L4 and 5 Alistair Heatherington - Water and Food Security

Question 1

What is water movement driven by?

How much of the water taken into the plant is lost though this?

Answer 1

Evapotranspiration
97%
eg an oak tree loses 120kg water daily

Question 2

What are 3 pathways water can move through a plant?

Answer 2

1. Transmembrane - cell-cell across membranes
2. Symplastic - intracellular, through cyttoplasm in plasmodesmata
3. Apoplastic - No cell entry, stopped y casparian strip so water must enter endodermal cells.

Question 3

What does the Casparian strip separate?

Answer 3

Root cortex (outer), endodermis (inside)

Question 4

Why is the casparian strip important?

Answer 4

Provides a selectivity filter, as water and minerals must pass through an endodermal cell before entering the xylem. Acts as a barrier to toxins and prevents water leaking back down from xylem.

Question 5

Why are stomata important?

Answer 5

Gives plant ability to withstand droughts
Controls evapotranspiration, and therefore the mineral and water supply.
Controls CO2 uptake, affecting photosynthesis and dry matter accumulation

Question 6

What affects how open the stomatal pore is?

Answer 6

Degree of turgor pressure in the guard cells

Question 7

What trade off does the plant face regarding stomata>

Answer 7

open - photosynthesis but water loss. Must balance plant needs.

Question 8

How big are stomata?

Answer 8

When fully open, aperture is up to 20micrometres

Question 9

What is a good measure to quantify how plants respond to environmental stimuli?

Answer 9

Stomatal aperture

Question 10

What 4 environmental stimuli would cause stomata to open?

Answer 10

High relative humidity
High light intensity
Blue light
Low CO2

Question 11

What 4 environmental stimuli would cause stomata to close?

Answer 11

ABA build up
Darkness
High CO2
Low relative humidity

Question 12

What is the role of Guard cells?

Answer 12

Integrate information from environmental signals to set best stomatal aperture to suit conditions balance the need of CO2 to restrict water loss.

Question 13

Upon stimulus, Plasma membrane events cause cytosolic coupling events. What 5 responses does this lead to, resulting in changes in stomatal aperture?

Answer 13

1. Alterations to ion transport - K and Cl enter guard cell, causing water to also enter, guard cell swells and opens pore.
2. Alterations to membrane trafficking - more membrane is made inside the guard cell, and added to outer membrane to allow swelling
3. Changes to metabolism - ATP is needed for these activities
4. Changes to gene expression
5. Changes to cytoskeleton

Question 14

How do plants respond to ABA

Answer 14

1. ABA receptors in cytosol of guard cell - PYR/PYL/RCAR, protein family that bind to ABA and are activated.
2. When activated, they activate other components in the signalling pathway. eg. SNRK2 - protein kinase, regulates the activity of ion channels in the membrane by phosphorylating them.
   3.Activated ion channels facilitate loss of anions from guard cells causing depolarisation of the membrane, activating K channels so loss of K+.
   Ca2+ is another intracellular messenger. Increase in Ca2+ detected by protein kinases which increase their activity.

Question 15

How do plants respond to relative humidity decrease?

Answer 15

Stomata close, causing temperature of leaf to increase

aba2 and ost1 mutants did not close stomata, so aba and ost genes must be involved in signal transduction.

Question 16

How can you identify plants which don’t respond to drop in relative humidity?

Answer 16

Use thermal imaging - when stomata open, leaf is cooler

Question 17

How do plants respond to CO2 conc changes.

Answer 17

ABA is involved in signalling.
Tested by Chater et al 2015 - Tested some ABA receptor mutants, found that double mutant which fails to make ABA had no response to changing CO2 levels.
Also other mutants had less response than WT.
Mechanism is not known

Question 18

evidence of stomatal evolution

Answer 18

Present in all plant lineages apart from liverworts, in fossils Cooksonia if the earliest plant with stomata, 420mya
Fossilized stomata look v similar to modern day ones.
Salaginella is a primitive lycophyte, can study stomata to compare to more modern - v similar response pathway.

Question 19

How does blue light induce stomatal opening?

Answer 19

Phot1 and phot2 photoreceptors in guard cells.
Protein kinase Blos1 activates H+ ATPase on membrane.
H+ pumped out of cell. Hyperpolarisation changes voltage of plasma membrane.
Activates inward K+ channels, K+ enters, H2O follows.
Stomata opens.

Question 20

How does Stomatal opening acquire ATP?

Answer 20

1. Mitochondrial oxidative phosphorylation

2. Chloroplastic photophosphorylation

Question 21

How do subcellular lipid droplets relate to stomatal aperture?
How can this be demonstrated?

Answer 21

Inverse correlation
In dark, Lipid droplets are abundant.
Can stain lipids with niol red to view.
Compare lipid content of WT and phot 1 and 2 mutants: in dark - no difference, by 4h light - 25% higher in mutants and smaller aperture. Difference in how lipids are metabolised.

Question 22

What lipids are metabolised to give ATP for stomatal opening?

Answer 22

TAGs - Tri Acyl Glycerols
Via a process of B oxidation, in the peroxisome.
Specific TAGs decline in guard cells during light induced opening of stomata at dawn

Question 23

What is the mechanism for B oxidation

Answer 23

SDP1 is an enzyme that breaks TAGs into 3xFA + glycerol, activated by blue light received by phot1/2.
FA transported into peroxisome by CTS1
B oxidation occurs inside peroxisome

OR: Feedback mechanism triggered by reduction of ATP
There are multiple mechanisms, plants show redundancy.

Question 24

How was the mechanism for B Oxidation tested>

Answer 24

Using sdp1-4 and sdp1-5 mutants (separate mutations in the same gene)
If b oxidation is interfered with, measure of H+ out of cell would reduce.
Found that all mutants reduce to the same amount as WT eventually but some much faster than others.

Can also test by triggering H+ ATPase w/o light, using fungal toxin which keeps ATPase in the active configuration.

Question 25

What are predictions for by 2030?

why?

Answer 25

50% increase in energy demand
50% increase food demand
30% increase water demand
Due to increasing popuation, urbanisation, goal to alleviate poverty and climate change.

Question 26

what is WUE and how is it determined?

Answer 26

Water Use efficiency

determined by net CO2 assimilation and water loss.

Question 27

How could controlling stomatal conductance be beneficial for WUE?

Answer 27

Transpiration increases linearly with stomatal conductance, whereas CO” assimilation saturates at high conductance.
So can maybe reduce conductance by controlling no. stomata, allowing transpiration to reduce but not CO2 assimilation.

Question 28

How do stomata form?

Answer 28

EPFs - Epidermal patterning factors, are secreted signals which bind to the cell surface receptor to activate an intracellular MAP kinase cascade, then phosphorylates a transcription factor required for cells to develop into stomata.

Question 29

What EPFs are involved?

Answer 29

EPF1 and EPF2 are inhibitors - double mutants have increased stomata whereas overexpressed EPF1/2 have reduced stomatal density.
STOMAGEN [EPFL9] - is an activator. Decreased EPFL9 have reduced SD.

Question 30

How has the impact of altering stomatal number on transpiration been shown

Answer 30

Use thermal imaging the compare water loss between different EFP mutants.
Mutants with high SD have more transpiration and are cooler.
Could use mutants to increase WUE
Found EPF2 OE = high WUE