plant and animal responses

• tannins

  • found in upper epidermis and make leaf taste bad
  • prevent infiltration by pathogenic microorganisms
• alkaloids

  • derived from amino acids
  • a feeding deterrent to animals - tasting bitter
  • located in growing tips and flowers
• pheromones

  • chemicals released by one individual and which can affect the behaviour or physiology of another
• tropism


  directional growth response in which the direction of the response is determined by the direction of the external stimulus
• phototropism


  shoots grow towards light (they are positively phototropic) which enables them to photosynthesise
• geotropism

  • roots grow towards the pull of gravity
  • this anchors them in the soil and helps them take up water which is needed for support (keeps them turgid) as a raw material for the plant and to help cool the plant.
  • minerals such as nitrate in water needed for synthesis of amino acids
• chemotropism

  • pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place
• thigmotropism

  • shoots of climbing plants wind around other plants or solid structures to gain support
• nastic responses

  non-directional responses to external stimuli
• • if plant responds towards a stimulus - positive tropic response (directional)
  • if plant responds away from a stimulus - negative tropic response (directional)
• cytokinins

  • promote cell division
  • delay leaf senescene
  • overcome apical dominance
  • promote cell expansion
• abscisic acid

  • inhibits seed germination and growth
  • causes stomatal closure when the plant is stressed by low water availibility
• auxins

  • promote cell elongation
  • inhibit growth of side shoots
  • inhibit leaf abscission (leaf fall)
• gibberellins

  • promote seed germination and growth of stems
• Ethene

  • promotes fruit ripening
• How plant hormones work

  • produced in a variety of tissues in the plant
  • hormones reach their target cells
  • they bind to receptors on plasma membrane
  • specific hormones have specific shapes which only bind to specific receptors with complementary shapes on membranes of particular cells
  • hormones can influence cell division, cell elongation and cell differentiation
• role of auxins

  • plant hormones responsible for growth
  • growth of plant upwards leads from the top apical stem
  • all buds below apical bud (lateral buds) are inhibited - known as apical dominance
• more auxin - slows lateral growth

  less auxin - speeds up lateral growth
• 1. when apical tip is removed auxin levels drop. lateral growth then occurs - cut end might have produced a hormone that promoted lateral growth
  2. with auxin transport inhibitor placing a ring of auxin transport inhibitor that prevents auxin growth on the stem also results in lateral growth
  3. auxin paste applied to a cut shoot, lateral buds did not grow
  4. scientist the applied an auxin transport inhibitor below apex of shoot and lateral buds grew
• abscisic acid


  lateral bud growth inhibition - slows growth outwards. high auxin levels encourage high abscistic acid levels
• cytokinins


  bud growth promotion - travel to where auxin is more concentrated
• role of auxins, cytokinins and abscisic acid to apical dominance ( with plant tip intact)

  1. high auxin levels
  2. high abscisic acid levels
  3. low cytokinins in lateral buds
  4. inhibit lateral bud growth
• role of auxins, cytokinins and abscisic acid to apical dominance ( with plant tip removed)

  1. low auxin levels
  2. low abscisic acid levels
  3. higher cytokinin levels in lateral buds
  4. lateral buds grow
• 1. testing gibberellic acid on different plants

  • fungal disease in Japan called bakanae was causing rice plants to grow very tall.
  • Gibberellic acid (GA3) was found in the fungus
  • Gibberellins responsible for stem elongation and seed germination
• 2. Testing GA1 levels

  • compared GA1 levels of tall pea plants an dwarf pea plants which were otherwise genetically identicsl
  • they found that plants with higher GA1 levels were taller
• 3. showing GA1 directly causes stem elongation

  • researchers need to know how GA1 is formed.
  • They worked out that Le gene was responsible for producing the enzyme that converted GA20 to GA1
  • researchers then chose a pea plant with a mutation that blocks Gibberellin production between ent-kaurene and GA12 - aldehyde in the pathway
  • those plants produce no gibberellin and grow to only about 1cm tall
• investigating the effect of plant hormones on phototropisms

  
• experiments to confirm the role of the shoot tip in producing a chemical messenger that controls phototropic responses

  
• Boysen Jensen's work 


  • confirmed that water and/or solutes need to be able to move backwards from the shoot tip for phototropism to happen
  • when a permeable gelatine block inserted behind shoot tip, shoot still showed positive phototropism
  • when impermeable mica block was inserted, there was no phototropic response