Download THE PLANT CELL WALL A. Introduction. Plant cell wall: a tough coat

Biol 458 Cell Wall p. 1
THE PLANT CELL WALL
A. Introduction.
Plant cell wall: a tough coat of polysaccharides and proteins,
which surrounds the plant cell external to the plasmalemma.
- the "extracellular" matrix, semi-rigid, integral to cell
- the major long-term sinks of fixed carbon in biosphere
(cellulose & lignin).
- dynamic: sensitive to environment, development, stresses
Biological importance of the CW:
- structural support for plants (via turgor and lignin)
- gives plant cells shape, tied closely to cell development
- gives protection from pathogens, acts as stress sensors
- facilitates water movement between cells (capillary action)
- primary contact with environment (positional signals?)
Practical importance of the CW:
- importance of industrial fiber (cotton, flax, hemp)
- health / dietary fiber (ß-glucans affect cholesterol, insulin)
- source of ‘solar’ energy - > biofuels from cellulose?
General features:
- primary CWs can expand, found all plant cells
- secondary CWs develop: only in specialized cells
- permeable to small molecules only
- larger molecules may go thru plasmodesmata (virus mov't)
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B. Cell wall components
- composed of cellulose micro- and macro-fibrils in a complex
matrix of carbohydrate and protein (ie fiberglass)
1. Microfibrils (cellulose)
- unbranched ß-(1->4) glucose polymer [-->cellobiose]
[note β anomers of glucose]
- stabilized by H-bonds (intra-, inter-molecular)
- 36 glucose chains --> microfibril - > coiled macrofibrils
- arrangement ("slinky") permits expansion
2. Matrix (very complex "glue", subdivided empirically, variable)
- contains pentoses, hexoses, deoxysugars - nomenclature
Hemicellulose (xyloglucan- KOH extracted)
- ß(1->4) glucose with xylose sidechains
- arabinose, galactose, fucose linked to xylose
- forms "adaptors" with cellulose and crosslinks microfibrils
- also contains: glucuronoarabinoxylan
Pectic polysaccharides (pectin - boiling H2O extracted)
- makes a "hydrated gel"
- rich in polygalacturonic acid
- abundant in middle lamella
- other sugars: arabinose, rhamnose {->arabinogalactans,
rhamnogalacturonan]
++
- carboxyl interaction with Ca , pectin methylesterase
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3. CW Proteins [structural & enzymes]:
Structural:
- heavily glycosylated, linked to polysacharides
- simple, repetive structures
- common structural CW proteins
• hydroxyproline-rich glycoproteins (HRGPs, (extensin)
- [...-ser-hyp-hyp-hyp-hyp-ser etc...tyr-lys-tyr] - glycosylated
• glycine-rich proteins (GRPs)
[...-gly-gly-X-...]
• proline-rich proteins (PRPs)
[pro, hyp]
• arabinogalactan-proteins (AGPs)
Enzymes - [see dynamic nature of CW]:
- ß-glucanase, polygalacturonase
- xyloglucan endotransglycosylase (XET)
- expansins
4. Model of polysaccharides in primary CW
- xyloglucans H-bond to microfibrils (MFs), crosslink MFs
- pectins make connections between MF's via xyloglucans
- pectins connect to CW proteins, act as hydrophilic filler
5. Secondary CW
- more xylan and less xyloglucan, much glucomannose
- in tracheids, fibers, sclereid cells (mechanical support)
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6. Lignin:
= high MW polymer, in matrix, derived from phenylalanine
- predominantly in xylem (wood)
- resist crushing forces
C. Summary of CW & Biosynthesis
- primary CW:
cellulose
20-30%
hemicellulose (xyloglucan) 25 %
pectic substances
30%
proteins
5-10%
- secondary CW: up to 90 % cellulose
- wood: 40 % cellulose, 30 %hemi-cellulose, 30 % lignin
- homo- vs hetero-polymers
Biosynthesis of cellulose and CW
Components all get exported
i) cellulose: rosette-shaped complex (CesA genes)
- microtubules, sucrose synthsas
ii) xyloglucans and pectic substances
- secretory pathway and assembly
iii) proteins - secreted via Golgi
NB: Assembly in situ
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D. Dynamic Nature of the Cell Wall
Example 1: Cell wall loosening during cell expansion
- CW is "plastic, not elastic"
- arrangement of MFs (slinky) -> microfibrils and microtubules
- enzymes to loosen matrix, turgor expands CW
i) ß(1->4)endoglucanase (
ii) xyloglucan endotransglycosylase (XET) - transfers
glycosidic bonds
iii) expansins - proteins that loosen xyloglucan/cellulose
H-bonds (pH activation & acid growth hypothesis)
- shown by hypocotyl and filter paper experiments
- after expansion (final cell size): - CW 'locked' into final
shape by HRGP crosslinks & desterification of pectins
Example 2: Defense against plant pathogens: the CW as a
barrier and detections system
- CW is a barrier
- Bacterial pathogen: extracellular, kill cells with toxins, CW
enzymes (pectin degrading - pectate lyase)
Fungal pathogens are classified as:
Necrotrophs (need dead cells for nutrients)
- often secrete polygalacturonases
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---> plant can secrete polygalacturonase inhibitory
proteins (PGIPs)
Biotrophs (in living cells, haustorium for nutrients)
- use penetration peg to get into cell
- "sneaky": actively suppress plant defense responses!
Plant defense against biotrophs:
- increased HRGP synthesis in CW
- crosslinking of PRPs (=disappearance from extracts)
- papillae formation at penetration (callose: ß1,3
glucose), CW proteins.
Defense induction by CW fragments
(How do plants know they are being invaded?)
i) plant-derived CW fragments:
- xyloglucan nonasaccharide
- oligogalacturonide (n=11-14)
ii) pathogen-derived CW fragments:
- hepta ß-glucoside
-oligochitin (oligo N-acetyl glucosamine)
Note high degree of specificity!
elicitors = molecules that stimulate plant defense
mechanisms
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The Plant CW is potential source of biofuels
- there is a huge amount of biomass in plant CWs (sugars)
- it is hard to get the energy stored in these molecules, and we
need to understand their structure and synthesis
1. Biofuels problem: need a transportable form of energy
2. CW sugars can be fermented to ethanol by microbes
3. Where are the bottlenecks?
- CWs evolved to resist microbial attack
- the sugars are not easily accessible to microbial enzymes
(especially crystalline cellulose)
- hemicellulose are very variable in composition
4. Attempts to modify the CW for cellulosic biofuels
i) decrease crystalline cellulose
ii) reduce H-bonding of xyloglucan, modify the structure
iii) less pentoses (arabinose, xylose), more hexose (mannose,
glucose)
iv) reduce lignin content
Can these by modified and still have a viable plant??
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