There are two types of cytokinins: adenine-type cytokinins, such as kinetin, zeatin, and 6-benzyl amino purine, and phenylurea-type cytokinins, such as diphenylurea and thidiazuron. Adenine-type cytokinins are represented by kinetin, zeatin, and 6-benzyl amino purine (TDZ).
The majority of adenine-type cytokinins are produced in the roots.
Additionally, cytokinins are produced by the Cambium and other actively dividing tissues.
phenylurea cytokinins have not been discovered in plants, according to the literature.
In addition to participating in local and long-distance signalling, cytokinins use the same transport mechanism as purines and nucleosides to get around. The xylem is typically the route by which cytokinins are transported. Plant growth hormones, cytokinins and auxin, work together to promote plant growth. Even though they are complementary, their effects are generally opposite.
Functions of Cytokinesis
Cytokinins are involved in a wide range of plant processes, including cell division and the development of shoot and root morphogenesis, among others. They are known to regulate axillary bud growth as well as apical dominance in the axillary bud. According to the “direct inhibition hypothesis,” these effects are caused by a change in the ratio of cytokinin to auxin in the body. a citation is required] According to this theory, auxin released by apical buds travels down shoots and inhibits the growth of axillary buds. This encourages the growth of the shoots while discouraging the formation of lateral branches. Cytokinin moves from the roots to the shoots, where it eventually signals the development of lateral buds. Simple experiments provide evidence in support of this theory. Because of the apical bud’s removal, the axillary buds are no longer inhibited, lateral growth increases and the plants become bushier as a result. The application of auxin to the cut stem a second time reduces lateral dominance. Furthermore, it has been demonstrated that cytokinin on its own does not affect parenchymal cell proliferation. When grown in the presence of auxin but not cytokinin, they grow in size but do not divide. Cytokinin and auxin are both required for cell expansion and differentiation when they are used together. When cytokinin and auxin are present in equal amounts, the parenchyma cells differentiate into a callus that is not differentiated. A higher ratio of cytokinin to auxin promotes the growth of shoot buds, whereas a higher ratio of auxin to cytokinin promotes the formation of roots.
Cellular cytokinins (Cytokinins) have been shown to delay the ageing of plant organs by inhibiting protein breakdown, stimulating protein synthesis, and assembling nutrients from nearby tissues.
When the senescence of tobacco leaves was controlled through genetic manipulation, researchers discovered that wild-type leaves turned yellow while transgenic leaves remained mostly green. A hypothesis was advanced on the effect that cytokinin has on enzymes involved in the regulation of protein synthesis and degradation.
Cytokinins have only recently been discovered to play a role in the pathogenesis of plants. For example, cytokinins have been shown to induce resistance against Pseudomonas syringe in Arabidopsis thaliana and Nicotiana tabacum, according to the literature. The cytokinins have also been suggested to have potential functions in the context of biological control of plant diseases. In this study, the production of cytokinins by Pseudomonas fluorescence G20-18 was discovered to be a critical determinant in achieving efficient control of the infection of A. thaliana with a P. syringe.
While the action of cytokinins in vascular plants has been described as pleiotropic, this class of plant hormones has been shown to specifically induce the transition from apical growth to growth in moss protonema by way of a three-faced apical cell. Specific single-cell differentiation is responsible for this bud induction, making cytokinin’s effect on this cell differentiation extremely specific.
Action
The signalling pathway for cytokinin in plants is mediated by a two-component phosphorelay system. Cytokinin binds to a histidine kinase receptor in the endoplasmic reticulum membrane, which signals the start of the signalling pathway. This results in the autophosphorylation of the receptor, with the phosphate being transferred to a phosphotransfer protein as a result of the autophosphorylation. The phosphotransfer proteins can then phosphorylate the type-B response regulators (RR), which are a transcription factor family that belongs to the RR family. Type-B RRs that have been phosphorylated and thus activated regulate the transcription of a large number of genes, including those encoding the type-A RRs. The type-A RRs are thought to exert a negative influence on the pathway.
Biosynthesis
ATP-isopentenyltransferase (IPT) is responsible for the first reaction in the biosynthesis of isoprene cytokinins, which occurs in the presence of adenosine phosphate. It may use ATP, ADP, or AMP as substrates, dimethylallyl pyrophosphate (DMAPP) or hydroxymethyl butenyl pyrophosphate (HMBPP) as prenyl donors, depending on the circumstances. Cytokinin biosynthesis is slowed down by this reaction, which is the rate-limiting step. The methylerythritol phosphate pathway is responsible for the production of the DMADP and HMBDP required for cytokinin biosynthesis (MEP).
Cytokinins can also be produced in plants and bacteria through the recycling of tRNAs.On the degradation of the tRNA with anticodons that begin with uridine and that contain an already-prenylated adenosine adjacent to the anticodon, the adenosine is released into the surrounding environment, where it acts as a cytokinin.
It is the enzyme tRNA-isopentenyltransferase that is responsible for the prenylation of these adenines. Auxin has been shown to regulate the biosynthesis of cytokinin in the laboratory.
Conclusion
Because cytokinins promote plant cell division and growth, they have been studied as potential agrochemicals since the 1970s. However, they have not yet been widely adopted, most likely due to the complex nature of their effects, which makes them difficult to understand.
According to one study, applying cytokinin to cotton seedlings resulted in a 5–10 percent increase in yield when the plants were grown in drought conditions. Some cytokinins are used in plant tissue culture and can also be used to promote the germination of seeds. Others are used in the treatment of cancer patients.