Oct. 26, 1999                                                                         Guest Lecturer: Elena Del Campillo

 Ref. Taiz and Zeiger (1998), chap. 15

I. PLANT CELL WALL: Cell wall: complex mixture of polysaccharides,

proteins, and lipids that are secreted by the cell and assembled into an

organized network linked together by a mixture of covalent and

non-covalent bonds.

Ia. PROPERTIES

The cell wall is not uniform and varies in appearance in different cell types.

The cell wall varies in composition according to the cell function.

The function of a cell is reflected in the structure of the cell wall.

The structure of plant cell wall is diverse and complex.

The individual sides of a wall surrounding a cell may vary in thickness

and architecture.

Ib. FUNCTIONS

Controls growth because growth is dependent on the ability of the cell wall to expand.

Determines the mechanical strength of plant structures (acts as an exoskeleton).

Allows high turgor pressure to develop and is required for normal water relations.

Is used as a storage area of large quantities of polysaccharides in

cells of specialized tissues like the endosperm or cotyledons in seed.

Acts as a diffusion barrier that limits the size of macromolecules that

can reach the plasma membrane from outside.

Cell wall components act as signaling molecules during cell

differentiation, recognition of pathogens.

 II. CELL WALL COMPONENTS

 Growing Plant Cell Walls Are Composed Of
        90% Carbohydrate Polysaccharides
        10% Protein.           The Proteins are themselves glycoproteins.

The carbohydrate composition is
    20% cellulose microfibrils and
    70-80% non-cellulosic polysaccharides.

 Non Cellulosic Polysaccharides Are Divided Into Two Groups:
    Pectic and Hemicelluloses.

Hemicelluloses: cellulose binding polymers, bind to cellulose

microfibrils by hydrogen bonds. Only strong alkali can extract them

from cell walls.

The substitutions in the backbone of hemicelluloses are important in

determining the formation of hydrogen bonds and a close packing to

cellulose.

Pectins form a hydrated gel network and are soluble in cold water. The

formation of the network involves ionic bridging of the COO groups of

PGA

PGA or polygalacturonic acid is an helical homopolymer of (1-4) a D

galacturonic acid. PGA is synthesized and secreted as a highly methyl

esterified forms. Methyl groups are removed by a cell wall enzyme

pectin methyl esterase. Unesterified gal a residues can condense by

cross linking with Ca2+ to form junctions zones linking two antiparallel

chains. The degree of esterification determines the cation-exchange

properties, the gel-forming ability which influence wall porosity.

Cell wall structural proteins

(Wounding, pathogen attack and treatment with elicitors increase

expression of these genes)

HRGP         Phloem, cambium, schlerids

PRP             Xylem, fibers and cortex

GRP             Xylem

 III. CELL WALL TYPES

The primary wall is defined as the wall that is produced by the cell

while growing in surface.

The secondary wall is defined as the part of the wall that is produced

after growth as ceased and its more compact.

Walls in tissues consist of two primaries. The outer most parts are

fused to form the Middle Lamellae, easily recognized by the absence of

cellulose.

The Middle Lamella have its origin in the cell plate that partition two

daughter cells after cytokinesis and it is composed mainly of pectic

substances.

 
IV. ORIGIN OF CELL WALL

New primary walls originate with the formation of the cell plate. The

cell plate forms when Golgi vesicles containing mainly pectins and ER

cisternae aggregate in the middle area of the dividing cells. Cell wall

maturation involves: synthesis, secretion, assembly, expansion, cross

linking, secondary wall formation.

Secondary walls form after expansion ceases. This wall are multilayered

and contain higher proportion of cellulose and xylan rather than

xyloglucan. The orientation of cellulose microfibrils is more neatly

aligned parallel to each other. Each layer has a particular cellulose

orientation. Lignin often is found in secondary walls. Lignin is a

phenolic polymer with a complex, irregular pattern of linkages. As

lignin form in the wall it displaces water from the matrix and form a

hidrofobic meshwork that bonds tightly to cellulose and prevents cell

enlargment. Lignin adds strength to cell walls and reduces the

susceptibility to pathogens and reduce the digestibility by animals.

Tracheids and fibers have secondary wall.

 
V. CELLULOSE SYNTHESIS

 Cellulose is a high molecular weight chain of (1,4)-b-D-glucan. The

repeating unit is considered to be cellobiose because the alternating

spatial configuration of the glucosidic bonds that link adjacent

glucose.

The extended glucan chain can interact with each other in a very precise

manner to form a rigid crystalline structure called microfibrils. The

adjacent glucan chains are tightly cross-linked by intermolecular

hydrogen bonds. About two-thirds of each microfibril may be considered

crystalline. The remaining one-third is less ordered, has no regular

hydrogen bonds between adjacent chains and is considered

paracrystalline. Part of the paracrystalline cellulose is localized at

the surface of the microfibrils.

Cellulose is synthesized at the plasma membrane by an enzyme complex

known as terminal complex or TC. The morphology of the complex is called

rosettes because they are circles of six particles. The TC subunit is

the site of catalysis and the repository of the major substrate UDP

glucose, as well as ions, nucleotides and effectors and regulators.

The quantity of glucans synthesized per TC subunit as well as the number

and arrangement of TC subunits determine the size and shape of the

microfibril.

Also the pattern of orientation of cellulose microfibril deposition is

affected by the location and orientation of microtubules possibly

through direct tracking of rosettes.

The plant TC catalytic subunit was isolated first from

Acetobacter Xylinum. This bacteria secretes large quantities of

cellulose as microfibrils from a row of synthethic sites along the

longitudinal axis of the cell. The microfibrils from each synthetic

site merge to form a large ribbon of cellulose. The ribbons tangle the

cells and form a floating pellicle that allows the non motile, strictly

aerobic bacteria to grow in the higher oxygen tension at the surface.

The catalytic subunit of the cellulose synthase of Acetobacter uses

UDP-glucose directly as substrate and a mutant blocked in the ability to

synthesize UDP-glucose from UDP-glucose pyrophosphorylase is cellulose

deficient.

The catalytic subunit of cellulose synthase in plants code for a

glycosyl transferase with two binding sites for UDP-glucose (sugar

donor). They are transferred to the non-reducing chain of the nascent

glucan acceptor. The two-site arrangement may explain why the

cellobiose disaccharide is the repeating subunit. The formation of

cellulose involves not only the synthesis but also the crystallization

of multiple glucans into microfibrils.

When cellulose microfibrils are synthesized it is secreted to the wall

that contains a high concentration of other polysaccharides that are

able to interact with the growing microfibril. Thus, hemicelluloses may

be entrapped during cellulose formation and alter microfibril

morphology.

Matrix polymers are synthesized in the Golgi and secreted in vesicles.

The enzyme responsible for the synthesis of hemicellulase and pectins

are sugar-nucleotides polysaccharide glycosyltransferases and they are

located in the membrane of the Golgi apparatus. Pectin and

hemicelluloses do not form crystalline structures because they are non

linear but branched structures. They have a partial order orientation

in the cell wall as a result of their physical tendency to become

aligned along the axis of cellulose.

VI. PROCESSES THAT ENTAIL CELL WALL CHANGES

VIb. ABSCISSION

Abscission is a natural process that allows plants to separate and shed

unwanted organs from the plant parent body. Plants can abscise almost

all aerial parts: such as leaves, flowers, fruits.

The shedding of a plant organ is the result of a cell separation

process which is confined (restricted) to few row or a narrow layer of

cells

located at precisely and genetically predefined positions usually at

the base of the organ to be shed. These specialized cells are referred

to as abscission cells (target cells) and they differentiate early in

plant development.

The fracture plane bisects many type of cells and not all the cells in

the fracture plane are involved in cell separation The cells of the

vascular tissue are mechanically disrupt

Prior to abscission the cells of the separation layer.

Abscission requires changes in gene expression in the target cells which

result in the localized synthesis and secretion of cell wall degrading

enzymes. These enzymes play an essential role in the enzymatic

loosening of polysaccharides and protein matrix of the wall.

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