Cyclophilin 37 maintains electron transport via the cytochrome b6/f complex under high light in Arabidopsis

Plants have evolved multiple mechanisms to cope with diverse types of light stress, particularly the regulation of the electron transport chain (ETC). Under high light (HL) conditions, the balance of electron flux in the ETC is disturbed, which leads to the overaccumulation of reactive oxygen species (ROS) and results in photodamage and photoinhibition. The cytochrome (Cyt) b6/f complex, which coordinates electron transfer between photosystems I and II (PSI and PSII), plays an essential role in regulating the ETC and initiating photoprotection. However, how the Cyt b6/f complex is maintained under HL conditions remains unclear. Here, we report that the activity of the Cyt b6/f complex is sustained by thylakoid-localized cyclophilin 37 (CYP37) in Arabidopsis (Arabidopsis thaliana). Compared with wild-type plants, cyp37 mutants displayed an imbalance in electron transport from Cyt b6/f to PSI under HL stress, which led to increased ROS accumulation, decreased anthocyanin biosynthesis, and increased chlorophyll degradation. Surprisingly, CYP37's role in regulating ETC balance was independent of photosynthesis control, which was indicated by a higher Y (ND), an indicator of P700 oxidation in PSI. Furthermore, the interaction between CYP37 and photosynthetic electron transfer A (PetA), a subunit of the Cyt b6/f complex, suggests that the central function of CYP37 is to maintain Cyt b6/f complex activity rather than to serve as an assembly factor. Our study provides insights into how plants balance electron flow between PSII and PSI via Cyt b6/f complex under HL. Cyclophilin 37, a maintenance factor of cytochrome b6/f, adapts to high light exposure via photosynthetic electron transfer balancing.

Automated summary

Plants have developed various mechanisms to deal with different types of light stress, with the regulation of the electron transport chain being particularly important. The cytochrome b6/f complex plays a crucial role in regulating the electron transport chain under high light conditions. The study reveals that thylakoid-localized cyclophilin 37 (CYP37) maintains the activity of the cytochrome b6/f complex in Arabidopsis. The interaction between CYP37 and photosynthetic electron transfer A (PetA) indicates that CYP37 functions as a maintenance factor of the cytochrome b6/f complex rather than an assembly factor.