Regulating Plant Growth & Fruit Ripening (HL) (HL IB Biology)

• Two plant hormones, auxin and cytokinin, interact together to ensure integration of root and shoot growth
  • Auxin is responsible for cell elongation and is produced in the shoots
  • Cytokinin is responsible for cell division and is produced in the roots
• Both hormones must be transported to the areas of the plant where they are not produced
  • Cytokinin is transported in the xylem tissue as direction is always from root to shoot
  • Auxin is transported in the phloem sap from shoot to root
• At certain concentrations the two hormones work together to ensure root and shoot growth is regulated; their activities can termed ‘complementary' due to the integration of their signaling
  • At low concentrations auxin limits the action of cytokinin
  • An increase in cytokinin level counteracts this inhibitory effect and leads to an inhibition of auxin signaling
  • At higher concentrations of both hormones, these interactions between cytokinin and auxin are prevented

• The production of ethylene in fruits is an example of a positive feedback loop
  • In positive feedback loops the original stimulus produces a response that causes the factor to deviate even more from the normal range
  • They enhance the effect of the original stimulus
• Ethylene (named ethene by International Union of Pure and Applied Chemistry, IUPAC) is a gas produced by fruit during the later stages of fruit ripening
• The gas can diffuse from one fruit to adjacent fruit which triggers further release of ethylene
• The effect is that all fruit ripens at the same time

Ethylene Positive Feedback Loop Diagram

The production of ethylene is an example of a positive feedback loop