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.