ABSTRACT
Plants are steadily threatened by a variety of pests and pathogens against which they have evolved different defense mechanisms. Preformed physical barriers
and toxic metabolites
are two parts of the defense. Other defense reactions are set in action only after attack by pathogens or pests. These include the formation of phytoalexins,
pathogenesis-related (PR) proteins,
papillae,
a strengthening of cell walls,
and the hypersensitive response (HR), which is thought to function by isolating and killing microorganisms in a small region of the plant by localized cell death.
