The Plant Cell wall

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The Plant Cell wall

• Structure of components

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The plant cell wall

• Primary Cell Wall
• Formed by growing cells.
• Usually considered to be relatively unspecialized
  and similar in structure in all cell types
• Secondary Cell Wall
• These are the cell walls that form after cell
  growth has ceased
• Can be highly specialized in structure and
  composition, such as the xylem
• Contain more cellulose and lignin replaces pectin
• Thicker than primary cell walls

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The Plant cell wall

• Critical to
• plant cell growth
• plant growth and development
• differentiation
• response to biotic and abiotic stress
• Impact human activities in many ways
• wood
• paper
• textile
• fuel
• food
• livestock feed
• brewing
• pharmaceuticals

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The Plant Cell wall

• The plant cell wall is a layer of structural
  material external to the protoplast, built from
  polysaccharides and proteins.
• The wall contains components for signaling and
  communication by symplastic continuity through
  plasmodesmata and maintains molecular connections
  with the plasma membrane and cytoskeleton

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The Plant Cell wall

• The cell wall is the organelle that ultimately
  controls the shape of plant cells and
  consequently of organs and whole organisms.
• It is sometimes naturally strengthened and made
  considerably more resistant to such abuses as
  pathogen infection by the release of specific
  oligosaccharides and enzymes and by overlaying or
  impregnation with cutin, suberin, waxes or silica

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Components of Primary Cell Walls Carbohydrates
Cellulose. Cross-linking Glucans Two main
classes. Pectin Three different
classes. Protein Hydroxyproline-Rich
Glycoproteins (HRGPs). Arabinogalactan Proteins
(AGPs). Glycine-rich Proteins
(GRPs). Proline-Rich Proteins (PRPs).
Solanaceous Lectins. Wall-localised
enzymes. Other Lignin and other
Phenolics. Water, fatty acids, waxes, micro- and
macro-nutrients (Inorganic ions mainly Ca
and B).
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Carbohydrates Cellulose Cross-linking
Glucans Xyloglucan (XG). Glucuronoarabinoxyl
an (GAX). Mannans, Glucomannans, Starch,
Callose Galactomannans. Pectin
Homogalacturonan (HGA). Rhamnogalacturona
n-I (RG-I). Rhamnogalacturonan-II (RG-II).
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Carbohydrate Components of Primary Cell Walls

• Cell wall polysaccharides are built from seven
  main sugars
• L-Rhamnose
• L-Fucose (both deoxyhexoses)
• L-Arabinose
• D-Xylose (both pentoses)
• D-Mannose
• D-Galactose
• D-Glucose (Hexoses).
• These sugars are coupled by glycosidic linkages
  to form complex polymers consisting of a backbone
  and, where applicable, associated side-chains.

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b-link
a-link
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Cellulose

• Linear glucan chains of unbranched
  (1-4)-b-linked-D-glucose in which every other
  glucose residue is rotated 180 with respect to
  its two neighbors and contrasts with other glucan
  polymers such as
• starch (1-4-a-glucan)
• callose (1-3-b-glucan).

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Cellulose

• This means that cellobiose, and not glucose, is
  the basic repeating unit of the cellulose
  molecule. Groups of 30 to 40 of these chains
  laterally hydrogen-bond to form crystalline or
  para-crystalline microfibrils.

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Cross-linking Glucans

• Xyloglucan
• Linear chains of (1-4)-b-D-glucan substituted
  with a-D-xylosyl residues upon three contiguous
  backbone glucosyl residues separated from the
  next repeat by one unsubstituted glucosyl
  residue.

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Cross-linking Glucans

• Xyloglucan
• The xylosyl residues may themselves be
  substituted further by the addition of
  b-D-galactose or a-L-arabinose to the C-2 of the
  xylosyl units.
• This serves to straighten the backbone
  facilitating closer packing to the cellulose

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Cross-linking Glucans

• Glucuronoarabinoxylan (GAX)
• Linear chains of (1-4)-b-D-xylose forming the
  backbone.
• With substitutions of single arabinose units at
  C-3 and single glucosyluronic acid (GlcA) residue
  at C-2.
• Many of the xylose residues carry C-2 or
  O-3-acetyl ester groups and approximately half
  the GlcA residues carry C-4-methyl ester groups

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Pectin

• Homogalacturonan (HGA)
• Linear chains of (1-4)-a-D-galactose units
• HGA chains can condense by cross-linking with
  Ca2 to form 'junction zones', linking two
  parallel or two anti-parallel chains.
• Can also contain methyl esters
• COOCH3 groups
• Not thought to be a separate molecule in its own
  right, but rather part of RG-I

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Pectin

• Rhamnogalacturonan-I (RG-I)
• The basic unit structure is composed of a
  backbone of alternating (1-4)-a-linked D-GalA and
  (1-2)-a-linked L-rhamnosyl residues which forms a
  contorted rod that is considerably more flexible
  than the helical HGA.
• The side chains of neutral arabinan, galactan and
  arabinogalactan are linked to the C-4 and/or C-3
  of the rhamnose
• Can also contain both methyl esters and acetyl
  ester groups
• OOCCH3

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RG-I Side chains

• Arabinans
• Mostly 5-linked arabinosyl units forming short
  helical chains. They can be connected to each
  other at virtually every free position, the C-2,
  C-3 and the C-5 forming a diverse group of
  polymers that are highly branched.

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RG-I Side chains

• Galactans
• Sometimes referred to as homogalactans generally
  have a backbone of (1-4)-b-linked galactosyl
  residues with (1-6)-b-linked galactosyl
  substitutions.

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RG-I Side chains

• Arabinogalactans (AGs)
• Type I Has a (1-4)-b-linked linear chain of
  D-galactosyl units onto which single arabinosyl
  units or short chains of (1-5)-a-linked
  L-arabinofuranosyl residues are connected,
  generally to C-3.
• These are only found in pectic fractions.

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RG-I Side chains

• Arabinogalactans (AGs)
• Type II Highly branched chains with backbones of
  (1-3)- and (1-6)-linked b-D-galactosyl units.
  The (1-3)-b-linkage is found in the internal
  domain of type II AG while the (1-6)-b- linkages
  are found mainly in the exterior portion. The
  chains are mostly terminated by (1-6)-a-linked
  L-arabinosyl residues.
• Also found in association with arabinogalactan
  proteins (AGPs) and xylans.

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Rhamnogalacturonan-II (RG-II)

• Minor but extremely highly conserved molecule.
• Provides a good example of the complexity that
  can result from the ability of polysaccharides to
  form branched structures
• Composed of a backbone consisting of 9 linear
  (1-4)-a-linked-D galacturonosyl residues to which
  four side chains are attached.

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Rhamnogalacturonan-II (RG-II)

• Contains a high proportion of rhamnose which is
  either (1-3)- and (1-2,3,4)- linked or as
  terminal units.
• Contains 11 different sugars including fucose,
  arabinose, galactose and apiose
• The apiose allows for borate diester bonds to
  form
• These add to cell wall strength

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How do these three classes of pectin form one
matrix?

• HGA regions of pectin chains can be bound
  together by calcium ions
• Forms egg-box junction zones
• This is only one level of aggregation

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How do these three classes of pectin form one
matrix?

• So
• Single HGA chains (green) join to give the
  egg-box junction areas (blue)
• This can also function as inter-junction segments
  between junction zones
• This allows room for the presence of both regions
  of RG-I and associated side chains as was as the
  presence of RG-II

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How do these three classes of pectin form one
matrix?

• Secondly
• The cable is formed by BOTH
• aggregation of egg-box junction zones (Pink)
• Calcium junctions forming where many ester
  groups are present
• This cable model accounts for all the chemical
  and structural properties of each class of pectin
  molecule

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Remember, all 3 types of pectin are thought to be
connected to each other in vitro.
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Other cell wall components

• Lignin
• A phenolic compound has an -OH group
• attached directly to a benzene ring.
• The primary cell walls of many, if not all,
• higher plants contain polymer-bound
• phenolics.
• Generally phenolics undergo oxidative coupling
  reactions.
• Such coupling reactions, which are probably
  catalysed within the cell wall itself by
  extracellular peroxidases, could play an
  important structural role by cross-linking the
  polymers to which the phenolics are bound.
• This leads to lignification of the cell wall

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Other cell wall components

• Proteins five types
• 1 Hydroxyproline-Rich Glycoproteins (HRGPs)
  These proteins have been shown to accumulate in
  the cell wall in response to the induction of
  systemic acquired resistance (SAR), mechanical
  wounding, ethylene, heat and almost any
  environmental stress.
• 2Arabinogalactan Proteins (AGPs) The function
  of AGPs has yet to be determined, although roles
  have been suggested as 'glues', lubricants
  between cells or as likely candidates for
  functioning in cell-cell recognition

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Other cell wall components

• Proteins five types
• 3 Glycine-rich Proteins (GRPs) These proteins
  are expressed in response to environmental
  stresses and wound healing.
• 4 Proline-Rich Proteins (PRPs) Involved in the
  association with nitrogen-fixing bacteria in the
  production of nodules.
• 5 Solanaceous Lectins Involved in sugar
  transport, stabilization of seed storage
  proteins, cell division, wound healing, and SAR.

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Primary cell wall architecture
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Primary cell wall architecture

• There are two basic types of primary cell wall
• Type I
• Dicots and all flowering plants
• Type II
• Found in the monocot grass families
• Vary in structural make up and classes of
  polysaccharide components present.
• NOT TO BE CONFUSED WITH SECONDARY CELL WALLS

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Type I cell walls

• Cellulose
• Cellulosic microfibrils are between 5 and 12 nm
  in diameter spaced 20 to 40 nm apart.
• There is room to house approximately four layers,
  or lamellae, of parallel-running cellulosic
  microfibrils between the plasma membrane and
  middle lamella.

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Type I cell walls

• Cross linking glucans
• The main one is xyloglucan with molecular lengths
  of up to 400 nm.
• These xyloglucans hydrogen-bond to two or more
  microfibrils to form a network, as well as
  coating the surface of the microfibril.
• .

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Type I cell walls

• Pectin Matrix
• Made up of HGA, RG-I with various side chains,
  and RG-II.
• Roughly the same amount of methyl esterified
  unesterified HGA found throughout this type of
  wall
• The junction zones contain calcium cross-links
• Located mostly in the middle lamella
• RG-II borate diesters are also present in low
  levels in type I plant cell walls.

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Type II cell walls

• Cellulose
• The same as in type-I walls.
• Cross linking glucans
• Small amounts of xyloglucan bind to the cellulose
  microfibrils, however GAXs are the principal
  polymers which interlock the microfibrils.
• Significant proportions of GAX and xyloglucan
  bind to each other or to the cellulose
  microfibrils via hydrogen bonding.
• GAXs are also cross-linked in walls by both
  esterified and etherified phenolic substances.

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Type II cell walls

• Pectin
• Contain very little pectin. Chemically these
  pectins are composed of both HGA and RG, but a
  highly substituted (HS)-GAX is closely associated
  with these pectins.
• Display a marked developmental preference for
  accumulating methyl esterified or unesterified
  HGAs in specific cell types.

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Summary

• The architecture, mechanisms, and function of
  plants depends completely on the structure of the
  cell wall
• The basic model of a primary cell wall is
• Network of cellulose microfibrils
• Cross-linking Glucans (xyloglucan) hydrogen bond
  to the cellulose microfibrils
• A third independent matrix of various types of
  pectin molecules
• There are two types of primary cell walls
• Type I generally dicots. Type two generally
  monocots

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Summary

• Primary Cell Wall
• Formed by growing cells.
• Usually considered to be relatively unspecialized
  and similar in structure in all cell types
• Secondary Cell Wall
• These are the cell walls that form after cell
  growth has ceased
• Can be highly specialized in structure and
  composition, such as the xylem
• Contain more cellulose and lignin replaces pectin
• Thicker than primary cell walls