What Are Cytokinins?

Cytokinins is a hormone that acts as a retarding agent in plant growth. This hormone also works with auxin to stimulate cell division and more.

There are five types of plant hormones. Each hormone has a specific function that is directly related to the concentration of the hormone, its site of action, and the developmental stage of the plant. The cytokinins hormone is the second hormone that synthesizes the roots and transports them to other organs.

Cytokinins is defined as a class of related plant hormones and retard aging and act in concert with auxin to stimulate cell division, influence the pathway of differentiation, and control apical dominance.

In the 1940's cytokinins were discovered by Johannes van Oberbeek. He was conducting trial-and-error experiments to find chemical additives that could enhance growth and development of plant cells. While he was working he found growth of plant cells could be stimulated by adding coconut milk. Then ten years later Folke Skoog and Carlos O. Miller induced tobacco cells that were being grown in a culture to divide by adding degraded samples of DNA. In each experiment the additives were forms of adenine. From this the growth regulators where names cytokinins because they stimulate cell division or cytokinesis.

Growing tissue are where cytokinins are produces. They reach their target area by moving up the xylem sap. Working with auxin the cytokinins influence the pathway of differentiation as well as stimulating growth. A sample of parenchyma tissue is cultivated without cytokinins, the cells grow very large but cannot divide. Then if just cytokinins are added there will be no effect. Yet if the cytokinins are added with auxin then the cells will divide. The differentiation of cells is based on the cytokinins to auxin ratio. If there is more auxin then cytokinins roots will form. If vise versa shoots buds develop from the callus. Yet if the ratios are equal the mass of cells will continue to grow but it remains an undifferentiated callus. These hormones stimulate RNA and protein synthesis.

The control of the apical dominance is an interaction between cytokinins and auxin. When the terminal bud suppresses the development of auxiliary buds. This cases is an example of when the two hormones are antagonistic. When the auxin transports down the shoot it restrains auxiliary buds from growing which allows the shoot to lengthen. Yet if cytokinins are entering the shoot systems they signal the auxiliary buds to start growing. Auxin cannot stop the buds from growing once it has begun.

The coordination between auxin and cytokinins allow a balance of growth in the shoots and roots systems. When the roots become more extensive they cytokines level increase and signals the system to form more branches. Once the roles are reversed in the development of the lateral roots; cytokinins inhibit root branching while auxin stimulates. Growth of the auxiliary buds are regulated by both auxin and cytokinins are still effected by one more hormone, ethylene.

Cytokinins can slow down the aging of some plant organs. This may be because they are inhibiting protein breakdown, by stimulating RNA and protein synthesis. If you took leaves from a plant and dipped them into a cytokinins solutions, they would stay green much longer. Fresh cut flowers are often sprayed with cytokinins spray to make them last longer

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