The establishment of Erwinia amylovora harpin-induced hypersensitive response (HR) in Nicotiana sylvestris was followed by infra-red thermography (IRT). Three to four hours after elicitation, the temperature decreased in the harpin-infiltrated zone associated to stomatal opening. The marked drop in temperature which reached 2 degrees C and preceded necrosis symptoms for several hours, is thus likely caused by higher transpiration. Neither of these effects was observed in a respiratory mutant, affected in complex I structure and function and over-expressing alternative oxidase, indicating that they are directly or indirectly mediated by mitochondrial function. However, as the HR establishment was similar in both wild type and mutant, cell death was either uncorrelated with the observed epidermal changes or occurred by a different signalling pathway in the two genotypes. IRT revealed a novel aspect of plant-pathogen interactions and could be applied to screen for mutants affected in elicitor signalling and/or for respiratory mutants.