The effects of plant hormones

• Why do plants need to respond to their environment?

  • Avoid abiotic stress
  • Avoid being eaten by herbivores (predation)
  • To ensure reproduction is successful (pollination, seed production)
  • Enhancing survival so greater chance of selective breeding
• Describe examples of biotic factors

  Biotic- living factors that affect plant growth/survival/population numbers

  • competition
  • predation
  • virus
  • nematodes
  • insects
• Abiotic factors?

  Abiotic- non living factors that affect plant growth/survival/population numbers

  • drought
  • sunscald
  • wind injury
  • chemical drift
  • nutrient deficiency
• Describe hormones and their general action

  • Chemical messengers
  • Transported away from site of manufacture to target cells or tissue
  • Produced by specific cells
  • Have a specific shape
  • Bind to receptors on target cell
  • Long lasting
  • Widespread effect
  • Receptor complementary to hormone shape
  • Carried in solution by diffusion, active transport or mass flow in xylem or phloem
• describe what is meant by synergism and antagonism

  • Synergy- amplifying each others effects greater response together than each on their own
  • Antagonistic- cancel out each other- balance determines the response of the plant
  • fine control over the responses of the plant can be achieved
• State the role of auxins in plants

  • Promote cell growth, elongation and differentiation
  • Inhibits growth of side shoots (promotes apical dominance)
  • Inhibits leaf fall (abscission)
  • stimulates the release of ethene
  • involved in fruit ripening
• State the role of gibberellins in the plant

  • Promotes seed germination
  • Growth of stems (stem elongation)
  • Flowering
• State the role of Cytokinins in plants

  Promotes cell division
• State the role of ethene in plants

  • Promotes fruit ripening
  • promotes leaf abscission
• What effect does auxin concentration have on the cell wall

  • Affects cell wall plasticity
  • More auxin means cell wall becomes more flexibile
• Where is auxin produced?

  In meristem tissue/cells
• Which hormone stimulates the release of ethene?

  Auxin
• Which hormones promote abscission?

  • Ethene
  • ABA
• Which hormone stimulates stomata closing and maintains dormancy of seeds and buds?

  Abscisic acid
• Which hormone prevents leaf abscission?

  Auxin
• How are gibberellins involved in seed germination?

  • Seed is watered
  • Embryo produces gibberellins
  • Gibberellins switch on genes which code for amylases and proteases, stimulating their production- digestive enzymes required for germination
  • The enzymes break down food stores required for germination- found in food stores such as cotyledons
  • Embryo uses these food stores to produce ATP and break through seed coat
• What hormone acts as an antagonist to gibberellin during seed germination?

  ABA
• state the role of ABA as a plant hormone

  • stimulates leaf abscission
  • stimulates seed dormancy
  • stimulates stomatal closure
  • responsible for cold protective responses (eg, antifreeze production)
• what is experimental evidence supporting the role of gibberellins in seed germination?

  • mutant varieties of seeds bred that don't have the gene which is required for gibberellin production. Seeds don't germinate, but if gibberellin is applied to seeds they will germinate
  • Gibberellin biosynthesis inhibitors- added to seeds, they won't germinate. If inhibition is removed/gibberellin applied, seeds germinate
• Describe the mechanism which allows auxin to maintain cell wall plasticity and how this plasticity disappears as the plant matures

  • higher concentration of auxin promotes apical shoot growth and means the cell walls become more flexible
  • auxin binds to receptors in the cell membrane, causing pH to drop to about 5
  • this maintains cell wall plasticity and elongation
  • when cell matures, enzymes destroy auxin
  • hormone levels fall
  • so pH rises and cell wall loses plasticity and becomes rigid and fixed in shape, cells no longer expand and grow
• what effects does high concentrations of auxin have on lateral shoots and what evidence is there to show this?

  • lateral shoot growth is inhibited
  • This is because lateral shoots are inhibited by the hormone that moves back down the stem, so they don't grow well
  • further down the stem, auxin concs are lower so the lateral shoots grow more strongly
  • evidence- removal of apical shoot- lateral shoots grow faster as no auxin produced at tip. If auxin applied to cut apical shoot, dominance is reasserted and lateral shoot growth supressed
• what effect do low auxin concs have on roots? what evidence is there for this?

  • they promote root growth
  • auxin produced at root tips and reaches roots in LOW concs from the apical shoot
  • if apical shoot is removed, [auxin] reaching roots reduced and root growth slows and stops
  • replacing auxin concs at apical shoot restores root growth
  • if auxin conc is too high it inhibits root growth

  
• what effects do gibberellin concs have on the growth of a plant stem? What evidence is there for this?

  • promotes stem elongation and internodal length
  • plants that produce little to no gibberellins have short stems
  • scientists breed dwarf varieties of plants where the gibberellin synthesis pathway is interrupted, and these plants are obviously small
  • however, shorter plants means less waste and makes plants less vulnerable to damage by weather and harvesting
• summary of the effects of [auxin] and [gibberellin] on seed germination, internodal elongation and apical dominance?

  AUXINS:

  • high concs promote apical dominance, prevent lateral shoot growth and prevent growth of roots
  • evidence- removing apical shoot
  • low concs promote root growth
  • evidence- removing apical shoot

  GIBBERELLINS:

  • high concs promote seed germination
  • evidence- mutant varieties and gibberellin biosynthesis inhibitors
  • high concs promote internodal growth
  • evidence- dwarf varieties - less waste produced by plant and plant less affected by damage