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Abstract
Photosynthetic species are subjected to a variety of environmental stresses, including suboptimal irradiance. In oxygenic photosynthetic organisms, a major effect of high light exposure is damage to the Photosystem II (PSII) reaction-center protein D1. This process even happens under low or moderate light. To cope with photodamage to D1, photosynthetic organisms evolved an intricate PSII repair and reassembly cycle, which requires the participation of different auxiliary proteins, including thiol/disulfide-modulating proteins. Most of these auxiliary proteins exist ubiquitously in oxygenic photosynthetic organisms. Due to differences in mobility and environmental conditions, land plants are subject to more extensive high light stress than algae and cyanobacteria. Therefore, land plants evolved additional thiol/disulfide-modulating proteins, such as Low Quantum Yield of PSII 1 (LQY1), to aid in the repair and reassembly cycle of PSII. In this study, we introduced an Arabidopsis thaliana homolog of LQY1 (AtLQY1) into the cyanobacterium Synechocystis sp. PCC6803 and performed a series of biochemical and physiological assays on AtLQY1-expressing Synechocystis. At a moderate growth light intensity (50 µmol photons m-2 s-1), AtLQY1-expressing Synechocystis was found to have significantly higher F v /F m, and lower nonphotochemical quenching and reactive oxygen species levels than the empty-vector control, which is opposite from the loss-of-function Atlqy1 mutant phenotype. Light response curve analysis of PSII operating efficiency and electron transport rate showed that AtLQY1-expressing Synechocystis also outperform the empty-vector control under higher light intensities. The increases in F v /F m, PSII operating efficiency, and PSII electron transport rate in AtLQY1-expressing Synechocystis under such growth conditions most likely come from an increased amount of PSII, because the level of D1 protein was found to be higher in AtLQY1-expressing Synechocystis. These results suggest that introducing AtLQY1 is beneficial to Synechocystis.
Highlights
Photosynthesis provides chemical energy for most life forms on earth
Measurements of different photosynthetic parameters consistently showed that Arabidopsis thaliana homolog of LQY1 (AtLQY1) expression improves Photosystem II (PSII) photochemical efficiency in Synechocystis cells grown at 50 μmol photons m-2 s-1
Further analysis of chlorophyll fluorescence parameters showed that AtLQY1-expressing Synechocystis grown at 50 μmol photons m-2 s-1 had higher Fv than the emptyvector control
Summary
Photosynthesis provides chemical energy for most life forms on earth. In oxygenic photosynthesis, which occurs in cyanobacteria, algae, and land plants, photosynthetic electron transport and ATP synthesis requires Photosystem II (PSII), cytochrome b6 f, Photosystem I (PSI), ATP synthase, as well as mobile electron carriers such as plastoquinone and plastocyanin (Allen et al, 2011; Nickelsen and Rengstl, 2013). The core subunits of PSII in cyanobacteria, algae, and land plants are similar except for the composition of light harvesting complexes (LHCs) and oxygenic evolving complexes (OECs) (Hankamer et al, 2001; Allen et al, 2011; Nickelsen and Rengstl, 2013). The LHCs of cyanobacterial and red algal PSII supercomplexes are termed phycobilisomes and consist of three types of phycobiliproteins: allophycocyanin (APC), phycocyanin (PC), and phycoerythrin (PE) (Stadnichuk et al, 2015). These phycobiliproteins contain one or multiple cysteine residues. PsbU and PsbV were lost during the evolution of green algae and land plants (Thornton et al, 2004)
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