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BL3303 > Plant transport > Flashcards

Plant transport Flashcards

1
Q

How is local and systematic transport achieved in plants?

A

local: through the plasmodesmata, channels directly connecting the cytoplasm of cells (cannot pass cell wall)
systematic: through xylem and phloem

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2
Q

What are the components of the plant cell wall?

A
  • 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
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3
Q

What are the properties of lignin?

A
  • 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
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4
Q

How is the cell wall established?

A
  • 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
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5
Q

What is the purpose of the cell wall?

A

Mechanical stabilisation

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6
Q

What is the apoplast

A

Cell wall and air spaces

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7
Q

What is the symplast

A

Everything inside the plasma membrane, continuous compartment throughout the organism

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8
Q

What are the plasmodesmata?

A
  • 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
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9
Q

What is the development of the plasmodesmata?

A

Laid down during cytokinesis and initially have a simple structure before becoming branched

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10
Q

What does the xlyem transport and how?

A
  • Transports water unidirectionally from roots to shoot
  • Uses capillary forces driven by evaporation
  • Air bubble formation can therefore kill the plant
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11
Q

What does the phloem transport and how?

A
  • 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)
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12
Q

Do the xylem and phloem interact?

A
  • In parralel but seperate vascular bundles

- Xylem can release water which is taken up by phloem to maintain pressure

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13
Q

What is a sink and a source?

A

Source - carbohydrate exporter
Sink - net carbohydrate importer
Changes during development but roots are always sinks

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14
Q

What are the components of the phloem?

A

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

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15
Q

What is the structure of the xylem?

A
  • Vessel elements and fibres for mechanical strength

- Lignified cell walls and ring like thickenings can give strength

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16
Q

Total flux =

A

diffuse transport + bulk flow + diffusive transport

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17
Q

What is Munchs hypothesis?

A

That connected tissues flow from high pressure to low pressure regions. In xylem due to tranpiration loss and in phloem through osmotic pressure

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18
Q

What are the properties of the sieve element: companion cell complex?

A
  • Linked by specialised plasmodesmata with a high size exclusion limit allowing free diffusion of macromolecules
  • Phloem loading is always apoplastic
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19
Q

Describe apoplastic phloem loading

A
  • 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
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20
Q

Describe symplastic phloem loading

A
  • 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
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21
Q

What are the different veins within a leaf?

A 
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
22
Q

How has the sink to source gradient in the leaf been shown?

A

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’

23
Q

How do plasmodesmata change from sink to source?

A
  • Simple in sink
  • Become branched with a large central cavity in source
  • ‘twinning’ at the transition
    note: unsure how this is regulated
24
Q

How has the change in plasmodesmata between sink and source been shown ?

A

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

25
Q

Why is phloem sampling hard?

A
  • Located deep within the tissue

- Cannot be cultured in isolation as de-differentiates, identity determined by positional cues

26
Q

How can the phloem be studied?

A
  • 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
27
Q

What components have been identified within phloem sap?

A
  • 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
28
Q

How has phloem RNA transport been shown?

A
  • grafts of different arabidopsis ecotypes with different RNA (singular nucleotide polymorphisms)
  • Do sequencing on either side of graft
29
Q

Have phloem mobile RNAs been found?

A
  • Over 2000 mobile RNAs

- Most mocinf from shoot to root, some the other way and some in both directions

30
Q

What is an argument fo and against phloem mobile RNAs?

A
  • 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
31
Q

Give an example of long distance signalling in the plant

A
  • 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
32
Q

What is cell-to-cell signalling?

A

Trafficking of molecules through the plasmodesmata

33
Q

What is cell-cell signalling?

A

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

34
Q

Where does cell-cell signalling occur?

A

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

35
Q

How do plant viruses spread through the plant?

A
  • 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
36
Q

What are small RNAs?

A

RNAs which are 21-24 nt long with a function of gene specific silencing

37
Q

What are the two types of small RNA silencing?

A

Post-translational - affects complimentary mRNA

Trancriptional - at chromatin conferring epigenetic mutations

38
Q

What are dicers?

A
  • 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)
39
Q

What are argonauts?

A

Load guide strand on to effect protein (argonaute) to confer sequence-specific silencing/methylation in the chromatin

40
Q

Why are perfectly paired double stranded RNAs targeted by siRNA?

A
  • 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
41
Q

Describe the miRNA pathway

A
  • 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
42
Q

How does RNA silencing spread in siRNAs?

A

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
43
Q

What have grafting studies shown with regards to siRNA movement?

A

When GFP-inverted repeat expressing shoots were grafted on to a GFP root the newly formed roots did not express GFP suggesting mobile RNA

44
Q

Why can miRNAs be classed as morphogens?

A
  • Distribution patterns can spatially restrict the activity of their targets
  • Form a gradient
45
Q

How are the upper and lower leaf areas distinct?

A
  • 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)
46
Q

How are leaves made at the shoot apical meristem?

A
  • 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
47
Q

How is the palisade mesophyll specified?

A
  • 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
48
Q

How is the spongy mesophyll specified?

A

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
49
Q

How does miRNA create a morphogen gradient in the root?

A
  • 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
50
Q

What is the role of miR399?

A
  • 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
51
Q

How are symplastic domains established during development?

A
  • 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

BL3303 (15 decks)

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