Conference Agenda

Abstract not available

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 and found that the level of O₂ reduction in leaves and isolated thylakoid membranes depends on the photoperiod at which plants are grown. Here, we used a set of Arabidopsis mutant plants affected in the stromal, membrane and lumenal thiol network to study the involvement of redox proteins (redoxins) 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 reconstituted assay allowed showing 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 is responsible for the increase in O₂ reduction. Furthermore, a differencial membrane localization of thioredoxins m and 2-Cys peroxiredoxin was found between short day and long day. 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.

Canonical plastidial thioredoxins (TRXs), consisting of a single TRX-fold domain, play crucial roles in the regulation of photosynthetic metabolism in relation with light conditions. The atypical TRX CDSP32 (Chloroplastic Drought-induced Stress Protein of 32 kDa), includes two TRX-fold domains with one atypical active site. We have previously shown that this TRX participates in stress responses via the supply of electrons to thiol-based antioxidant enzymes. Nonetheless, up to now it was unclear how the atypical structure and active site of CDSP32 could determine its function, and whether this TRX could play additional roles in the regulation of photosynthesis.

Here, we further characterized potato lines modified for CDSP32 expression to clarify its physiological roles. We show that CDSP32 is involved in the response to saline-alkaline conditions, a type of stress that been investigated only recently in plants. Most importantly, we unveil its participation in the regulation of photosynthesis, and in particular of the chloroplast ATP-synthase activity. The D1 and PsbO subunits of photosystem II and the γ subunit of the ATP-synthase are less abundant in plants overexpressing CDSP32, while plants lacking CDSP32 expression show an impairment in the early steps of photosynthesis activation during dark/light transitions. We show that this is due to a modified redox regulation of the plastidial ATP-synthase activity, thus revealing the involvement of CDSP32 in the control of the photosynthetic machinery and in the regulation of the trans-thylakoid membrane potential. This is in agreement with the previously reported interaction in planta between CDSP32 and the ATP-synthase γ subunit. Our structure modelling analyses confirm the likely interaction between the two partners and suggest that the atypical structure of CDSP32 could determine its specificity towards the γ subunit. We also show that CDSP32 homologues and redox-regulated γ subunits are only found in the eukaryotic green lineage and not in other photosynthetic organisms, supporting a possible coevolution of the two partners. These results could stimulate further investigations on the requirements for fine redox tuning of the ATP-synthase activity in different classes of photosynthetic organisms.

Photosystem II (PSII) assembly is a stepwise process involving the transient formation of intermediate PSII complexes with different protein compositions. The assembly process is facilitated by assembly factors, such as the lipoprotein Psb27, which form intermediate complexes with a specific subset of PSII subunits. Using cryo-electron microscopy, we have solved the structure of a partially functional PSII assembly intermediate from the thermophilic cyanobacterium Thermosynechococcus vestitus BP-1 (formerly known as T. elongatus BP-1) at 2.94 Å resolution [1]. It contains three assembly factors (Psb27, Psb28, Psb34) and provides detailed insights into their molecular function. The structure also shows how the PSII active site is prepared for the incorporation of the Mn4CaO5 cluster, which performs the unique water splitting reaction. Recently, we have succeeded in the isolation and subsequent cryo-EM analysis of additional PSII assembly intermediates.

1. Zabret J, Bohn S, Schuller SK, Arnolds O, Möller M, Meier-Credo J, Liauw P, Chan A, Tajkhorshid E, Langer JD, Stoll R, Krieger-Liszkay A, Engel BD, Rudack T, Schuller JM, Nowaczyk MM (2021) Nature Plants 7, 524-538