Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616, U.S.A.
The hypersensitive reaction (HR) is an inducible plant response that is associated with disease resistance. It is characterized by rapid, localized cell death at the site of infection and is believed to inhibit the spread of invading pathogens. We have isolated a cDNA clone, designated Hra32 (for hypersensitive reaction associated), corresponding to an RNA transcript that accumulates in bean during an HR. The predicted protein product of the Hra32 cDNA is an approximately 17 kDa protein of 161 amino acids, with four putative EF-hand calcium-binding domains. The temporal pattern of Hra32 transcript accumulation correlated closely with the onset of the HR in bean after inoculation with incompatible Pseudomonas syringae pv. tabaci and pv. tomato and with tobacco necrosis virus. Hra32 transcript also accumulated in bean in response to compatible P. syringae pv. phaseolicola and was correlated with necrotic cell death associated with disease lesion formation. A more transient pattern of Hra32 transcript accumulation occurred in bean in response to general stimuli that did not result in the HR or host cell death. These treatments included infiltration with a P. syringae pv. tabaci Hrp¯ mutant, P. syringae pv. tabaci cells treated with kanamycin, Escherichia coli, P. fluorescens, or glutathione, and in response to wounding. Thus, there was differential accumulation of the Hra32 transcript in response to specific stimuli resulting in the HR, compared with general stimuli that did not result in cell death. We hypothesize that the Hra32 product may be a component of the pathway that leads to hypersensitive cell death.