Fast chlorophyll a fluorescence induction (OJIP) phenotyping of Arabidopsis single and higher order mutants depleted of proteins involved in donor and acceptor side function of photosystem II

Arabidopsis single and multiple knockout mutants generated for the Oxygen Evolving Complex (OEC) proteins of Photosystem II (PSII) contributed in the past to understand specific roles of these subunits in the functioning of PSII. Here, by applying a comparative phenotyping approach based on fast chlorophyll a fluorescence induction and JIP-test to the single psbr (ΔR) and psbo1 (ΔO), and multiple psbq1-psbq2 (ΔQ), psbq1-psbq2-psbr (ΔQR) and psbo1-psbp2-psbq1-psbq2-psbr (ΔOQR) Arabidopsis knockout mutants we dissected the defects occurring at different steps of the photosynthetic flux induced by these mutations. The psbq1 psbq2 mutations induced a clear impairment of the PSII donor side, as evidenced by the lowered F_WOC index in ΔQ. Conversely, the psbr mutation led to a predominant impairment of the PSII acceptor side, as all JIP-test parameters associated with the electron transfer further than Q_A (ET_O/RC, ET_O/TR_O, ET_O/ABS) were negatively affected in ΔR, possibly due to a defect in the exchange between reduced and oxidized quinones at the Q_B site. These mutations in ΔQR had an additive effect on the perturbation of the function of PSII. The psbo1 mutation induced dramatic effects on the PSII donor side, as evidenced by the strong decrease of the F_WOC index and the presence of a pronounced positive K-band in the OJIP transient in ΔO, leading to a marked reduction of the PSII photochemistry. The higher order mutant ΔOQR shared many characteristics with ΔO, indicating that the effect of the psbo1 mutation is preponderant even in the presence of additional psbq1 psbq2 and psbr mutations.