Enhanced expression and activation of the alternative oxidase during infection of Arabidopsis with Pseudomonas syringae pv tomato

Cyanide-resistant ("alternative") respiration was studied in Arabidopsis du ring incompatible and compatible infection with Pseodomonas syringae pv tom ato DC3000. Total leaf respiration increased as the leaves became necrotic, as did the cyanide-resistant component that was sensitive to salicylhydrox amic acid. Infiltration of leaves with an avirulent strain rapidly induced alternative oxidase (AOX) mRNA, whereas the increase was delayed in the com patible combination. The increase in mRNA correlated with the increase in A OX protein. Increased expression was confined to the infected leaves, in co ntrast to the pathogenesis-related protein-1, which was induced systemicall y. Virtually all of the AOX protein was in the reduced (high-activity) form . Using transgenic NahG and mutant npr1-1 and etr1-1 plants, we established that the rapid induction of the AOX was associated with necrosis and that ethylene, but not salicylic acid, was required for its induction. Increased pyruvate levels in the infected leaves suggested that increased substrate levels were respired through the alternative pathway; however, in the contr ol leaves and the infected leaves, respiration was not inhibited by salicyl hydroxamic acid alone. Increased respiration appeared to be associated prim arily with symptom expression rather than resistance reactions.