- Skoog & co-workers showed that callus formation occurred when internodal segments of tobacco are cultured. In the nutrient medium in addition to auxin, they provided extracts of yeast, vascular tissues, coconut milk or DNA.
- Miller first discovered it from autoclaved Herring sperm DNA. He named it as kinetin.
- Letham first obtained natural cytokinin from maize grain & coconut milk, it is referred as Zeatin.
- Site of synthesis: Major source is roots & regions where rapid cell division occurs like meristems, root tip, buds, young leaves & developing fruits.
- Transported through the xylem.
- Derived from Adenine, it is 6-furfuryl aminopurine.
Physiological effects of cytokinin:
- A true cell division factor required for cytokinesis. It can induce cell division in permanent cells along with auxin. It helps in the formation of inter-fascicular cambium. Used in tissue culture along with auxin.
- Can cause cell elongation by increasing permeability of cell membrane & thus increasing water uptake by the cell.
- Morphogenesis: given by Skoog & Miller. They showed that when pith of tobacco is cultured in the presence of auxin & cytokinin, callus formation occurred. When the concentration of auxin is increased root started developing & when the concentration of cytokinin is increased shoot & bud formation occurred. It forms plantlets that can grow into new plants.
- Promotes the growth of lateral buds thus suppressing apical dominance.
- It induces the formation of new leaves, chloroplast in leaves, lateral shoot formation & adventitious shoot formation.
- Capable of breaking dormancy of seeds.
- It promotes the formation of female flowers.
- Delay of senescence (Richmond-Lang effect): it delays senescence of leaves & organs by mobilizing the nutrients, reducing degradation of chlorophyll and protein. Thus preventing leaf fall and yellowing of leaves.