Description:
<jats:title>Summary</jats:title><jats:p>The plastid terminal oxidase <jats:styled-content style="fixed-case">PTOX</jats:styled-content> is a plastohydroquinone:oxygen oxidoreductase that is important for carotenoid biosynthesis and plastid development. Its role in photosynthesis is controversially discussed. Under a number of abiotic stress conditions, the protein level of <jats:styled-content style="fixed-case">PTOX</jats:styled-content> increases. <jats:styled-content style="fixed-case">PTOX</jats:styled-content> is thought to act as a safety valve under high light protecting the photosynthetic apparatus against photodamage. However, transformants with high <jats:styled-content style="fixed-case">PTOX</jats:styled-content> level were reported to suffer from photoinhibition. To analyze the effect of <jats:styled-content style="fixed-case">PTOX</jats:styled-content> on the photosynthetic electron transport, tobacco expressing <jats:styled-content style="fixed-case">PTOX</jats:styled-content>‐1 from <jats:italic>Chlamydomonas reinhardtii</jats:italic> (Cr‐<jats:styled-content style="fixed-case">PTOX</jats:styled-content>1) was studied by chlorophyll fluorescence, thermoluminescence, P700 absorption kinetics and <jats:styled-content style="fixed-case">CO</jats:styled-content><jats:sub>2</jats:sub> assimilation. Cr‐<jats:styled-content style="fixed-case">PTOX</jats:styled-content>1 was shown to compete very efficiently with the photosynthetic electron transport for <jats:styled-content style="fixed-case">PQH</jats:styled-content><jats:sub>2</jats:sub>. High pressure liquid chromatography (<jats:styled-content style="fixed-case">HPLC</jats:styled-content>) analysis confirmed that the <jats:styled-content style="fixed-case">PQ</jats:styled-content> pool was highly oxidized in the transformant. Immunoblots showed that, in the wild‐type, <jats:styled-content style="fixed-case">PTOX</jats:styled-content> was associated with the thylakoid membrane only at a relatively alkaline <jats:styled-content style="fixed-case">pH</jats:styled-content> value while it was detached from the membrane at neutral <jats:styled-content style="fixed-case">pH</jats:styled-content>. We present a model proposing that <jats:styled-content style="fixed-case">PTOX</jats:styled-content> associates with the membrane and oxidizes <jats:styled-content style="fixed-case">PQH</jats:styled-content><jats:sub>2</jats:sub> only when the oxidation of <jats:styled-content style="fixed-case">PQH</jats:styled-content><jats:sub>2</jats:sub> by the cytochrome <jats:italic>b</jats:italic><jats:sub><jats:italic>6</jats:italic></jats:sub><jats:italic>f</jats:italic> complex is limiting forward electron transport due to a high proton gradient across the thylakoid membrane.</jats:p>