Cytokinins have a molecular structure similar to adenine. Naturally occurring zeatin, isolated first from corn is the most active of the cytokinins. Cytokinins are found in the sites of active cell division in plants such as in root tips, seeds, fruits and leaves. Cytokinins are a smaller group of related compounds. They are transported in the xylem and work in the presence of auxin to promote cell division. Differing cytokinin: auxin ratios change the nature of organogenesis. If kinetin is high and auxin low, shoots are formed; if kinetin is low and auxin high, roots are formed.

Biosynthesis and metabolism
Cytokinin biosynthesis occurs through the biochemical modification of adenine.

The process by which they are synthesized is as follows:
i. A product of the mevalonate pathway called isopentyl pyrophosphate is isomerized. This isomer can then react with adenosine monophosphate with the aid of an enzyme called isopentenyl AMP synthase.
ii. The result is isopentenyl adenosine-5′-phosphate (isopentenyl AMP). This product can then be converted to isopentenyl adenosine by removal of the phosphate by phosphatase and further converted to isopentenyl adenine by removal of the ribose group.

Role of cytokinins

  1. They promote lateral bud development, which is retarded by auxin.
  2. They delay the senescence of leaves and promote the expansion of cotyledons.
  3. They stimulate cell division, morphogenesis (shoot initiation/bud formation) in tissue culture; stimulate leaf expansion resulting from cell enlargement.
  4. They may enhance stomatal opening in some species.
  5. They promote the conversion of etioplasts into chloroplasts via stimulation of chlorophyll synthesis.

Transport

  1. Transport of cytokinins occur its via xylem (transpiration stream).
  2. Zeatin ribosides are the main transport form; converted to the free base or glucosides in the leaves.

Physiological effects

  1. Cell division The formation, maintenance and growth of the shoot apical meristem involve cytokinin. Cytokinins are positive regulators of cell division in the shoot apical meristem and negative regulators of cell division in the root apical meristem. They are involved in the regulation of both G1-S and G2-M transitions.
  2. Morphogenesis (shoot and root initiation) in the callus is stimulated by cytokinins and auxin. If the ratio of auxin to cytokinin is high or when auxin is present alone, root formation may initiate by callus. Conversely, a high cytokinin to auxin promotes shoot formation. Roughly proliferation of undifferentiated callus is promoted by equal amounts of auxin and cytokinin.
  3. Crown gall In dicot plants, cytokinin and auxin also induce crown gall formation. Plant tissues originate crown galls that have been infected with Agrobacterium tumefaciens. T-DNA which is a small portion of the Ti plasmid is transferred into the nuclear DNA of the host plant cell. Genes necessary for the biosynthesis of cytokinin and auxin are carried by T-DNA, as well as a member of a class unusual nitrogen containing compounds called opines. These genes present on T-DNA are expressed in the plant cells, leading to hormone synthesis and unregulated proliferation of the cells to form the gall.

Chloroplasts maturation
The conversion of etioplasts into chloroplasts is promoted by cytokinins via stimulation of chlorophyll synthesis. Dark-grown seedlings are said to be etiolated. The internodes of etiolated seedlings are more elongated and chloroplast does not mature. The proplastids of dark-brown seedlings develop into etioplasts instead of maturing as chloroplasts, which contain photochlorophyll instead of chlorophyll and do not synthesize most of the enzymes and structural proteins required for the formation of the chloroplast thylakoid system and photosynthesis machinery. Chloroplasts mature directly from the proplastids present in the embryo when seedlings germinate in the light.
Senescence Delay in leaf senescence is caused by cytokinins. Energy dependent genetically programmed aging process, which leads to organs/plant death, is known as senescence. This is an essential process of the growth and development of plants. As plant senescence, they produce enzymes which are necessary to recycle materials and re-route the subunits to areas for use by active growth elsewhere. The mechanism of cytokinins by which they are able to delay senescence is not clear, but there is some evidence that cytokinins exert a role in mobilizing nutrients. Onset of senescence is marked by a transient rise in respiration called climacteric in some fruits. The hormones, abscisic acid and ethylene are responsible for senescence.

Cytokinins Signaling Pathway
Cell division, cell expansion in cotyledon, development of chloroplast and etioplast, suppression of apical dominance and senescence and differentiation of in vitro culture cells are induced by all cytokinins. However, at the molecular level very little is known about the mechanism of cytokinin action. Cytokinins are perceived by histidine kinases and transduced by a two components signaling system. The detail about cytokinin signaling has been elucidated in Arabidopsis. In the Arabidopsis, cytokinin signal transduction pathway, hybrid histidine protein kinases (AHKs) serve as cytokinin receptors and histidine phosphotransfer proteins (AHPs) transmit the signal from (AHKs) to nuclear response regulators (ARRs), which can activate or repress transcription. Histidine phosphotransfer proteins act as signaling shuttles between the cytoplasm and nucleus in a cytokinin dependent manner. The short signaling circuit reaches the nuclear target genes by enabling nuclear response regulators or ARR1, ARR2 and ARR10 as transcription activators. The cytokinin inducible ARR4, ARR5, ARR6 and ARR7 genes encode transcription repressors that mediate a negative feedback loop in cytokinin signaling.