The reaction center (RC) of photosystem II (PSII), which is composed of D1, D2, PsbI, and cytochrome b559 subunits, forms at an early stage of PSII biogenesis. However, it is largely unclear how these components assemble to form a functional unit. In this work, we show that synthesis of the PSII core proteins D1/D2 and formation of the PSII RC is blocked specifically in the absence of ONE-HELIX PROTEIN1 (OHP1) and OHP2 proteins in Arabidopsis (Arabidopsis thaliana), indicating that OHP1 and OHP2 are essential for the formation of the PSII RC. Mutagenesis of the chlorophyll-binding residues in OHP proteins impairs their function and/or stability, suggesting that they may function in the binding of chlorophyll in vivo. We further show that OHP1, OHP2, and HIGH CHLOROPHYLL FLUORESCENCE244 (HCF244), together with D1, D2, PsbI, and cytochrome b559, form a complex. We designated this complex the PSII RC-like complex to distinguish it from the RC subcomplex in the intact PSII complex. Our data imply that OHP1, OHP2, and HCF244 are present in this PSII RC-like complex for a limited time at an early stage of PSII de novo assembly and of PSII repair under highlight conditions. In a subsequent stage of PSII biogenesis, OHP1, OHP2, and HCF244 are released from the PSII RC-like complex and replaced by the other PSII subunits. Together with previous reports on the cyanobacterium Synechocystis, our results demonstrate that the process of PSII RC assembly is highly conserved among photosynthetic species. PSII is a multisubunit pigment-protein complex present in the thylakoid membranes of oxygenic photosynthetic organisms (Nelson and Yocum, 2006; Nelson and Junge, 2015). It captures light energy to extract electrons from water and drive the transfer of electrons to plastoquinone, resulting in the production of oxygen (Nelson and Junge, 2015). A series of structural analyses revealed that the PSII structure is highly conserved among cyanobacteria and chloroplasts, except for the light-harvesting system (Umena et al., 2011; Su et al., 2017). The photochemical reaction center (RC) is the smallest unit having photochemical activity in PSII and is composed of D1, D2, PsbI, as well as the cytochrome (Cyt) b559 aand b-subunits (PsbE and PsbF, respectively). In addition, the PSII RC contains several cofactors, including six chlorophylls, two pheophytins a, the plastoquinones Q A and Q B , one heme, and one b-carotene molecule (Nelson and Yocum, 2006), which are required for energy transfer, charge separation, and electron transfer. Surrounding the PSII RC are the CP47 and CP43 subunits, which bind chlorophyll a and b-carotene molecules. CP43 and D1 also provide ligands for the CaMn 4 cluster that is essential for water oxidation in the oxygen-evolving complex of PSII (Shen, 2015). In addition to these core subunits, PSII contains at least 11 subunits with low molecular mass (Shi et al., 2012). The phycobilisomes and chlorophyll a/b-binding complex (LHCII) associated with PSII core complexes in cyanobacteria and chloroplasts, respectively...

The color of bracts generally turns yellow or black from green during cereal grain development. However, the impact of these phenotypic changes on photosynthetic physiology during black bract formation remains unclear. Two oat cultivars (Avena sativa L.), ‘Triple Crown’ and ‘Qinghai 444’, with yellow and black bracts, respectively, were found to both have green bracts at the heading stage, but started to turn black at the flowering stage and become blackened at the milk stage for ‘Qinghai 444’. Their photosynthetic characteristics were analyzed and compared, and the key genes, proteins and regulatory pathways affecting photosynthetic physiology were determined in ‘Triple Crown’ and ‘Qinghai 444’ bracts. The results show that the actual PSII photochemical efficiency and PSII electron transfer rate of ‘Qinghai 444’ bracts had no significant changes at the heading and milk stages but decreased significantly (p < 0.05) at the flowering stage compared with ‘Triple Crown’. The chlorophyll content decreased, the LHCII involved in the assembly of supercomplexes in the thylakoid membrane was inhibited, and the expression of Lhcb1 and Lhcb5 was downregulated at the flowering stage. During this critical stage, the expression of Bh4 and C4H was upregulated, and the biosynthetic pathway of p-coumaric acid using tyrosine and phenylalanine as precursors was also enhanced. Moreover, the key upregulated genes (CHS, CHI and F3H) of anthocyanin biosynthesis might complement the impaired PSII activity until recovered at the milk stage. These findings provide a new insight into how photosynthesis alters during the process of oat bract color transition to black.