Abstract

Summary Early events occurring during the hypersensitive resistance response (HR) were examined using the avrRpm1 / RPM1 gene‐for‐gene interaction in Arabidopsis challenged by Pseudomonas syringae pv. tomato . Increases in cytosolic Ca 2+ were measured in whole leaves using aequorin‐mediated bioluminescence. During the HR a sustained increase in Ca 2+ was observed which was dependent on the presence of both a functional RPM1 gene product and delivery of the cognate avirulence gene product AvrRpm1. The sequence‐unrelated avirulence gene avrB , which also interacts with RPM1 , generated a significantly later but similarly prolonged increase in cytosolic Ca 2+ . Accumulation of H 2 O 2 at reaction sites, as revealed by electron microscopy, occurred within the same time frame as the changes in cytosolic Ca 2+ . The NADPH oxidase inhibitor diphenylene iodonium chloride did not affect the calcium signature, but did block H 2 O 2 accumulation and the HR. By contrast, the calcium‐channel blocker LaCl 3 suppressed the increase in cytosolic Ca 2+ as well as H 2 O 2 accumulation and the HR, placing calcium elevation upstream of the oxidative burst.