Tag: plants and animal hormones

Cytokinins are the plant hormones which has manifold effects on plants. It includes delaying of senescence, active cell division and so on. Cytokinins increase cell division by stimulating proteins needed for mitosis. Mitosis is non-sexual cell division that occurs in all living things producing additional cells for body growth. The active principle responsible for inducing cell division was isolated first from the extracts of yeasts. Such a substance was called kientin, later the name was changed and called as cytokinin.

Cytokinins are the phytohormones which can delay plant senescence by the stimulation of cell division. Cytokinins also prevent the destruction of chlorophyll and consequently the plant parts do not turn yellow. This is known as Richmond Lang effect after the workers who discovered it.

Cytokinins are phytohormones which have specific effects on cytokinesis and were discovered as kinetin (a modified form of adenine, a purine) from the autoclaved herring sperm DNA. Kinetin does not occur naturally in plants. Search for natural substances with cytokinin-like activities led to the isolation of zeatin from corn-kernels and coconut milk. Since, the discovery of zeatin, several naturally occurring cytokinins and some synthetic compounds with cell division promoting activity, have been identified. Natural cytokinins are synthesised in regions where rapid cell division occurs, for example, root apices, developing shoot buds, young fruits etc.

One effect of cytokinins on plant cells is to delay the aging process. Plant cells, like all living cells, go through a natural aging process known as senescence. Senescence is accelerated in cells of plant parts that are cut, such as flower stems. Botanists think that plants must have a continual supply of cytokinins from the roots. Cut stems, of course, lose their source of cytokinins and therefore age rapidly. When cytokinins are sprayed on leaves of a cut stem of many species, they remain green, whereas unsprayed leaves turn yellow and die.

During the 1940s and 1950s, researchers were trying to find substances that might induce plant cells to divide in tissue culture, a technique in which cells are isolated from plants and grown in a nutrient medium. They discovered that cells would not divide without a substance found in coconut milk. Because coconut milk has a complex chemical composition, investigators did not chemically identify the division-inducing substance for some time. Finally, researchers isolated an active substance from a different source, aged DNA from herring sperm. They called it cytokinin because it induces cytokinesis, or cytoplasmic division. In 1963, researchers identified the first plant cytokinin, zeatin, in corn. Since then, similar molecules have been identified in other plants. Biologists have also synthesized several cytokinins. Cytokinins are structurally similar to adenine, a purine base that is part of DNA and RNA molecules.

The ageing process in plants is known as senescence. Cytokinins delay the process of scenescence. The ability of cytokinins to delay the process of senescence is known as Richmond-Lang effect.

One important effect of cytokinins on plant cells is to delay the aging process. Plant cells, like all living cells, go through a natural aging process known as senescence. Senescence is accelerated in cells of plant parts that are cut, such as flower stems. Botanists think that plants must have a continual supply of cytokinins from the roots. Cut stems, of course, lose their source of cytokinins and therefore age rapidly. When cytokinins are sprayed on leaves of a cut stem of many species, they remain green, whereas unsprayed leaves turn yellow and die.

Cytokinins promote cell division and differentiation of young, relatively unspecialized cells into mature, more specialized cells in intact plants. They are a required ingredient in any plant tissue culture medium and must be present for cells to divide. In tissue culture, cytokinins interact with auxin during the formation of plant organs such as roots and stems. For example, in tobacco tissue culture a high ratio of auxin to cytokinin induces root formation, whereas a low ratio of auxin to cytokinin induces shoot formation. Cytokinins and auxin also interact in the control of apical dominance. Here, their relationship is antagonistic: auxin inhibits the growth of axillary buds, and cytokinin promotes their growth.

Cytokinin causes delay of senescence called Richmond Lang effect. It checks disappearance of chlorophyll and degradation of protein synthesis in leaves. 
Senescense of leaves leads to yellowing and finally leads to the fall from the plant.  If a young excised leaf is kept in water, it slowly changes its color to yellow and dies.  If such leaves are provided with cytokinin, the yellowing is significantly delayed and such an effect is called Richmond and Langs effect; named after the discoverers.