Coordination of a mitochondrial superoxide burst during the hypersensitive response to bacterial pathogen in Nicotiana tabacum

Abstract

We characterized responses of Nicotiana tabacum to pathovars of the bacterial pathogen Pseudomonas syringae. These included a compatible response associated with necrotic cell death (pv. tabaci), an incompatible response that included hypersensitive response (HR) cell death (pv. maculicola) and an incompatible response that induced defences but lacked the HR (pv. phaseolicola). Signalling molecules (salicylic acid, nitric oxide, H 2O 2) known to induce the stress responsive tobacco Aox1a gene [that encodes the mitochondrial electron transport chain (ETC) component alternative oxidase (AOX)] accumulated preferentially during the HR, but this did not elevate Aox1a transcript or AOX protein, while the transcript and protein were strongly elevated during the defence response to pv. phaseolicola. In addition, matrix manganese superoxide dismutase (MnSOD) activity declined during the HR, unlike its response to the other pathovars, and unlike the response of other superoxide dismutase (SOD) enzymes. Finally, the HR (but not the response to pv. phaseolicola or pv. tabaci) was accompanied by an early and persistent mitochondrial superoxide (O 2-) burst prior to cell death. We propose that a coordinated response of the major ETC mechanism to avoid O 2- generation (AOX) and the sole enzymatic means to scavenge mitochondrial O 2- (MnSOD) is important in the determination of cell fate during responses to pathogen. This work shows that the hypersensitive response of tobacco leaf to the bacterial pathogen Pseudomonas syringae is preceded by an early and persistent burst of superoxide in the mitochondrion. This burst appears to be sustained by suppression of the major mitochondrial mechanism to avoid superoxide generation by the electron transport chain (alternative oxidase) and suppression of the sole enzymatic means to scavenge mitochondrial superoxide (manganese superoxide dismutase). Based on these studies, it is suggested that alternative oxidase and manganese superoxide dismutase are uniquely positioned to impact both the strength and specificity of reactive oxygen species (ROS) signals in the mitochondrion and that coordination of their activities represents a potential means to construct multiple distinct mitochondrial ROS signatures. © 2011 Blackwell Publishing Ltd.