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16.4.3.3: Cytokinins

Cytokinins are plant hormones that are derivatives of the purine adenine. They are not to be confused with cytokines.

They were discovered as an absolutely essential ingredient in medium for growing plant cells in culture. Without cytokinins in the medium, plant cells will not divide by mitosis. Cytokinins have been implicated in many plant activities; usually along with some other plant hormone such as auxin or ethylene.

Among these:

  • mitosis
  • chloroplast development
  • differentiation of the shoot meristem
  • stimulating the development of lateral buds and therefore branching
  • differentiation of the tissues of the root
  • leaf formation
  • leaf senescence

One of the clearest examples of cytokinin activity occurs in the germination of seeds. The endosperm of monocot seeds, such as corn (maize), contains large stores of the precursor to the cytokinin zeatin (right). When the corn kernel germinates, zeatin moves from the endosperm to the root tip where it stimulates vigorous mitosis.

Steps in cytokinin signaling

The following are the steps in cytokinin signaling:

  • A cytokinin, like zeatin, binds to a receptor protein embedded in the plasma membrane of the cell.
  • The internal portion of the receptor then attaches a phosphate group to a protein in the cytosol.
  • This protein moves into the nucleus where
  • it activates one or more nuclear transcription factors.
  • These bind to the promoters of genes.
  • Transcription of these genes produces mRNAs that move out into the cytosol.
  • Translation of these mRNAs produces the proteins that enable the cell to carry out its cytokine-induced function.

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