The Biotrophic form of Pathogenesis


LIFE CYCLE

 

Virulence determinants are different in biotrophic forms of
 pathogenesis

 

Toxins

 

1.   Mostly small peptides

2.   Unlike fungal toxins, these do not exhibit host-specificity

3.   Also unlike fungal toxins, virulence, not pathogenicity functions.

 

·       A toxins are produced by non-pathogenic strains of P.syringae

·       Some biosynthetic genes may be mutated without loss of

pathogenicity.

 

4.   Syringomycin / coronatine are part synthesised by non-  

ribosomal  means.

 

Toxins Produced by Pseudomonas syringae pathovars

 

Toxin

Pathovar

Function or Target

Symptoms
Syringomycin

syringae
Forms pores in plasma membrane

 
Necrosis
Syringopeptin

syringae
Coronatine

e.g. tomato,

Molecular mimic of the plant signal, jasmonic acid.

 
Chlorosis
Tagetitoxin

tagetis

Inhibitor of chloroplast RNA polymerase

 
Chlorosis
Phaseolotoxin

phaseolicola

Inhibitor of ornthine carbamoyltransferase

 

Chlorosis
Tabtoxin

tabaci
Inhibitor of glutamine synthase
Chlorosis

 

 

Phaseolotoxin – causes halo’s in French bean blight

 

-        a tripeptide – ornithine- alanine – arginine (with a

 phosphosulphinyl group).

 

-        ornthine carbamoyltransferase – normally converts

ornithine to citrulline – an arginine precursor.

-        Inhibiton by  Phaseolotoxin depletes citrulline-

arginine – Elevates ornithine.

-        Changes the permeability of membranes

 

Tabtoxin

 

Tabtoxin minus mutants show reduced virulence – causing

necrotic spots without chlorosis.   

 

A dipeptide composed of threonine and the unique amino acid – tabtoxinine.

 

Hydrolysed in plant cell and tab-toxinine inhibits glutamine synthase.  

 

 

 

 

 

 

 

 

 

 

 


Reduces glutamine levels and increased the concentration

of ammonia.

 

Ammonia inhibits photosythesis and destroys the thylakoid

membrane of the chloroplast. This lead to chlorosis and eventually

necrosis.

 

Also suppresses plant defences.

 

·       Extracellular polysaccharide (EPS).

 

 

 

 

 

 

 

 

 

 

 


In Pseudomonas (Ralstonia) solanacearum

  

1      Extracellular polysaccharide occurring as a capsular material surrounding the bacterial cell or may be spread as a fluvial slime,

2     
Linear polymer of three types of sugars e.g.

N-aceylgalactosamine .

d-
 
 

 


3      Therefore a highly charged polymer…viscous.

4      Produced to high levels in planta.

5      Functions

 

·       Protects bacteria from desiccation

·       Concentrates minerals and nutrients

·       Enhances attachment to surfaces

·       Acts a virulence determinant, sustaining water-soaking

and blocks xylem resulting in wilt