Naranjo, B. | bioRxiv Science

bioRxiv2023

A pgr5 suppressor screen uncovers a distinct mechanism safeguarding the cytochrome b6f complex from damage through PGR5

PROTON GRADIENT REGULATION5 (PGR5) is thought to promote cyclic electron flow (CEF) and its deficiency causes increased photosensitivity of photosystem I (PSI), leading to lethality under fluctuating light (FL). By screening for suppressor mutations that rescue FL lethality of pgr5 plants, we identified a portfolio of mutations affecting 12 photosynthesis-related proteins. Six are required for proper PSII function, one (CcdA) promotes cytochrome (cyt) b6f assembly, and another (PAA1) provides plastocyanin with its copper cofactor. Two other mutations are associated with the chloroplast FBPase cFBP1. This, together with targeted knockout of other genes in the pgr5 background, suggests three pathways to restore FL viability: (i) reduced electron flow to PSI due to defects in PSII, cyt b6f or plastocyanin but not PSI, (ii) increased electron flow from PSI due to inactivation of ACHT2, a regulator of cFBP1 activity, and (iii) hyperactivity of the NDH-dependent CEF due to inactivation of cFBP1. The remaining two suppressor mutations affected the cyt b6f complex. PFSC1 controls cyt b6f accumulation at early developmental stages. DEIP1/NTA1, previously suggested to be essential for cyt b6f assembly, appears to protect cyt b6f from deleterious effects of PGR5, since plants lacking both DEIP1/NTA1 and PGR5 are viable and accumulate cyt b6f.

plant biology↗

bioRxiv2023

A complex and dynamic redox network regulating oxygen reduction at photosystem I

Thiol-dependent redox regulations of enzyme activities play a central role in regulating photosynthesis. Beside the regulation of metabolic pathways, alternative electron transport has been shown to be subjected to thiol-dependent regulation. We investigated the regulation of O2 reduction at photosystem I. The level of O2 reduction in leaves and isolated thylakoid membranes depends on the photoperiod in which plants are grown. We used a set of Arabidopsis mutant plants affected in the stromal, membrane and lumenal thiol network to study the redox protein partners involved in regulating O2 reduction. Light-dependent O2 reduction was determined in leaves and in thylakoids of plants grown in short day and long day conditions using a spin-trapping EPR assay. In wild type samples from short day, ROS generation was twice the amount of that in samples from long day, while this difference was abolished in several redoxin mutants. An in vitro reconstitution assays showed that thioredoxin m, NADPH-dependent reductase C (NTRC) and NADPH are required for high O2 reduction levels in long day thylakoids. Using isolated photosystem I, we also show that reduction of a PSI protein is responsible for the increase in O2 reduction. Furthermore, differences in the membrane localization of thioredoxins m and 2-Cys peroxiredoxin were demonstrated between thylakoids of short day and long day plants. Finally, we propose a model of redox regulation of O2 reduction according to the reduction power of the stroma and the capabilities of the different thiol-containing proteins to form a network of redox interactions.

biochemistry↗