Cytokinin
Cytokinins or cytokinins are a group of plant hormones with multiple effects on growth and development of plants. The name is derived from its stimulating effect on cytokinesis from (Greek kytos = cell and kinesis = movement). As the phytohormones cytokinins are ubiquitous (everywhere) distributed in plants, but are also found in animal tissues and in bacteria, fungi and algae. Cytokinins in plant tissue are in free form or bound to specific transfer ribonucleic acids (tRNA ) before. The concentration of cytokinins in plant tissue is extremely low and the plant organ and its development depends.
The most important natural cytokinins are cis -and trans- zeatin, zeatin riboside and ribotide, dihydrozeatin and N6- isopentenyl adenine, and other derivatives of adenine with a C5 isoprenoid side chain ( R) on the exocyclic nitrogen atom N6.
Zeatin was isolated as the first natural cytokinin in immature maize grains and is widely distributed in higher plants. In addition to this basic cytokinin also occur corresponding ribonucleosides (9- ribosides ) and ribonucleotides (9- riboside -5'- monophosphate ) and N-7 and N-9 and 4- O- glucosides in plants.
Of the synthetic cytokinins, the so-called organic compounds with cytokinin activity, are particularly kinetin (6- furfurylaminopurine ) and Benzylaminoadenin and some N, N- Diphenylharnstoffderivate of importance, such as kinetin in the Phytohormonforschung as a model substance.
Synthesis
The first step of cytokinin biosynthesis is the transfer of an isopentenyl group ( DMAPP, HMBDP ) on an adenosine unit ( ATP / ADP / AMP). In plants, a transfer of DMAPP ( Dimethylallydiphosphat ) on ATP / ADP, bacteria, however, is made as transferred, eg Agrobacterium tumefaciens HMBDP ( 1-hydroxy -2-methyl -2-( E)- butenyl 4 -diphosphate ) to AMP. Be enzymatically catalyzed steps through different IPT ( isopentenyl transferase ).
The biosynthesis of natural cytokinins is closely associated with the RNA metabolism and occurs either de novo from low molecular weight compounds on the purine ring or through the release of cytokines by reducing Cytokininhaltiger t- RNA. Hauptbiosyntheseort are the root tip and young fruits and seeds. The transport takes place in the xylem ( vascular tissue ).
Effect
The effects of cytokinins overlap with those of other phytohormones. Characteristic are the stimulation of cytokinesis ( cell division, mitosis) in callus and the promotion of elongation of the plant.
Cytokinins regulate in close interaction with other phytohormones, especially auxins and abscisic acid, and environmental factors such as light diverse plant growth and differentiation processes. This also includes the delay of the aging process in plants.
While the effect of cytokinins in vascular plants is often described as pleiotropic, they induce the protonema of mosses bud formation. This transition from apical growth ( tip growth) to grow by three dashing apical cell goes back to the differentiation capacity of a single cell and represents a specific cytokinin
The function of cytokinins in plant defense has been described in recent studies. Thus, for example, could be shown that cytokinins resistance to Pseudomonas syringae in Arabidopsis thaliana and Nicotiana tabacum induce.