Alternative oxidase impacts the plant response to biotic stress by influencing the mitochondrial generation of reactive oxygen species

Abstract

Previously, we showed that inoculation of tobacco with Pseudomonas syringae incompatible pv. maculicola results in a rapid and persistent burst of superoxide (O2-) from mitochondria, no change in amount of mitochondrial alternative oxidase (AOX) and induction of the hypersensitive response (HR). However, inoculation with incompatible pv. phaseolicola resulted in increased AOX, no O2- burst and no HR. Here, we show that in transgenic plants unable to induce AOX in response to pv. phaseolicola, there is now a strong mitochondrial O2- burst, similar to that normally seen only with pv. maculicola. This interaction did not however result in a HR. This indicates that AOX amount is a key determinant of the mitochondrial O2- burst but also that the burst itself is not sufficient to induce the HR. Surprisingly, the O2- burst normally seen towards pv. maculicola is delayed in plants lacking AOX. This delay is associated with a delayed HR, suggesting that the burst does promote the HR. A O2- burst can also be induced by the complex III inhibitor antimycin A (AA), but is again delayed in plants lacking AOX. The similar mitochondrial response induced by pv. maculicola and AA suggests that electron transport is a target during HR-inducing biotic interactions. It was previously shown that the hypersensitive response (HR) of tobacco leaf to the bacterial pathogen Pseudomonas syringae is preceded by an early and persistent burst of superoxide in the mitochondrion. Here, we show that the amount of mitochondrial alternative oxidase (AOX), an electron transport chain component able to dampen superoxide and nitric oxide generation by the respiratory chain, is a critical determinant of this superoxide burst. Evidence is also provided that this burst promotes the HR and that the electron transport chain, perhaps Complex III, is a target during HR-inducing biotic stress. The work provides a compelling example of a biological process, in this case a response to bacterial pathogen, in which AOX appears to play a central role in defining a mitochondrial reactive oxygen species signature. © 2012 Blackwell Publishing Ltd.