Involvement of the chloroplast plastoquinone pool in the Mehler reaction

Abstract

Light-dependent oxygen reduction in the photosynthetic electron transfer chain, i.e. the Mehler reaction, has been studied using isolated pea thylakoids. The role of the plastoquinone pool in the Mehler reaction was investigated in the presence of dinitrophenyl ether of 2-iodo-4-nitrothymol (DNP-INT), the inhibitor of plastohydroquinone oxidation by cytochrome b6/f complex. Oxygen reduction rate in the presence of DNP-INT was higher than in the absence of the inhibitor in low light at pH 6.5 and 7.6, showing that the capacity of the plastoquinone pool to reduce molecular oxygen in this case exceeded that of the entire electron transfer chain. In the presence of DNP-INT, appearance of superoxide anion radicals outside thylakoid membrane represented approximately 60% of the total superoxide anion radicals produced. The remaining 40% of the produced superoxide anion radicals was suggested to be trapped by plastohydroquinone molecules within thylakoid membrane, leading to the formation of hydrogen peroxide (H O ). To validate the reaction of superoxide anion radical with plastohydroquinone, xanthine/xanthine oxidase system was integrated with thylakoid membrane in order to generate superoxide anion radical in close vicinity of plastohydroquinone. Addition of xanthine/xanthine oxidase to the thylakoid suspension resulted in a decrease in the reduction level of the plastoquinone pool in the light. The obtained data provide additional clarification of the aspects that the plastoquinone pool is involved in both reduction of oxygen to superoxide anion radicals and reduction of superoxide anion radicals to H O . Significance of the plastoquinone pool involvement in the Mehler reaction for the acclimation of plants to light conditions is discussed.

Oxidation of the plastoquinone pool in chloroplast thylakoid membranes by superoxide anion radicals

“…To study the effect of on the redox state of PQ‐pool it is necessary to provide the supply of to the membrane, using the X‐XO system in close proximity to the membrane surface. These conditions were explored in our previous study , where it was in particular found, that the incubation of thylakoids for 24/48 h at 5 °C after isolation facilitated such contact, and ESR experiments with the SASL‐16 spin probe revealed that such incubation resulted in changes of thylakoid membrane surface. The thylakoids preserved approximately 80% of photosynthetic activity after 48 h followed the isolation.…”

Antioxidant and signaling functions of the plastoquinone pool in higher plants

“…in the stroma, saturated already in low light (Mubarakshina et al , Borisova‐Mubarakshina et al ). Based on these data and on the data presented in (Mubarakshina and Ivanov , Borisova‐Mubarakshina et al , Vetoshkina et al ) it was proposed that Reaction 1 (interaction of O 2 •− with PQH 2 ) was the leading reaction producing ‘the membrane H 2 O 2 ’ molecules for the long‐term acclimation of plants to high light conditions (Ivanov et al ). For this reaction, O 2 •− presumably is generated within the thylakoid membrane by phyllosemiquinone (PhQ •− ) of PSI (Kozuleva et al ).…”

“…Early experimental studies provided the indirect data of the antioxidant activity of the reduced PQ in term of scavenging of O 2 •– (Kruk et al , Ivanov et al , Vetoshkina et al ). Recently, the reaction of O 2 •− with PQH 2 molecules (Reaction 1) in the PQ pool of thylakoid membrane was demonstrated (Borisova‐Mubarakshina et al ).…”

“…This review describes an important role of plastoquinone‐9 (PQ) in protection of the photosynthetic apparatus of plant cells under various environmental conditions. In the light PQ is involved in electron transfer in photosynthetic electron transport chain being an electron carrier between photosystem II (PSII) and cytochrome b 6 f complex, in the Mehler reaction (Mubarakshina and Ivanov , Vetoshkina et al ) that is the reduction of molecular oxygen to hydrogen peroxide via superoxide anion radical as well as in cyclic electron flows around photosystem I (PSI) (Munekage et al ).…”

From light capture to metabolic needs, oxygenic photosynthesis is an ever‐expanding field of study in plants, algae and cyanobacteria

“…The molecular mechanism of 1 O 2 generation in PSI is unknown but it has been demonstrated that 1 O 2 in PSII is generated by charge recombination reactions of the primary radical pair, yielding the triplet state of chlorophyll. In addition to electron transport reactions at and within the photosystems, the plastoquinone pool is a source of O 2 •− /H 2 O 2 (Vetoshkina et al ). While light‐dependent ROS production damages PSII and PSI either directly or via the release of reactive electrophile species (Roach et al ), ultra‐violet (UV) and eventually blue light may directly damage the Mn cluster of PSII and cause photoinhibition.…”