Plant ECM Flashcards
(33 cards)
How is the plant cell wall different from the animal ECM?
The plant CELL WALL is analogous to the animal ECM and employs some common principles but has v different physical and chemical properties.
Has to allow growth and support the cell. The cells of higher plants don’t move relative to one another.
What are the four functions of the plant cell wall
- Structural support
- Mechanical resistance to turgor pressure from the protoplast (inside the membrane).
- Defence against disease and physical attack.
- Mechanical and chemical signals from the environment.t
What are the implications of the plant cell wall for growth and function?
6 points
key words phagocytosis daughter cells plant cell movement growth vectors controlled
Rigid cell wall defines shape
Cell wall doesn’t allow phagocytosis
Daughter cells are conjoined at birth by a plant cell wall
Plant cells generally cannot move or grow past each other
Growth vectors of individual cells determine shape of individual organs
Controlled weakening of the cell wall is needed for growth
What are features of the primary cell wall
4 points
In relatively young, growing cells
Flexible, malleable, expandable
Formed between cells upon division
Mostly polysaccharide
How are new cell walls created?
Hint: talk about cell division in plants
By cell division - formed within the cell plate
Land plants and their immediate ancestors have unique mode of cell division
In animal cytokinesis, the actin ring constricts
In plants,
the cell plate divides cell, forming a cell wall inside the cell cell plate (within the plasma membrane)
Plants form two unique microtubule arrays during cell division
There is a phragmoplast that guides formation of the cell plate and the cell plate expands centrifugally - contains a nascent cell wall .
The preprophase band (PPB) disassembles but leaves a mark in the membrane.
Predicts the future division plane
How does cell plate formation work?
Golgi-dervied vesicles carrying cell wall polysaccharides travel to the mid-plane on microtubules
Vesicle fusion forms a tubular network and then a disc
Relies on two unique microtubule arrays for guidance - phragmoplast
What is the composition of the primary cell wall?
Fun fact - plant cell walls do not have protein filaments…
90% polysaccharides
- Cellulose 20-30%
- Hemicellulose 30-40%
- Pectins 30%
Proteins 10%
Lignins —
Name some of the common sugars in cell wall polysaccharides?
D-xylopyranose - pentose D- glucopyranose - hexose D-mannopyranose - hexose 4 0 methylD glucoronopyranose - uronic acid L-arabinofuranose - pentose D-galactopyranose - hexose
Name the three components in plant cell wall architecture and what they do
Cellulose (20-30%) - forms large microfibrils providing tensile strength
Hemicellulose -
Pectin
Talk about cellulose
The bonds in it
Cellulose fibre structure
Polymer of beta 1-4 linked glucose
** Most abundant biopolymer on earth…
Each polymer forms part of a micro crystalline array of 18-10,000 of H-bonded cellulose chains in microfibrils and fibrils
They interact via H bonds
Microfibrils, with a crystalline core inside
Cellulose’s crytalinity accounts for its insolubility, acid resistance, and high tensile strength - similar to that of steel.
Talk about how cellulose appeared early in plant evolution
Bacterial (endosymbiotic origin?)
Secondary gain,
useful for a newly autotrophic eukaryote
cell wall passively resists turgor
‘Lysosome’ freed to evolve into a large vacuole
How is cellulose synthesised?
1980 - discovery of rosettes in pm
Hypothesis - cellulose is made at the surface of the pm by multimeric enzymatic “terminal complexes” called “rosettes”
Biochemical analysis of rosettes proved impossible
Genetic analysis in Arabidopsis thaliana solved the problem :)
What did the radial swelling1 rsw1 mutant of Arabidopsis show?
rsw1 mutant has 30% less cellulose than WT
RSW1 gene encodes a CesA-related protein
There is stunted growth of the mutant
Plant RSW 1 similar to Bacterial CesA - bacterial cellulose synthase
How was it proven that terminal rosettes contain CESA protein ?
Proven by immune-gold electron microscopy
Each rosette has 6 particles
Each particle has 3-6 CESA protein
Each CESA protein makes one cellulose polymer
18-36 chains = minimal microfibril
Why is hemicellulose called hemicellulose?
‘Half cellulose’ because they have glucan backbone.
Meshwork cross links and separates cellulose fibrils
e.g. xyloglucans, heteromannans, heteroxylans, ad mixed-linkage glucan
What is the structure of xyloglucan?
Side chains lie along one side of beta 1-4 glucose polymer backbone
the other side can interact with cellulose microfibrils
XGs are thought to span between ceullulose microfibrils
and may act as a GLUE forming mechanical ‘hotspots’
How is hemicellulose made?
Hemicelluloses are made in the golgi apparatus and transported in secretory vesicles that fuse with the PM
Made by variants of plant-specific CesA enzyme family
What are pectins?
They form an independent gel
Pectins are made in the Golgi apparatus and transported in secretory vesicles that fuse with the PM
Pectins are abundant in the cell plate. What is taking place here?
During expansion they remain abundant in the central zone of the wall - middle lamella
Middle lamella is the ‘glue’ between the cells
What are the two major pectins?
PGA and RG-I
each can be part of a single covalent polymer
Polygalacturonic acid (PGA) - 60% of pectin
Rhamagalacturonan I (RGI) + diverse side chain sugars (galactan, arabinose)
50% of cellulose fibril surface is covered by pectin
What is a recent view of the possible arrangement of the polymers in plant cell walls?
Based on structural studies and mutant phenotypes
Cellulose fibrils
Hemicellulose joining cellulose fibrils in mechanical hot spots
Hemicellulose loosely bound to cellulose
Mobile pectic network loosely cross-linking cellulose fibrils
What are the structural and signalling proteins in the cell wall? - function is to regulate different polymers
There are three types:
HRGPs
PRPs and GRPs
AGPs
Heavily glycosylated
Hydroxyproline rich proteins e.g. extensin
inserted radially in expanding cell wall
interlock the separated microfibrils and thereby arrest further spreading of the cellulose microfibrils
Proline rich proteins and glycine rich proteins
particularly prevalent in vasculature
cross-linking role
Arabinogalactan proteins
Membrane anchored proteins coated in complex polysaccharides
structural as well as important signalling role
Cell growth requires carefully regulated loosening of the cell wall. How does this happen?
Degree and spatial pattern of loosening
T, P, symbol with vertical line through O
The role can be formalised:
Stress relaxation:
- stress is imposed by turgor pressure (P)
- cell wall has a yield threshold (y), pressure above which the wall yields to pressure
Rate of expansion is determined by wall extensibility
Lockhart equation: R = dV/V*dt = O(P-Y)
How do cells actively control the parameter of wall extensibility (this determines the rate of expansion)
Regulating ‘polymer creep’
Slippage of polymers
