Photosystem I of the moss Physcomitrella patens has special properties, including the capacity to undergo non-photochemical fluorescence quenching. We studied the organization of photosystem I under different light and carbon supply conditions in wild-type moss and in moss with the lhcb9 (light-harvesting complex) knockout genotype, which lacks an antenna protein endowed with red-shifted absorption forms. Wild-type moss, when grown on sugars and in low light, accumulated LHCB9 proteins and a large form of the photosystem I supercomplex, which, besides the canonical four LHCI subunits, included a LHCII trimer and four additional LHC monomers. The lhcb9 knockout produced an angiosperm-like photosystem I supercomplex with four LHCI subunits irrespective of the growth conditions. Growth in the presence of sublethal concentrations of electron transport inhibitors that caused oxidation or reduction of the plastoquinone pool prevented or promoted, respectively, the accumulation of LHCB9 and the formation of the photosystem I megacomplex. We suggest that LHCB9 is a key subunit regulating the antenna size of photosystem I and the ability to avoid the over-reduction of plastoquinone: this condition is potentially dangerous in the shaded and sunfleck-rich environment typical of mosses, whose plastoquinone pool is reduced by both photosystem II and the oxidation of sugar substrates.
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Genomic sequence data from this article can be found in the GenBank/EMBL data libraries (https://www.ncbi.nlm.nih.gov/refseq/) and Phytozome v.12.1.6 (Plant Comparative Genomics portal of the Department of Energy’s Joint Genome Institute) (https://phytozome.jgi.doe.gov/pz/portal.html) under the following accession numbers: XM_001756491 (Pp1s252_28V6.1) for lhcb9.1 and XM_001779101 (Pp1s23_96V6.2) for lhcb9.2. Protein sequence data identified by MS can be found in the online database UniProt (https://www.uniprot.org/). All the accession numbers are indicated in Supplementary Table 3.
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The research was supported by the Marie Curie Actions Initial Training Networks S2B (675006-SE2B) to R.B., A.P., EM. A., E.J.B. and R.K and by grant LO1204 (Sustainable Development of Research in the Centre of the Region Hana) from the National Program of Sustainability I from the Ministry of Education, Youth and Sports, Czech Republic to L.N. and R.K.
Supplementary Figures 1–7 and Supplementary Table 1.
Polypeptide composition of PSI–LHCI–Megacomplex and PSI–LHCI. List of proteins identified by MS analysis in the PSI–LHCI–Megacomplex and in the PSI–LHCI bands in WT and in the corresponding region of lhcb9 KO. Number of PSMs, Mascot score and number of peptides identified are shown for each sample analysed. Sp indicates a protein belonging to the list of common laboratory contaminants included in the database.
Mass spectrometry data. List of peptide spectra matches (PSMs) of the unique peptides identified by MS for the subunits shown in Figure 5 in the PSI–LHCI.
Mass spectrometry data. List of peptide spectra matches (PSMs) in PSI–LHCI–Megacomplex (b) bands in WT and in the corresponding region of lhcb9 KO.