The plastome-encoded PsaJ subunit is required for efficient Photosystem I excitation, but not for plastocyanin oxidation in tobacco
Résumé
The functions of several small subunits of the large photosynthetic multiprotein complex photosystem I (PSI) are not yet understood. To elucidate the function of the small plastome-encoded PsaJ subunit, we have produced knock-out mutants by chloroplast transformation in tobacco (Nicotiana tabacum). PsaJ binds 2 chlorophyll-a molecules and is localized at the periphery of PSI, close to both the Lhca2 and Lhca3 docking sites and the plastocyanin binding site. Tobacco psaJ knock-out lines do not display a visible phenotype. Despite a 25% reduction in the content of redox-active PSI, neither growth rate nor assimilation capacity are altered in the mutants. In vivo, redox equilibration of plastocyanin and PSI is as efficient as in the wild type, indicating that PsaJ is not required for fast plastocyanin oxidation. However, PsaJ is involved in PSI excitation: Altered 77 K chlorophyll-a fluorescence emission spectra and reduced accumulation of Lhca3 indicate that antenna binding and exciton transfer to the PSI reaction center are impaired in {Delta}psaJ mutants. Under limiting light intensities, growth of {Delta}psaJ plants is retarded and the electron transport chain is far more reduced than in the wild type, indicating that PSI excitation might limit electron flux at sub-saturating light intensities. In addition to defining in vivo functions of PsaJ, our data may also have implications for the interpretation of the crystal structure of PSI.
Domaines
ChimieOrigine | Fichiers produits par l'(les) auteur(s) |
---|