A theoretical framework of the hybrid mechanism of photosystem II photodamage

A model of time-dependent macromolecular and elemental composition of phytoplankton

Phytoplankton Chl:C:N:P ratios are important from both an ecological and a biogeochemical perspective. We show that these elemental ratios can be represented by a phytoplankton physiological model of low complexity that includes major cellular macromolecular pools. In particular, our model resolves time-dependent intracellular pools of chlorophyll, proteins, nucleic acids, carbohydrates/lipids, and N and P storage. Batch culture data for two diatom and two prasinophyte species are used to constrain parameters that represent specific allocation traits and strategies. A key novelty is the simultaneous estimation of physiological parameters for two phytoplankton groups of such different sizes. The number of free parameters is reduced by assuming (i) allometric scaling for maximum uptake rates, (ii) shared half-saturation constants for synthesis of functional macromolecules, (iii) shared exudation rates of functional macromolecules across the species. The rationale behind this assumption is that across the different species, the same or similar processes, enzymes, and metabolites play a role in key physiological processes. For the turnover numbers of macromolecular synthesis and storage exudation rates, differences between diatoms and prasinophytes need to be taken into account to obtain a good fit. Our model fits suggest that the parameters related to storage dynamics dominate the differences in the C:N:P ratios between the different phytoplankton groups. Since descriptions of storage dynamics are still incomplete and imprecise, predictions of C:N:P ratios by phytoplankton models likely have a large uncertainty.

Investigation of singlet-oxygen-responsive genes in the cyanobacterium Synechocystis PCC 6803

Singlet oxygen (1O2) is an important reactive oxygen species whose formation by the type-II, light-dependent, photodynamic reaction is inevitable during photosynthetic processes. In the last decades, the recognition that 1O2 is not only a damaging agent, but can also affect gene expression and participates in signal transduction pathways has received increasing attention. However, contrary to several other taxa, 1O2-responsive genes have not been identified in the important cyanobacterial model organism Synechocystis PCC 6803. By using global transcript analysis we have identified a large set of Synechocystis genes, whose transcript levels were either enhanced or repressed in the presence of 1O2. Characteristic 1O2 responses were observed in several light-inducible genes of Synechocystis, especially in the hli (or scp) family encoding HLIP/SCP proteins involved in photoprotection. Other important 1O2-induced genes include components of the Photosystem II repair machinery (psbA2 and ftsH2, ftsH3), iron homeostasis genes isiA and idiA, the group 2 sigma factor sigD, some components of the transcriptomes induced by salt-, hyperosmotic and cold-stress, as well as several genes of unknown function. The most pronounced 1O2-induced upregulation was observed for the hliB and the co-transcribed lilA genes, whose deletion induced enhanced sensitivity against 1O2-mediated light damage. A bioreporter Synechocystis strain was created by fusing the hliB promoter to the bacterial luciferase (lux), which showed its utility for continuous monitoring of 1O2 concentrations inside the cell.

Excitation Energy Transfer between Higher Excited States of Photosynthetic Pigments: 1. Carotenoids Intercept and Remove B Band Excitations

Chlorophylls (Chls) are known for fast, subpicosecond internal conversion (IC) from ultraviolet/blue-absorbing ("B" or "Soret" states) to the energetically lower, red light-absorbing Q states. Consequently, excitation energy transfer (EET) in photosynthetic pigment-protein complexes involving the B states has so far not been considered. We present, for the first time, a theoretical framework for the existence of B-B EET in tightly coupled Chl aggregates such as photosynthetic pigment-protein complexes. We show that according to a Förster resonance energy transport (FRET) scheme, unmodulated B-B EET has an unexpectedly high range. Unsuppressed, it could pose an existential threat: the damage potential of blue light for photochemical reaction centers (RCs) is well-known. This insight reveals so far undescribed roles for carotenoids (Crts, this article) and Chl b (next article in this series) of possibly vital importance. Our model system is the photosynthetic antenna pigment-protein complex (CP29). Here, we show that the B → Q IC is assisted by the optically allowed Crt state (S2): The sequence is B → S2 (Crt, unrelaxed) → S2 (Crt, relaxed) → Q. This sequence has the advantage of preventing ∼39% of Chl-Chl B-B EET since the Crt S2 state is a highly efficient FRET acceptor. The B-B EET range and thus the likelihood of CP29 to forward potentially harmful B excitations toward the RC are thus reduced. In contrast to the B band of Chls, most Crt energy donation is energetically located near the Q band, which allows for 74/80% backdonation (from lutein/violaxanthin) to Chls. Neoxanthin, on the other hand, likely donates in the B band region of Chl b, with 76% efficiency. Crts thus act not only in their currently proposed photoprotective roles but also as a crucial building block for any system that could otherwise deliver harmful "blue" excitations to the RCs.

Exposed anthocyanic leaves of Prunus cerasifera are special shade leaves with high resistance to blue light but low resistance to red light against photoinhibition of photosynthesis

BACKGROUND AND AIMS

The photoprotective role of foliar anthocyanins has long been ambiguous: exacerbating, being indifferent to or ameliorating the photoinhibition of photosynthesis. The photoinhibitory light spectrum and failure to separate photo-resistance from repair, as well as the different methods used to quantify the photo-susceptibility of the photosystems, could lead to such a discrepancy.

METHODS

We selected two congeneric deciduous shrubs, Prunus cerasifera with anthocyanic leaves and Prunus triloba with green leaves, grown under identical growth conditions in an open field. The photo-susceptibilities of photosystem II (PSII) and photosystem I (PSI) to red light and blue light, in the presence of lincomycin (to block the repair), of exposed leaves were quantified by a non-intrusive P700+ signal from PSI. Leaf absorption, pigments, gas exchange and Chl a fluorescence were also measured.

KEY RESULTS

The content of anthocyanins in red leaves (P. cerasifera) was >13 times greater than that in green leaves (P. triloba). With no difference in maximum quantum efficiency of PSII photochemistry (Fv/Fm) and apparent CO2 quantum yield (AQY) in red light, anthocyanic leaves (P. cerasifera) showed some shade-acclimated suites, including lower Chl a/b ratio, lower photosynthesis rate, lower stomatal conductance and lower PSII/PSI ratio (on an arbitrary scale), compared with green leaves (P. triloba). In the absence of repair of PSII, anthocyanic leaves (P. cerasifera) showed a rate coefficient of PSII photoinactivation (ki) that was 1.8 times higher than that of green leaves (P. triloba) under red light, but significantly lower (-18 %) under blue light. PSI of both types of leaves was not photoinactivated under blue or red light.

CONCLUSIONS

In the absence of repair, anthocyanic leaves exhibited an exacerbation of PSII photoinactivation under red light and a mitigation under blue light, which can partially reconcile the existing controversy in terms of the photoprotection by anthocyanins. Overall, the results demonstrate that appropriate methodology applied to test the photoprotection hypothesis of anthocyanins is critical.

Heavy metal-resistant rhizobacteria fosters to alleviate the cadmium toxicity in Arabidopsis by upregulating the plant physiological responses

This study was designed to investigate the role of Morganella morganii strains in alleviating Cd stress in Arabidopsis seedlings under controlled conditions. Both M. morganii strains ABT3 (ON316873) and ABT9 (ON316874) strains isolated from salt-affected areas showed higher resistance against Cd and possess plant growth-promoting traits such as nitrogen fixation, indole-acetic acid production, ammonia production, phosphate solubilization, and, catalase, gelatinase and protease enzyme production. Plant inoculation assay showed that varying concentration of Cd (1.5 mM and 2.5 mM) significantly reduced Arabidopsis growth, quantum yield (56.70%-66.49%), and chlorophyll content (31.90%-42.70%). Cd toxicity also triggered different associations between lipid peroxidation (43.61%-69.77%) and enzymatic antioxidant mechanisms. However, when both strains were applied to the Arabidopsis seedlings, the shoot and root length and fresh and dry weights were improved in the control and Cd-stressed plants. Moreover, both strains enhanced the resistance against Cd stress by increasing antioxidant enzyme activities [catalase (19.47%-27.39%) and peroxidase (37.50%-48.07%)]that ultimately cause a substantial reduction in lipid peroxidation (27.71%-41.90%). Both strains particularly ABT3 also showed positive results in improving quantum yield (73.84%-98.64%) and chlorophyll content (41.13%-48.63%), thus increasing the growth of Arabidopsis seedlings. The study suggests that PGPR can protect plants from Cd toxicity, and Cd-tolerant rhizobacterial strains can remediate heavy metal polluted sites and improve plant growth.

The energy cost of repairing photoinactivated photosystem II: an experimental determination in cotton leaf discs

Photosystem II (PSII), which splits water molecules at minimal excess photochemical potential, is inevitably photoinactivated during photosynthesis, resulting in compromised photosynthetic efficiency unless it is repaired. The energy cost of PSII repair is currently uncertain, despite attempts to calculate it. We experimentally determined the energy cost of repairing each photoinactivated PSII in cotton (Gossypium hirsutum) leaves, which are capable of repairing PSII in darkness. As an upper limit, 24 000 adenosine triphosphate (ATP) molecules (including any guanosine triphosphate synthesized at the expense of ATP) were required to repair one entire PSII complex. Further, over a 7-h illumination period at 526-1953 μmol photons m^-2  s^-1 , the ATP requirement for PSII repair was on average up to 4.6% of the ATP required for the gross carbon assimilation. Each of these two measures of ATP requirement for PSII repair is two- to three-fold greater than the respective reported calculated value. Possible additional energy sinks in the PSII repair cycle are discussed.

Advances in the Understanding of the Lifecycle of Photosystem II

Photosystem II is a light-driven water-plastoquinone oxidoreductase present in cyanobacteria, algae and plants. It produces molecular oxygen and protons to drive ATP synthesis, fueling life on Earth. As a multi-subunit membrane-protein-pigment complex, Photosystem II undergoes a dynamic cycle of synthesis, damage, and repair known as the Photosystem II lifecycle, to maintain a high level of photosynthetic activity at the cellular level. Cyanobacteria, oxygenic photosynthetic bacteria, are frequently used as model organisms to study oxygenic photosynthetic processes due to their ease of growth and genetic manipulation. The cyanobacterial PSII structure and function have been well-characterized, but its lifecycle is under active investigation. In this review, advances in studying the lifecycle of Photosystem II in cyanobacteria will be discussed, with a particular emphasis on new structural findings enabled by cryo-electron microscopy. These structural findings complement a rich and growing body of biochemical and molecular biology research into Photosystem II assembly and repair.

Molecular origins of induction and loss of photoinhibition-related energy dissipation qI

Photosynthesis fuels life on Earth using sunlight as energy source. However, light has a simultaneous detrimental effect on the enzyme triggering photosynthesis and producing oxygen, photosystem II (PSII). Photoinhibition, the light-dependent decrease of PSII activity, results in a major limitation to aquatic and land photosynthesis and occurs upon all environmental stress conditions. In this work, we investigated the molecular origins of photoinhibition focusing on the paradoxical energy dissipation process of unknown nature coinciding with PSII damage. Integrating spectroscopic, biochemical, and computational approaches, we demonstrate that the site of this quenching process is the PSII reaction center. We propose that the formation of quenching and the closure of PSII stem from the same event. We lastly reveal the heterogeneity of PSII upon photoinhibition using structure-function modeling of excitation energy transfer. This work unravels the functional details of the damage-induced energy dissipation at the heart of photosynthesis.