Active plant defense mechanism are comparable to the immune system of vertebrate animals, althought the cellular and molecular bases are fundamentally different. Both,however,are triggered in reaction to intrusion, implying that the host has some means of recognizing the presence of a foreign organism, the most dramatic example of an inducible plant defense reaction is the hypersensitive response. In the hypersensitive response,cells undergo rapid necrosis---that is, they become diseased and die---after being penetrated by as parasite itself subsequently ceases to grow and is therefore restricted to one or a few cells around the entry site,several theories have been put forward to explain the basis of hypersensitive resistance.