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.