Cell wall composition and its defense mechanism against pathogens.

Submitted by Sheikh Mansoor Shafi

Department of Biochemistry

Skuast Jammu.

The cell wall has a number of functions

Protects the cell against pathogens

Lends the cell stability

Determines its shape

Influences its development

Counterbalances the osmotic pressure

Components of cell walls

Cellulose

Pectic substances

Hemicellulose

Lignin

Proteins

Plant cell wall

Middle lamella

Primary cell wall

Secondary cell wall

Cell wall composition

Middle lamella first layer formed during cell division rich in

pectin. Joins adjacent cells & holds them together

Primary cell wall composed of pectin, hemicellulose, &

glycoproteins. The layer consists of framework of cellulose

microfibrils in a gel like matrix, it is thin flexible &

extensible

Secondary cell wall it is extremely rigid & provides strength.

Composed of cellulose, hemicellulose & lignin

Sugars present in cell wall are

Pentoses (xylose, Arabinose,apiose

Hexoses (Galactose,Glucose and mannose)

Uronic acid ( Galactouronic acid & Glucounonic acid)

Deoxy sugars ( Rhamnose and Fucose)

Cellobiose (Glycosyl + Glucose)

Composition

Plant cell walls consist of carbohydrate, protein, and aromatic compounds

and are essential to the proper growth and development of plants.

The carbohydrate components make up 90% of the primary wall, and are

critical to wall function. There is a diversity of polysaccharides that make up

the wall and that are classified as one of three types: cellulose, hemicellulose,

or pectin.

The pectins, which are most abundant in the plant primary cell walls and the

middle lamellae, are a class of molecules defined by the presence of

galacturonic acid.

.

The pectic polysaccharides include the galacturonans

(homogalacturonan, substituted galacturonans, and RG-II) and

rhamnogalacturonan-I. Galacturonans have a backbone that

consists of a-1,4-linked galacturonic acid.

The identification of glycosyltransferases involved in pectin

synthesis is essential to the study of cell wall function in plant

growth and development

Resistence

Cuticle and cell wall thickness may influence resistance to certain

pathogens

The presence of secondary cell walls in sclerenchyma, xylem or

older plant tissue often retards pathogen development

Many pathogens enter through wounds, natural openings or are

introduced by vectors.

Molecular components that serve essential functions for the fitness

or survival of microbes are often highly conserved. For plants,

detection of such microbial fingerprints also referred to as pathogen-

associated molecular patterns (PAMPs), is a warning of impending

attack

In addition to sensing PAMPs, the ability to sense a compro-

mised “self ” by detecting damage-associated molecular patterns

(DAMPs) such as released plant cell wall fragments is a central

part of plant defense

Pathogen-associated molecular pattern perception is medi- ated

by ligand-binding surface-exposed transmembrane pattern-

recognition receptors (PRRs) of either the receptor-like kinase

(RLK) or receptor-like proteins (RLPs) families.

Fig. 7. Illustration of the fungal stealth infection strategy using a-1,3-glucan. (A) After perforation of the plant cuticle, the cell wall of non-pathogenic fungi is partly digested byplant apoplastic enzymes and PAMPs are released. Recognition of the released PAMPs by the plant PRRs activates PTI, that attacks the invading fungi. (B) Fungal plant pathogensrecognize plant factor(s) and accumulate a-1,3-glucan at the surface of the cell wall. Surface a-1,3-glucan interferes with the access of plant apoplastic enzymes the to cell walls ofinfectious structures. Consequently, PAMPs release is suppressed and the pathogens invade plant

cells without evoking PTI.

The lignin monomers

(coneferyl alcohol, coumeryl alcohol &

sinapyl alcohol ) arise from the aromatic

amino acid phenylalanine and tyrosine via

the phenylpropanoid pathway .Deposited

in secondary walls of most plant cells

Extremely resistant to microbial

degradation increased lignification in cell

wall in response to pathogen infection,

especially in the incompatible host-

parasite interaction.

Fungal cell wall and plant cell wall are elicitors.

Cell-wall-degrading enzymes

produced by plant pathogens

Enzymes that degrade pectic

substances

Pectin methylesterase

Polygalacturonases

Pectin lyase

Rhamnogalacturonanases

Enzymes that degrade pectic substances

Pectin lyase (transeliminase)

Pectin lyase splits a-1,4-linkage between

methylgalacturonides (pectin).

Pectin methylesterase (PME)

Degrade pectin and pectinic acid to form

pectate. Pectate is more soluble and easily

be degraded by other pectate degrading

enzymes.

Pectate lyase Pectate lyases split the a-(1-4) linkage

between galacturonosyl residues in pectate

(homogalacturonan, HGA).

Polygalacturonase

(pectin glycosidase, PG) polygalacturonase

attacks a-1,4-glycosidic bonds of pectate.

Enzymes that degrade hemicelluloses

Xylanase

b-(1,4)-linkage xylans are degraded by endo- and exo-xylase to oligomer or xylose.

Glucanase

b-(1, 3)-glucan is a minor component of plant tissue, but it is important in plant disease resistance because it occurs primarily in cell wall appositions and papillae in the form of callose in response to fungal penetration.

Many pathogens produce b-(1, 3)-glucanase to degrade b-(1, 3)-glucan

Galactanase

Degrading arabinogalactan

Enzymes that degrade the cell wall are most useful for

necrotrophic pathogens with wide host range.

Cell-wall-degrading enzymes produced by plant pathogens

Enzymes that degrade cellulose Complete degradation of native cellulose to glucose requires

three enzymes. b-1,4-glucanase Cellobiohydrolase b-glucosidase

Enzymes that degrade lignin

Lignin peroxidase Manganese-dependent peroxidase,

Tyrosinase Enzymes that degrade proteins Protease