Plant responses to internal + external stimuli

Question 1

Briefly describe how plant cells respond to stimuli

Answer 1

1. hormone or enviro stimulus binds to receptor
2. relay proteins + 2nd messengers in cascade of transduction events
3. activation of cellular responses

Question 2

How were plant growth regulators discovered?

Answer 2

Coleoptile bent towards the light

Phototropic response only when tip is illuminated

Phototropic response only when tip separated by permeable barrier

Question 3

Describe the conclusions formed when auxin was discovered

Answer 3

Derived from coleoptile tip
Diffusible
Present at higher conc on dark side of shoot

Question 4

What is required for plants to respond to hormones?

Answer 4

Different hormones must interact in the right ratios and balance

Question 5

What does IAA stand for?

What is it, briefly?

Answer 5

Indole acetic acid

A common auxin

Question 6

How is auxin transported?

Answer 6

Tip to base polar transport

Question 7

What are auxins?

Answer 7

Any substance that promotes elongation of coleoptiles

Question 8

What does auxin do?

Answer 8

Stimulates cell elongation in young developing shoots
by binding to auxin receptors + transporters at the plasma membrane
-> permits fluxes in auxin conc in target cells

Question 9

What can high auxin conc do?

Answer 9

Inhibits growth by inducing the production of other hormones
e.g. ethylene which inhibits growth

Question 10

What is the acid growth hypothesis?

Answer 10

Ability of plant cells + cell walls to elongate or expand quickly at low pH.
This form of growth does not involve an increase in cell no.

Question 11

Describe the acid growth hypothesis

Answer 11

1. auxin increases proton pump activity 
= pumps H+ into cell wall from cytoplasm
2. cell wall becomes more acidic 
3. low pH activates expansins which separate microfibrils from cross-linking polysaccharides 
4. cleaving allows microfibrils to slide
5. cell can elongate

Question 12

What does the polar transport of auxin play a role in?

Answer 12

Pattern formation of developing plant
- reducing auxin flow from shoot of a branch stimulates growth in lower branches (=apical dominance)

Leaf venation pattern

Phyllotaxy

Question 13

What is IBA?

What does it do?

Answer 13

Indolbutyric acid
= an auxin

Stimulates adventitious root formation

Question 14

What are synthetic auxins used for?

Answer 14

> preventing leaf abscission
preventing fruit drop
promoting flowering + fruiting
controlling weeds

Question 15

What is 2,4-D?

What is it used for?

Answer 15

A synthetic auxin

Overdose can kill plants
= herbicide

Question 16

What do cytokinins do?

Answer 16

Stimulate cytokinesis

cell division

Question 17

What are all cytokinins similar to?

Answer 17

Adenine

Question 18

Where is cytokinin synthesised?

Answer 18

Mainly in roots

also embryos + fruit

Question 19

How is cytokinin transported?

Answer 19

Non-polar fashion

through xylem, phloem + parenchyma cells

Question 20

Which hormone to cytokinins work with?

Answer 20

Auxin

• to regulate cell division _ differentiation
• ratio is important*

Question 21

What do high concs of auxin alone promote?

What about high concs of cytokinin alone?

Answer 21

Cell expansion

Cell division

Question 22

What is apical dominance?

Answer 22

A terminal bud’s ability to suppress development of axillary buds

Question 23

How are auxins and cytokinins involved in apical dominance?

Answer 23

Auxins - maintaining dominance of apical bud

Cytokinins - stimulate axillary buds

Cytokinin acts antagonistically to auxin in apical dominance

Question 24

What is the direct inhibition hypothesis?

Answer 24

Auxin from apical buds travels down shoots to inhibit axiliary bud growth
–> promotes shoot growth, + restricts lateral branching

Cytokinin moves from the roots into the shoots, eventually signaling lateral bud growth

Question 25

How do cytokinins delay leaf senescence?

Answer 25

> inhabit protein degradation > stimulate RNA + protein synthesis > mobilise nutrients from surrounding tissues

Question 26

What are the roles of gibberellins (GAs)?

Answer 26

Enhance cell elongation + cell division | Can induce dwarfing mutants to grow tall

Question 27

Which hormone is responsible for 'bolting' during the switch from vegetative to reproductive growth?

Answer 27

Gibberellins

Question 28

Which 2 hormones must be present in most plants for fruit to develop?

Answer 28

Auxins + Gibberellins

Question 29

Which hormone is sprayed onto Thompson seedless grapes? | Why?

Answer 29

Gibberellins No seeds to produce the hormone

Question 30

How are gibberellins involved in germination?

Answer 30

After taking on water... 1. GAs send signal to aleurone 2. Aleurone secretes alpha-amylase + other enzymes 3. Sugars + other nutrients are consumed

Question 31

What does ABA stand for?

Answer 31

Abscisic acid

Question 32

Where is ABA produced + how is it transported?

Answer 32

Most cells Non-polar fashion

Question 33

What is the overall role of ABA?

Answer 33

Slows growth - acts as antagonist to other hormones

Question 34

What are the 2 effects of ABA?

Answer 34

Seed dormancy - stops germination at inappropriate times of year etc Drought tolerance

Question 35

How is seed dormancy broken?

Answer 35

When ABA is removed by heavy rain, light or prolonged cold

Question 36

What determines whether a seed remains dormant or germinates?

Answer 36

Ratio of GA to ABA

Question 37

How is ABA involved in drought tolerance?

Answer 37

ABA accumulation causes stomata to close rapidly | due to massive efflux of K+ ions

Question 38

What are the effects of ethylene?

Answer 38

> response to stresses > senescence > leaf abscission > fruit ripening

Question 39

What is ethylene synthesis inhibited by?

Answer 39

CO2

Question 40

What can high auxin levels stimulate the synthesis of?

Answer 40

Ethylene

Question 41

What is the triple response to mechanical stress?

Answer 41

Slowing stem elongation , thickening the stem + horizontal growth in order to avoid obstacles

Question 42

Which hormone induces the triple repose to mechanical stress?

Answer 42

Ethylene

Question 43

What is a burst in ethylene production associated with?

Answer 43

Apoptosis

Question 44

What does a change in the auxin-ethylene balance lead to?

Answer 44

Leaf abscission

Question 45

What is the ripening of fruit?

Answer 45

Enzymatic breakdown of cell wall components + conversion of starches + acids to sugars

Question 46

Which hormone leads to fruit ripening?

Answer 46

Ethylene

Question 47

What is the effect of light on morphology called?

Answer 47

Photomorphogenesis

Question 48

What are the 2 main classes of photoreceptor?

Answer 48

Blue-light | Red/far-red (phytochromes)

Question 49

What do blue-light photoreceptor control?

Answer 49

Hypocotyl elongation Stomatal opening Phototropism

Question 50

What do phytochrome control?

Answer 50

``` Seed germination Shade avoidance Photoperiodism Seasonal responses Flowering ```

Question 51

How does phytochrome work?

Answer 51

Red light causes conformational change in protein from Pr to Pfr -> responses: seed germination, flowering control etc

Question 52

What is phytochrome reversed by?

Answer 52

Far-red light

Question 53

What does phytochrome conversion mark?

Answer 53

Sunrise + sunset = thus provides biological clock w/ environmental cues

Question 54

What is photoperiodism?

Answer 54

A physiological response to photoperiod | = environmental stimulus that enables plants to sense time of yr

Question 55

What is crucial for the control of flowering?

Answer 55

The length of the dark period

Question 56

What are the other internal + external stresses that plants respond to?

Answer 56

Gravitropism Thigmotropism Abiotic stresses - drought, flooding, salt, heat, cold Biotic - herbivores, pathogens