ribose too before the compound has any true activity. Plants carry out this reaction easily and rapidly, so ribosides appear to have activity on their own, but this is an artifact. Cells in culture require the cytokinin to be freethese sometimes lack the enzymes to cleave the ribose, so supplied ribosides are inactive and the free-base must be supplied in the medium.The indigen cytokinins also occur as modified bases in RNA and deoxyribonucleic acid strands. In fact cis-zeatin (a less active form) is found in many tRNA molecules in almost all living cells of all species The finish to which the free cytokinin pool is altered by conjugation with other nucleotides or released from nucleotide polymers is not clear.CYTOKININS DEGRADATIONIn addition to synthesis and conjugation, the pools of cytokinins can be altered by degradation. Below is how one natural cytokinin is made inactiveREFERENCEShttp// www.plant-hormones.info/cytokinins.htmKieber JJ (March 2002). Tribute to Folke Skoog Recent Advances in our catch of Cytokinin Biology. J. Plant Growth Regul. 21 (1) 12. doi10.1007/s003440010059. PMID11981613.Campbell, Neil A. Reece, Jane B. Urry, Lisa Andrea. Cain, Michael L. Wasserman, Steven Alexander. Minorsky, ray of light V. Jackson, Robert Bradley (2008). Biology (8th ed.). San Francisco Pearson, Benjamin Cummings. pp.82730.Chen CM, Ertl JR, Leisner SM, Chang CC (July 1985).GOVERNMENT COLLEGE UNIVERSITY,LAHOREPage 1