topic 9.3

Question 1

what is auxin

Answer 1

• auxin is a plant hormone that plays a role in the cell elongation, promotion of root growth and suppression of lateral buds to promote apical dominance

Question 2

what effect do auxins have on cell walls

Answer 2

• they affect the ability of cell walls to stretch

Question 3

where are auxins synthesized and how do they travel down a plant

Answer 3

• synthesised in the meristems
• diffuse down the plant

Question 4

cell elongation

Answer 4

1. auxin binds to receptors on cell membrane
2. hydrogen ion pump is activated so pH decreases
3. enzyme breaks bonds between cellulose microfibrils
4. water enters by osmosis and cell elongates

Question 5

how does elongation stop

Answer 5

• eventually enzymes destroy the auxin’s - stopping elongation

Question 6

root growth

Answer 6

• auxin’s actively transported down the plant towards the roots
• the more the auxin’s are transported down the stem, the more the root growth

Question 7

what are tropisms

Answer 7

• they are directional growth responses to specific environmental cues

Question 8

what are the two type of tropisms

Answer 8

• phototropism
• gravitropism

Question 9

phototropism

Answer 9

• plant shoots grow toward light unilaterally
• positive phototropism
• plant roots grow away from light
• negative phototropism

when light hits the shoots of the plant, auxin’s move to shaded side and promote elongation towards the light

Question 10

gravitropism

Answer 10

• plant shoots grow against gravity
• negative gravitropism
• plant roots grow in the direction of gravity
• positive gravitropism

auxin’s move down the plant and promote elongation

Question 11

what are cytokinins

Answer 11

• cytokinins promote cell division in apical meristems and cambium, promote lateral bud development and work with ethene in abscission of leaves

Question 12

high levels of cytokinin?

Answer 12

• keeps the leave healthy and alive

Question 13

low levels of cytokinin

Answer 13

• leaf dies and falls

Question 14

gibberellins

Answer 14

• stimulate elongation of cells, growth of fruit and breaking dormancy in seeds

Question 15

how do gibberellins germinate

Answer 15

1. the embryo secretes gibberellins that diffuse into the aleurone layer
2. aleurone layer produces amylase to digest the carbohydrate stores of endosperm
3. products from endosperm are used to make new cells and germinate

Question 16

synergy

Answer 16

• when two hormones work together complimenting each other and giving greater response together

Question 17

antagonism

Answer 17

• when two hormones have opposite effects and the balance between then determines response

Question 18

apical dominance

Answer 18

• this is when one lead shoot grows bigger and faster than the others

Question 19

auxin - apical dominance

Answer 19

• high auxin levels from dominant shoot inhibits lateral bud growth
• when the shoot grows further away, the inhibition of auxin is reduced (cytokinin dominant)
• as level of auxin reduces down the stem, lateral buds start to develop
• if main shoot is removed, source of auxin is removed so cytokinin is dominant → lateral buds can grow

Question 20

cytokinin and auxin work…

Answer 20

antagonistacally

Question 21

what is phytochrome

Answer 21

• a plant pigment that reacts differently with different type of light and affects the responses of plants

Question 22

what are the two forms of phytochrome pigment

Answer 22

Pr and Pfr

Question 23

what converts Pr to Pfr

Answer 23

red light

Question 24

what converts Pfr to Pr

Answer 24

far red light

Question 25

how is Pr converted to Pfr

Answer 25

- a seedling germinates and makes Pr - it breaks through the surface of the soil and is exposed to red light - this causes Pr to be converted Pfr

Question 26

when do short day plants flower

Answer 26

autumn

Question 27

short day plants

Answer 27

- short days and long nights - not a lot of light - **high levels of Pfr inhibit flowering** - longer nights so most Pfr is converted to Pr - Pfr levels **fall** - hence flowering can occur - short daytime = Pr →Pfr; Pfr inhibits flowering - long nightime = Pfr ## Footnote flower in spring and autumn

Question 28

long day plants

Answer 28

- long days and short nights - lots of light - high levels of Pfr stimulate flowering - short nights so little Pfr is converted back to Pr - Pfr levels maintained are high - hence flowering occurs - long daytime = Pr → Pfr; Pfr stimulates flowering - nightime = more Pr than Pfr ## Footnote flower in summer

Question 29

photomorphogenesis

Answer 29

- the process by which the form and development of a plant is controlled by the levels of and type of light

Question 30

ehat are the two forms phytochrome converts between

Answer 30

- biologically inactive Pr which absorbs red light - biologically active Pfr absorbs far red light

Question 31

etiolated plant

Answer 31

- all phytochrome is in the Pr form - these grow rapidly using food reserves in attempt to reach light - tall and thin - small yellow leaves - little root growth

Question 32

what does phytochrome control

Answer 32

- the changes that take place when a plant becomes etiolated - the reverse when it reaches light when germinating seeds break through soil are controlled by phytochrome

Question 33

germination - phytochrome

Answer 33

- phytochrome is synthesised as Pr - when a seedling energes from a seed underground it only contains Pr as there is no light to produce Pfr - seedling shows characteristics of etiolation - stem lengthens quickly, doesn't thicken, little root growth, no leaf growth, no chloropyll - when shoot breaks through surface, stem elongation slows down, leaves open, chloropyll forms, seedling starts to photosynthesise - changes are caused by Pfr

Question 34

how does phytochrome act as a transcription factor

Answer 34

- involved in switching genes on and off in the nucleus of plant cells

Question 35

phytochrome as a transcription factor

Answer 35

- Pr is converted to Pfr in the presence of light - Pfr moves into the nucleus through pores in the nuclear membrane - Pfr binds to protein phytochrome interacting factor, PIF3, in nucleus - PIF3 **activates gene transcription** when it is bound to Pfr - genes activated by PIF3 control different aspects of growth and development in plants

Question 36

what is PIF3

Answer 36

- PIF3 is a transcription factor which only binds to Pfr