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Biology Module 5 > Plant Hormones > Flashcards

Plant Hormones Flashcards

1
Q

what do plants respond to

A
  • abiotic (non-living)
  • biotic (living)
  • external stimuli
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2
Q

What are the type of chemical defences

A

Tannins
Alkaloids
Pheromones

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3
Q

Describe Tannins

A

these are toxic to microorganisms and large herbivores, in leaves they are found in the upper epidermis and make the lead taste bad in roots they prevent infiltration by pathogenic microorganisms

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4
Q

Describe Alkaloids

A

derived from amino acids, in plants they are a feeding deterrent to animals and taste bitter, they are located in the growing tips and the flowers and peripheral cell layers of stems and roots

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5
Q

Describe Pheromones

A

these are chemicals which are released by one individual and which can affect the behaviour and physiology of another

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6
Q

name the type of responses

A
  • phototropism
  • geotropism
  • chemotropism
  • thigmotropism
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7
Q

describe phototropism

A

this is when the shoot grows towards light as they are positively phototropic this enables them to photosynthesise

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8
Q

describe geotropism

A

this is when roots grow towards the pull of gravity, this anchors them in the soil and helps them take up water this is needed for support as it keeps the cell turgid and as a raw material for photosynthesis to help cool the plant, also minerals such as nitrates

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9
Q

describe chemotropism

A

one a flower pollen tubes grow down the style attracted by chemicals towards the ovary where fertilisation can take place

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10
Q

describe thigmotropism

A

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

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11
Q

if a plant responds towards a stimulus…

A

it is a positive tropic response

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12
Q

if a plant responds away from a stimulus…

A

it is still a tropic response but it is negative rather that positive

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13
Q

what is a non directional responses

A
  • Non – directional responses to external stimuli are nastic responses, Mimosa pudica responds to touch with a sudden folding of the leaves this is thigmonasty
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14
Q

what do hormones do in the plant

A
  • Hormones coordinate plant responses to environmental stimuli, chemical responses that can be transported away from their site of manufacture to act in other parts of the plant
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15
Q

What produces hormones

A
  • Produced by cells in a variety of tissues in the plant
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16
Q

How do hormones work

A
  • When the hormones reach their target cells they bind to receptors on the plasma membrane, specific hormones have specific shapes which can only bind to specific receptors with complementary shapes on the membranes of particular cells, specific binding makes sure that the hormones only act on correct tissue
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17
Q

some hormones have different effects on ……

A

different tissues some can amplify each other’s effects and some can even cancel out each other’s effects

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18
Q

How do plant hormones travel around the plant

A
  • Active transport
  • Diffusion
  • Mass flow into the phloem sap or in xylem vessels
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19
Q

names some hormones

A
  • Cytokinins
  • Abscisic acid
  • Auxins for example IAA
  • Gibberellins
  • Ethene
20
Q

Describe what the cytokinins does

A
  1. Promote cell division
  2. Delay leaf senescene
  3. Overcome apical dominance
  4. Promote cell expansion
21
Q

Describe what Abscisic acid does

A
  1. Inhibit seed germination and growth

2. Causes stomatal closure when the plant is stressed by low water availability

22
Q

Describe what Auxins IAA does

A
  1. Promote cell elongation
  2. Inhibit growth of side-shoots
  3. Inhibit leaf abscission (leaf falling)
23
Q

Describe what Gibberellins does

A
  1. Promote seed germination and growth of stems
24
Q

Describe what Ethene does

A
  1. Promotes fruit ripening
25
Q

Define Apical Dominance

A

nhibition of lateral buds further down the shoot by chemicals produced by the apical bud at the tip of a plant shoot

26
Q

Define auxins

A

plant hormones which are responsible for regulating plant growth

27
Q

Define gibberellins

A

plant hormones which are responsible for control of stem elongation and seed germination

28
Q

what happens if you break the shoot tip

A
  1. If you break the shoot tip, this is the part that the auxins are in, the plant starts to grow side branches from lateral buds that were previously dormant, therefore researchers suggested that auxins from the apical bud prevent the lateral buds from growing therefore when the tip is removed
29
Q

how did they test that the auxin was in the tip

A
  • To test this scientists applied a paste containing auxins to the cut end of the shoot and the lateral buds did not grow, however the manipulation of the plants could have an unexpected effect upon exposure to oxygen cells on the cut end of the stem could produce a hormone that promoted lateral bud growth because of this they applied a ring of auxin transport inhibitor below the apex of the shoot, the lateral buds grew
  • Therefore, they suggested that a normal auxin level inhibits growth whereas a low auxin level promotes growth,
30
Q

third variable that affects auxin level and growth inhibition

A
  • However, the two variables auxin levels and growth inhibition may have no effect on each other but could both be affected by a third variable
  • scientists now realised that when they cut the tip of the auxin levels increase so now they think two other hormones are involved these are
  • abscisic acid – inhibits bud growth, high auxin in the shoot may keep abscisic acid levels high in the bud, when the tip is removed abscisic acid levels drop and the bud starts to grow
  • cytokinins promote bud growth – directly apply cytokinin to buds can override the apical dominance effect, high levels of auxin make the shoot apex a sink for cytokinin’s produced in the root, most of the cytokinin goes to the shoot apex, when the apex is removed cytokinin spreads evenly around the plant
31
Q

How did they test for gibberellins

A
  • Scientists tested gibberellic acid on many different plants, when they applied to dwarf varieties of plants they grew taller
  • Gibberellic acid is responsible for plant stem growth
  • But just because GA3 can cause stem elongation it does not mean that it does so in nature, this is because the experiment does not work within concentrations of gibberellins naturally found in plants and in parts of the plants that gibberellin molecules normally reach
  • To meet this criteria they compared GA1 levels of tall pea plants and dwarf pea plants which were otherwise genetically identical, they found that plants with higher G1 levels were taller
  • To show that GA1 directly causes stem growth they need to know how GA1 is formed, they worked out that the le gene was responsible for producing the enzyme that converted GA20 to GA1
  • Then the researchers chose a pea plant with a mutation that blocks gibberellin production between ent-kaurene and GA12-aldehyde in the pathway shown those plants produce no gibberellin and grow to only about 1cm tall, the researchers grafted a shoot onto a homozygous le plant and it grew tall
  • A shoot with no GA20 of its own does have the enzyme to convert GA20 to GA1 and it can use the unused GA20 from the normal plant because the shot grew tall this confirmed that GA1 causes stem elongation
  • Cause internodes by stimulating cell elongation (loosening cell walls) and cell division ( by stimulating production of a protein that controls the cell cycle)
32
Q

Describe seed germination

A
  • Gibberellins also promote seed germination, this is when the seed absorbs water, the embryo releases gibberellin which travels to the aleurone layer in the endosperm region of the seed
  • Gibberellin enables the production of amylase which can break starch into glucose this provides substrate for respiration for the embryo and so it grows
  • Glucose is also used for protein synthesis
33
Q

Where is a plant does growth occurs

A
  • Apical meristems are at the tips or apices of roots and shoots and are responsible for the roots and shoots getting longer
  • Lateral bud meristems are found in the buds these can give rise to side shoots
  • Lateral meristems forming a cylinder near the outside of the roots and shoots are responsible for the roots and shoots getting wider
  • In some plants intercalary meristems are located between the nodes where leaves and buds branch off the stem, growth between the nodes is responsible for the shoot getting longer
34
Q

Phototropic responses can be investigated

A
  • Uses an experimental plant and a control plant
  • The control plant is illuminated from all sides while the experimental plant has illumination from one side, shoots and roots are marked every 2mm at the start
  • Shoot has bent towards the light because the shady side of the shoot has elongated more that the illuminated side, the mean and standard deviation of the lengths between the marks has increased on the shady side
35
Q

Investigate geotropic responses

A
  • Control plant is constantly spun by a machine called a klinostat to ensure the effect of gravity is applied equally to all sides of the plant, for the experimental plant the klionstat is not switched on so gravity is only applied to one side
  • In experimental plant the roots bend downwards because the upper side of the root has elongated more than the lower side, the shoot bends upwards because the lower side, the shoot bends upwards because the lower side of the shoot has elongated more than the upper side,
  • In the control plant both root and shoot grow horizontally,
  • Take care not to handle main plugs, switches, lamps or the kilnostat with wet hands
36
Q

Phototropism

- Are plant hormones involved

A
  • Darwin experiments confirmed that the shoot tip was responsible for phototropic responses
  • Boysen-Jensen work confirmed that the water and/or solutes need to be able to move backwards from the shoot tip for phototropism to happen
  • When a permeable gelatine block was inserted behind the shoot tip the shoot still showed positive phototropism
  • When an impermeable mica block was inserted there was no phototropic response
37
Q

Photoropism

How did they confirm the chemical messenger auxin

A
  • To confirm the role of auxin as the chemical messenger agar blocks impregnated with different concentrations of auxin give the same result
  • Series of block of different concentrations of auxin created by serial dilution gives shoot curvature in proportion to the amount of auxin
  • Auxin produced at the apex of the shoot, the auxin travels to the cells in the zone of elongation causing them to elongate and make the shoot grow
  • When light was equal on all sides the auxin simply promotes shoot growth evenly
  • However combining the conclusions from the experiment above light shining on one side of the shoot appears to cause auxins to be transported to the shaded side causing the cells there to elongate more quickly making the shoot bend toward the light
  • The extent to which cells elongate is proportional to the concentration of auxins
38
Q

Describe how auxin works

A
  • Auxin increases the stretchiness of the cell wall by promoting active transport of H+ by an ATPase enzyme on the plasma membrane into the cell wall
  • The low Ph provides optimum conditions for wall-loosening enzymes to work, these enzymes break bonds within cellulose so the walls become less rigid and can expand as the cell takes in water
  • How light redistributes auxin is still uncertain, two enzymes have been identified these are phototropin 1 and phototropin 2 – these are promoted by blue light
  • Blue light is the main component of white light that causes the phototropic response, therefore there is lots of phototropin 1 activity on the light side but progressively less on the dark side
  • The gradient is thought to cause the redistribution of auxins through their effect on PIN proteins, these transmembrane proteins can be found dorsally ventrally or laterally on the plasma membrane of cells – control the efflux of auxin from each cell therefore they send auxin in different directions in the shoot depending on their location on the plasma membrane
  • PIN proteins is controlled by a different molecule called PINOID, one theory is the idea that phototropins affect the acitivty of PINOIDS which then affects PIN activity but this may only be for pulse induced phototropism
39
Q

Auxin in geotropic responses of roots

A
  • In a root lying flat auxin accumulates on the lower side this inhibits cell elongation and the upper side continues to grow and the root bends downwards
  • The effect of auxin in roots is in contrast to that in the shoot where auxin promotes cell elongation on the lower side making the shoot lying flat bend upwards
  • Root and shoot cells in the elongation zone exhibit difference responses to the same concentrations of auxin
40
Q

Commercial uses of auxins

A
  • Used to prevent lead and fruit drop and to promote flowering
  • Take cuttings – dip the end of a cutting in rooting powder before planting it encourages root growth, rooting powder contains auxin and talcum powder
  • Seedless fruit – treating unpollinated flowers with auxin can promote growth of seedless fruit, applying auxin promotes ovule growth which triggers automatic production of auxin by tissues in the developing fruit to help complete the developmental process
  • Herbicide – auxins are used as herbicides to kill weeds, this is because they are manmade, plants find them difficult to break down and they can act within the plant for longer, they promote shoot growth so much that the stem cannot support itself
41
Q

Commercial uses of cytokinins

A
  • As cytokinins can delay leaf senescence they are sometimes used to prevent yellowing of lettuce leaves after they have been picked
  • Used in tissue culture to mass produce plants, promote bud and shoot growth from small pieces of tissue taken from a parent plant, produces a short shoot with lots of side branches that can then be split into lots of small plants which are then growth separately
42
Q

Commercial uses of ethene

A
  • Solution can be sprayed and is easily absorbed releasing ethene slowly inside of the plant
  • Speeds up fruit ripening in apples tomatoes and citrus fruits
  • Promote fruit drop in cotton, cherry and walnut
  • Promote female sex expression in cucumbers reducing the change of self-pollination and increasing yield
  • Promote lateral growth in some plants and yield compact flowering stems
  • Preventing ethene synthesis can prevent fruit from ripening good for shipping
  • Silver salts can also inhibit ethene and increase shelf life
43
Q

commercial uses of gibberllins

A
  • fruit production
  • brewing
  • sugar production
  • plant breeding
44
Q

describe fruit production

A

Delay senescence in citrus fruit, therefore extending the time fruits can be left unpicked and making them last longer in the shop, Acting with cytokinins can make apple elongate to improve shape, Grape stalks elongate therefore they are less compact and get bigger than if they are compact where they cannot grow

45
Q

describe brewing

A

To make beer you need malt, this is usually produced in a malt house in a brewery, when barley seeds germinate the aleurone layer of the seed produces amylase enzymes that break down stored starch into maltose.Genes for amylase production are switched on by naturally occurring gibberellins and adding gibberellins speed up the process, malt is then produced by drying and grinding up the seeds

46
Q

describe sugar production

A

Spraying sugar cane with gibberellins stimulates growth between the nodes making the stems elongate, this is useful because the sugar cane stores sugar in the cells of the internodes making the sugar available from each plant

47
Q

describe plant breeding

A

Plants breeders job is to produce plants with desired characteristics, by breeding together other plants usually over many generations, in conifer plants this can take a long time as they don’t become reproductively active for a long time, Gibberellins speed up the process by inducing seed formation on young trees, Seed companies that want to harvest seeds from biennial plants can add gibberellins to induce seed production, Stopping plants making gibberellins is also useful, praying with gibberellin synthesis inhibitors can keep flowers short and stocky and ensures that the internodes of crop plants stay short helping to prevent lodging, this happens in wet summers when stems bend over because of the weight of the water collected on the ripened seed heads making the crop difficult to harvest

Biology Module 5 (12 decks)

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