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

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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>O</mml:mi> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> <mml:mrow><mml:mo>·</mml:mo> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:math> ), which may react with the PQ-pool. Here, using isolated thylakoids, we show for the first time the oxidation of the PQ-pool by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>O</mml:mi> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> <mml:mrow><mml:mo>·</mml:mo> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:math> . The xanthine-xanthine oxidase system was used to supply <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>O</mml:mi> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> <mml:mrow><mml:mo>·</mml:mo> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:math> 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  <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi>O</mml:mi> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> <mml:mrow><mml:mo>·</mml:mo> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:math> produced in Photosystem I with reduced PQ (plastohydroquinone) creates hydrogen peroxide, which serves as a messenger that signals the redox state of the PETC.