Composition of the cell wall:

Functionally cell wall is divided into 3 regions, viz.,

1. middle lamella (made of pectins),
2. primary wall (cellulose, pectic substances) and
3. secondary cell wall (entirely cellulose).

Fig : Composition of cell wall

• Middle lamella acts as intercellular cement which binds the cells together in tissue system.
• Pectin or pectic substances are major chemical constituents of wall layers and entire middle lamella, where as in other layers, cellulose is found in good amounts.
• Main components of cell wall are pectic substances, cellulose, hemicelluloses, lignin and small quantity of protein.
• The epidermis of plants is covered by cuticle, whose major chemical substance is cutin in addition to cuticular wax.

Cuticular wax:

• Plant waxes are found as granular or rod like projections or as a continuous layer outside / within the cuticle.
• Cuticular waxes are made up of long chain molecules of paraffin, hydrocarbons, alcohols, ketones and acids.
• Most of the fungi and parasitic higher plants penetrate wax layers by means of mechanical force alone.

Cutin:

• It is an insoluble polyester of unbranched derivatives of C 16 and C 18 hydroxy fatty acids.
• Cutinases break cutin molecules and release monomers as well as oligomers from insoluble cutin polymer.
• Ex: Colletotrichum gloeosporioides, Sphaerotheca pannosa, Venturia inaequalis, Helminthosporium victoriae.

Pectic substances:

• These are major components of middle lamella (intercellular cement that holds in place the cells of plant tissues).
• They also make up a large portion of primary cell wall in which they form an amorphous gel filling the spaces between cellulose microfibrils.
• Pectic substances are of three types, namely, pectic acid (non methylated units), pectinic acid (<75% methylated galacturonan units) and pectin (>75% methylated units).
• Term protopectin is used to denote substances which are soluble in water and upon restricted hydrolysis yields pectinic acid.
• Pectin degradation results in liquefaction of the pectic substances and weakening of cell walls, leading to tissue maceration.
• Ex: Soft rot bacterium, Erwinia cara

Cellulose:

• Cellulose is a polysaccharide, made of chains of β-D-glucopyranose units.
• Cellulose is insoluble in crystalline form (native form), and soluble in amorphous form (modified cellulose).
• Cellulose is degraded by cellulases.
• Cellulase degrading enzymes play a role in softening and degradation of cell wall material and facilitate easy penetration and spread of pathogen in the host.
• Ex: Basidiomycetes fungi

Hemicellulose:

• These are the major constituents of primary cell wall and also seen in middle lamella and secondary cell wall.
• Hemicellulases degrade hemicelluloses and depending on the monomer released from polymer on which they act, they are termed as xylanase, galactanase, glucanase, arabinase, mannose, and so on.
• Ex: Sclerotinia sclerotiorum, Sclerotinia fructigena.

Lignin:

• Lignin is found in the middle lamella, as well as in the secondary cell wall of xylem vessels and the fibres that strengthen plants.
• White rot fungi (Basidiomecetes) secrete one or more ligninases which enable them to utilize lignin. Ex: Xylaria, Chaetomium, Alternaria, Cephalosporium, etc.

Cell wall proteins:

• Cell wall proteins are similar to other proteins, except that they are rich in aminoacid, hydroxy proline.
• Proteins are degraded by means of enzymes, proteases or proteinases or peptidases.

Lipids:

• Various types of lipids occur in all plant cells.
• The most important ones are phospholipids and glycolipids.
• These lipids contain fatty acids, which may be saturated or unsaturated.
• Lipolytic enzymes, called lipases (phospholipases, glycolipases) hydrolyze lipids and release fatty acids.

Starch:

• Starch is the main reserve polysaccharide found in plant cells.
• It is a glucose polymer and exists in two forms: amylose, a linear molecule, and amylopectin, a highly branched molecule. Starch is degraded by enzyme, amylases.