Hormone intro & HW assignment

Auxins

Giberellins

Abscisic acid

Ethylene

Cytokinins

 


Cytokinins

  • resembling adenine which promote cell division and have other similar functions to kinetin
  • The most common form of cytokinin in plants is called zeatin which was isolated from corn
  • 200 +natural and synthetic cytokinins
  • concentrations are highest in meristematic regions and areas of growth within roots, young leaves, developing fruits, and seeds

Discovery: Miller accidentally added degraded DNA to initiate cell division in tobacco callus cultures. The degraded adenine turned out to be cytokinin ( not actually produced in plants but similar to natural cytokinins), which is responsible for cell division.

Effects of Cytokinins:

  • Cellular Division -- Cytokinins stimulate cellular division by speeding the transition of cells from the G2 phase to the M phase of the cell cycle. All this is dependent on the presence of auxin.
    1. IAA alone -- cell expansion
    2. Kinetin alone -- little effect
    3. IAA + Kinetin -- rapid cell division
  • Effects on Cotyledons -- It promotes cellular division and expansion in cotyledons. Increase in wall elasticity from cytokinin results in cellular expansion.
  • Organogenesis (the formation of organs) -- Cytokinins and auxins affect organogenesis.
    1. High cytokinin/auxin ratios favor the formation of shoots.
    2. Low cytokinin/auxin ratios favor the formation of roots.
  • Senescence -- Cytokinins delay chlorophyll breakdown in detached leaves, by preventing those genes from being turned off that stimulate the formation of chlorophyll. Promotes the conversion of etioplasts into chloroplasts via stimulation of chlorophyll synthesis.

This only occurs in detached leaves. Roots are loaded with cytokinins, and therefore the transport of these cytokinins to the leaves cause the delay in senescence.

  • May enhance stomatal opening in some species
  • Stimulates the growth of lateral buds-release of apical dominance.