Chemical Quenching of Singlet Oxygen by Plastoquinols and Their Oxidation Products in Arabidopsis

Prenylquinols (tocochromanols and plastoquinols) serve as efficient physical and chemical quenchers of singlet oxygen ( O ) formed during high light stress in higher plants. Although quenching of O by prenylquinols has been previously studied, direct evidence for chemical quenching of O by plastoquinols and their oxidation products is limited in vivo. In the present study, the role of plastoquinol-9 (PQH -9) in chemical quenching of O was studied in Arabidopsis thaliana lines overexpressing the SOLANESYL DIPHOSPHATE SYNTHASE 1 gene (SPS1oex) involved in PQH -9 and plastochromanol-8 biosynthesis. In this work, direct evidence for chemical quenching of O by plastoquinols and their oxidation products is presented, which is obtained by microscopic techniques in vivo. Chemical quenching of O was associated with consumption of PQH -9 and formation of its various oxidized forms. Oxidation of PQH -9 by O leads to plastoquinone-9 (PQ-9), which is subsequently oxidized to hydroxyplastoquinone-9 [PQ(OH)-9]. We provide here evidence that oxidation of PQ(OH)-9 by O results in the formation of trihydroxyplastoquinone-9 [PQ(OH) -9]. It is concluded here that PQH -9 serves as an efficient O chemical quencher in Arabidopsis, and PQ(OH) -9 can be considered as a natural product of O reaction with PQ(OH)-9. The understanding of the mechanisms underlying O chemical quenching provides information on the role of plastoquinols and their oxidation products in the response of plants to photooxidative stress.