Biochemical defense mechanisms:

A) Pre-existing chemical defenses:

a) Inhibitors released by the plant in its environment:

• Plants exude a variety of leaf and root exudates which contain aminoacids, sugars, glycosides, organic acids,etc.
• The inhibitory substances directly affect micro-organisms or encourage certain groups to dominate the environment which may act as antagonists to pathogen.
• Eg : Tomato leaves secrete exudates which are inhibitory to Botrytis cinerea.
• Red scales of red onion contain the phenolic compounds, protocatechuic acid and catechol, which diffuse out to the surface and inhibits the conidial germination of onion smudge fungus, Colletotrichum circinans.
• Resistant varieties of apple secrete waxes on the leaf surface which prevents the germination of Podosphaera leucotricha (powdery mildew of apples).
• Chlorogenic acid present in sweet potato, potato and carrot inhibits Ceratocystis fimbriata.

b) Inhibitors present in plant cells before infection:

• Several phenolic compounds, in cells of young fruits, leaves or seeds are responsible for the resistance of young tissues to Botrytis.
• Saponins have antifungal membranolytic activity which excludes fungal pathogens that lack saponinases. Ex: Tomatine in tomato and Avenacin in oats.

B) Post inflectional or induced defense mechanisms:

a) Phytoalexins :

• Phytoalexins are toxic antimicrobial substances produced after stimulation by phytopathogenic micro-organisms or by chemical or mechanical injury.
• Phytoalexins are produced by healthy cells adjacent to localized damaged or necrotic cells in response to materials diffusing from the infected cells.

Characteristics of phytoalexins :

1. Fungitoxic and bacteriostatic at low concentrations.
2. Produced in host plants in response to stimulus (elicitors) and metabolic products.
3. Absent in healthy plants
4. Remain close to the site of infection.
5. Produced in quantities proportionate to the size of inoculum.
6. Produced in response to the weak or non-pathogens than pathogens
7. Produced within 12-14 hours reaching peak around 24 hours after inoculation.
8. Host specific rather than pathogen specific.

b) Hypersensitive response (HR) :

• The hypersensitive response is a localized induced cell death in the host plant at the site of infection by a pathogen, thus limiting the growth of pathogen.
• HR is initiated by the recognition of specific pathogen-produced signal molecules, known as elicitors.
• Recognition of the elicitors by the host results in altered cell functions leading to the production of defense related compounds.

The most common new cell functions and compounds include:

• A rapid burst of oxidative reactions
• Increased ion movement, especially of K + and H + through cell membrane
• Disruption of membranes and loss of cell compartmentalization
• Cross-linking of phenolics with cell wall components and strengthening of plant cell wall
• Production of antimicrobial substances such as phytoalexins and pathogenesis-related proteins (such as chitinases).

c) Plantibodies:

• Transgenic plants have been produced which are genetically engineered to incorporate into their genome, and to express foreign genes, such as mouse genes that produce antibodies against certain plant pathogens.
• Such antibodies, encoded by animal genes, but produced in and by the plant, are called plantibodies.
• Ex: Transgenic plants producing plantibodies against coat protein of viruses, such as, artichoke mottle crinkle virus have been produced.