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

Abstract

The plastoquinone (PQ)-pool in chloroplast thylakoid membranes is a key electron carrier in the photosynthetic electron transport chain (PETC), and its redox state plays an essential role in the control of plant metabolism. Oxygen reduction in thylakoid membranes produces superoxide anion radicals ( ), which may react with the PQ-pool. Here, using isolated thylakoids, we show for the first time the oxidation of the PQ-pool by . The xanthine-xanthine oxidase system was used to supply externally to the thylakoid membrane and the redox state of the PQ-pool was monitored by tracking chlorophyll a fluorescence. We propose that, in vivo, the reaction of produced in Photosystem I with reduced PQ (plastohydroquinone) creates hydrogen peroxide, which serves as a messenger that signals the redox state of the PETC.

Photo‐reducible plastoquinone pools in chloroplasts of Tradescentia plants acclimated to high and low light

“…In control leaves, after sufficiently long dark‐adaptation (≥ 5 min) almost all PQ molecules (including PQ A ) become oxidized due to chlororespiration . The ETC interaction with molecular oxygen may occur via the chloroplast plastoquinol terminal oxidase (PTOX) and/or due to PQH 2 oxidation through superoxide radicals . O 2 ‐dependent modulation of F 0 has been observed in Chlamidomonas reinhardtii .…”

A commonly used photosynthetic inhibitor fails to block electron flow to photosystem I in intact systems

“…In this experiment, DNP-INT failed to block the transient re-reduction of P700 caused by actinic light pulses in both the leaf and thylakoid samples from 2 μM to 500 μM concentration. That is 1000x the published minimum concentration required and 100x the concentration generally reported when DNP-INT is used to truncate PSI from the PETC (Khorobrykh and Ivanov 2002; Borisova-Mubarakshina, Naydov and Ivanov 2018; Krieger-Liszkay, Kienzler and Johnson 2000). Another important question was to assure the purity of the DNP-INT we had procured.…”

“…This was interpreted as strong evidence that DNP-INT blocks electron flow via PSI to MV, and therefore the chemical effectively truncates PSI from the PETC. Furthermore, similarity in O 2 uptake rates observed between untreated and DNP-INT treated samples suggested the presence of superoxide forming pathways, similar to that in PSI, also associated with the reduced PQ pool and cyt - b6f complex (Khorobrykh and Ivanov 2002; Mubarakshina and Ivanov 2010; Borisova-Mubarakshina, Naydov and Ivanov 2018).…”

“…The results from O 2 electrode measurements with isolated thylakoids were interpreted as the ability of DNP-INT to allow semiquinone formation, whilst simultaneously blocking cyt- f reduction (Mubarakshina and Ivanov 2010), which made DNP-INT a critical tool in truncating the PETC at Cyt- b6f . It has resulted in the wide use of DNP-INT as an alternative to DBMIB for truncation of PSI from the PETC, particularly in works to explore reactive oxygen species (ROS) formation and scavenging pathways involving both reduced PQ-pool and semiquinones (Khorobrykh and Ivanov 2002; Mubarakshina and Ivanov 2010; Borisova-Mubarakshina, Naydov and Ivanov 2018; Heyno and others 2009). Based on these characterizations, use of DNP-INT has provided a range of significant findings in plant (Stepien and Johnson 2009) and algal physiology (Barbagallo, Finazzi and Forti 1999), bioenergetics (Malnoe and others 2011), bio-fuel applications (Mus and others 2005), biochemical characterizations of ETC components (Krieger-Liszkay, Kienzler and Johnson 2000) and efforts to determine sites and activity of ROS formation/quenching (Khorobrykh and Ivanov 2002; Mubarakshina and Ivanov 2010; Borisova-Mubarakshina, Naydov and Ivanov 2018; Heyno and others 2009; Vetoshkina and others 2017).…”

Antioxidant and signaling functions of the plastoquinone pool in higher plants

“…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 work was conducted using isolated thylakoids from pea plants in the presence of dinitrophenylether of 2‐iodo‐4‐nitrothymol (DNP‐INT), the inhibitor of PQH 2 oxidation by cytochrome b 6 f ‐complex.…”