Light harvesting complex I is essential for Photosystem II photoprotection under variable light conditions in Arabidopsis thaliana

Higher plant Photosystem I (PSI) includes a peripheral antenna system (LHCI) composed of four light-harvesting proteins (Lhca1-Lhca4). The LHCI system is highly conserved, suggesting that it plays a specific role within the LHC family. In order to elucidate the specific function of LHCI, the phenotype of an Arabidopsis mutant devoid of the whole LHCI system was studied over a range of conditions, including rapid changes in irradiation. dLhca plants displayed stunted growth and reduced thylakoid lumen acidification respect to wild type, suggesting that the lack of LHCI affected electron transport rate. Under rapidly changing light intensity, growth rate of the mutant was further reduced and the redox balance of the photosynthetic electron chain impaired. Instead, under constant, excess light regime, the Delta Lhca plants did not suffer enhanced photooxidation vs. wild type, implying LHCI optimizes the flow rate through the electron transport chain by maintaining high PSI activity at all irradiances. We conclude that a complete PSI supercomplex, including LHCI, is crucial for the dynamic regulation of PQ redox state and therefore for PSII photoprotection.