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 (1O2) formed during high light stress in higher plants. Although quenching of 1O2 by prenylquinols has been previously studied, direct evidence for chemical quenching of 1O2 by plastoquinols and their oxidation products is limited in vivo. In the present study, the role of plastoquinol‐9 (PQH2‐9) in chemical quenching of 1O2 was studied in Arabidopsis thaliana lines overexpressing the SOLANESYL DIPHOSPHATE SYNTHASE 1 gene (SPS1oex) involved in PQH2‐9 and plastochromanol‐8 biosynthesis. In this work, direct evidence for chemical quenching of 1O2 by plastoquinols and their oxidation products is presented, which is obtained by microscopic techniques in vivo. Chemical quenching of 1O2 was associated with consumption of PQH2‐9 and formation of its various oxidized forms. Oxidation of PQH2‐9 by 1O2 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 1O2 results in the formation of trihydroxyplastoquinone‐9 [PQ(OH)3‐9]. It is concluded here that PQH2‐9 serves as an efficient 1O2 chemical quencher in Arabidopsis, and PQ(OH)3‐9 can be considered as a natural product of 1O2 reaction with PQ(OH)‐9. The understanding of the mechanisms underlying 1O2 chemical quenching provides information on the role of plastoquinols and their oxidation products in the response of plants to photooxidative stress.
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