5.5.2 controlling plant growth Flashcards
apical dominance
inhibition of lateral buds further down shoot by chemicals produced by apical bud at tip of plant shoot
auxins
plant hormones which are responsible for regulating plant growth
gibberellins
plant hormones responsible for control of stem elongation & seed germination
(auxins) what happens if you break the shoot tip off a plant
plant starts to grow side branches from lateral buds that were previously dormant
apex
shoot tip
what was suggested about auxins from the apical bud
prevent lateral buds growing
- how did researchers test their hypothesis that the auxins from apical buds stop lateral buds growing?
- what are the issues with this?
- how did researchers deal with these issues?
applied paste containing auxins to cut end of shoot = lateral buds didn’t grow
issues:
- manipulation of plants could have unexpected effect
- upon exposure, cells in cut end of stem could have produced hormone that promoted lateral growth
solution:
- applied ring of auxin transport inhibitor below apex of shoot & lateral buds grew
what did scientists suggest about auxin levels
normal = inhibit growth
low = promote growth
why is it suggested that 2 other hormones are involved in lateral bud growth
a scientist found auxin levels in lateral buds of kidney beans increased when shoot tip cut off
(auxins) two other hormones potentially involved & how?
- abscisic acid = inhibits bud growth
- high auxin in root may keep abscisic acid levels high in bud
- when tip (source of auxin) removed = acid levels drop & bud starts to grow - cytokinins = promote bud growth
- directly applying can override apical dominance effect
- high levels of auxin = shoot apex becomes sink for cytokinins produced in roots
- apex removed = cytokinin spreads evenly around plant
what 2 processes are gibberellins involved in
- stem elongation
- seed germination
research conducted - giberellins effect on stem elongation (unnatural concentrations)
- scientists tested gibberellic acid on many different plants
- when applied to dwarf varieties of plants (eg. peas, maize, rosette plants) they grew taller
research conducted - giberellins effect on stem elongation (concentrations found naturally in plants)
- researchers compared GA1 levels of tall pea plants & dwarf pea plants (otherwise genetically identical)
- found plants with higher GA1 levels were taller
what did researchers require to demonstrate how GA1 (member of gibberellin family) directly causes stem growth
- needed to know how GA1 is formed
- found Le gene responsible for producing enzyme that converts GA20 to GA1
- tall pea plants were homozygous dominant for Le allele
- dwarf pea plants homozygous recessive for Le allele (le)
further research conducted on gibberellins effect on stem elongation (after understanding how hormone is made)
- researchers chose pea plant with mutation blocking gibberellin production between ent-kaurene & GA12-aldehyde in pathway = produce no gibberellin & grow to ~1cm tall
- researchers grafted shoot onto homozygous le plant (cannot convert GA20 to GA1) & it grew tall
- shoot (no GA20) has enzyme to convert GA20 to GA1, so used unused GA20 from normal plant
- confirmed GA1 causes stem elongation as shoot grw tall
what have further studies shown regarding gibberellin’s effect on stem elongation
shown gibberellins cause growth in internodes by stimulating cell elongation & division
how are gibberellins involved in seed germination
- when seed absorbs water, embryo releases gibberellin
- this travels to aleurone layer in endosperm region of seed
- gibberellin enables production of amylase = breaks starch into glucose
- provides substrate for respiration for embryo, so it grows
- glucose also used for protein synthesis
