Impact of hydrogen peroxide on the redistribution of antenna complexes between photosystems in higher plants

One of the acclimatory mechanisms of photosynthetic organisms to changing light conditions is the redistribution of antenna complexes between photosystems, the process known as state transitions. This process allows the amount of light energy absorbed by each photosystem to be regulated. Numerous studies have demonstrated that state transitions are inhibited under high light intensity; however, the exact mechanism of this inhibition remains unclear. In the present study, the effect of H₂O₂ at various concentrations on the state transition process was investigated using functionally active thylakoids isolated from Arabidopsis leaves. Additionally, the specific stage of this process affected by H₂O₂ was evaluated. To assess state transitions, low-temperature chlorophyll a fluorescence spectra (F, from 650 to 780 nm) were measured, and the F745/F685 ratio was calculated as an indicator of state transition activity. It was shown that the addition of H₂O₂ led to the inhibition of state transitions in low light. The addition of H₂O₂ to thylakoids under low light conditions resulted in a decreased accumulation of phosphorylated Lhcb1 and Lhcb2 proteins, which are involved in state transitions. This indicates that the inhibition of state transitions is likely a consequence of inhibited activity of the STN7 kinase. It is important to note that H₂O₂ at the concentrations used did not affect the rate of electron transport, indicating that the inhibition of STN7 kinase activity is not associated with a suppression of the photosynthetic electron transport chain functioning. Moreover, the study demonstrates the selective effect of H₂O₂ on the activity of the STN7 kinase: no decrease in the level of the phosphorylated photosystem II D1 protein, the substrate of the STN8 kinase, was observed upon H₂O₂ treatment. Thus, this work provides the first evidence of the H₂O₂-dependent inhibitory mechanism of STN7 kinase activity and, consequently, of the state transition process.