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.