A kaleidoscope of photosynthetic antenna proteins and their emerging roles

Relationship between non-photochemical quenching efficiency and the energy transfer rate from phycobilisomes to photosystem II

The chromophorylated PBLcm domain of the ApcE linker protein in the cyanobacterial phycobilisome (PBS) serves as a bottleneck for Förster resonance energy transfer (FRET) from the PBS to the antennal chlorophyll of photosystem II (PS II) and as a redirection point for energy distribution to the orange protein ketocarotenoid (OCP), which is excitonically coupled to the PBLcm chromophore in the process of non-photochemical quenching (NPQ) under high light conditions. The involvement of PBLcm in the quenching process was first directly demonstrated by measuring steady-state fluorescence spectra of cyanobacterial cells at different stages of NPQ development. The time required to transfer energy from the PBLcm to the OCP is several times shorter than the time it takes to transfer energy from the PBLcm to the PS II, ensuring quenching efficiency. The data obtained provide an explanation for the different rates of PBS quenching in vivo and in vitro according to the half ratio of OCP/PBS in the cyanobacterial cell, which is tens of times lower than that realized for an effective NPQ process in solution.

Integrating ecology, physiology and transcriptomics reveals the response of Zostera marina to rusting of iron transplantation frame

Frame Transplantation System (FTS) is considered an efficient method for seagrass restoration, but the effect of the rusting of iron frame on seagrass restoration remains unclear. We transplanted Zostera marina plants using iron FTS treated with fluorocarbon paint (painted treatment, PT) and traditional unpainted iron FTS (unpainted treatment, UT) under controlled mesocosm conditions for 24 days. Our results showed that the survival rate of Z. marina under the UT was significantly 31.2 % lower than that of the plants under the PT. Soluble sugar content in Z. marina rhizomes under the UT was significantly 2.19 times higher than that of the plants under the PT. Transcriptome analysis revealed differentially expressed genes (DEGs) involved in photosynthesis, metabolism and signal transduction functions. The results provide valuable data that could prove helpful in the development of efficient restoration techniques for Z. marina beds.

The structural basis for light acclimation in phycobilisome light harvesting systems systems in Porphyridium purpureum

Photosynthetic organisms adapt to changing light conditions by manipulating their light harvesting complexes. Biophysical, biochemical, physiological and genetic aspects of these processes are studied extensively. The structural basis for these studies is lacking. In this study we address this gap in knowledge by focusing on phycobilisomes (PBS), which are large structures found in cyanobacteria and red algae. In this study we focus on the phycobilisomes (PBS), which are large structures found in cyanobacteria and red algae. Specifically, we examine red algae (Porphyridium purpureum) grown under a low light intensity (LL) and a medium light intensity (ML). Using cryo-electron microscopy, we resolve the structure of ML-PBS and compare it to the LL-PBS structure. The ML-PBS is 13.6 MDa, while the LL-PBS is larger (14.7 MDa). The LL-PBS structure have a higher number of closely coupled chromophore pairs, potentially the source of the red shifted fluorescence emission from LL-PBS. Interestingly, these differences do not significantly affect fluorescence kinetics parameters. This indicates that PBS systems can maintain similar fluorescence quantum yields despite an increase in LL-PBS chromophore numbers. These findings provide a structural basis to the processes by which photosynthetic organisms adapt to changing light conditions.

Far-red photosynthesis: Two charge separation pathways exist in plant Photosystem II reaction center

An alternative charge separation pathway in Photosystem II under the far-red light was proposed by us on the basis of electron transfer properties at 295 K and 5 K. Here we extend these studies to the temperature range of 77-295 K with help of electron paramagnetic resonance spectroscopy. Induction of the S2 state multiline signal, oxidation of Cytochrome b559 and ChlorophyllZ was studied in Photosystem II membrane preparations from spinach after application of a laser flashes in visible (532 nm) or far-red (730-750 nm) spectral regions. Temperature dependence of the S2 state signal induction after single flash at 730-750 nm (Tinhibition ~ 240 K) was found to be different than that at 532 nm (Tinhibition ~ 157 K). No contaminant oxidation of the secondary electron donors cytochrome b559 or chlorophyllZ was observed. Photoaccumulation experiments with extensive flashing at 77 K showed similar results, with no or very little induction of the secondary electron donors. Thus, the partition ratio defined as (yield of YZ/CaMn4O5-cluster oxidation):(yield of Cytb559/ChlZ/CarD2 oxidation) was found to be 0.4 at under visible light and 1.7 at under far-red light at 77 K. Our data indicate that different products of charge separation after far-red light exists in the wide temperature range which further support the model of the different primary photochemistry in Photosystem II with localization of hole on the ChlD1 molecule.

The cards you have been dealt: How an intertidal green macroalga absorbs blue-green light

Light-harvesting complex II (LHCII) is vital for many photosynthetic organisms to harvest light and safely dissipate excess energy. The LHCII found in Bryopsis corticulans is uniquely adapted to absorb blue-green light. In this issue of Structure, Li et al. determine the structural bases of light absorbance by B. corticulans LHCII, providing insight into the diversity of light-harvesting strategies in nature.

Resilient and sustainable production of peanut (Arachis hypogaea) in phosphorus-limited environment by using exogenous gamma-aminobutyric acid to sustain photosynthesis

Globally, many low to medium yielding peanut fields have the potential for further yield improvement. Low phosphorus (P) limitation is one of the significant factors curtailing Arachis hypogaea productivity in many regions. In order to demonstrate the effects of gamma-aminobutyric acid (GABA) on peanuts growing under P deficiency, we used a pot-based experiment to examine the effects of exogenous GABA on alleviating P deficiency-induced physiological changes and growth inhibition in peanuts. The key physiological parameters examined were foliar gas exchange, photochemical efficiency, proton motive force, reactive oxygen species (ROS), and adenosine triphosphate (ATP) synthase activity of peanuts under cultivation with low P (LP, 0.5 mM P) and control conditions. During low P, the cyclic electron flow (CEF) maintained the high proton gradient (∆pH) induced by low ATP synthetic activity. Applying GABA during low P conditions stimulated CEF and reduced the concomitant ROS generation and thereby protecting the foliar photosystem II (PSII) from photoinhibition. Specifically, GABA enhanced the rate of electronic transmission of PSII (ETRII) by pausing the photoprotection mechanisms including non-photochemical quenching (NPQ) and ∆pH regulation. Thus, GABA was shown to be effective in restoring peanut growth when encountering P deficiency. Exogenous GABA alleviated two symptoms (increased root-shoot ratio and photoinhibition) of P-deficient peanuts. This is possibly the first report of using exogenous GABA to restore photosynthesis and growth under low P availability. Therefore, foliar applications of GABA could be a simple, safe and effective approach to overcome low yield imposed by limited P resources (low P in soils or P-fertilizers are unavailable) for sustainable peanut cultivation and especially in low to medium yielding fields.

Analysis of state 1-state 2 transitions by genome editing and complementation reveals a quenching component independent from the formation of PSI-LHCI-LHCII supercomplex in Arabidopsis thaliana

BACKGROUND

The light-harvesting antennae of photosystem (PS) I and PSII are pigment-protein complexes responsible of the initial steps of sunlight conversion into chemical energy. In natural environments plants are constantly confronted with the variability of the photosynthetically active light spectrum. PSII and PSI operate in series but have different optimal excitation wavelengths. The prompt adjustment of light absorption by photosystems is thus crucial to ensure efficient electron flow needed to sustain downstream carbon fixing reactions. Fast structural rearrangements equilibrate the partition of excitation pressure between PSII and PSI following the enrichment in the red (PSII-favoring) or far-red (PSI-favoring) spectra. Redox imbalances trigger state transitions (ST), a photoacclimation mechanism which involves the reversible phosphorylation/dephosphorylation of light harvesting complex II (LHCII) proteins by the antagonistic activities of the State Transition 7 (STN7) kinase/TAP38 phosphatase enzyme pair. During ST, a mobile PSII antenna pool associates with PSI increasing its absorption cross section. LHCII consists of assorted trimeric assemblies of Lhcb1, Lhcb2 and Lhcb3 protein isoforms (LHCII), several being substrates of STN7. However, the precise roles of Lhcb phosphorylation during ST remain largely elusive.

RESULTS

We inactivated the complete Lhcb1 and Lhcb2 gene clades in Arabidopsis thaliana and reintroduced either wild type Lhcb1.3 and Lhcb2.1 isoforms, respectively, or versions lacking N-terminal phosphorylatable residues proposed to mediate state transitions. While the substitution of Lhcb2.1 Thr-40 prevented the formation of the PSI-LHCI-LHCII complex, replacement of Lhcb1.3 Thr-38 did not affect the formation of this supercomplex, nor did influence the amplitude or kinetics of PSII fluorescence quenching upon state 1-state 2 transition.

CONCLUSIONS

Phosphorylation of Lhcb2 Thr-40 by STN7 alone accounts for ≈ 60% of PSII fluorescence quenching during state transitions. Instead, the presence of Thr-38 phosphosite in Lhcb1.3 was not required for the formation of the PSI-LHCI-LHCII supercomplex nor for re-equilibration of the plastoquinone redox state. The Lhcb2 phosphomutant was still capable of ≈ 40% residual fluorescence quenching, implying that a yet uncharacterized, STN7-dependent, component of state transitions, which is unrelated to Lhcb2 Thr-40 phosphorylation and to the formation of the PSI-LHCI-LHCII supercomplex, contributes to the equilibration of the PSI/PSII excitation pressure upon plastoquinone over-reduction.