Water oxidation in photosystem II (PSII) is still insufficiently understood and is assumed to involve HCO3−. A Chlamydomonas mutant lacking a carbonic anhydrase associated with the PSII donor side shows impaired O2 evolution in the absence of HCO3−. The O2 evolution for saturating, continuous illumination ( R O2) was slower than in the wild type, but was elevated by HCO3− and increased further by Cah3. The R O2 limitation in the absence of Cah3/HCO3− was amplified by H2O/D2O exchange, but relieved by an amphiphilic proton carrier, suggesting a role of Cah3/HCO3− in proton translocation. Chlorophyll fluorescence indicates a Cah3/HCO3− effect at the donor side of PSII. Time‐resolved delayed fluorescence and O2‐release measurements suggest specific effects on proton‐release steps but not on electron transfer. We propose that Cah3 promotes proton removal from the Mn complex by locally providing HCO3−, which may function as proton carrier. Without Cah3, proton removal could become rate limiting during O2 formation and thus, limit water oxidation under high light. Our results underlie the general importance of proton release at the donor side of PSII during water oxidation.

Jasmonates are plant hormones that are involved in the regulation of different aspects of plant life, wherein their functions and molecular mechanisms of action in wheat are still poorly studied. With the aim of gaining more insights into the role of jasmonic acid (JA) in wheat growth, development, and responses to environmental stresses, we have generated transgenic bread wheat plants overexpressing Arabidopsis 12-OXOPHYTODIENOATE REDUCTASE 3 (AtOPR3), one of the key genes of the JA biosynthesis pathway. Analysis of transgenic plants showed that AtOPR3 overexpression affects wheat development, including germination, growth, flowering time, senescence, and alters tolerance to environmental stresses. Transgenic wheat plants with high AtOPR3 expression levels have increased basal levels of JA, and up-regulated expression of ALLENE OXIDE SYNTHASE, a jasmonate biosynthesis pathway gene that is known to be regulated by a positive feedback loop that maintains and boosts JA levels. Transgenic wheat plants with high AtOPR3 expression levels are characterized by delayed germination, slower growth, late flowering and senescence, and improved tolerance to short-term freezing. The work demonstrates that genetic modification of the jasmonate pathway is a suitable tool for the modulation of developmental traits and stress responses in wheat.

It is found that dark reduction of photooxidized primary electron donor P ₈₇₀ ⁺ in reaction centres from purple anoxygenic bacteria (two non-sulphur Fe-oxidizing Rhodovulum iodosum and Rhodovulum robiginosum , Rhodobacter sphaeroides R-26 and sulphur alkaliphilic Thiorhodospira sibirica ) is accelerated upon the addition of Mn ²⁺ jointly with bicarbonate (30–75 mM). The effect is not observed if Mn ²⁺ and HCO ₃ ⁻ have been replaced by Mg ²⁺ and HCO ₂ ⁻ , respectively. The dependence of the effect on bicarbonate concentration suggests that formation of Mn ²⁺ –bicarbonate complexes, Mn(HCO ₃ ) ⁺ and/or Mn(HCO ₃ ) ₂ , is required for re-reduction of P ₈₇₀ ⁺ with Mn ²⁺ . The results are considered as experimental evidence for a hypothesis on possible participation of Mn–bicarbonate complexes in the evolutionary origin of oxygenic photosynthesis in the Archean era.

Photosystem II (PSII)-enriched membranes retain the original PSII architecture in contrast to PSII cores or PSII supercomplexes, which are usually isolated from Chlamydomonas reinhardtii. Here, we present data that fully characterize the structural and functional properties of PSII complexes in isolated PSII-enriched membranes from C. reinhardtii. The preparations were isolated from wild-type (WT) and CAH3-deficient mutant cia3 as the influence of CAH3 on the PSII function was previously proposed. Based on the equal chlorophyll content, the PSII-enriched membranes from WT and cia3 have the same amount of reaction centers (RCs), cytochrome b559, subunits of the water-oxidizing complex, Mn ions, and carotenes. They differ in the ratio of other carotenoids, the parts of low/intermediate redox forms of cytochrome b559, and the composition of outer light-harvesting complexes. The preparations had 40% more chlorophyll molecules per RC compared to higher plants. Functionally, PSII-enriched membranes from WT and cia3 show the same photosynthetic activity at optimal pH 6.5. However, the preparations from cia3 contained more closed RCs even at pH 6.5 and showed more pronounced suppression of PSII photosynthetic activity at shift pH up to 7.0, established in the lumen of dark-adapted cells. Nevertheless, the PSII photosynthetic capacities remained the same.