L4 and 5 Alistair Heatherington - Water and Food Security Flashcards
What is water movement driven by?
How much of the water taken into the plant is lost though this?
Evapotranspiration
97%
eg an oak tree loses 120kg water daily
What are 3 pathways water can move through a plant?
- Transmembrane - cell-cell across membranes
- Symplastic - intracellular, through cyttoplasm in plasmodesmata
- Apoplastic - No cell entry, stopped y casparian strip so water must enter endodermal cells.
What does the Casparian strip separate?
Root cortex (outer), endodermis (inside)
Why is the casparian strip important?
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.
Why are stomata important?
Gives plant ability to withstand droughts
Controls evapotranspiration, and therefore the mineral and water supply.
Controls CO2 uptake, affecting photosynthesis and dry matter accumulation
What affects how open the stomatal pore is?
Degree of turgor pressure in the guard cells
What trade off does the plant face regarding stomata>
open - photosynthesis but water loss. Must balance plant needs.
How big are stomata?
When fully open, aperture is up to 20micrometres
What is a good measure to quantify how plants respond to environmental stimuli?
Stomatal aperture
What 4 environmental stimuli would cause stomata to open?
High relative humidity
High light intensity
Blue light
Low CO2
What 4 environmental stimuli would cause stomata to close?
ABA build up
Darkness
High CO2
Low relative humidity
What is the role of Guard cells?
Integrate information from environmental signals to set best stomatal aperture to suit conditions balance the need of CO2 to restrict water loss.
Upon stimulus, Plasma membrane events cause cytosolic coupling events. What 5 responses does this lead to, resulting in changes in stomatal aperture?
- Alterations to ion transport - K and Cl enter guard cell, causing water to also enter, guard cell swells and opens pore.
- Alterations to membrane trafficking - more membrane is made inside the guard cell, and added to outer membrane to allow swelling
- Changes to metabolism - ATP is needed for these activities
- Changes to gene expression
- Changes to cytoskeleton
How do plants respond to ABA
- ABA receptors in cytosol of guard cell - PYR/PYL/RCAR, protein family that bind to ABA and are activated.
- 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.
How do plants respond to relative humidity decrease?
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.
How can you identify plants which don’t respond to drop in relative humidity?
Use thermal imaging - when stomata open, leaf is cooler
How do plants respond to CO2 conc changes.
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
evidence of stomatal evolution
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.
How does blue light induce stomatal opening?
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.
How does Stomatal opening acquire ATP?
- Mitochondrial oxidative phosphorylation
2. Chloroplastic photophosphorylation
How do subcellular lipid droplets relate to stomatal aperture?
How can this be demonstrated?
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.
What lipids are metabolised to give ATP for stomatal opening?
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
What is the mechanism for B oxidation
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.
How was the mechanism for B Oxidation tested>
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.
