Arabidopsis PsbP-Like Protein 1 Facilitates the Assembly of the Photosystem II Supercomplexes and Optimizes Plant Fitness under Fluctuating Light.

In green plants, photosystem II (PSII) forms multisubunit supercomplexes (SCs) containing a dimeric core and light-harvesting complexes (LHCs). In this study, we show that Arabidopsis thaliana PsbP-like protein 1 (PPL1) is involved in the assembly of the PSII SCs and is required for adaptation to changing light intensity. PPL1 is a homolog of PsbP protein that optimizes the water-oxidizing reaction of PSII in green plants and is required for the efficient repair of photodamaged PSII; however, its exact function has been unknown. PPL1 was enriched in stroma lamellae and grana margins and associated with PSII subcomplexes including PSII monomers and PSII dimers, and several LHCII assemblies, while PPL1 was not detected in PSII-LHCII SCs. In a PPL1 null mutant (ppl1-2), assembly of CP43, PsbR and PsbW was affected, resulting in a reduced accumulation of PSII SCs even under moderate light intensity. This caused the abnormal association of LHCII in ppl1-2, as indicated by lower maximal quantum efficiency of PSII (Fv/Fm) and accelerated State 1 to State 2 transitions. These differences would lower the capability of plants to adapt to changing light environments, thereby leading to reduced growth under natural fluctuating light environments. Phylogenetic and structural analyses suggest that PPL1 is closely related to its cyanobacterial homolog CyanoP, which functions as an assembly factor in the early stage of PSII biogenesis. Our results suggest that PPL1 has a similar function, but the data also indicate that it could aid the association of LHCII with PSII.