5.5.3 plant responses: investigating tropisms

Question 1

define geotropism

Answer 1

directional growth in response to gravity

Question 2

define phototropism

Answer 2

directional growth in response to light

Question 3

where does plant growth happen

Answer 3

• apical meristems = tips of roots/shoots & responsible for roots/shoots getting longer
• lateral bud meristems = found in buds & give rise to side shoots
• lateral meristems = form cylinder near outside of roots/shoots & responsible for roots/shoots getting wider
• (some plants) intercalary meristems located between nodes (leaves/buds branch off) & growth between nodes is responsible for shoot getting longer

Question 4

how can phototropic responses be investigated

Answer 4

• experimental plant & control plant (with 10 replicates)
• control plant illuminated from all sides
• experimental plant illumination on 1 side
• in each plant, the shoots/roots are marked every 2mm at the start

Question 5

result of phototropic responses investigation

Answer 5

• (after several days) the shoot has bent more towards the light, as the shaded side has elongated more
• mean & standard deviation of the lengths between the marks has increased on the shady side

Question 6

how can geotropic responses be investigated

Answer 6

• control plant constantly spun (slowly) by machine called a klinostat = ensures effect of gravity applied equally to all sides
• for experimental plant, the klinostat is not switched on, so gravity only applied to 1 side

Question 7

result of geotropic responses investigation

Answer 7

• in experimental plant, the root bends downwards (as upper side of root has elongated more than lower side) & the shoot bends upwards (as lower side of shoot has elongated more than upper side)
• in control plant, the root & shoot grow horizontally

Question 8

investigating the effect of plant hormones on phototropisms

Answer 8

experiments confirmed a chemical messenger from shoot tip is responsible for phototropic responses:

• darwin = confirmed shoot tip was responsible for phototropic responses
• boysen-jensen = confirmed water &/or solutes need to be able to move backwards from shoot tip for phototropism to occur (when permeable gelatine block inserted behind shoot tip, shoot still showed positive phototropism BUT when impermeable mica block inserted, no phototropic response)
• went = showed chemical messenger existed & could stimulate phototropic effect artificially (placed shoot tip on agar where auxin diffused into agar block, placed on plant in place of shoot & stimulated phototropic response)

Question 9

how was auxin confirmed as the chemical messenger

Answer 9

• agar blocks impregnated with different concentrations of auxin give same results
• using series of blocks of different auxin concentrations created by serial dilution = gives shoot curvature in proportion to amount of auxin

Question 10

where is auxin produced & where does it travel

Answer 10

• produced at apex of shoot
• auxin travels to cells in zone of elongation = makes them elongate, causing the shoot to grow
• when light equal on all sides, the auxin promotes shoot growth evenly

Question 11

what happens to auxin when light shines on one side of the shoot

Answer 11

• causes auxins to be transported to shaded side = causes cells to elongate more quickly
• makes shoot bends towards light
• extent to which cells elongate is proportional to concentration of auxins

Question 12

the mechanism of auxin’s effect (how does auxin work?)

Answer 12

• auxin increases stretchiness of the cell wall by promoting active transport of H+ by ATPase enzyme on plasma membrane, into cell wall
• resulting low pH = optimum conditions for wall-loosening enzymes (expansins) to work
• these enzymes break bonds within cellulose (& increased H+ ions also disrupt H bonds within cellulose) = cell walls become less rigid & can expand as cell takes in water

Question 13

how does light cause redistribution of auxin

Answer 13

= still unclear

• 2 enzymes identified (phototropin 1 & phototropin 2) = activity is promoted by blue light
• blue light is the main component of white light that causes phototropic response (eg. lots of phototropin 1 activity on light side but progressively less towards dark side)
• this gradient thought to cause redistribution of auxins due to effective PIN proteins

Question 14

what are PIN proteins

Answer 14

• transmembrane proteins found dorsally, ventrally or laterally on plasma membranes of cells
• control effluent of auxin from each cell (essentially sending auxin in different directions in shoot) depending on their location on the plasma membrane

Question 15

what is the activity of PIN proteins controlled by

Answer 15

= PINOID

• 1 theory suggests phototropins (1/2) affect activity of PINOID, which then affects PIN activity
• recent evidence (from arabidopsis) suggests this may only work for pulse-induced phototropism (short bursts of light), with another independent mechanism able to operate in continuous light

Question 16

how is auxin involved in geotropic responses of roots

Answer 16

• in a root lying flat, went discovered auxin accumulates on lower side = inhibits cell elongation
• the upper side continues to grow & root bends downwards
• happens because root & shoot cells in the elongation zone exhibit different responses to same concentrations of auxin