Photosystem I photoinhibition attenuates LHCI-dependent light-harvesting activity in spinach leaves

The over-reduced state of photosystem I causes photoinhibition, and changes in electron transport activity and core-subunit content have been studied to examine the degree of photosystem I photoinhibition. However, the involvement of peripheral subunits, such as light-harvesting complex I, in photosystem I photoinhibition has not been thoroughly evaluated. Here, we aimed to determine whether the light-harvesting functions of the light-harvesting complex I are altered by photosystem I photoinhibition in spinach leaves. To this end, we developed a method to estimate the functional antenna size of light-harvesting complex I by measuring the far-red light-dependent absorption change in the oxidized P700 reaction center chlorophyll in leaves in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and methyl viologen. We analyzed kinetics using the double Gompertz model. Subsequently, we found that the rate constant and relative rate for P700+ induction (kmajor and Vmajor) increased linearly with an increase in the illuminated far-red light intensity. The Arabidopsis lhca4 mutant demonstrated that kmajor and Vmajor are suppressed compared to wild-type plants, suggesting that these parameters reflect light-harvesting activity depending on light-harvesting complex I. Following photoinhibitory treatment of photosystem I, spinach leaves showed reduced kmajor and Vmajor. The 77 K fluorescence emission spectra analysis showed blue-shifted fluorescence at approximately 740 nm after photosystem I photoinhibition, suggesting that the light energy transfer process from light-harvesting complex I to the PSI-core is disturbed because of photosystem I photoinhibition. From these observations, we propose that the light-harvesting complex I, as well as its core subunits, are targets of photosystem I photoinhibition.