Plant hormones and growth in plants Flashcards
what is tropisms?
directional growth in response to environmental cues
why is plant responses slower than that in animals?
they have no CNS
what are the 4 main plant hormones?
auxins
gibberellin
ethene
ABA (abscisic acid)
whats the role of auxins?
- cell elongation
- prevent leaf fall (abscission)
- maintain apical dominance
- involved in tropisms
- stimulate release of ethene
- involved in fruit ripening
what is the role of gibberellin?
- cause stem elongation
- trigger mobilisation of food stores in a seed at
germination - stimulate pollen tube growth in fertilisation
What is the role of ethene?
- causes fruit ripening
- promotes abscission in deciduous trees
Whats the role of ABA (abscisic acid)?
- maintains dominancy of seeds and buds
- stimulates cold protective responses- eg.
production of antifreeze - stimulates stomatal closing
why do plants produce these chemicals?
signal to other species- eg. to protect themselves from attack by insects
communicate with other plants
produce chemical defences against herbivores
Why are scientists still unsure about some aspects of plant responses?
plant hormones work at very low concentrations- isolating them and measuring changes is difficult
multiple interactions between the different chemical control systems = difficult to isolate the role of a single chemical in a specific response
Explain the role of hormones in the process of seed germination
seed absorbs water, embryo activated= gibberellins produced, stimulates production of enzymes that break down food stores (cotyledons in dicot seeds, endosperm in monocot seeds), these food stores used to produce ATP, to help grow and break out of seed coat
gibberellins also thought to switch on genes coding for amylase + protease = the digestive enzymes required for germination
ABA acts as antagonist to gibberellins (interferes with the action of gibberellins) = the relative levels of both these hormones determining which seeds will germinate.
Whats the evidence for the role of hormones in seed germination?
mutant variations of seeds produced without gene enabling them to make gibberellins= do not germinate, if gibberellins supplied to seeds externally= then germinate
if gibberellin biosynthesis inhibitors are applied to seeds = do not germinate, cannot make gibberellins needed for them to break dormancy. If inhibition removed or gibberellins applied, the seed germinates.
Where are auxins produced?
EG. indoleacetic acid (IAA)
are growth stimulants made in cells at tip of roots and shoots and in meristems
they can move up from rood or down from shoot in transport tissues, and move from cell to cell
the effect of the auxin depends on its conc and any interactions it has with other hormones
what 3 effects do auxins have on plant growth?
stimulate growth of main apical shoot
presence of auxins means cell wall stretches more easily- affects plasticity- auxins bind to receptors in cell membrane= decrease in PH to 5 = optimum PH for enzymes needed to keep walls very plastic and flexible.. The level of this hormone decreases with time, meaning cells become more rigid ad fixed in shape and size so they can no longer expand and grow.
role in apical dominance-
growth in the main shoot stimulated by the auxin produced at the tip so it grows quickly
auxins supress the growth of lateral shoots,, lower down the stem the auxin conc is lower= can grow laterally
if apical shoot is removed, the auxin-producing cells are gone= no auxin = lateral shoots are freed from dominance and grow faster. if auxin is applied artificially to the cut apical shoot, apical dominance is reasserted.
Low conc of auxins = slow root growth
up to a given conc, this is true
auxin produced by root tips and travels from growing shoots (in low concs)
if apical shoot is removed, amount of auxin reaching roots is greatly reduced, root growth slows and stops
replacing the auxin artificially at the cut apical shoot restores growth of roots
high auxin conc= inhibit root growth
how were gibberellins discovered?
they are produced by a fungus that affects rice, the infected seedlings grew extremely tall and thin
was then found that plants produce gibberellins-
there are over 100 naturally produced gibberellins
How and why do plants breed dwarf species of plants (involving gibberellins)?
HOW- the gibberellin synthesis pathway is interrupted
WHY- stems are much shorter, reduces waste and makes the plant less vulnerable to damage by weather and harvesting
