cytokinin

                  CH2OH
        C        /
        H2      /
     H / \    C
     N/   \ // \
     |     C   CH3
     |     H
     |
     |
     |
    //\   N
   //  \/  \\
  N    ||   \\
  \\   ||    /
   \\  / \  /
     N    N
          H

A representation of a basic cytokinin molecule.

Working in an unassuming little lab at Cold Spring Harbor, NY Johannes van Overbeek stumbled across a major discovery in the 1940s. He found that plant growth could be substantially stimulated by adding coconut milk to them. Unfortunately, he had no basis for why this happened. Nearly ten years later Folke Skoog and Carlos Miller of the University of Wisconsin made tobacco cells divide by adding a solution containing degraded DNA. This turned out to be van Overbeek's key, as the only similar ingredient between the two seemingly unrelated solutions was adenine. These and similar growth stimulators were named cytokinins because they induced cytokinesis. The most common variety found in plants is zeatin (first discovered in Zea Mays, common name Corn) and a second cytokinin frequently used in lab work is kinetin.

Following research immediately delved into the complex relation between cytokinins and auxin, the other major plant growth hormone. Timeless studies have measured the affect of varying concentrations of cytokinins and auxin in solution, the results are as follows:

• No chemicals added: Cells in culture will grow large, but will not divide.
• Cytokinins only: No change.
• Cytokinins and Auxin: Cells will now divide.
• Concentration of Cytokinins equals that of Auxin: Cells will grow, but in an undifferentiated manner. No cell specialization.
• [Cytokinins] greater than [Auxin]: Shoot buds will develop.
• [Cytokinins] less than [Auxin]: Roots will develop.

The fascinating relation of cytokinins and auxin grows greater when one considers how the two chemical hormones are utilized in a plant. Auxin is generally concentrated in the tip of the terminal bud of the plant, and suppresses the budding of new branches of the plant. Adversely, cytokinins are based much closer to the roots, and will encourage branching and budding, which will most likely begin near the bottom of the plant, where the hold of the auxin is least powerful. Once a plant begins budding, auxin can do very little to suppress it and in some cases will have no further effect on that aspect of the plant's growth.

The final stunning fact about cytokinins is that they appear to prevent aging. Leaves picked off of trees and plants and placed in a solution containing cytokinins will remain green for a much longer period of time than those placed in pure water or a water-sucrose (the runner up) solution. Cytokinin sprays are used in this manner to keep cut flowers fresh and can also have an affect on fruits and vegetables as per their shelf life in markets. Unfortunately, this final application is not as of yet approved by the FDA.

Plant hormones

auxin   cytokinins   gibberellin
  abscisic acid   ethylene

Source
Biology, Third Edition by Campbell, Chapter 35: Control Systems in Plants