Lecture 2: Cell Walls

1
Lecture 2 Cell Walls

• Functions
• Structure and synthesis
• Structural components
• Cellulose microfibrils
• Structure
• Biosynthesis

• Primary walls
• Secondary walls
• Patterns of cell expansion, Multinet Hypothesis
• Cell wall loosening
• Cessation of cell wall expansion
• Summary

2
Functions of Cell Walls

• Primary functions
• Regulating cell volume
• Determining cell shape
• Cell-cell adhesion

• Secondary functions forms specialized
  structures for
• Transport
• Defense
• Reproduction

• Important for human economics
• Paper, textiles, fibers (cotton, flax), charcoal,
  lumber
• Plastics, film, coatings, gels, thickeners...

3
Functions of Cell Walls (cont.)

• Cell walls are essential!
• When cell wall is removed protoplasts adopt a
  spherical shape

• Cell walls are dynamic

Protoplasts
4
Structure and Synthesis of Cell Walls
Architecture

• Cell walls are heterogeneous structures
• Composed of layers lamellae analogous to
  layers in plywood
• Each cell secretes its own wall junction between
  the walls is the Middle lamella
• Cell wall is penetrated by plasmodesmata
  connecting neighboring cells

Plasmodesmata
5
Structural Components of Cell Walls
Cell walls are primarily composed of sugar
polymers - polysaccharides
6
Structural Components of Cell Walls
7
Where are cell wall components synthesized in the
cell?
Cellulose microfibrils are synthesized at the
plasma membrane.
Electron micrograph showing Golgi stacks and
vesicles containing xyloglucan and glycosylated
proteins
Synthesis of cell wall polysaccharides by enzymes
in the Golgi apparatus. Polysaccharides are
secreted to the wall by fusion of membrane
vesicles to the plasma membrane.
8
Schematic diagram of major structural components
of the primary cell wall
(Hemicelluloses)

• Cellulose fibrils are cross-linked (by
  hemicelluloses) within and between lamellae
• A primary wall 50 nm thick has 5-10 lamellae of
  cellulose microfibrils

9
Structural model of a cellulose microfibril
10
Structural model of a cellulose microfibril
11
How are cellulose microfibrils synthesized?
Cellulose fibrils are composed of linear chains
of glucose Fibrils are synthesized from
UDP-Glucose Each fibril contains multiple
cellulose chains hydrogen bonded together (18-36
cellulose chains in higher plants)
Model of cellulose synthesis
12
Cellulose synthesis is catalyzed by protein
complexes- terminal complexes (or particle
rosettes) - that are embedded in the plasma
membrane

• Terminal complexes are comprised of hexameric
  rosettes
• Rosettes are thought to be the smallest unit of
  cellulose synthase complex

13
Microtubules are thought to direct cellulose
synthase
14
Cellulose synthase genes (CES) have been
identified based on homology to prokaryotic
cellulose biosynthetic enzymes
15
Does CesA encode a cellular synthase?

• Several mutations have been identified that
  cause call wall defects.
• rsw1 (radially swollen 1) is mutated in CesA1
  and causes root swelling,
• stunted growth and lethality in severe alleles
• - CesA has been immunolocalized to the terminal
  complexes

16
Arabidopsis mutant screens revealed phenotypes of
many CES genes
17
Arabidopsis CesA mutants have defects in the
glycosyltransferase catalytic domain
18
Model for cellulose microfibril biosynthesis
A
In this model, at least two types of CesA
polypeptides, ? and ß, are required for
spontaneous rosette assembly. Two different types
of isoform can be distinguished, ?1 which
interacts with two ß isoforms only, and ?2
interacting with another ?2 isoform and two ß
isoforms.
B
Plant Cell Physiol (2002) 43, 1407-1420
19
Other structural proteins and cell wall
cross-linking
Pectins - are negatively charged
polysaccharides that give compressive strength
to the cell wall - bind Ca2 to form a
gel (used in making jelly) - differentially
localized within cell walls - modulate porosity,
pH, ion concentrations within the wall
Hydroxyproline
Hydroxyproline-rich glycoproteins (HRGP) -
predicted multimeric, rod-shape structure
Proline-rich proteins (PRP) - predicted
multimeric, rod-shape structure
Glycine-rich proteins (GRP) - predicted ß-sheet
structure
Arabinogalactan proteins (AGP) - more than 90
of mass of AGPs may be sugars (primarily
galactose and arabinose) - may function in cell
adhesion and cell signaling during cell
differentiation
20
Primary cell walls originated de novo during the
final stages of cell division when cell plate
separates daughter cells
Cell plate forms when Golgi vesicles and ER
cisternae aggregate in spindle midzone area of
dividing cell
This aggregation is organized by the
phragmoplast, a complex assembly of microtubules,
membranes and vesicles that forms during late
anaphase or early telophase
Membranes of vesicles fuse with each other and
with the lateral plasma membrane to become the
new plasma membrane separating the daughter cells
Contents of the vesicles are the precursors from
which the new middle lamella and primary wall are
assembled
21
Secondary walls
Many plant cells synthesize secondary walls after
the cell has completely elongated (i.e. after
cell expansion has ceased)
Primary walls 15-30 cellulose No lignins
Secondary walls 50-95 cellulose Lignins present
22
Secondary walls (?) are found in various cell
types
Collenchyma (Medicago stem)
Fruit stone cell (e.g. pear)
Stomatal guard cell
23
Secondary walls make up the bulk in woody plants
Tracheary elements develop reinforced cell walls
that can form in a variety of patterns
24
Secondary walls are multilayered and thickness
and orientation of cellulose fibrils may differ
in these layers
Lumen
25
How does the plant cell direct wall growth?
Patterns of cell expansion
Isotropic (randomly oriented cellulose
microfibrils)
Anisotropic (transverse cellulose microfibrils)
26
Cell wall deposition continues as cells enlarge
The Multinet hypothesis for wall extension
Newly synthesized cellulose microfibrils are
continually deposited on the inner surface of the
wall in transverse orientation. As cell
elongation proceeds, the older (outer layers) are
progressively thinned and weakened. The
cellulose microfibrils of these outer layers are
passively rearranged to a longitudinal
orientation. Thus, the wall mechanical
properties are determined by the inner layers.
27
Cell wall loosening
To make room for new microfibrils, cell wall
needs to be loosened Two modes of extending
walls Cleavage of hydrogen bonds between
fibrils by expansins Glucanases and other
hydrolytic enzymes cleave hemicellulose cross
bridges, e.g. XET xyloglucan
endotransglycosylase, a wall-loosening
enzyme Growing cell walls extend much faster at
acidic pH than at neutral pH ? acid growth
proton extrusion by plasma membrane H-ATPase
acidifies the wall, activating expansin example
initiation of root hair
28
Cell Expansion Wall loosening H2O uptake

• Original cell
• Increase in wall extensibility and reduction in
  turgor (P)
• Water uptake
• Increase in cell volume
• Wall becomes rigid (increased cross linking) and
  turgor pressure increases
• Wall thickens

Cell expands incrementally by repeating steps 1-6
29
Acid-induced growth is mediated by expansins
Extensometer
30
Acid-induced growth is mediated by expansins
31
Cessation of cell elongation, wall degradation
and defense
Cessation of cell elongation due to cell wall
rigidification caused by an increase in number
of cross links. Hydrolytic enzymes may degrade
mature cell walls completely or selectively
during fruit ripening, seed germination and
formation of abscission layers. Oxidative cross
linking in response to pathogen attack. Pathogen
attack may release cell wall fragments that may
act as cell signaling agents.
32
Summary

• Architecture, mechanics, and function of plants
  depend crucially on structure
• of cell wall
• Primary cell walls are synthesized in growing
  cells
• Primary wall is network of cellulose
  microfibrils, hemicelluloses, pectins
• and structural proteins
• Cellulose microfibrils are synthesized on
  particle rosettes and secreted
• into wall via Golgi apparatus
• Secondary walls are deposited in certain cells
  (xylem, sclerenchyma) after
• cell expansion ceases
• - Secondary walls have higher content of
  cellulose, different hemicelluloses
• and lignin instead of pectin
• After leaving the meristem, plant cells elongate
  greatly this is controlled by
• pH and wall-loosening proteins such as
  expansins, glucanases
• - Hydrolytic enzymes degrade mature cell walls