Incubation of Petunia hybrida cells with H202 leadsto an increase in alternative oxidase activity measured after 24 h. This increased activity is accompanied by an increase in alternative oxidase protein. A model is presented for the regulation of alternative oxidase protein synthesis in which active oxygen species and especially H202 play a crucial role as second messengers in the signal transducing pathway from the mitochondria to the nucleus. It is proposed that also the induction of the alternative oxidase by salicylic acid is mediated via H202.

In the past few years, knowledge of the nature and regulation of the alternative oxidase in plant mitochondria has increased greatly. The protein has been characterized and mechanisms that regulate its activity have been described. The consequences of these regulatory mechanisms are that in vivo the cytochrome pathway and the alternative pathway may compete for electrons. The implications for the interpretation of the ‘Bahr and Bonner’ inhibitor titrations, formerly used to estimate the partitioning of electrons over the two pathways, are discussed. It is proposed that activation and engagement of the alternative oxidase may keep Q reduction levels low in order to prevent harmful high levels of free radical production. A model is presented for the regulation of alternative oxidase protein induction, involving a signalling function of active oxygen species.

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Current understanding of the structure and function of the plant alternative oxidase is reviewed. In particular, the role of the oxidase in the protection of tissues against oxidative stress is developed.