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A LHCB9-dependent photosystem I megacomplex induced under low light in Physcomitrella patens.

Autor: Pinnola A., Alboresi A., Nosek L., Semchonok D., Rameez A., Trotta A., Barozzi F., Kouřil R., Dall’Osto L., Aro E.-M., Boekema E.L., Bassi R.Published: Nature Plants 4(11), 910-919Year: 2018

The moss Physcomitrella patens has photosystem I with special properties including the capacity of undergoing non-photochemical fluorescence quenching. We have studied photosystem I organization under different light and carbon supply conditions in wild-type and the lhcb9 KO genotype, which lacks an antenna protein endowed with red-shifted absorption forms. Wild-type moss, when grown on sugars and low light, accumulated the LHCB9 proteins and a large form of photosystem I supercomplex which, besides the canonical four LHCI subunits, also includes a LHCII trimer and 4 additional LHC monomers. The lhcb9 KO produced an angiosperm-like photosystem I supercomplex with 4 LHCI subunits irrespective from growth conditions. Growth in the presence of sub-lethal concentrations of electron transport inhibitors, causing oxidation or reduction of the plastoquinone pool, respectively prevented or promoted the accumulation of LHCB9 and the formation of the photosystem I megacomplex. We suggest that LHCB9 is a key subunit regulating photosystem I antenna size and ability to avoid over-reduction of plastoquinone: this is a condition potentially dangerous in the shaded and sunflecks-rich environment typical of mosses, whose plastoquinone pool is reduced by both the photosystem II and the oxidation of sugar substrates.

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