VL 36 (Marcus Grebe) Flashcards
Importance of the plant cell wall:
- Establishment of counterpressure to protoplast H2O-potential → “exoskeleton”
- mech. tissue stability
- conductive elements, e.g. xylem (→ transpiration)
- hold cells together
- determines, limits elongation growth
- contributes to H2O balance (cell wall determines the relationship between turgor pressure – cell volume)
- diffusion barrier, limiting in size to macromolecules that can reach the plasma membrane
- structural barrier to pathogen infection
chem.-molecular cell wall composition:
Carbohydrates:
* Pectins
* Hemicelluloses
* Cellulose
Proteins:
* Structural proteins
* Modifying enzymes
Hydrophobic polymers:
* Lignins
* cutins
* suberines
Pectins
–> Group of Carbohydrates in cell wall
- abundant in middle lamella, primary wall
- protopectin is mixture of: galacturonans + rhamnogalacturonans
Homogalacturonan molecules of protopectin:
- Alpha-galacturonic acid polymer = homogalacturonan
- No interaction through methylester; methylated = esterified
- Cell wall thickening by salt bridges (Ca2+/Mg2+)
Pectin Methyl Esterase (PME) catalyzes the transformation to non-esterified pectin, e.g. on the flank of the pollen tube:
- pollen tube; grow at tip
- golgi: pectin synthesis → vesicle transport via actin to tip
- PME (= modifying enzyme) active at tip; cleaves ester group → anion → salt bridges built → thicker, stiffer cell wall
Rhamnogalacturonan molecules of protopectin:
Hemicelluloses:
- Location: primary wall
- pentoses (e.g. D-xylose, L-arabinose) + hexoses (e.g. D-/glucose/galactase/mannose)
- golgi: pectin synthesis → vesicle transport via actin to tip
Example: Xyloglucan
–> Picture
Cellulose: only beta-1,4-glucan chain
Modification of Xyloglucan Polymers by Xyloglucan Endotransglucosylase (X
- Xyloglucan secreted into cell wall
→cross-connection between different xyloglucan chains;
hybrid product
→enhance cell wall structure
Cellulose:
- in primary (10%), secondary wall (94%)
- linear beta-1,4 glucan; beta-D-glucose (or its dimer)
- up to 15,000 glucose SU; straightened out
- synthesized by cellulose synthase (in cell wall Cellulose-Synthase complexes)
Structural protein of the cell wall:
- carbohydrates attached to proteins
- →interaction with other cell wall
carbohydrate molecules
A hydroxyproline-rich glycoprotein of tomato:
Main Structural Components of the Primary Cell Wall and their Likely Arrangement
Structure of primary cell wall:
- Stabilisation via o H-bonds
–> Ca2+/Mg2+ bridges o Pectins
–> carbohydrate
Secondary cell wall:
- H-bonds between cellulose molecules, hemicellulose, pectins
- Cellulose: parallel arrangement of MTs, microfibrils
→semicrystalline structure - MTs
–> under plasmamembrane
–> parallel arrangement due to MAP65/PLEIADE cross-links
Vectorial Cellulose Synthesis is determined by cortical MTs:
Cellulose synthase (CESA)
* in plasma membrane (transmembrane)
* catalyzes: cellulose microfibril synthesis
* catalytic units (cellulose synthase (CesA) proteins) guided by MTs o required molecule - activated glucose: UDPG
→ glucose removed
by cellulose synthase
→ UDP
→ bound to beta-1,4-glucan chain
(growing end)
* Cellulose Synthase Complexes (CSC) assembled in golgi
Rosette complexes – number of SU not finally clarified:
- hexameric, 6 units
- each unit composed of 3 cellulse
- synthase SU
- →microfibrils in cross section: 18-~36 cellulose molecules but also deviations: e.g. so far CSCs SU number not observed in cotton
Orientation of newly deposited cellulose microfibrils:
→cell shape determined by microfibril arrangement
* random microfibril distribution
→cell rounds up
* transverse microfibrils
→cells can ́t laterally extend
Initiation of cell wall precursor delivery during cell plate formation
- Cell division: delivers membrane vesicles from TGN (with pectin, hemicellulose, cellulose synthase) via MTs → cell middle → fuse to cell plate → expand
Formation of Middle lamella and primary wall at cytokinesis end:
- vesicle transport from TGN along MTs → cell center
- →form: phragmoplast (= set of MTs, actin filaments, vesicles) getting round by MT-(de)polymerisation
→ vesicles added ad cell plate periphery - cell plate fusion with parental membrane
- cell plate + golgi vesicles → middle lamella (= cell wall layer that is shared between two daughter cells)
- primary wall between plasma membrane – middle lamella
Formation of primary plasmodesmata:
- pore in plasma membrane
→molecule transport - axial center with cylinder of compressed ER = desmotubule
- cell-cell connections between cell symplasts
Microfibrils determine growth direction:
Modification of wall properties: Expansins
- Secreted expansins→solubilize H-bonds between microfibrils
- Higher activity at acidic pH (around pH 5.5)
Cellulose Synthase Interactive1 protein (CSI1/POM-POM2) binds to both MT and Cellulose Synthase Complexes (CSC):
- binds MTs, CSC
- tethering: exocyst complex on plasma membrane + vesicle
→vesicle pulled to membrane - CSI1: cytosolic protein; CSC-guidance along cortical MTs;
(binds MTs→catches CSC during tethering + docking process; CSC pushed to MTs)
Thickening of secondary cell wall by cellulose and lignin:
- MAP17, CSC helps bunching
- accumulation of cellulose microfibrils → ribs of secondary
cell wall - lignin synthesis makes bunches hydrophobic
- lignin inserted → cell die; H2O-impermeable
