Plant hormones

• Plant chemical defences include:

  1. Tannins- toxic to microbes and larger herbivores. In upper epidermis of leaves to make them taste bad. Prevent inflammation by pathogens in roots
  2. Alkaloids- bitter in taste to deter feeding. Found in growing tips and flowers and peripheral cell layers of stems and roots
  3. Pheromones- affects behaviour and physiology of other animals
• tropisms are directional growth responses
• phototropism is when there is growth towards light (positive) or away from light (negative)
• geotropism/gravitropism is when there is growth towards the pull of gravity (positive) or against the pull of gravity (negative)
• chemotropism is when there is growth towards a chemical e.g. pollen tubes grow down the style
• thigmotropism is when there is growth in response to contact with an object e.g. shoots of climbing plants winding around solid structures for support
• non-directional responses to stimuli are nastic responses e.g. Mimosa pudica responds to touch by suddenly folding its leaves which prevents herbivory as it reduces the surface area of leaves and causes small insects to fall off. This is thigmonasty
• an example of an auxin is IAA (Indoleacetic acid)
• a kilnostat is used to investigate geotropism (acts as the control) as it applies gravity equally on all sides
• auxins promote cell elongations by

  1. increasing the stretchiness of the cell wall
  2. promotes active transport of hydrogen ions into cell wall (by ATPase enzyme in cell membrane)
  3. Low pH provides optimum conditions for wall-loosening enzymes (expansins)
  4. expansins break bonds in cellulose (H+ also disrupts H bonds) so cell wall becomes less rigid and can expand as the cell takes in water
• auxins move to the shady side of plants because more phototropins have active on the light side (activated by blue light). This creates a gradient which redistributes auxin molecules via PIN proteins (transmembrane channel proteins) which are controlled by PINOID molecules. One theory suggests phototropins affect PINOID activity which affects PIN activity.
• what are the two hormones associated with apical dominance?

  abscisic acid and cytokinins
• explain the role of abscisic acid (ABA) in apical dominance

  it inhibits the growth of lateral buds. High concentrations of auxins may maintain high concentrations of ABA. when the apical bud is removed, auxin concentration decreases, causing ABA levels to fall. This causes lateral bud growth.
• explain the role of cytokinins in apical dominance

  high concentrations of auxin make the tip a sink for cytokinins (made in the roots). When tip is intact, most cytokinins move to the tip. When the tip is removed, cytokinins are more evenly distributed. Their presence in the lateral bud causes growth.
• explain the first experiment done to investigate apical dominance


  tip of shoot was removed, causing lateral bud growth. auxin paste applied to tip and lateral bud growth was inhibited. Not necessarily due to auxin alone. Could be due to exposure of other factors e.g. oxygen
• explain the second experiment done to investigate apical dominance

  Done by Thimann and Skoog. They applied a ring of auxin transport inhibitor below intact shoot apex. Lateral buds grew, further evidence that auxin inhibits lateral bud growth.
• explain the third experiment done to investigate apical dominance

  done by Gocal (1991). found that removal of apex leads to an increase in auxin concentration at the lateral bud.
• who investigated phototropism and what did they do?

  Investigated by Darwin (1880) who observed that shoot grew towards sunlight. Cut off tip of shoot and saw no growth. Shaded tip and saw no directional growth.
• who investigated role of water and solutes in order to display phototropism, what did they do, what did this prove?

  Boysen-Jensen (1913) inserted a sheet of mica (impermeable to water) through the shaded side of coleoptile. There was no curvature response. They then inserted it into the light side of the shoot tip and there as a curvature response. They then cut the tip off and placed a block of gelatine (permeable) between the stem and shoot tip. Normal curvature response was seen?

  This proved water and solutes need to be able to move backwards from the shoot tip to display phototropism.
• Who demonstrated a chemical messenger was involved in displaying phototropism and what did they do?

  Went (1926) placed coleoptile tips on agar and cut blocks from it. He then placed the block on one side a coleoptile which had its tip removed in the dark. The shoot displayed a curvature response. The more blocks he added, the greater the angle of the curvature response.
• gibberellins are responsible for:

  1. plant stem growth
  2. seed germination
  3. growth of side shoots
• explain experimental evidence for role of gibberellin on plant growth

  scientist used dwarf and tall varieties of peas (genetically identical apart form the Le gene)

  tall plants= LeLe

  dwarf plants= lele

  they measured levels of GA1 and found taller plants had more.

  they found the Le gene codes for an enzyme that converts GA20 to GA1
• explain the role of gibberellins in seed germination

  1. seed absorbs water
  2. causes production of GA by embryo
  3. GA has na effect in the aleurone layer
  4. aleurone layer produces amylase that breaks down starch into maltose to provide energy for growing embryo
• how are cytokinins involved in leaf abscission?

  they prevent senescence. they make sure leaves are sinks for the phloem. if cytokinin levels drop then the leaf ages and is then shed
• explain the role of auxins in leaf abscission

  auxins prevent abscission. They act on the abscission zone, as auxin levels fall over time, the leaf is more sensitive to ethene
• explain the role of ethene in leaf abscission

  causes an abscission layer to grow at the base of the petiole (leaf stalk) which is made of thin-walled cells. The balance of auxin and ethene causes an increase in cellulase which weakens the layer further, causing the leaf to fall.

  Before the leaf falls a protective layer forms which contains cells with lots of Suberin in their walls, preventing entry of pathogens
• explain how abscisic acid causes stomatal closure

  1. produced in roots when soil has decreased water potential
  2. translocated to leaves
  3. ABA binds to receptors on guard cell plasma membrane
  4. a cascade of events occurs which results in opening of calcium ion channels
  5. causes calcium ions to enter cell. pH also raises as proton pumps are inhibited
  6. this causes potassium ions, nitrates and chlorides to leave the cell
  7. the water potential in the guard cell increases and water moves out by osmosis
  8. guard cells become placid then close