Plant mitochondria are proposed to act as signaling organelles in the orchestration of defense responses to biotic stress and acclimation responses to abiotic stress. However, the primary signal(s) being generated by mitochondria and then interpreted by the cell are largely unknown. Recently, we showed that mitochondria generate a sustained burst of superoxide (O 2(-)) during particular plant-pathogen interactions. This O 2(-) burst appears to be controlled by mitochondrial components that influence rates of O 2(-) generation and scavenging within the organelle. The O 2(-) burst appears to influence downstream processes such as the hypersensitive response, indicating that it could represent an important mitochondrial signal in support of plant stress responses. The findings generate many interesting questions regarding the upstream factors required to generate the O 2(-) burst, the mitochondrial events that occur in support of and in parallel with this burst and the downstream events that respond to this burst.
Keywords: alternative oxidase; biotic stress; manganese superoxide dismutase; mitochondrial electron transport chain; nitric oxide; organelle signaling; salicylic acid; superoxide burst.