The thylakoid membrane system of higher-plant chloroplasts consists of interconnected subdomains of appressed and non-appressed membrane bilayers, known as grana and stroma lamellae, respectively. The shaping of the cylindrical grana stacks is mediated, in a dosage-dependent manner, by CURT1 protein complexes, which facilitate membrane curvature at the grana margins, the interface between grana and stroma lamellae. Although grana stacks are highly conserved among land plants, the functional relevance of grana stacking remains unclear. Here we show that CURT1-related alteration of thylakoid ultrastructure reduces photosynthetic efficiency and plant fitness under adverse, controlled and natural light conditions. Plants that lack CURT1 are impaired in adjusting grana diameter, which compromises the effectiveness of regulatory mechanisms like PSII repair cycle and state transitions. Interestingly, CURT1A suffices to induce thylakoid membrane curvature in planta, and thylakoid hyper-bending that is observed in CURT1A overexpressor lines. We suggest that CURT1 oligomerisation is regulated at the post-translational level in a light-dependent fashion and CURT1-mediated thylakoid plasticity is critical for plant fitness and plays an important role in fine-tuning photosynthesis under challenging growth conditions.

Thylakoid phosphorylation is predominantly mediated by the protein kinases STN7 and STN8. While STN7 primarily catalyzes LHCII phosphorylation, which enables LHCII to migrate from photosystem (PS) II to PSI, STN8 mainly phosphorylates PSII core proteins. The reversible phosphorylation of PSII core proteins is thought to regulate the PSII repair cycle and PSII supercomplex stability, and play a role in modulating the folding of thylakoid membranes. Earlier studies clearly demonstrated a considerable substrate overlap between the two STN kinases, raising the possibility of a balanced interdependence between them at either the protein or activity level. Here, we show that such an interdependence of the STN kinases on protein level does not seem to exist as neither knock-out nor overexpression of STN7 or STN8 affects accumulation of the other. STN7 and STN8 are both shown to be integral thylakoid proteins that form part of molecular supercomplexes, but exhibit different spatial distributions and are subject to different modes of regulation. Evidence is presented for the existence of a second redox-sensitive motif in STN7, which seems to be targeted by thioredoxin f. Effects of altered STN8 levels on PSII core phosphorylation, supercomplex formation, photosynthetic performance and thylakoid ultrastructure were analyzed in Arabidopsis thaliana using STN8-overexpressing plants (oeSTN8). In general, oeSTN8 plants were less sensitive to intense light and exhibited changes in thylakoid ultrastructure, with grana stacks containing more layers and reduced amounts of PSII supercomplexes. Hence, we conclude that STN8 acts in an amount-dependent manner similar to what was shown for STN7 in previous studies. However, the modes of regulation of the STN kinases appear to differ significantly.