Plant

• Tropisms


  Directional growth responses of plants
• Phototropism


  Growth response to light, shoots grow towards light, roots grow away from light
• Geotropism


  Growth response to gravity, roots exhibit positive geotropism, shoots exhibit negative geotropism
• Genetically engineered organisms


  Used to synthesise human insulin instead of using animal-derived versions, resulting in lower risk of rejection and infection. Also, bacteria are easy to grow, making manufacturing cheaper than extracting from animals.
• Plant growth responses


  • Tropisms
  • Phototropism
  • Geotropism
  • Chemotropism
• Phototropism


  Growth response to light, shoots exhibit positive phototropism (grow towards light), roots exhibit negative phototropism (grow away from light)
• Geotropism


  Growth response to gravity, roots exhibit positive geotropism (grow with gravity), shoots exhibit negative geotropism (oppose gravity)
• Plant growth regulators


  • Auxins
  • Gibberellins
  • Abscisic acid
  • Ethene
• Auxins


  Promote cell elongation by transporting hydrogen ions into cell walls, lowering pH and enabling expansins to loosen cellulose
• Shoot illuminated from one side

  Auxins move towards shaded side, causing elongation and bending towards light
• Leaf abscission


  Dropping of leaves controlled by auxin, ethene and cytokinins. As leaf ages, auxin and cytokinin decrease, ethene increases, leading to breakdown of cell walls in abscission layer and leaf drop.
• Apical dominance


  Growth of side shoots inhibited by auxin, abscisic acid and cytokinins produced in the apex. When apex is removed, abscisic acid levels drop, allowing side shoot growth.
• Commercial uses of plant hormones

  • Auxins - rooting powder, seedless fruit growth, herbicides, fruit drop
  • Gibberellins - delay senescence, elongate apples, grape stalks, malt production, sugar cane yield, conifer seed formation, prevent lodging
  • Cytokinins - prevent lettuce leaf yellowing, promote shoot growth
  • Ethene - speed up ripening, promote lateral growth, fruit drop