The HR appears to be a form of PCD in plants. Firstly, the appearance of the HR is genetically controlled (see below) and second, purified HR-inducing factors from bacteria called harpins will not induce the HR unless the plant tissue is transcriptionally active (40). In addition, HR-inducing bacteria will not cause the HR if
protein synthesis is blocked in the plant (41). In the search for a signal for HR induction, several groups have determined that H202 is rapidly produced by plant cells in culture during the HR in a phenomenon termed the oxidative burst [reviewed by Medhy (42)].