Plant interactions with bacterial Pathogens
Handout 1: Luis Mur [email protected]
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
HH
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
OHHOOH
O(H)
OHHO
cellulose
OOO
o
o
o
o
o
o
o
CH 2
O
CH 2
O
Furthersubstituentsugars: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
PHASEOTOXIN Phaseolotoxin 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
HH
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