Abstract
Mitochondrial respiration plays an important role in optimizing photosynthetic efficiency in plants. As yet, the mechanisms by which plant mitochondria sense and respond to changes in the environment are unclear, particularly when exposed to light. Here we describe the characterization of the Chlamydomonas reinhardtii mutant stm6, which was identified on the basis of impaired state transitions, a mechanism that regulates light harvesting in the chloroplast. The gene disrupted in stm6, termed Moc1, encodes a homologue of the human mitochondrial transcription termination factor (mTERF). MOC1 is targeted to the mitochondrion, and its expression is up-regulated in response to light. Loss of MOC1 causes a high light-sensitive phenotype and disrupts the transcription and expression profiles of the mitochondrial respiratory complexes causing, as compared with wild type, light-mediated changes in the expression levels of nuclear and mitochondrial encoded cytochrome c oxidase subunits and ubiquinone-NAD subunits. The absence of MOC1 leads to a reduction in the levels of cytochrome c oxidase and of rotenone-insensitive external NADPH dehydrogenase activities of the mitochondrial respiratory electron transfer chain. Overall, we have identified a novel mitochondrial factor that regulates the composition of the mitochondrial respiratory chain in the light so that it can act as an effective sink for reductant produced by the chloroplast.
Highlights
In plant cells, respiration within the mitochondrion provides ATP and serves an important role in the maintenance of an appropriate reduction/oxidation state within the chloroplast [1]
A key signalling molecule in the chloroplast thylakoid membrane is plastoquinone (PQ) which according to its redox state appears to control gene transcription in the chloroplast [7] and to regulate light-harvesting which in turn appears to control the ratio of linear to cyclic electron flow in algal chloroplasts [8]
Isolation of state transition mutants To identify genes involved in state transitions within C. reinhardtii, we screened a library of potentially tagged nuclear mutants that was generated through the random insertion of plasmid pArg7.8, carrying the Arg7 gene, into the nuclear genome of the arginine auxotrophic strain, CC1618 [27,28]
Summary
Respiration within the mitochondrion provides ATP and serves an important role in the maintenance of an appropriate reduction/oxidation (redox) state within the chloroplast [1]. Because the redox state of the plastoquinone pool is affected by mitochondrial respiration in the dark [9], and possibly in the light, the mitochondrion has the capacity to influence both chloroplast function and development. Important evidence for the role of the respiratory chain in optimising photosynthesis [10,11,12,13] and the development of the chloroplast [14] has come from analysis of mutants lacking various respiratory complexes, as well as through the use of various mitochondrial-specific inhibitors [15]
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