Plant Hormones: Cytokinins

CHAPTER: 15(D)

Plant Hormones: Cytokinins

Cytokinins:

•      Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots.

•      They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. 

•      There are two types of cytokinins: adenine-type cytokinins represented by kinetin, zeatin, and 6-benzylaminopurine, and phenylurea-type cytokinins like diphenylurea and thidiazuron (TDZ). 

•      Most adenine-type cytokinins are synthesized in roots Cambium and other actively dividing tissues also synthesize cytokinins.

•      No phenylurea cytokinins have been found in plants. 

•      Cytokinins participate in local and long-distance signaling, with the same transport mechanism as purines and nucleosides. 

•      Typically, cytokinins are transported in the xylem.

•      Cytokinins act in concert with auxin, another plant growth hormone.

Biosynthesis of Cytokinins:

•      Adenosine phosphate-isopentenyltransferase (IPT) catalyses the first reaction in the biosynthesis of isoprene cytokinins.

•      It may use ATP, ADP, or AMP as substrates and may use dimethylallyl diphosphate (DMAPP) or hydroxymethylbutenyldiphosphate (HMBDP) as prenyl donors. 

•      DMAPP and HMBDP used in cytokinin biosynthesis are produced by the methylerythritol phosphate pathway (MEP).

•      Cytokinins can also be produced by recycled tRNAs in plants and bacteria. The prenylation of these adenines is carried out by tRNA-isopentenyltransferase.

•      Auxin is known to regulate the biosynthesis of cytokinin.

Physiological roles of Cytokinins:

•      Accelerates cell division, cell elongation and morphogenesis.

•      Initiation in protein and nucleic acid metabolism.

•      Counteract the influence of apical dominance.

•      Helps to delay senescence.

•      Provide resistance to plant injured by high temperature and low temperature.

•      Can break seed dormancy and promotes germination.

Practical Applications of Cytokinins in Agriculture:

1. Accelerate the induction of flowering of Short-Day plants.
2. Accelerate the development of fruits.
3. Accelerate tissue culture.
4. Stimulates root initiation.
5. Breaking the dormancy of certain light sensitive crop seeds.
6. Delay in senescence in vegetables and keeps the cut flowers and vegetables fresh for a long period.