Mechanisms of Photodamage and Protein Turnover in Photoinhibition

Harnessing photosynthesis to produce electricity using cyanobacteria, green algae, seaweeds and plants

The conversion of solar energy into electrical current by photosynthetic organisms has the potential to produce clean energy. Life on earth depends on photosynthesis, the major mechanism for biological conversion of light energy into chemical energy. Indeed, billions of years of evolution and adaptation to extreme environmental habitats have resulted in highly efficient light-harvesting and photochemical systems in the photosynthetic organisms that can be found in almost every ecological habitat of our world. In harnessing photosynthesis to produce green energy, the native photosynthetic system is interfaced with electrodes and electron mediators to yield bio-photoelectrochemical cells (BPECs) that transform light energy into electrical power. BPECs utilizing plants, seaweeds, unicellular photosynthetic microorganisms, thylakoid membranes or purified complexes, have been studied in attempts to construct efficient and non-polluting BPECs to produce electricity or hydrogen for use as green energy. The high efficiency of photosynthetic light-harvesting and energy production in the mostly unpolluting processes that make use of water and CO2 and produce oxygen beckons us to develop this approach. On the other hand, the need to use physiological conditions, the sensitivity to photoinhibition as well as other abiotic stresses, and the requirement to extract electrons from the system are challenging. In this review, we describe the principles and methods of the different kinds of BPECs that use natural photosynthesis, with an emphasis on BPECs containing living oxygenic photosynthetic organisms. We start with a brief summary of BPECs that use purified photosynthetic complexes. This strategy has produced high-efficiency BPECs. However, the lifetimes of operation of these BPECs are limited, and the preparation is laborious and expensive. We then describe the use of thylakoid membranes in BPECs which requires less effort and usually produces high currents but still suffers from the lack of ability to self-repair damage caused by photoinhibition. This obstacle of the utilization of photosynthetic systems can be significantly reduced by using intact living organisms in the BPEC. We thus describe here progress in developing BPECs that make use of cyanobacteria, green algae, seaweeds and higher plants. Finally, we discuss the future challenges of producing high and longtime operating BPECs for practical use.

Microbial Inoculation Improves Growth, Nutritional and Physiological Aspects of Glycine max (L.) Merr

Considering a scenario where there is a low availability and increasing costs of fertilizers in the global agricultural market, as well as a finitude of important natural resources, such as phosphorus (P), this study tested the effect of the inoculation of rhizospheric or endophytic microorganisms isolated from Hymenaea courbaril and Butia purpurascens on the growth promotion of Glycine max (L.) Merr. The tests were conducted in a controlled greenhouse system, and the effects of biofertilization were evaluated using the following parameters: dry biomass, nutritional content, and photochemical and photosynthetic performance of plants. Seed biopriming was performed with four bacterial and four fungal isolates, and the results were compared to those of seeds treated with the commercial product Biomaphos^®. Overall, microbial inoculation had a positive effect on biomass accumulation in G. max, especially in strains PA12 (Paenibacillus alvei), SC5 (Bacillus cereus), and SC15 (Penicillium sheari). The non-inoculated control plants accumulated less nutrients, both in the whole plant and aerial part, and had reduced chlorophyll index and low photosynthetic rate (A) and photochemical efficiency. Strains PA12 (P. alvei), SC5 (B. cereus), and 328EF (Codinaeopsis sp.) stood out in the optimization of nutrient concentration, transpiration rate, and stomatal conductance. Plants inoculated with the bacterial strains PA12 (P. alvei) and SC5 (B. cereus) and with the fungal strains 328EF (Codinaeopsis sp.) and SC15 (P. sheari) showed the closest pattern to that observed in plants treated with Biomaphos^®, with the same trend of direction of the means associated with chlorophyll index, (A), dry mass, and concentration of important nutrients such as N, P, and Mg. We recommend the use of these isolates in field tests to validate these strains for the production of biological inoculants as part of the portfolio of bioinputs available for G. max.

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.

NaCl Pretreatment Enhances the Low Temperature Tolerance of Tomato Through Photosynthetic Acclimation

Plants often need to withstand multiple types of environmental stresses (e.g., salt and low temperature stress) because of their sessile nature. Although the physiological responses of plants to single stressor have been well-characterized, few studies have evaluated the extent to which pretreatment with non-lethal stressors can maintain the photosynthetic performance of plants in adverse environments (i.e., acclimation-induced cross-tolerance). Here, we studied the effects of sodium chloride (NaCl) pretreatment on the photosynthetic performance of tomato plants exposed to low temperature stress by measuring photosynthetic and chlorophyll fluorescence parameters, stomatal aperture, chloroplast quality, and the expression of stress signaling pathway-related genes. NaCl pretreatment significantly reduced the carbon dioxide assimilation rate, transpiration rate, and stomatal aperture of tomato leaves, but these physiological acclimations could mitigate the adverse effects of subsequent low temperatures compared with non-pretreated tomato plants. The content of photosynthetic pigments decreased and the ultra-microstructure of chloroplasts was damaged under low temperature stress, and the magnitude of these adverse effects was alleviated by NaCl pretreatment. The quantum yield of photosystem I (PSI) and photosystem II (PSII), the quantum yield of regulatory energy dissipation, and non-photochemical energy dissipation owing to donor-side limitation decreased following NaCl treatment; however, the opposite patterns were observed when NaCl-pretreated plants were exposed to low temperature stress. Similar results were obtained for the electron transfer rate of PSI, the electron transfer rate of PSII, and the estimated cyclic electron flow value (CEF). The production of reactive oxygen species induced by low temperature stress was also significantly alleviated by NaCl pretreatment. The expression of ion channel and tubulin-related genes affecting stomatal aperture, chlorophyll synthesis genes, antioxidant enzyme-related genes, and abscisic acid (ABA) and low temperature signaling-related genes was up-regulated in NaCl-pretreated plants under low temperature stress. Our findings indicated that CEF-mediated photoprotection, stomatal movement, the maintenance of chloroplast quality, and ABA and low temperature signaling pathways all play key roles in maintaining the photosynthetic capacity of NaCl-treated tomato plants under low temperature stress.

Enzymes degraded under high light maintain proteostasis by transcriptional regulation in Arabidopsis

Photoinhibitory high light stress in plants leads to increases in markers of protein degradation and transcriptional up-regulation of proteases and proteolytic machinery, but protein homeostasis (proteostasis) of most enzymes is largely maintained under high light, so we know little about the metabolic consequences of it beyond photosystem damage. We developed a technique to look for rapid protein turnover events in response to high light through ¹³C partial labeling and detailed peptide mass spectrometry. This analysis reveals a light-induced transcriptional program for nuclear-encoded genes, beyond the regulation of photosystem II, to replace key protein degradation targets in plants and ensure proteostasis under high light stress.

Photoinhibitory high light stress in Arabidopsis leads to increases in markers of protein degradation and transcriptional up-regulation of proteases and proteolytic machinery, but proteostasis is largely maintained. We find significant increases in the in vivo degradation rate for specific molecular chaperones, nitrate reductase, glyceraldehyde-3 phosphate dehydrogenase, and phosphoglycerate kinase and other plastid, mitochondrial, peroxisomal, and cytosolic enzymes involved in redox shuttles. Coupled analysis of protein degradation rates, mRNA levels, and protein abundance reveal that 57% of the nuclear-encoded enzymes with higher degradation rates also had high light–induced transcriptional responses to maintain proteostasis. In contrast, plastid-encoded proteins with enhanced degradation rates showed decreased transcript abundances and must maintain protein abundance by other processes. This analysis reveals a light-induced transcriptional program for nuclear-encoded genes, beyond the regulation of the photosystem II (PSII) D1 subunit and the function of PSII, to replace key protein degradation targets in plants and ensure proteostasis under high light stress.

A new method for separate evaluation of PSII with inactive oxygen evolving complex and active D1 by the pulse-amplitude modulated chlorophyll fluorometry

A method that separately quantifies the PSII with inactive oxygen-evolving complex (OEC) and active D1 retaining the primary quinone acceptor (QA )-reducing activity from the PSII with damaged D1 in the leaf was developed using PAM fluorometry. It is necessary to fully reduce QA to obtain F m , the maximum fluorescence. However, QA in PSII with inactive OEC and active D1 would not be fully reduced by a saturating flash. We used the acceptor-side inhibitor DCMU to fully reduce QA . Leaves of cucumber (Cucumis sativus L.) were chilled at 4°C in dark or illuminated with UV-A to selectively inactivate OEC. After these treatments, F v /F m , the maximum quantum yield, in the leaves vacuum-infiltrated with DCMU were greater than those in water-infiltrated leaves. In contrast, when the leaves were illuminated by red light to photodamage D1, F v /F m did not differ between DCMU- and water-infiltrated leaves. These results indicate relevance of the present evaluation of the fraction of PSII with inactive OEC and active D1. Several examinations in the laboratory and glasshouse showed that PSII with inactive OEC and active D1 was only rarely observed. The present simple method would serve as a useful tool to clarify the details of the PSII photoinhibition.

Overexpression mutants reveal a role for a chloroplast MPD protein in regulation of reactive oxygen species during chilling in Arabidopsis

Reactive oxygen species (ROS) contribute to cellular damage in several different contexts, but their role during chilling damage is poorly defined. Chilling sensitivity both limits the distribution of plant species and causes devastating crop losses worldwide. Our screen of chilling-tolerant Arabidopsis (Arabidopsis thaliana) for mutants that suffer chilling damage identified a gene (At4g03410) encoding a chloroplast Mpv17_PMP22 protein, MPD1, with no previous connection to chilling. The chilling-sensitive mpd1-1 mutant is an overexpression allele that we successfully phenocopied by creating transgenic lines with a similar level of MPD1 overexpression. In mammals and yeast, MPD1 homologs are associated with ROS management. In chilling conditions, Arabidopsis overexpressing MPD1 accumulated H2O2 to higher levels than wild-type controls and exhibited stronger induction of ROS response genes. Paraquat application exacerbated chilling damage, confirming that the phenotype occurs due to ROS dysregulation. We conclude that at low temperature increased MPD1 expression results in increased ROS production, causing chilling damage. Our discovery of the effect of MPD1 overexpression on ROS production under chilling stress implies that investigation of the nine other members of the Mpv17_PMP22 family in Arabidopsis may lead to new discoveries regarding ROS signaling and management in plants.

Leaf structure and photosynthesis in Populus alba under naturally fluctuating environments

The ability to modulate photosynthesis is essential for plants to adapt to fluctuating growing conditions. Populus species show high tolerance to various and highly variable environments. To understand their response strategies against fluctuating environments, this study investigated the morphological and physiological differences of white poplar (Populus alba) leaves when grown in a phytotron, glasshouse, and field. Our results show that the palisade cells were elongated in the field, which would enhance intercellular CO2 exchange. Photosynthetic capacity was the highest in the field leaves, as shown by higher electron transport rates (1.8 to 6.5 times) and carbon assimilation rates (2.7 to 4.2 times). The decrease of PSI acceptor-side limitation and increase of PSI donor-side limitation suggests changes in PSI redox status may contribute to photoprotection. This plasticity of white poplar allows adjusting its structure and photosynthesis under fluctuating conditions, which may partly enable its outstanding tolerance against environmental changes.

Quantifying the long-term interplay between photoprotection and repair mechanisms sustaining photosystem II activity

The photosystem II reaction centre (RCII) protein subunit D1 is the main target of light-induced damage in the thylakoid membrane. As such, it is constantly replaced with newly synthesised proteins, in a process dubbed the 'D1 repair cycle'. The mechanism of relief of excitation energy pressure on RCII, non-photochemical quenching (NPQ), is activated to prevent damage. The contribution of the D1 repair cycle and NPQ in preserving the photochemical efficiency of RCII is currently unclear. In this work, we seek to (1) quantify the relative long-term effectiveness of photoprotection offered by NPQ and the D1 repair cycle, and (2) determine the fraction of sustained decrease in RCII activity that is due to long-term protective processes. We found that while under short-term, sunfleck-mimicking illumination, NPQ is substantially more effective in preserving RCII activity than the D1 repair cycle (Plant. Cell Environ. 41, 1098-1112, 2018). Under prolonged constant illumination, its contribution is less pronounced, accounting only for up to 30% of RCII protection, while D1 repair assumes a predominant role. Exposure to a wide range of light intensities yields comparable results, highlighting the crucial role of a constant and rapid D1 turnover for the maintenance of RCII efficiency. The interplay between NPQ and D1 repair cycle is crucial to grant complete phototolerance to plants under low and moderate light intensities, and limit damage to photosystem II under high light. Additionally, we disentangled and quantified the contribution of a slowly-reversible NPQ component that does not impair RCII activity, and is therefore protective.

Low N level increases the susceptibility of PSI to photoinhibition induced by short repetitive flashes in leaves of different rice varieties

The recovery from photoinhibition is much slower in photosystem (PS) I than in PSII; therefore, the susceptibility of PSI to photoinhibition is important with respect to photosynthetic production under special physiological conditions. Previous studies have shown that repetitive short-pulse (rSP) illumination selectively induces PSI photoinhibition. Depending on the growth light intensity or the variety/species of the plant, PSI photoinhibition is different, but the underlying mechanisms remain unknown. Here, we aimed to clarify whether the differences in the susceptibility of PSI to photoinhibition depend on environmental factors or on rice varieties and which physiological properties of the plant are related to this susceptibility. We exposed mature leaves of rice plants to rSP illumination. We examined the effects of elevated CO₂ concentration and low N during growth on the susceptibility of PSI to photoinhibition and compared it in 12 different varieties. We fitted the decrease in the quantum yield of PSI during rSP illumination and estimated a parameter indicating susceptibility. Low N level increased susceptibility, whereas elevated CO₂ concentration did not. The susceptibility differed among different rice varieties, and many indica varieties showed higher susceptibility than the temperate japonica varieties. Susceptibility was negatively correlated with the total chlorophyll content and N content. However, the decrease in P m ' value, an indicator of damaged PSI, was positively correlated with chlorophyll content. This suggests that in leaves with a larger electron transport capacity, the overall PSI activity may be less susceptible to photoinhibition, but more damaged PSI may accumulate during rSP illumination.

Responses of Linear and Cyclic Electron Flow to Nitrogen Stress in an N-Sensitive Species Panax notoginseng

Nitrogen (N) is a primary factor limiting leaf photosynthesis. However, the mechanism of N-stress-driven photoinhibition of the photosystem I (PSI) and photosystem II (PSII) is still unclear in the N-sensitive species such as Panax notoginseng, and thus the role of electron transport in PSII and PSI photoinhibition needs to be further understood. We comparatively analyzed photosystem activity, photosynthetic rate, excitation energy distribution, electron transport, OJIP kinetic curve, P700 dark reduction, and antioxidant enzyme activities in low N (LN), moderate N (MN), and high N (HN) leaves treated with linear electron flow (LEF) inhibitor [3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU)] and cyclic electron flow (CEF) inhibitor (methyl viologen, MV). The results showed that the increased application of N fertilizer significantly enhance leaf N contents and specific leaf N (SLN). Net photosynthetic rate (P n) was lower in HN and LN plants than in MN ones. Maximum photochemistry efficiency of PSII (F v/F m), maximum photo-oxidation P700+ (P m), electron transport rate of PSI (ETRI), electron transport rate of PSII (ETRII), and plastoquinone (PQ) pool size were lower in the LN plants. More importantly, K phase and CEF were higher in the LN plants. Additionally, there was not a significant difference in the activity of antioxidant enzyme between the MV- and H2O-treated plants. The results obtained suggest that the lower LEF leads to the hindrance of the formation of ΔpH and ATP in LN plants, thereby damaging the donor side of the PSII oxygen-evolving complex (OEC). The over-reduction of PSI acceptor side is the main cause of PSI photoinhibition under LN condition. Higher CEF and antioxidant enzyme activity not only protected PSI from photodamage but also slowed down the damage rate of PSII in P. notoginseng grown under LN.

Impact of streetlights on physiology, biochemistry and diversity of urban bryophyte: a case study on moss Semibarbula orientalis

The use of artificial light at night is a very basic symbol of urbanization and has distorted many ecological, biochemical and physiological phenomena in plants, which have settled for millions of years in the biological system. Continuous illumination of light significantly alters the circadian rhythm of all organisms. The present study was focused to understand the effects of continuous light (CL) on the biochemistry and physiology of moss Semibarbula orientalis. It was observed that H2O2 accumulation and activities of chlorophyllase, phenylalanine ammonia-lyase, superoxide dismutase and catalase enzymes significantly enhanced in plants growing under streetlights. Similarly, plants under CL showed a marked reduction in photosynthetic performance. Specific fluxes (ABS/RC, TR/RC, ET/RC), phenomenological fluxes (ABS/CS, TR/CS, ET/CS), density of photosystem-II, quantum yield of photosynthesis and chlorophyll concentration markedly declined in plants growing under streetlights. Depletion in performance indices (PIcs and PIabs) and primary and secondary photochemistry [PHIO/(1 − PHIO) and PSIO/(1 − PSIO)] were also noticed, which indicated failure of adaptive strategies of photosystem-II, resulting in the loss of biomass of S. orientalis. Biomass decline is also shown by a decrease in coverage, which reduces the bryophyte species richness of the chosen locations. Present studies clearly indicate that artificial light at night drastically affects the moss population. The reduction in the dominating species, S. orientalis, improves species evenness and results in a slow growth rate.

Chloroplast ROS and stress signaling

Chloroplasts overproduce reactive oxygen species (ROS) under unfavorable environmental conditions, and these ROS are implicated in both signaling and oxidative damage. There is mounting evidence for their roles in translating environmental fluctuations into distinct physiological responses, but their targets, signaling cascades, and mutualism and antagonism with other stress signaling cascades and within ROS signaling remain poorly understood. Great efforts made in recent years have shed new light on chloroplast ROS-directed plant stress responses, from ROS perception to plant responses, in conditional mutants of Arabidopsis thaliana or under various stress conditions. Some articles have also reported the mechanisms underlying the complexity of ROS signaling pathways, with an emphasis on spatiotemporal regulation. ROS and oxidative modification of affected target proteins appear to induce retrograde signaling pathways to maintain chloroplast protein quality control and signaling at a whole-cell level using stress hormones. This review focuses on these seemingly interconnected chloroplast-to-nucleus retrograde signaling pathways initiated by ROS and ROS-modified target molecules. We also discuss future directions in chloroplast stress research to pave the way for discovering new signaling molecules and identifying intersectional signaling components that interact in multiple chloroplast signaling pathways.

Photosystem II photoinhibition and photoprotection in a lycophyte, Selaginella martensii

The Lycophyte Selaginella martensii efficiently acclimates to diverse light environments, from deep shade to full sunlight. The plant does not modulate the abundance of the Light Harvesting Complex II, mostly found as a free trimer, and does not alter the maximum capacity of thermal dissipation (NPQ). Nevertheless, the photoprotection is expected to be modulatable upon long-term light acclimation to preserve the photosystems (PSII, PSI). The effects of long-term light acclimation on PSII photoprotection were investigated using the chlorophyll fluorometric method known as "photochemical quenching measured in the dark" (qP_d ). Singularly high-qP_d values at relatively low irradiance suggest a heterogeneous antenna system (PSII antenna uncoupling). The extent of antenna uncoupling largely depends on the light regime, reaching the highest value in sun-acclimated plants. In parallel, the photoprotective NPQ (pNPQ) increased from deep-shade to high-light grown plants. It is proposed that the differences in the long-term modulation in the photoprotective capacity are proportional to the amount of uncoupled LHCII. In deep-shade plants, the inconsistency between invariable maximum NPQ and lower pNPQ is attributed to the thermal dissipation occurring in the PSII core.

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.

Biotechnology under extreme conditions: Lichens after extreme UVB radiation and extreme temperatures produce large amounts of hydrogen

The present study demonstrates biotechnological applications of the lichen Pleurosticta acetabulum, specifically the production of large amounts of hydrogen even after the lichen exposure to extreme conditions such as a) extreme UVB radiation (1.7 mW/cm² = 1000 J m^-2 min^-1) over different time periods (4, 20 & 70 h) and b) combined exposure of the lichen to high intensity UVB radiation and extreme low (-196 °C) or extreme high temperatures (+70 °C). The results highlight that the extremophilic and polyextremophilic behavior of lichens both in dehydrated and in regenerated form, under extreme conditions not necessarily recorded on earth, is compatible with their biotechnological uses. The lichen viability was measured using fluorescence induction techniques (OJIP-test), which record changes in the molecular structure and function of the photosynthetic mechanism, while its ability to produce molecular hydrogen was measured through thermal conductivity gas chromatography (GC-TCD) analysis. Hydrogen is a promising fuel for the future. The exciting result of a lichen micro-ecosystem is its ability to expel its moisture and remain in an inactive state, protecting itself from extreme conditions and maintaining its ability to high yield hydrogen production in a closed system, with the sole addition of water and without the need for additional energy. Our results expand the potential use of lichens for future biotechnological applications in extreme Earth environments, but also in environments on other planets, such as Mars, thus paving the way for astrobiotechnological applications.

Analysis of the changes of electron transfer and heterogeneity of photosystem II in Deg1-reduced Arabidopsis plants

Deg1 protease functions in protease and chaperone of PSII complex components, but few works were performed to study the effects of Deg1 on electron transport activities on the donor and acceptor side of PSII and its correlation with the photoprotection of PSII during photoinhibition. Therefore, we performed systematic and comprehensive investigations of electron transfers on the donor and acceptor sides of photosystem II (PSII) in the Deg1-reduced transgenic lines deg1-2 and deg1-4. Both the maximal quantum efficiency of PSII photochemistry (F_v/F_m) and the actual PSII efficiency (Φ_PSII) decreased significantly in the transgenic plants. Increases in nonphotochemical quenching (NPQ) and the dissipated energy flux per reaction center (DI₀/RC) were also shown in the transgenic plants. Along with the decreased D1, CP47, and CP43 content, these results suggested photoinhibition under growth light conditions in transgenic plants. Decreased Deg1 caused inhibition of electron transfer on the PSII reducing side, leading to a decline in the number of Q_B-reducing centers and accumulation of Q_B-nonreducing centers. The Tm of the Q band shifted from 5.7 °C in the wild-type plant to 10.4 °C and 14.2 °C in the deg1-2 and deg1-4 plants, respectively, indicating an increase in the stability of S₂Q_A^¯ in transgenic plants. PSIIα in the transgenic plants largely reduced, while PSIIβ and PSIIγ increased with the decline in the Deg1 levels in transgenic plants suggesting PSIIα centers gradually converted into PSIIβ and PSIIγ centers in the transgenic plants. Besides, the connectivity of PSIIα and PSIIβ was downregulated in transgenic plants. Our results reveal that downregulation of Deg1 protein levels induced photoinhibition in transgenic plants, leading to loss of PSII activities on both the donor and acceptor sides in transgenic plants. These results give a new insight into the regulation role of Deg1 in PSII electron transport.

Photosystem I Inhibition, Protection and Signalling: Knowns and Unknowns

Photosynthesis is the process that harnesses, converts and stores light energy in the form of chemical energy in bonds of organic compounds. Oxygenic photosynthetic organisms (i.e., plants, algae and cyanobacteria) employ an efficient apparatus to split water and transport electrons to high-energy electron acceptors. The photosynthetic system must be finely balanced between energy harvesting and energy utilisation, in order to limit generation of dangerous compounds that can damage the integrity of cells. Insight into how the photosynthetic components are protected, regulated, damaged, and repaired during changing environmental conditions is crucial for improving photosynthetic efficiency in crop species. Photosystem I (PSI) is an integral component of the photosynthetic system located at the juncture between energy-harnessing and energy consumption through metabolism. Although the main site of photoinhibition is the photosystem II (PSII), PSI is also known to be inactivated by photosynthetic energy imbalance, with slower reactivation compared to PSII; however, several outstanding questions remain about the mechanisms of damage and repair, and about the impact of PSI photoinhibition on signalling and metabolism. In this review, we address the knowns and unknowns about PSI activity, inhibition, protection, and repair in plants. We also discuss the role of PSI in retrograde signalling pathways and highlight putative signals triggered by the functional status of the PSI pool.

Characterization of Nannochloropsis oceanica Rose Bengal Mutants Sheds Light on Acclimation Mechanisms to High Light When Grown in Low Temperature

A barrier to realizing Nannochloropsis oceanica's potential for omega-3 eicosapentaenoic acid (EPA) production is the disparity between conditions that are optimal for growth and those that are optimal for EPA biomass content. A case in point is temperature: higher content of polyunsaturated fatty acid, and especially EPA, is observed in low-temperature (LT) environments, where growth rates are often inhibited. We hypothesized that mutant strains of N. oceanica resistant to the singlet-oxygen photosensitizer Rose Bengal (RB) would withstand the oxidative stress conditions that prevail in the combined stressful environment of high light (HL; 250 μmol photons m-2 s-1) and LT (18°C). This growth environment caused the wild-type (WT) strain to experience a spike in lipid peroxidation and an inability to proliferate, whereas growth and homeostatic reactive oxygen species levels were observed in the mutant strains. We suggest that the mutant strains' success in this environment can be attributed to their truncated photosystem II antennas and their increased ability to diffuse energy in those antennas as heat (non-photosynthetic quenching). As a result, the mutant strains produced upward of four times more EPA than the WT strain in this HL-LT environment. The major plastidial lipid monogalactosyldiacylglycerol was a likely target for oxidative damage, contributing to the photosynthetic inhibition of the WT strain. A mutation in the NO10G01010.1 gene, causing a subunit of the 2-oxoisovalerate dehydrogenase E1 protein to become non-functional, was determined to be the likely source of tolerance in the RB113 mutant strain.

Critical Role of NdhA in the Incorporation of the Peripheral Arm into the Membrane-Embedded Part of the Chloroplast NADH Dehydrogenase-Like Complex

The chloroplast NADH dehydrogenase-like (NDH) complex mediates ferredoxin-dependent plastoquinone reduction in the thylakoid membrane. In angiosperms, chloroplast NDH is composed of five subcomplexes and further forms a supercomplex with photosystem I (PSI). Subcomplex A (SubA) mediates the electron transport and consists of eight subunits encoded by both plastid and nuclear genomes. The assembly of SubA in the stroma has been extensively studied, but it is unclear how SubA is incorporated into the membrane-embedded part of the NDH complex. Here, we isolated a novel Arabidopsis mutant chlororespiratory reduction 16 (crr16) defective in NDH activity. CRR16 encodes a chloroplast-localized P-class pentatricopeptide repeat protein conserved in angiosperms. Transcript analysis of plastid-encoded ndh genes indicated that CRR16 was responsible for the efficient splicing of the group II intron in the ndhA transcript, which encodes a membrane-embedded subunit localized to the connecting site between SubA and the membrane subcomplex (SubM). To analyze the roles of NdhA in the assembly and stability of the NDH complex, the homoplastomic knockout plant of ndhA (ΔndhA) was generated in tobacco (Nicotiana tabacum). Biochemical analyses of crr16 and ΔndhA plants indicated that NdhA was essential for stabilizing SubA and SubE but not for the accumulation of the other three subcomplexes. Furthermore, the crr16 mutant accumulated the SubA assembly intermediates in the stroma more than that in the wild type. These results suggest that NdhA biosynthesis is essential for the incorporation of SubA into the membrane-embedded part of the NDH complex at the final assembly step of the NDH-PSI supercomplex.

A Systematic Survey of the Light/Dark-dependent Protein Degradation Events in a Model Cyanobacterium

Light is essential for photosynthetic organisms and is involved in the regulation of protein synthesis and degradation. The significance of light-regulated protein degradation is exemplified by the well-established light-induced degradation and repair of the photosystem II reaction center D1 protein in higher plants and cyanobacteria. However, systematic studies of light-regulated protein degradation events in photosynthetic organisms are lacking. Thus, we conducted a large-scale survey of protein degradation under light or dark conditions in the model cyanobacterium Synechocystis sp. PCC 6803 (hereafter referred to as Synechocystis) using the isobaric labeling-based quantitative proteomics technique. The results revealed that 79 proteins showed light-regulated degradation, including proteins involved in photosystem II structure or function, quinone binding, and NADH dehydrogenase. Among these, 25 proteins were strongly dependent on light for degradation. Moreover, the light-dependent degradation of several proteins was sensitive to photosynthetic electron transport inhibitors (DCMU and DBMIB), suggesting that they are influenced by the redox state of the plastoquinone (PQ) pool. Together, our study comprehensively cataloged light-regulated protein degradation events, and the results serve as an important resource for future studies aimed at understanding light-regulated processes and protein quality control mechanisms in cyanobacteria.

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Light-/dark-regulated protein degradation events in a model Cyanobacterium were identified.

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Seventy-nine proteins displayed light-regulated degradation.

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Thirty-one proteins displayed dark-regulated degradation.

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Multiple light-regulated protein degradation events were regulated by the redox state of the plastoquinone pool.

Enhanced Secretions of Algal Cell-Adhesion Molecules and Metal Ion-Binding Exoproteins Promote Self-Flocculation of Chlorella sp. Cultivated in Municipal Wastewater

The mechanism of self-flocculation remains unclear, partially impeding its efficiency enhancement and commercial application of microalgae-based municipal wastewater (MW) bioremediation technology. This study revealed the contributions of exoproteins [PN, proteins in extracellular polymeric substances (EPS)] to the separation of indigenous microalgae from treated MW. Compared to the low light intensity group, the high light intensity (HL) group produced Chlorella sp. with 4.3-fold higher self-flocculation efficiencies (SE). This was attributed to the enriched biological functions and positional rearrangement of increased PN within 2.9-fold higher EPS. Specifically, a total of 75 PN was over-expressed in the HL group among the 129 PN identified through label-free proteomics. The algal cell-adhesion molecules (Algal-CAMs) and metal-ion-binding PN were demonstrated as two dominant contributors promoting cell adhesion and bridging, through function prediction based on the contained domains. The modeled 3D structure showed that Algal-CAMs presented less hydrophilic α-helix abundance and were distributed in the outermost position of the EPS matrix, further facilitating microalgal separation. Moreover, the 10.1% lower hydrophily degree value, negative interfacial free energy (-19.5 mJ/m²), and 6.8-fold lower energy barrier between cells also supported the observed higher SE. This finding is expected to further fill the knowledge gap of the role of PN in microalgal self-flocculation and promote the development of biomass recovery from the microalgae-wastewater system.

Cryo-EM photosystem I structure reveals adaptation mechanisms to extreme high light in Chlorella ohadii

Photosynthesis in deserts is challenging since it requires fast adaptation to rapid night-to-day changes, that is, from dawn's low light (LL) to extreme high light (HL) intensities during the daytime. To understand these adaptation mechanisms, we purified photosystem I (PSI) from Chlorella ohadii, a green alga that was isolated from a desert soil crust, and identified the essential functional and structural changes that enable the photosystem to perform photosynthesis under extreme high light conditions. The cryo-electron microscopy structures of PSI from cells grown under low light (PSI_LL) and high light (PSI_HL), obtained at 2.70 and 2.71 Å, respectively, show that part of light-harvesting antenna complex I (LHCI) and the core complex subunit (PsaO) are eliminated from PSI_HL to minimize the photodamage. An additional change is in the pigment composition and their number in LHCI_HL; about 50% of chlorophyll b is replaced by chlorophyll a. This leads to higher electron transfer rates in PSI_HL and might enable C. ohadii PSI to act as a natural photosynthesiser in photobiocatalytic systems. PSI_HL or PSI_LL were attached to an electrode and their induced photocurrent was determined. To obtain photocurrents comparable with PSI_HL, 25 times the amount of PSI_LL was required, demonstrating the high efficiency of PSI_HL. Hence, we suggest that C. ohadii PSI_HL is an ideal candidate for the design of desert artificial photobiocatalytic systems.

A theoretical framework of the hybrid mechanism of photosystem II photodamage

Photodamage of photosystem II is a significant physiological process that is prevalent in the fields of photobiology, photosynthesis research and plant/algal stress. Since its discovery, numerous efforts have been devoted to determine the causes and mechanisms of action of photosystem II photodamage. There are two contrasting hypotheses to explain photodamage: (1) the excitation pressure induced by light absorption by the photosynthetic pigments and (2) direct photodamage of the Mn cluster located at the water-splitting site, which is independent of excitation pressure. While these two hypotheses seemed mutually exclusive, during the last decade, several independent works have proposed an alternative approach indicating that both hypotheses are valid. This was termed the dual hypothesis of photosystem II photodamage, and it postulates that both excess excitation and direct Mn photodamage operate at the same time, independently or in a synergic manner, depending on the type of sample, temperature, light spectrum, or other environmental stressors. In this mini-review, a brief summary of the contrasting hypotheses is presented, followed by recapitulation of key discoveries in the field of photosystem II photodamage of the last decade, and a synthesis of how these works support a full hybrid framework (operation of several mechanisms and their permutations) to explain PSII photodamage. All these are in recognition of Prof. Wah Soon Chow (the Australian National University), one of the key proposers of the dual hypothesis.

Singlet oxygen damages the function of Photosystem II in isolated thylakoids and in the green alga Chlorella sorokiniana

Singlet oxygen (¹O₂) is an important damaging agent, which is produced during illumination by the interaction of the triplet excited state pigment molecules with molecular oxygen. In cells of photosynthetic organisms ¹O₂ is formed primarily in chlorophyll containing complexes, and damages pigments, lipids, proteins and other cellular constituents in their environment. A useful approach to study the physiological role of ¹O₂ is the utilization of external photosensitizers. In the present study, we employed a multiwell plate-based screening method in combination with chlorophyll fluorescence imaging to characterize the effect of externally produced ¹O₂ on the photosynthetic activity of isolated thylakoid membranes and intact Chlorella sorokiniana cells. The results show that the external ¹O₂ produced by the photosensitization reactions of Rose Bengal damages Photosystem II both in isolated thylakoid membranes and in intact cells in a concentration dependent manner indicating that ¹O₂ plays a significant role in photodamage of Photosystem II.

Photosystem I in low light-grown leaves of Alocasia odora, a shade-tolerant plant, is resistant to fluctuating light-induced photoinhibition

When intact green leaves are exposed to the fluctuating light, in which high light (HL) and low light (LL) alternate, photosystem I (PSI) is readily damaged. This PSI inhibition is mostly alleviated by the addition of far-red (FR) light. Here, we grew Alocasia odora, a shade-tolerant species, at several light levels and examined their photosynthetic traits in relation to the fluctuating light-induced PSI inhibition. We found that, even in the absence of FR, PSI in LL-grown leaves was resistant to the fluctuating light. LL leaves showed higher chlorophyll (Chl) contents on leaf area basis, lower Chl a/b ratios, lower cytochrome f/P700 ratios, and lower PSII/PSI excitation ratios assessed by the 77 K fluorescence. Also, P700 in the HL phase of the fluctuating light was more oxidized. The results of the regression analyses of the PSI photoinhibition to these traits indicate that the lower electron flow rate to P700 and more excitation energy transfer to PSI protect PSI in LL-grown leaves. Both of these contribute oxidization of P700 to the efficient quencher form P700⁺. These features may be common in LL-grown shade-tolerant species, which are often exposed to strong sunflecks in their natural habitats.

Diverse Biosynthetic Pathways and Protective Functions against Environmental Stress of Antioxidants in Microalgae

Eukaryotic microalgae have been classified into several biological divisions and have evolutionarily acquired diverse morphologies, metabolisms, and life cycles. They are naturally exposed to environmental stresses that cause oxidative damage due to reactive oxygen species accumulation. To cope with environmental stresses, microalgae contain various antioxidants, including carotenoids, ascorbate (AsA), and glutathione (GSH). Carotenoids are hydrophobic pigments required for light harvesting, photoprotection, and phototaxis. AsA constitutes the AsA-GSH cycle together with GSH and is responsible for photooxidative stress defense. GSH contributes not only to ROS scavenging, but also to heavy metal detoxification and thiol-based redox regulation. The evolutionary diversity of microalgae influences the composition and biosynthetic pathways of these antioxidants. For example, α-carotene and its derivatives are specific to Chlorophyta, whereas diadinoxanthin and fucoxanthin are found in Heterokontophyta, Haptophyta, and Dinophyta. It has been suggested that AsA is biosynthesized via the plant pathway in Chlorophyta and Rhodophyta and via the Euglena pathway in Euglenophyta, Heterokontophyta, and Haptophyta. The GSH biosynthetic pathway is conserved in all biological kingdoms; however, Euglenophyta are able to synthesize an additional thiol antioxidant, trypanothione, using GSH as the substrate. In the present study, we reviewed and discussed the diversity of microalgal antioxidants, including recent findings.

The desert green algae Chlorella ohadii thrives at excessively high light intensities by exceptionally enhancing the mechanisms that protect photosynthesis from photoinhibition

Although light is the driving force of photosynthesis, excessive light can be harmful. One of the main processes that limits photosynthesis is photoinhibition, the process of light-induced photodamage. When the absorbed light exceeds the amount that is dissipated by photosynthetic electron flow and other processes, damaging radicals are formed that mostly inactivate photosystem II (PSII). Damaged PSII must be replaced by a newly repaired complex in order to preserve full photosynthetic activity. Chlorella ohadii is a green microalga, isolated from biological desert soil crusts, that thrives under extreme high light and is highly resistant to photoinhibition. Therefore, C. ohadii is an ideal model for studying the molecular mechanisms underlying protection against photoinhibition. Comparison of the thylakoids of C. ohadii cells that were grown under low light versus extreme high light intensities found that the alga employs all three known photoinhibition protection mechanisms: (i) massive reduction of the PSII antenna size; (ii) accumulation of protective carotenoids; and (iii) very rapid repair of photodamaged reaction center proteins. This work elucidated the molecular mechanisms of photoinhibition resistance in one of the most light-tolerant photosynthetic organisms, and shows how photoinhibition protection mechanisms evolved to marginal conditions, enabling photosynthesis-dependent life in severe habitats.

Trophic Transition Enhanced Biomass and Lipid Production of the Unicellular Green Alga Scenedesmus acuminatus

Microalgal heterotrophic cultivation is an emerging technology that can enable producing high cell-density algal cell cultures, which can be coupled with photoautotrophic cultivation for valuable chemicals such as lipids manufacturing. However, how the heterotrophically grown algal cells respond to the lipid-inducing conditions has not been fully elucidated so far. In this study, when the heterotrophically grown Scenedesmus acuminatus cells were subjected to the high light (HL) and nitrogen-limited (NL) conditions, both the biomass and lipid productivity were enhanced as compared to that of the photoautotrophically grown counterparts. The chlorophyll a fluorometry analysis showed that the Fv/Fm and Y(II) of the heterotrophically grown cells subjected to the HL and NL conditions was recovered to the maximum value of 0.75 and 0.43, respectively, much higher than those of the photoautotrophically grown cells under the same stress conditions. Transcriptomic analysis revealed that heterotrophically grown cells fully expressed the genes coding for the photosystems proteins, including the key photoprotective proteins D1, PsbS, light-harvesting-complex (LHC) I and LHC II. Meanwhile, downregulation of the carotenoid biosynthesis and upregulation of the glycolysis/gluconeogenesis, tricarboxylic acid (TCA) cycle and oxidative phosphorylation pathways were observed when the heterotrophically grown cells were subjected to the HL and N-limited conditions for lipid production. It was deduced that regulation of these pathways not only enhanced the light utilization but also provided the reducing power and ATP by which the biomass accumulation was significantly elevated. Besides, upregulation of the acetyl-CoA carboxylase/biotin carboxylase, digalactosyl diacylglycerol synthase and diacylglycerol acyltransferase 2 encoding genes may be attributable to the enhanced lipid production. Understanding the cellular responses during the trophic transition process could guide improvement of the strength of trophic transition enhancing microalgal biomass and lipid production.

Dynamic Changes in Protein-Membrane Association for Regulating Photosynthetic Electron Transport

Photosynthesis has to work efficiently in contrasting environments such as in shade and full sun. Rapid changes in light intensity and over-reduction of the photosynthetic electron transport chain cause production of reactive oxygen species, which can potentially damage the photosynthetic apparatus. Thus, to avoid such damage, photosynthetic electron transport is regulated on many levels, including light absorption in antenna, electron transfer reactions in the reaction centers, and consumption of ATP and NADPH in different metabolic pathways. Many regulatory mechanisms involve the movement of protein-pigment complexes within the thylakoid membrane. Furthermore, a certain number of chloroplast proteins exist in different oligomerization states, which temporally associate to the thylakoid membrane and modulate their activity. This review starts by giving a short overview of the lipid composition of the chloroplast membranes, followed by describing supercomplex formation in cyclic electron flow. Protein movements involved in the various mechanisms of non-photochemical quenching, including thermal dissipation, state transitions and the photosystem II damage–repair cycle are detailed. We highlight the importance of changes in the oligomerization state of VIPP and of the plastid terminal oxidase PTOX and discuss the factors that may be responsible for these changes. Photosynthesis-related protein movements and organization states of certain proteins all play a role in acclimation of the photosynthetic organism to the environment.

Connecting Chlorophyll Metabolism with Accumulation of the Photosynthetic Apparatus

Chlorophyll (Chl) is indispensable for photosynthesis. In association with Chl-binding proteins (CBPs), it is responsible for light absorption, excitation energy transfer, and charge separation within the photosynthetic complexes. By contrast, photoexcitation of free Chl and its metabolic intermediates generates hazardous reactive oxygen species (ROS). While antagonistic activities of Chl synthesis and catabolism have been mostly elucidated, the tight synchronization of these metabolic activities with the formation and dismantling of the photosynthetic complexes is poorly understood. Recently, a set of auxiliary factors were identified to adjust metabolic activities and provide accurate amounts of Chl for pigment-protein complexes. Here, we review current knowledge of post-translational coordination of Chl formation, breakdown, and turnover with the assembly and disassembly of various CBPs and highlight future research perspectives.

Over-expression of an electron transport protein OmcS provides sufficient NADH for D-lactate production in cyanobacterium

BACKGROUND

An efficient supply of reducing equivalent is essential for chemicals production by engineered microbes. In phototrophic microbes, the NADPH generated from photosynthesis is the dominant form of reducing equivalent. However, most dehydrogenases prefer to utilize NADH as a cofactor. Thus, sufficient NADH supply is crucial to produce dehydrogenase-derived chemicals in cyanobacteria. Photosynthetic electron is the sole energy source and excess electrons are wasted in the light reactions of photosynthesis.

RESULTS

Here we propose a novel strategy to direct the electrons to generate more ATP from light reactions to provide sufficient NADH for lactate production. To this end, we introduced an electron transport protein-encoding gene omcS into cyanobacterium Synechococcus elongatus UTEX 2973 and demonstrated that the introduced OmcS directs excess electrons from plastoquinone (PQ) to photosystem I (PSI) to stimulate cyclic electron transfer (CET). As a result, an approximately 30% increased intracellular ATP, 60% increased intracellular NADH concentrations and up to 60% increased biomass production with fourfold increased D-lactate production were achieved. Comparative transcriptome analysis showed upregulation of proteins involved in linear electron transfer (LET), CET, and downregulation of proteins involved in respiratory electron transfer (RET), giving hints to understand the increased levels of ATP and NADH.

CONCLUSIONS

This strategy provides a novel orthologous way to improve photosynthesis via enhancing CET and supply sufficient NADH for the photosynthetic production of chemicals.

Chloroplast lipid biosynthesis is fine-tuned to thylakoid membrane remodeling during light acclimation

Reprogramming metabolism, in addition to modifying the structure and function of the photosynthetic machinery, is crucial for plant acclimation to changing light conditions. One of the key acclimatory responses involves reorganization of the photosynthetic membrane system including changes in thylakoid stacking. Glycerolipids are the main structural component of thylakoids and their synthesis involves two main pathways localized in the plastid and the endoplasmic reticulum (ER); however, the role of lipid metabolism in light acclimation remains poorly understood. We found that fatty acid synthesis, membrane lipid content, the plastid lipid biosynthetic pathway activity, and the degree of thylakoid stacking were significantly higher in plants grown under low light compared with plants grown under normal light. Plants grown under high light, on the other hand, showed a lower rate of fatty acid synthesis, a higher fatty acid flux through the ER pathway, higher triacylglycerol content, and thylakoid membrane unstacking. We additionally demonstrated that changes in rates of fatty acid synthesis under different growth light conditions are due to post-translational regulation of the plastidic acetyl-CoA carboxylase activity. Furthermore, Arabidopsis mutants defective in one of the two glycerolipid biosynthetic pathways displayed altered growth patterns and a severely reduced ability to remodel thylakoid architecture, particularly under high light. Overall, this study reveals how plants fine-tune fatty acid and glycerolipid biosynthesis to cellular metabolic needs in response to long-term changes in light conditions, highlighting the importance of lipid metabolism in light acclimation.

Perspectives on improving light distribution and light use efficiency in crop canopies

Plant stands in nature differ markedly from most seen in modern agriculture. In a dense mixed stand, plants must vie for resources, including light, for greater survival and fitness. Competitive advantages over surrounding plants improve fitness of the individual, thus maintaining the competitive traits in the gene pool. In contrast, monoculture crop production strives to increase output at the stand level and thus benefits from cooperation to increase yield of the community. In choosing plants with higher yields to propagate and grow for food, humans may have inadvertently selected the best competitors rather than the best cooperators. Here, we discuss how this selection for competitiveness has led to overinvestment in characteristics that increase light interception and, consequently, sub-optimal light use efficiency in crop fields that constrains yield improvement. Decades of crop canopy modeling research have provided potential strategies for improving light distribution in crop canopies, and we review the current progress of these strategies, including balancing light distribution through reducing pigment concentration. Based on recent research revealing red-shifted photosynthetic pigments in algae and photosynthetic bacteria, we also discuss potential strategies for optimizing light interception and use through introducing alternative pigment types in crops. These strategies for improving light distribution and expanding the wavelengths of light beyond those traditionally defined for photosynthesis in plant canopies may have large implications for improving crop yield and closing the yield gap.

Evidences for a Nutritional Role of Iodine in Plants

Little is known about the role of iodine in plant physiology. We evaluated the impact of low concentrations of iodine on the phenotype, transcriptome and proteome of Arabidopsis thaliana. Our experiments showed that removal of iodine from the nutrition solution compromises plant growth, and restoring it in micromolar concentrations is beneficial for biomass accumulation and leads to early flowering. In addition, iodine treatments specifically regulate the expression of several genes, mostly involved in the plant defence response, suggesting that iodine may protect against both biotic and abiotic stress. Finally, we demonstrated iodine organification in proteins. Our bioinformatic analysis of proteomic data revealed that iodinated proteins identified in the shoots are mainly associated with the chloroplast and are functionally involved in photosynthetic processes, whereas those in the roots mostly belong and/or are related to the action of various peroxidases. These results suggest the functional involvement of iodine in plant nutrition.

Transcription initiation as a control point in plastid gene expression

The extensive processing and protein-assisted stabilization of transcripts have been taken as evidence for a viewpoint that the control of gene expression had shifted entirely in evolution from transcriptional in the bacterial endosymbiont to posttranscriptional in the plastid. This suggestion is however at odds with many observations on plastid gene transcription. Chloroplasts of flowering plants and mosses contain two or more RNA polymerases with distinct promoter preference and division of labor for the coordinated synthesis of plastid RNAs. Plant and algal plastids further possess multiple nonredundant sigma factors that function as transcription initiation factors. The controlled accumulation of plastid sigma factors and modification of their activity by sigma-binding proteins and phosphorylation constitute additional transcriptional regulatory strategies. Plant and algal plastids also contain dedicated one- or two-component transcriptional regulators. Transcription initiation thus continues to form a critical control point at which varied developmental and environmental signals intersect with plastid gene expression.

Light-Dependent Translation Change of Arabidopsis psbA Correlates with RNA Structure Alterations at the Translation Initiation Region

mRNA secondary structure influences translation. Proteins that modulate the mRNA secondary structure around the translation initiation region may regulate translation in plastids. To test this hypothesis, we exposed Arabidopsis thaliana to high light, which induces translation of psbA mRNA encoding the D1 subunit of photosystem II. We assayed translation by ribosome profiling and applied two complementary methods to analyze in vivo RNA secondary structure: DMS-MaPseq and SHAPE-seq. We detected increased accessibility of the translation initiation region of psbA after high light treatment, likely contributing to the observed increase in translation by facilitating translation initiation. Furthermore, we identified the footprint of a putative regulatory protein in the 5' UTR of psbA at a position where occlusion of the nucleotide sequence would cause the structure of the translation initiation region to open up, thereby facilitating ribosome access. Moreover, we show that other plastid genes with weak Shine-Dalgarno sequences (SD) are likely to exhibit psbA-like regulation, while those with strong SDs do not. This supports the idea that changes in mRNA secondary structure might represent a general mechanism for translational regulation of psbA and other plastid genes.

Untargeted Metabolomics Unveil Changes in Autotrophic and Mixotrophic Galdieria sulphuraria Exposed to High-Light Intensity

The thermoacidophilic red alga Galdieria sulphuraria has been optimizing a photosynthetic system for low-light conditions over billions of years, thriving in hot and acidic endolithic habitats. The growth of G. sulphuraria in the laboratory is very much dependent on light and substrate supply. Here, higher cell densities in G. sulphuraria under high-light conditions were obtained, although reductions in photosynthetic pigments were observed, which indicated this alga might be able to relieve the effects caused by photoinhibition. We further describe an extensive untargeted metabolomics study to reveal metabolic changes in autotrophic and mixotrophic G. sulphuraria grown under high and low light intensities. The up-modulation of bilayer lipids, that help generate better-ordered lipid domains (e.g., ergosterol) and keep optimal membrane thickness and fluidity, were observed under high-light exposure. Moreover, high-light conditions induced changes in amino acids, amines, and amide metabolism. Compared with the autotrophic algae, higher accumulations of osmoprotectant sugars and sugar alcohols were recorded in the mixotrophic G. sulphuraria. This response can be interpreted as a measure to cope with stress due to the high concentration of organic carbon sources. Our results indicate how G. sulphuraria can modulate its metabolome to maintain energetic balance and minimize harmful effects under changing environments.

The Arabidopsis NOT4A E3 ligase promotes PGR3 expression and regulates chloroplast translation

Chloroplast function requires the coordinated action of nuclear- and chloroplast-derived proteins, including several hundred nuclear-encoded pentatricopeptide repeat (PPR) proteins that regulate plastid mRNA metabolism. Despite their large number and importance, regulatory mechanisms controlling PPR expression are poorly understood. Here we show that the Arabidopsis NOT4A ubiquitin-ligase positively regulates the expression of PROTON GRADIENT REGULATION 3 (PGR3), a PPR protein required for translating several thylakoid-localised photosynthetic components and ribosome subunits within chloroplasts. Loss of NOT4A function leads to a strong depletion of cytochrome b₆f and NAD(P)H dehydrogenase (NDH) complexes, as well as plastid 30 S ribosomes, which reduces mRNA translation and photosynthetic capacity, causing pale-yellow and slow-growth phenotypes. Quantitative transcriptome and proteome analysis of the not4a mutant reveal it lacks PGR3 expression, and that its molecular defects resemble those of a pgr3 mutant. Furthermore, we show that normal plastid function is restored to not4a through transgenic PGR3 expression. Our work identifies NOT4A as crucial for ensuring robust photosynthetic function during development and stress-response, through promoting PGR3 production and chloroplast translation.

PSB33 protein sustains photosystem II in plant chloroplasts under UV-A light

Plants can quickly and dynamically respond to spectral and intensity variations of the incident light. These responses include activation of developmental processes, morphological changes, and photosynthetic acclimation that ensure optimal energy conversion and minimal photoinhibition. Plant adaptation and acclimation to environmental changes have been extensively studied, but many details surrounding these processes remain elusive. The photosystem II (PSII)-associated protein PSB33 plays a fundamental role in sustaining PSII as well as in the regulation of the light antenna in fluctuating light. We investigated how PSB33 knock-out Arabidopsis plants perform under different light qualities. psb33 plants displayed a reduction of 88% of total fresh weight compared to wild type plants when cultivated at the boundary of UV-A and blue light. The sensitivity towards UV-A light was associated with a lower abundance of PSII proteins, which reduces psb33 plants' capacity for photosynthesis. The UV-A phenotype was found to be linked to altered phytohormone status and changed thylakoid ultrastructure. Our results collectively show that PSB33 is involved in a UV-A light-mediated mechanism to maintain a functional PSII pool in the chloroplast.

Fast Diffusion of the Unassembled PetC1-GFP Protein in the Cyanobacterial Thylakoid Membrane

Biological membranes were originally described as a fluid mosaic with uniform distribution of proteins and lipids. Later, heterogeneous membrane areas were found in many membrane systems including cyanobacterial thylakoids. In fact, cyanobacterial pigment-protein complexes (photosystems, phycobilisomes) form a heterogeneous mosaic of thylakoid membrane microdomains (MDs) restricting protein mobility. The trafficking of membrane proteins is one of the key factors for long-term survival under stress conditions, for instance during exposure to photoinhibitory light conditions. However, the mobility of unbound 'free' proteins in thylakoid membrane is poorly characterized. In this work, we assessed the maximal diffusional ability of a small, unbound thylakoid membrane protein by semi-single molecule FCS (fluorescence correlation spectroscopy) method in the cyanobacterium Synechocystis sp. PCC6803. We utilized a GFP-tagged variant of the cytochrome b₆f subunit PetC1 (PetC1-GFP), which was not assembled in the b₆f complex due to the presence of the tag. Subsequent FCS measurements have identified a very fast diffusion of the PetC1-GFP protein in the thylakoid membrane (D = 0.14 - 2.95 µm²s^-1). This means that the mobility of PetC1-GFP was comparable with that of free lipids and was 50-500 times higher in comparison to the mobility of proteins (e.g., IsiA, LHCII-light-harvesting complexes of PSII) naturally associated with larger thylakoid membrane complexes like photosystems. Our results thus demonstrate the ability of free thylakoid-membrane proteins to move very fast, revealing the crucial role of protein-protein interactions in the mobility restrictions for large thylakoid protein complexes.

Effect of exogenously-applied abscisic acid, putrescine and hydrogen peroxide on drought tolerance of barley

The objective of this study was to identify the effect of abscisic acid (ABA), putrescine (Put) and hydrogen peroxide (H2O2) foliar pre-treatment on drought tolerance of barley. Despite water limitation, ABA-sprayed plants preserved increased water content, photosynthetic efficiency of PSII (ΦPSII) and CO2 assimilation rate (Pn) compared to untreated stressed plants. The ABA-treated plants presented also the lowest rate of lipid peroxidation (MDA), lowered the rate of PSII primary acceptor reduction (1 – qP) and increased the yield of regulated energy dissipation (NPQ) with higher accumulation of PGRL1 (PROTON GRADIENT REGULATION LIKE1) protein. These plants preserved a similar level of photochemical efficiency and the rate of electron transport of PSII (ETRII) to the well-watered samples. The significantly less pronounced response was observed in Put-sprayed samples under drought. Additionally, the combined effects of drought and H2O2 application increased the 1 – qP and quantum yield of non-regulated energy dissipation in PSII (ΦNO) and reduced the accumulation of Rubisco activase (RCA). In conclusion, ABA foliar application allowed to balance water retention and preserve antioxidant capacity resulting in efficient photosynthesis and the restricted risk of oxidative damage under drought. Neither hydrogen peroxide nor putrescine has been able to ameliorate drought stress as effectively as ABA.

LHCSR3-Type NPQ Prevents Photoinhibition and Slowed Growth under Fluctuating Light in Chlamydomonas reinhardtii

Natural light intensities can rise several orders of magnitude over subsecond time spans, posing a major challenge for photosynthesis. Fluctuating light tolerance in the green alga Chlamydomonas reinhardtii requires alternative electron pathways, but the role of nonphotochemical quenching (NPQ) is not known. Here, fluctuating light (10 min actinic light followed by 10 min darkness) led to significant increase in NPQ/qE-related proteins, LHCSR1 and LHCSR3, relative to constant light of the same subsaturating or saturating intensity. Elevated levels of LHCSR1/3 increased the ability of cells to safely dissipate excess light energy to heat (i.e., qE-type NPQ) during dark to light transition, as measured with chlorophyll fluorescence. The low qE phenotype of the npq4 mutant, which is unable to produce LHCSR3, was abolished under fluctuating light, showing that LHCSR1 alone enables very high levels of qE. Photosystem (PS) levels were also affected by light treatments; constant light led to lower PsbA levels and F_v/F_m values, while fluctuating light led to lower PsaA and maximum P700⁺ levels, indicating that constant and fluctuating light induced PSII and PSI photoinhibition, respectively. Under fluctuating light, npq4 suffered more PSI photoinhibition and significantly slower growth rates than parental wild type, whereas npq1 and npq2 mutants affected in xanthophyll carotenoid compositions had identical growth under fluctuating and constant light. Overall, LHCSR3 rather than total qE capacity or zeaxanthin is shown to be important in C. reinhardtii in tolerating fluctuating light, potentially via preventing PSI photoinhibition.

Blue Light Mediates Chloroplast Avoidance and Enhances Photoprotection of Vanilla Orchid

Vanilla orchid, which is well-known for its flavor and fragrance, is cultivated in tropical and subtropical regions. This shade-loving plant is very sensitive to high irradiance. In this study, we show that vanilla chloroplasts started to have avoidance movement when blue light (BL) was higher than 20 μmol m-2s-1 and significant avoidance movement was observed under BL irradiation at 100 μmol m-2s-1 (BL100). The light response curve indicated that when vanilla was exposed to 1000 μmol m-2s-1, the electron transport rate (ETR) and photochemical quenching of fluorescence (qP) were significantly reduced to a negligible amount. We found that if a vanilla orchid was irradiated with BL100 for 12 days, it acquired BL-acclimation. Chloroplasts moved to the side of cells in order to reduce light-harvesting antenna size, and chloroplast photodamage was eliminated. Therefore, BL-acclimation enhanced vanilla orchid growth and tolerance to moderate (500 μmol m-2s-1) and high light (1000 μmol m-2s-1) stress conditions. It was found that under high irradiation, BL-acclimatized vanilla maintained higher ETR and qP capacity than the control without BL-acclimation. BL-acclimation induced antioxidant enzyme activities, reduced ROS accumulation, and accumulated more carbohydrates. Moreover, BL-acclimatized orchids upregulated photosystem-II-associated marker genes (D1 and PetC), Rubisco and PEPC transcripts and sustained expression levels thereof, and also maximized the photosynthesis rate. Consequently, BL-acclimatized orchids had higher biomass. In short, this study found that acclimating vanilla orchid with BL before transplantation to the field might eliminate photoinhibition and enhance vanilla growth and production.

A study of light-induced stomatal response in Arabidopsis using thermal imaging

Thermal imaging was used to study the early stage response to light-induced heating of Arabidopsis thaliana leaves. Time-series thermograms provided a spatial and temporal characterization of temperature changes in Arabidopsis wild type and the ost1-2 mutant rosettes exposed to excessive illumination. The initial response to high light, defined by the exponential increase in leaf temperature of ost1-2 gave an increased thermal time constant compared to wild type plants. The inability to regulate stomata in ost1-2 resulted in enhanced stomatal conductance and transpiration rate. Under strong irradiation, a significant decline in the efficiency of photosystem II was observed. This study evaluates infrared thermography kinetics and determines thermal time constants in particular, as an early and rapid method for diagnosing the prime indicators of light stress in plants under excessive light conditions.

Label-free comparative proteomic and physiological analysis provides insight into leaf color variation of the golden-yellow leaf mutant of Lagerstroemia indica

GL1 is a golden-yellow leaf mutant that cultivated from natural bud-mutation of Lagerstroemia indica and has a very low level of photosynthetic pigment under sunlight. GL1 can gradually increase its pigment content and turn into pale-green leaf when shading under sunshade net (referred as Re-GL1). The mechanisms that cause leaf color variation are complicated and are not still unclear. Here, we have used a label-free comparative proteomics to investigate differences in proteins abundance and analyze the specific biological process associated with mechanisms of leaf color variation in GL1. A total of 245 and 160 proteins with different abundance were identified in GL1 vs WT and GL1 vs Re-GL1, respectively. Functional classification analysis revealed that the proteins with different abundance mainly related to photosynthesis, heat shock proteins, ribosome proteins, and oxidation-reduction. The proteins that the most significantly contributed to leaf color variation were photosynthetic proteins of PSII and PSI, which directly related to photooxidation and determined the photosynthetic performance of photosystem. Further analysis demonstrated that low jasmonic acid content was needed to golden-yellow leaf GL1. These findings lay a solid foundation for future studies into the molecular mechanisms that underlie leaf color formation of GL1. BIOLOGICAL SIGNIFICANCE: The natural bud mutant GL1 of L. indica is an example through changing leaf color to cope with complex environment. However, the molecular mechanism of leaf color variation are largely elusive. The proteins with different abundance identified from a label-free comparative proteomics revealed a range of biological processes associated with leaf color variation, including photosynthesis, oxidation-reduction and jasmonic acid signaling. The photooxidation and low level of jasmonic acid played a primary role in GL1 adaptation in golden-yellow leaf. These findings provide possible pathway or signal for the molecular mechanism associated with leaf color formation and as a valuable resource for signal transaction of chloroplast.

The role of chloroplast protein remodeling in stress responses and shaping of the plant peptidome

In addition to photosynthesis, chloroplasts perform a variety of important cellular functions in the plant cell, which can, for example, regulate plant responses to abiotic and biotic stress conditions. Under stress, intensive chloroplast protein remodeling and degradation can occur, releasing large numbers of endogenous peptides. These protein-derived peptides can be found intracellularly, but also in the plant secretome. Although the pathways of chloroplast protein degradation and the types of chloroplast proteases implicated in this process have received much attention, the role of the resulting peptides is less well understood. In this review we summarize the data on peptide generation processes during the remodeling of the chloroplast proteome under stress conditions and discuss the mechanisms leading to these changes. We also review the experimental evidence which supports the concept that peptides derived from chloroplast proteins can function as regulators of plant responses to (a)biotic stresses.