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

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.