Formation mechanisms of superoxide radical and hydrogen peroxide in chloroplasts, and factors determining the signalling by hydrogen peroxide

Reduction of O₂ molecule to superoxide radical, O₂^•-, in the photosynthetic electron transport chain is the first step of hydrogen peroxide, H₂O₂, production in chloroplasts in the light. The mechanisms of O₂ reduction by ferredoxin, by the components of the plastoquinone pool, and by the electron transfer cofactors in PSI are analysed. The data indicating that O₂^•- and H₂O₂ can be produced both outside and within thylakoid membrane are presented. The H₂O₂ production in the chloroplast stroma is described as a result of either dismutation of O₂^•- or its reduction by stromal reductants. Formation of H₂O₂ within thylakoid membrane in the reaction of O₂^•- with plastohydroquinone is examined. The significance of both ways of H₂O₂ formation for specificity of the signal being sent by photosynthetic electron transport chain to cell adaptation systems is discussed.