Arabidopsis plants lacking PsbQ and PsbR subunits of the oxygen-evolving complex show altered PSII super-complex organization and short-term adaptive mechanisms

The oxygen-evolving complex of eukaryotic photosystem II (PSII) consists of four extrinsic subunits, PsbO (33 kDa), PsbP (23 kDa), PsbQ (17 kDa) and PsbR (10 kDa), encoded by seven nuclear genes, PsbO1 (At5g66570), PsbO2 (At3g50820), PsbP1 (At1g06680), PsbP2 (At2g30790), PsbQ1 (At4g21280), PsbQ2 (At4g05180) and PsbR (At1g79040). Using Arabidopsis insertion mutant lines, we show that PsbP1, but not PsbP2, is essential for photoautotrophic growth, whereas plants lacking both forms of PsbQ and/or PsbR show normal growth rates. Complete elimination of PsbQ has a minor effect on PSII function, but plants lacking PsbR or both PsbR and PsbQ are characterized by more pronounced defects in PSII activity. Gene expression and immunoblot analyses indicate that accumulation of each of these proteins is highly dependent on the presence of the others, and is controlled at the post-transcriptional level, whereas PsbO stability appears to be less sensitive to depletion of other subunits of the oxygen-evolving complex. In addition, comparison of levels of the PSII super-complex in wild-type and mutant leaves reveals the importance of the individual subunits of the oxygen-evolving complex for the supramolecular organization of PSII and their influence on the rate of state transitions.