Role of toxins and enzymes in plant diseases

A pathogen secretes toxins, enzymes, growth regulators and polysaccharides, which are used as weapons during pathogenesis. Some of the important weapons are listed here:

Toxins involved in plant diseases:

Tentoxin

Tentotoxin is secreted by Alternaria alternata. This is a cyclic tetrapeptide that induces chlorosis in hosts by binding a chloroplast-coupling factor involved in energy transfer. It also inhibits light dependent phosphorylation of ADP to form ATP.

Tabtoxin

Tabtoxin is produced by the bacterium Pseudomonas syringae pv. tabaci causing “wildfire disease of tobacco”. This is a dipeptide. Tabtoxin inhibits the enzyme glutamine synthetase.

Fusicoccin

Fusicoccin is a wilt toxin produced by a non-vascular pathogen, Fusicoccum amygdali causing bud cankers on shoots of almond and peach trees. This toxin is readily translocated by the transpiration stream from the invaded bark tissues to the leaves, where it induces stomatal guard cells to open irreversibly. This causes uncontrolled water loss and thus the leaf blade wilts and withers.

Fusaric acid

Fusaric acid is a vivotoxin secreted by Fusarium moniliforme, Fusarium oxysporum, and Fusarium heterosporum. It is a pyridine-carboxylic acid. This toxin causes growth inhibition and injury in my vegetable crops. This toxin alone cannot cause wilt.


Enzymes involved in plant diseases:

Cutinases

During invasion, pathogens secrete cutinases to dissolve the outer waxy cuticle.

Pectinases

Middle lamella between the plant cells is composed of pectic substances. During piercing the cells pathogens used pectin degrading substances often termed pectinases or pectolytic enzymes as weapons.

Cellulases

In plants cellulose makes the major framework and is the principal component of the cell wall. Cellulases are secreted by many pathogens as a weapon. These enzymes digest the cell wall and collapse the tissues.
Pectinases and cellulases are chief enzymes involved in damping-off of seedlings caused by Pythium and other fungi.


Hormones involved in plant diseases:

Auxins

Auxins are plant growth hormones responsible for cell elongation. In a number of plant diseases e.g., Corn smut (Ustilago maydis), clubroot of crucifers (Plasmodiophora brassicae), crown gall (Agrobacterium tumefaciens) altered level of auxin have been implicated.

Ethylene

Ethylene production in infected tissues can be dramatically induced. This induction is largely dependent on activation of the ethylene biosynthetic pathway in plant tissues.


Content first created on 26-01-2023
last updated on 26-01-2023