Glycolate Induces Redox Tuning Of Photosystem II in Vivo: Study of a Photorespiration Mutant

Bicarbonate removal from the nonheme iron at the acceptor side of photosystem II (PSII) was shown recently to shift the midpoint potential of the primary quinone acceptor Q A to a more positive potential and lowers the yield of singlet oxygen ( 1 O 2 ) production. The presence of Q A - results in weaker binding of bicarbonate, suggesting a redox-based regulatory and protective mechanism where loss of bicarbonate or exchange of bicarbonate by other small carboxylic acids may protect PSII against 1 O 2 in vivo under photorespiratory conditions. Here, we compared the properties of Q A in the Arabidopsis ( Arabidopsis thaliana ) photorespiration mutant deficient in peroxisomal HYDROXYPYRUVATE REDUCTASE1 ( hpr1-1 ), which accumulates glycolate in leaves, with the wild type. Photosynthetic electron transport was affected in the mutant, and chlorophyll fluorescence showed slower electron transport between Q A and Q B in the mutant. Glycolate induced an increase in the temperature maximum of thermoluminescence emission, indicating a shift of the midpoint potential of Q A to a more positive value. The yield of 1 O 2 production was lowered in thylakoid membranes isolated from hpr1-1 compared with the wild type, consistent with a higher potential of Q A /Q A - In addition, electron donation to photosystem I was affected in hpr1-1 at higher light intensities, consistent with diminished electron transfer out of PSII. This study indicates that replacement of bicarbonate at the nonheme iron by a small carboxylate anion occurs in plants in vivo. These findings suggested that replacement of the bicarbonate on the nonheme iron by glycolate may represent a regulatory mechanism that protects PSII against photooxidative stress under low-CO 2 conditions.