Plant transport Flashcards
How is local and systematic transport achieved in plants?
local: through the plasmodesmata, channels directly connecting the cytoplasm of cells (cannot pass cell wall)
systematic: through xylem and phloem
What are the components of the plant cell wall?
- Cellulose which forms parallel semi-crystalline aligned fibres
- Hemicellulose which is strongly modified flexible cross-linking cellulose fibres with heavily modified sugar chains
- Lignin
- Various other proteins and components
What are the properties of lignin?
- Formed by random and spontaneous polymerisation which causes monomers to bind together
- Makes cell walls rigid and stiff
- Consists of phenolytic components which are toxic to most organisms and so defensive
How is the cell wall established?
- Primary cell wall laid down during cytokinesis at the new division plane
- When the cell is fully developed it then produces more cell wall components to form a secondary cell wall which can grow larger than the cell itself and become fortified with lignin
What is the purpose of the cell wall?
Mechanical stabilisation
What is the apoplast
Cell wall and air spaces
What is the symplast
Everything inside the plasma membrane, continuous compartment throughout the organism
What are the plasmodesmata?
- Channels with a continuous cytoplasm and cell membrane which lines the cells filled with a strand of tubular endoplasmic reticulum (energetically unstable, still not sure what for)
- Neck region and central cavity
- only 2-3nm of free space for things to diffuse across
What is the development of the plasmodesmata?
Laid down during cytokinesis and initially have a simple structure before becoming branched
What does the xlyem transport and how?
- Transports water unidirectionally from roots to shoot
- Uses capillary forces driven by evaporation
- Air bubble formation can therefore kill the plant
What does the phloem transport and how?
- Transports nutrients throught the plant (mainly sugar) to non-photosynthetic parts of the plant
- Direction changes according to metabolic development
- Uses osmotic forces (high concentration of solutes causing import of water)
Do the xylem and phloem interact?
- In parralel but seperate vascular bundles
- Xylem can release water which is taken up by phloem to maintain pressure
What is a sink and a source?
Source - carbohydrate exporter
Sink - net carbohydrate importer
Changes during development but roots are always sinks
What are the components of the phloem?
Sieve elements - tube that solutes flow in, loose mitochondria and nucleus, ER pressed to the side
Companion cells - Support the sieve elements, filled with metabolically active cytoplasm
What is the structure of the xylem?
- Vessel elements and fibres for mechanical strength
- Lignified cell walls and ring like thickenings can give strength
Total flux =
diffuse transport + bulk flow + diffusive transport
What is Munchs hypothesis?
That connected tissues flow from high pressure to low pressure regions. In xylem due to tranpiration loss and in phloem through osmotic pressure
What are the properties of the sieve element: companion cell complex?
- Linked by specialised plasmodesmata with a high size exclusion limit allowing free diffusion of macromolecules
- Phloem loading is always apoplastic
Describe apoplastic phloem loading
- Companion cells use transporters (sucrose exporters SWEETs and importers SUCs) to actively transport solutes
- Solutes move into adjeascent sieve components through the plasmodesmata by the pressure created by the osmotic flow
- Few plasmodesmata between SE:CC and surrounding cells
Describe symplastic phloem loading
- Plasmodesmata between SE:CC and neighbouring cells, companion cells sometimes referred to as intermediary cells
- Companion cells convert any sucrose arriving through plasmodesmata into raffinose-family oligosaccharides
- Maintain sucrose gradient meaning that it is still taken up continuously into the cell
- RFOs are too large to flow back through plasmodesmata, trapped in floem creating pressure gradient (polymer trap model)
- Main crop using this is cucurbits
What are the different veins within a leaf?
class 1 - lamina veins which branch out from the midriff class 2 and 3 - phloem unloading major veins class 4 and 5 - minor veins responsible for phloem loading
How has the sink to source gradient in the leaf been shown?
Phloem unloading experiment using fluorescent tracers - use CFDA which is membrane permeable which is cleaved into fluorescent CF which is impermeable when in the symplast
- Show halo of fluorescence at base but none at tip which is a ‘source’
How do plasmodesmata change from sink to source?
- Simple in sink
- Become branched with a large central cavity in source
- ‘twinning’ at the transition
note: unsure how this is regulated
How has the change in plasmodesmata between sink and source been shown ?
Expression of GFP SUC2 promotor (macromolecule, but expressed in the companion cells)
- In sink leaf expressed everywhere
- In source leaf limited to phloem as plasmodesmata are closed s that substrates may not escape
Plants can therefore regulate which parts or the organism are connected by open plasmodesmata where nutrients become distributed
Why is phloem sampling hard?
- Located deep within the tissue
- Cannot be cultured in isolation as de-differentiates, identity determined by positional cues
How can the phloem be studied?
- Phloem exudate (sap), however there is the argument that this also samples from other cells
- More reliable method is through aphids which have long stylid, sever this and you can sample a drop of pure sap
What components have been identified within phloem sap?
- Ca sensors and signals which may be responsible for phloem closing
- molecular chaperones an RNA binding proteins
- flowering locus T involved in flowering regulation
- P proteins which dtect phloem damage
- Hormones
- Viruses
How has phloem RNA transport been shown?
- grafts of different arabidopsis ecotypes with different RNA (singular nucleotide polymorphisms)
- Do sequencing on either side of graft
Have phloem mobile RNAs been found?
- Over 2000 mobile RNAs
- Most mocinf from shoot to root, some the other way and some in both directions
What is an argument fo and against phloem mobile RNAs?
- Could just be moving through open plasmodesmata in companion cells
- However some might be functional as they flow against source-sink
- Inducing hairpin like structures also induces motility
Give an example of long distance signalling in the plant
- Constans expression (clock control) is activated during increased daylight hours andn activates flowering locus t (FT) expression
- This protein traffics into the phloem and serves as a slorigenic stimulus that controls the shoot apical meristem (SAM) to influpresence meristem (MI) transition
What is cell-to-cell signalling?
Trafficking of molecules through the plasmodesmata
What is cell-cell signalling?
Apoplastic signalling by receptor ligand mediated interactions. Here a molecule difuses through the apoplast and binds to the plasma membrane bound receptors activating downsteam signalling cascades
Where does cell-cell signalling occur?
At the shoot and root apical meristems where the membranes are thinner
e. g Knotted in SAM
- RNA only expressed in L2 layer
- Mobile protein found in L1 and L2 layers
How do plant viruses spread through the plant?
- Start out in a few cells via a vector, moving through the plasmodesmata before entering the phloem and becoming systemic
- Travel less quickly to source
- Encode transporter mover proteins which target the plasmodesmata, binding to nucleic acids and opening them for transport
- Capsid protein is also needed for phloem movement
- Polymerisation of RNA binding pushes through plasmodesmata
- Some use protein tubule to move spherical virions through permanently modified plasmodesmata
What are small RNAs?
RNAs which are 21-24 nt long with a function of gene specific silencing
What are the two types of small RNA silencing?
Post-translational - affects complimentary mRNA
Trancriptional - at chromatin conferring epigenetic mutations
What are dicers?
- Double strand specific RNAses which cut RNA to length of 21, 22 or 24 nt long dependent on the isoform
- Then separated into guide stand and passenger strand (degraded)
What are argonauts?
Load guide strand on to effect protein (argonaute) to confer sequence-specific silencing/methylation in the chromatin
Why are perfectly paired double stranded RNAs targeted by siRNA?
- Avoided in eukaryotes therefore a sign of viruses and retrotransposons
- Cell can also make these through RNA dependent RNA polymerase which concerts mRNA double strand into target for dicer
- Therefore provides protection from foreign genetic elements
Describe the miRNA pathway
- Targets non-perfectly paired RNA which contains mismatches and loops, MIR gene encodes RNA trancipt with no open reading frames and hairpin structure
- miRNAs are always 21 nt long
- Role in developmental and physiological regulation
- Always function through post-translational silencing
How does RNA silencing spread in siRNAs?
Look at whole GFP expressing plant, infiltrating leaf with GFP-encoding inverted repeat RNA which is double stranded and induce silencing of GFP expression
- Soon after silencing spread to surrounding cells
- Then spreads systematically to upper leaves
- mobile factor likely double stranded RNA
What have grafting studies shown with regards to siRNA movement?
When GFP-inverted repeat expressing shoots were grafted on to a GFP root the newly formed roots did not express GFP suggesting mobile RNA
Why can miRNAs be classed as morphogens?
- Distribution patterns can spatially restrict the activity of their targets
- Form a gradient
How are the upper and lower leaf areas distinct?
- Upper side has palisade mesophyll filled with chloroplasts and is the site of photosynthesis (adaxial)
- Lower side has spongy mesophyll and is the site of gas exchange (abaxial)
How are leaves made at the shoot apical meristem?
- Distinct group of cells in a dome
- Divide and signal to other cells surroudning to differentiate radially into leaves
- These first form bulbs called leaf promordia which already have a well established axis
How is the palisade mesophyll specified?
- Trancription factor fabulosa regulated by miR166
- knockout of miR166, binding site of fabulosa or arganoute 1 causes entire leaf to be palisade mesophyll
- Promotor for miR166 only on outer layer of adaxial surface however actual RNA forms gradient
How is the spongy mesophyll specified?
Transcription factor arf3 and tasiRNA (transacting silencing)
- MIR gene cleaved to be miRNA which targets Tas gene for amplification of the signal
- RNA dependent RNA polymerase makes long RNA strand which is chopped up by dicer and loaded into abother form of arganoute
How does miRNA create a morphogen gradient in the root?
- miR166 enoded in the endodermis of the root and is expressed inwards and outwards
- Gradient in opposite direction by protein shortroot (SHR) which is only found in the vascular bundle, however spreads to switch on miR166 which regulate fabulosa expression
What is the role of miR399?
- Switched on during phosphate starvation (needed for ATP/DNA) to switch on phosphate uptake mechanisms
- Switches of ubiquitin-conjugating E2 allowing phosphate accumulation genes to switch on
- Phosphate accumulation can only happen if there is Pho2 knowckout in root (graft study) however will happen on both sides with overexpression of miR399
How are symplastic domains established during development?
- Differential size exclusion limits for what can move within symplastic fields is regulated by stomata
- Shown as genetically encoded GFP that is smaller is highly mobile during all stages, bigger is restricted quickly
