Plant interactions with bacterial Pathogens

Handout 1: Luis Mur lum@aber.ac.uk

users.aber.ac.uk/lum

From Agrios G.N.: Plant Pathology (4thed.)

The variety of plant - bacterial interactions

NECROTROPHIC BACTERIAL INTERACTIONS

Cell-wall polymers

(A) Cellulose Unbranched polymer (1000-1500 Units) of glucose residues joined at C1-4

.

H O

CH 2OH

OHH

O

H

H

O

H

OH

O

CH 2OH

H OH

H

H

OH

1

23

4

5

6

O

H

OH

CH 2OH

H OH

H

H

OH

(B) Matrix Polymers -"molecular glue"

(1) hemicellulases - mainly xyloglucans . Xylose C5 sugar in pyranose (C5 ring) form

CO

OH1

23

4

5

OH

OHHO

CH 2

O

OH

OH

OHHO

OH

O(H)

OHHO

cellulose

OOO

o

o

o

o

o

o

o

CH 2

O

CH 2

O

Further

substituent

sugars:

e.g. galactose

(2) pectic polygalacturonic acids:

OH

HO

C=OO

O

OH

1

23

4

5

6

O -

OH

O

C=OO

OH

OH

O -

C1-4 linkedgalacturonic acids

occasional rhamnose sugars in chain

OH

C=O O

O

OH

O -

OH

O

C=O O

OH

O -

OH

C=O O

O

OH

OH

O

C=O O

OH

OH

O

O

Ca 2+

O O

2-hydroxycinnamic acid

CHCHCOOH

A MODEL FOR PRIMARY CELL WALL ASSEMBLY

microfilament

- O

- O

- O

- O

- O

- O

HO +

HO d+ HO

d+ HO d+

hydrogen-bonding with glucan-spine of hemicellulase

HO +

HO +

O

co-valent bonds with acidic pectins

O

polygalacturonic acids

Degrading the Cell wall: Erwinia carotovora

· Pectic lyases attach the 1-4 glucosidic linkage by -elimination to produce unsaturated products.

OH

C=OO

OOH

OOH

C=OO

OH

=O

OH OH

PECTICLYASE

· Polygalacturonidase cleaves pectate polymers by hydrolysis -

OH

C=OO

OOH

OOH

C=OO

OH

OHPolygalacturondiase

OH

HO

BUT resistance to Erwinia in e.g. potato is associated with the methylation of pectate. So some strains of Erwinia also produce a methyltransferase

Pectic methyltransferases -

OH

C=OO

OOH

OOH

C=OO

OH

CH 2 OH

Methyltransferase

PECTIN PECTATE

+ H

+ H20+H

Another type of virulence function : Quorum sensing

But first – Quorum sensing in Vibrio fischeri.

The range of acyl homoserine lactams

(AHL) associated with plant

pathogens

Quorum sensing in Erwinia.

BIOTROPHIC BACTERIAL INTERACTIONS

Toxin Pathovar Function or Target Symptoms

Syringomycin syringae

Syringopeptin syringae

Forms pores in plasma membrane Necrosis

Coronatine e.g. tomato, Molecular Mimic of the plant

signal, jasmonic acid.

Chlorosis

Tagetitoxin tagetis Inhibitor of chloroplast RNA

polymersase

Chlorosis

Phaseolotoxin phaseolicola Inhibitor of ornthine

carbamoyltransferase

Chlorosis

Tabtoxin tabaci Inhibitor of glutamine synthase Chlorosis

Toxins Produced by P. syringae pathovars

PHASEOTOXINPhaseolotoxin is produced by P. syringae p.v. phaseolicola which cauyse halo blight on legumes and bacterial canker. It consists of a sulphodiaminophosphinyl moiety linked to a tri-peptide consisting of ornithine, alanine and homoarginine (Fig. 1).

Fig. 1. The structure of phaseolotoxin. Plant peptidase cleave phasolotoxin (arrow) to release the alanine and homooarginine residues, a reaction that results in octicidine (Psorn) formation.

Phaseolotoxin competitively inhibit ornithine carbamoyl transferase (OCTase), a critical enzyme in the Urea cycle which converts ornithine and carbamoyl phosphate to citrulline (Fig.2). Psorn (N-[N’sulphodiaminophosphyinyl) – L- orthinine)is an irreversible inhibitor of OCTase. Inhibition of OCTasa casues an accumulation of ornithine and a defiency in the intracellular pools of argnine, leading to necrosis.

Fig. 2. Mechanism of action of octicidine (Psorn), the toxic moiety Of phaseolotoxin.

Vascular Wilts : Ralstonia (Pseudomonas) solanacearum

N-acetylglucosamide monomers of EPS in Pseudomonas solanaceraum.

H O

H

O

H

H

O

H

OH

O

CH 2 OH

H OH

H

H

NH

1

23

4

5

6

C=O

CH3

O

H

OH

CH 2 OH

H OH

H

H

NH

C=O

CH3

CH 2 OH

Amine group