Plant Growth Hormone: Cytokinin
Growth and development of an organism depends upon the internal and external factors supporting the growth of organism.
The external environmental cues are essential for the stimulation of growth and development for reproduction and survival efficiency of these environmental cues (i.e.) stimulus process the internal response of the organism.
The response is manifested and cell functions corresponding to the response is recorded. Such stimulus and response are well coordinated in the plant body by the chemical messengers – the hormones.
Hormones acts as a mediator for carrying and transferring information for the coordination of physiological, metabolic and chemical activity.
For example: the growth of coleoptile of Phalaris canariensis towards the light is the coordinated movement for external stimulus light (i.e.) the phototropism is mediated by the hormone auxin. This was the initial discovery for the presence of phytohormones in plant system regulating the function of whole plant body.
These chemical compounds enhance cell communication and integrate the multicellular organism to organized as a single unit.
Further studies were undergoing for the deducing the mechanism and variety of hormones coordinating plant body.
Between 1950 – 1960 a group of five hormones were identified to maintain the plant homeostasis. These hormones were combinedly termed as ” Classical Five” they are: AUXIN, CYTOKININ, ETHYLENE, GIBBERELLIN, ABSCISIC ACID.
Along with classical five there are brassanosteroids and jasmonic acid. These set of hormones are termed as “Plant Growth Regulators” as they have an active role in regulating growth and development rather than a broad action spectrum.
Hormones are sensitive, specific, low concentration action and are naturally occurring in plant species. These important characteristic makes it an ideal small molecule chemical messengers and regulators.
The mode of action is receptor mediated and are transported to different regions by vascular tissues(i.e.) xylem and phloem. Hormones like ethylene are volatile, hence they are diffused throughout the plant body.
Discovery of cytokinin was first started from Haberlandt, German plant physiologist in 1913 that the phloem extracts are used to cause cell division in tubers.
In 1921, from another experiment he found that cell division is promoted by soluble factor.
In 1940’s and 1950’s the cell culture techniques were practiced, where the addition of auxins did not yield cell division. but on addition of autoclaved herring sperm initiated rapid cell division.
The substance that promoted the cell division was named Kinetin. Later naturally occurring zeatin were discovered.
Functions of Cytokinin
1. Promotes cell division in callus and tissue
2. Auxin and Cytokinin combines together to form a shoot bud vs root growth in tissue culture and stem cuttings
3. Regulate Apical Dominance and lateral root initiation
4. Retard senescence and chlorophyll degradation
5. Present in major parts of plant
IPP is the key for cytokine synthesis.
IPP isomerize with DMAPP.
DMAPP condense with AMP give rise to iso – pentenyl adenosine mono phosphate (iPMP).
iPMP is precursor for all naturally occurring Cytokinins.
iPMP is hydroxylated at C4 of the side chain and forms zeatin riboside.
In further steps, Ribose and phosphate are cleaved to yield Zeatin.
Site of synthesis: primarily synthesized in meristematic tissues.
Regulation of Cytokinin Levels
1. It is regulated by conjugation of Cytokinins
2. Irreversible inactivation