Differential FeS cluster photodamage plays a critical role in regulating excess electron flow through photosystem I.

The photosynthetic electron flux from photosystem I (PSI) is mainly directed to NADP+ and CO2 fixation, but a fraction is always shared between alternative and cyclic electron transport. Although the electron transfer from P700 to ferredoxin, via phylloquinone and the FeSX, FeSB and FeSA clusters, is well characterized, the regulatory role of these redox intermediates in the delivery of electrons from PSI to NADP+, alternative and cyclic electron transport under environmental stress remains elusive. Here we provide evidence for sequential damage to PSI FeS clusters under high light and subsequent slow recovery under low light in Arabidopsis thaliana. Wild-type plants showed 10-35% photodamage to their FeSA/B clusters with increasing high-light duration, without much effect on P700 oxidation capacity, FeSX function or CO2 fixation rate, and without additional oxygen consumption (O2 photoreduction). Parallel FeSA/B cluster damage in the pgr5 mutant was more pronounced at 50-85%, probably due to weak photosynthetic control and low non-photochemical quenching. Such severe electron pressure on PSI was also shown to damage the FeSX clusters, with a concomitant decrease in P700 oxidation capacity and a decrease in thylakoid-bound ferredoxin in the pgr5 mutant. The results from wild-type and pgr5 plants reveal controlled damage of PSI FeS clusters under high light. In wild-type plants, this favours electron transport to linear over alternative pathways by intact PSI centres, thereby preventing reactive oxygen species production and probably promoting harmless charge recombination between P700+ and FeSX- as long as the majority of FeSA/B clusters remain functional.

Phytic Acid Maintains Peripheral Neuron Integrity and Enhances Survivability against Platinum-Induced Degeneration via Reducing Reactive Oxygen Species and Enhancing Mitochondrial Membrane Potential.

Phytic acid (PA) has been reported to possess anti-inflammatory and antioxidant properties that are critical for neuroprotection in neuronal disorders. This raises the question of whether PA can effectively protect sensory neurons against chemotherapy-induced peripheral neuropathy (CIPN). Peripheral neuropathy is a dose-limiting side effect of chemotherapy treatment often characterized by severe and abnormal pain in hands and feet resulting from peripheral nerve degeneration. Currently, there are no effective treatments available that can prevent or cure peripheral neuropathies other than symptomatic management. Herein, we aim to demonstrate the neuroprotective effects of PA against the neurodegeneration induced by the chemotherapeutics cisplatin (CDDP) and oxaliplatin. Further aims of this study are to provide the proposed mechanism of PA-mediated neuroprotection. The neuronal protection and survivability against CDDP were characterized by axon length measurements and cell body counting of the dorsal root ganglia (DRG) neurons. A cellular phenotype study was conducted microscopically. Intracellular reactive oxygen species (ROS) was estimated by fluorogenic probe dichlorofluorescein. Likewise, mitochondrial membrane potential (MMP) was assessed by fluorescent MitoTracker Orange CMTMRos. Similarly, the mitochondria-localized superoxide anion radical in response to CDDP with and without PA was evaluated. The culture of primary DRG neurons with CDDP reduced axon length and overall neuronal survival. However, cotreatment with PA demonstrated that axons were completely protected and showed increased stability up to the 45-day test duration, which is comparable to samples treated with PA alone and control. Notably, PA treatment scavenged the mitochondria-specific superoxide radicals and overall intracellular ROS that were largely induced by CDDP and simultaneously restored MMP. These results are credited to the underlying neuroprotection of PA in a platinum-treated condition. The results also exhibited that PA had a synergistic anticancer effect with CDDP in ovarian cancer in vitro models. For the first time, PA's potency against CDDP-induced PN is demonstrated systematically. The overall findings of this study suggest the application of PA in CIPN prevention and therapeutic purposes.

Electrical stimulation enhances mitochondrial trafficking as a neuroprotective mechanism against chemotherapy-induced peripheral neuropathy.

Electrical stimulation (ESTIM) has shown to be an effective symptomatic treatment to treat pain associated with peripheral nerve damage. However, the neuroprotective mechanism of ESTIM on peripheral neuropathies is still unknown. In this study, we identified that ESTIM has the ability to enhance mitochondrial trafficking as a neuroprotective mechanism against chemotherapy-induced peripheral neuropathies (CIPNs). CIPN is a debilitating and painful sequalae of anti-cancer chemotherapy treatment which results in degeneration of peripheral nerves. Mitochondrial dynamics were analyzed within axons in response to two different antineoplastic mechanisms by chemotherapy drug treatments paclitaxel and oxaliplatin in vitro. Mitochondrial trafficking response to chemotherapy drug treatment was observed to decrease in conjunction with degeneration of distal axons. Using low-frequency ESTIM, we observed enhanced mitochondrial trafficking to be a neuroprotective mechanism against CIPN. This study confirms ESTIM enhances regeneration of peripheral nerves by increased mitochondrial trafficking.

Mitochondrial trafficking as a protective mechanism against chemotherapy drug-induced peripheral neuropathy: Identifying the key site of action.

AIMS: Chemotherapy induced peripheral neuropathy (CIPN) is a common side effect seen in patients who have undergone most chemotherapy treatments to which there are currently no treatment methods. CIPN has been shown to cause axonal degeneration leading to Peripheral Neuropathy (PN), which can lead to major dosage reduction and may prevent further chemotherapy treatment due to oftentimes debilitating pain. Previously, we have determined the site-specific action of Paclitaxel (PTX), a microtubule targeting agent, as well as the neuroprotective effect of Fluocinolone Acetonide (FA) against Paclitaxel Induced Peripheral Neuropathy (PIPN). MAIN METHODS: Mitochondrial trafficking analysis was determined for all sample sets, wherein FA showed enhanced anterograde (axonal) mitochondrial trafficking leading to neuroprotective effects for all samples. KEY FINDINGS: Using this system, we demonstrate that PTX, Monomethyl auristatin E (MMAE), and Vincristine (VCR), are toxic at clinically prescribed levels when treated focally to axons. However, Cisplatin (CDDP) was determined to have a higher toxicity when treated to cell bodies. Although having different targeting mechanisms, the administration of FA was determined to have a significant neuroprotective effect for against all chemotherapy drugs tested. SIGNIFICANCE: This study identifies key insights regarding site of action and neuroprotective strategies to further development as potential therapeutics against CIPN. FA was treated alongside each chemotherapy drug to identify the neuroprotective effect against CIPN, where FA was found to be neuroprotective for all drugs tested. This study found that treatment with FA led to an enhancement in the anterograde movement of mitochondria based on fluorescent imaging.

A review on current brain organoid technologies from a biomedical engineering perspective.

Brain organoids are 3D cytoarchitectures resembling the embryonic human brain. This review focuses on current advancements in biomedical engineering methods to develop organoids such as pluripotent stem cells assemblies, quickly aggregated floating culture, hydrogel suspension, microfluidic systems (both photolithography and 3D printing), and brain organoids-on-a-chip. These methods have the potential to create a large impact on neurological disorder studies by creating a model of the human brain investigating pathogenesis and drug screening for individual patients. 3D brain organoid cultures mimic not only features of patients' unknown drug reactions, but also early human brain development at cellular, structural, and functional levels. The challenge of current brain organoids lies in the formation of distinct cortical neuron layers, gyrification, and the establishment of complex neuronal circuitry, as they are critically specialized, developmental aspects. Furthermore, recent advances such as vascularization and genome engineering are in development to overcome the barrier of neuronal complexity. Future technology of brain organoids is needed to improve tissue cross-communication, body axis simulation, cell patterning signals, and spatial-temporal control of differentiation, as engineering methods discussed in this review are rapidly evolving.

Fluocinolone Acetonide Enhances Anterograde Mitochondria Trafficking and Promotes Neuroprotection against Paclitaxel-Induced Peripheral Neuropathy.

Paclitaxel (PTX)-induced peripheral neuropathy (PIPN) is a debilitating health condition which is a result of degeneration of peripheral nerves found in extremities. Currently, there are no established treatment methods that can prevent or protect from PIPN. Fluocinolone acetonide (FA) has been recently identified as a potential candidate for protection from PIPN. However, the fundamental mechanism of action is still unknown. In this study, we showed that enhanced anterograde mitochondrial movement in dorsal root ganglion (DRG) cells has a major role in FA-mediated neuroprotection in PIPN. In this study, cells were treated with PTX or FA along with their combination followed by mitochondrial fluorescence staining. Somal (proximal) and axonal (distal) mitochondria were selectively stained using a microfluidic compartmentalized chamber with different MitoTrackers blue and red, respectively, which we termed, the two-color staining approach. Results revealed that axons were protected from degeneration by the PTX effect when treated along with FA. PTX exposure alone resulted in low mitochondrial mobility in DRG cells. However, cotreatment with PTX and FA showed significant enhancement of anterograde trafficking of somal (proximal) mitochondria to distal axons. Similarly, cotreatment with FA restored mitochondrial mobility significantly. Overall, this study affirms that increasing mitochondrial recruitment into the axon by cotreatment with FA can be a worthwhile strategy to protect or prevent PIPN. The proposed two-color staining approach can be extended to study trafficking for other neuron-specific subcellular organelles.

The Encapsulation of Bioactive Plant Extracts into the Cellulose Microfiber Isolated from G. optiva Species for Biomedical Applications.

Agricultural waste-based cellulose fibers have gained significant interest for a myriad of applications. Grewia optiva (G. optiva), a plant species, has been widely used for feeding animals, and the small branches' bark is used for making rope. Herein, we have extracted cellulose fibers from the bark of G. optiva species via chemical treatments (including an alkaline treatment and bleaching). The gravimetric analysis revealed that the bark of G. Optiva contains cellulose (63.13%), hemicellulose (13.52%), lignin (15.13%), and wax (2.8%). Cellulose microfibre (CMF) has been synthesized from raw fibre via chemical treatment methods. The obtained cellulose fibers were crosslinked and employed as the matrix to encapsulate the bioactive plant extracts derived from the root of Catharanthus roseus (C. roseus). The microscopic images, XRD, FTIR, and antibacterial/antioxidant activity confirmed the encapsulation of natural extracts in the cellulose microfiber. The microscopic images revealed that the encapsulation of the natural extracts slightly increased the fiber's diameter. The XRD pattern showed that the extracted cellulose microfiber had an average crystalline size of 2.53 nm with a crystalline index of 30.4% compared to the crystalline size of 2.49 nm with a crystalline index of 27.99% for the plant extract incorporated membrane. The water uptake efficiency of the synthesized membrane increased up to 250%. The antimicrobial activity of the composite (the CMF-E membrane) was studied via the zone inhibition against gram-positive and gram-negative bacteria, and the result indicated high antibacterial activity. This work highlighted G. optiva-derived cellulose microfiber as an optimum substrate for antimicrobial scaffolds. In addition, this paper first reports the antimicrobial/antioxidant behavior of the composite membrane of the C. roseus extract blended in the G. optiva microfiber. This work revealed the potential applications of CMF-E membranes for wound healing scaffolds.

A Review on the Technological Advances and Future Perspectives of Axon Guidance and Regeneration in Peripheral Nerve Repair.

Despite a significant advance in the pathophysiological understanding of peripheral nerve damage, the successful treatment of large nerve defects remains an unmet medical need. In this article, axon growth guidance for peripheral nerve regeneration was systematically reviewed and discussed mainly from the engineering perspective. In addition, the common approaches to surgery, bioengineering approaches to emerging technologies such as optogenetic stimulation and magnetic stimulation for functional recovery were discussed, along with their pros and cons. Additionally, clear future perspectives of axon guidance and nerve regeneration were addressed.

Polypyrrole Nanotunnels with Luminal and Abluminal Layered Double Hydroxide Nanosheets Grown on a Carbon Cloth for Energy Storage Applications.

The structural design of transition metal-based electrode materials with gigantic energy storage capabilities is a crucial task. In this work, we report an assembly of thin layered double hydroxide (LDH) nanosheets arrayed throughout the luminal and abluminal parts of polypyrrole tunnels fastened onto both sides of a carbon cloth as a battery-type energy storage system. Electron microscopy images reveal that the resulting electrode (NiCo-LDH@H-PPy@CC, where H-PPy@CC represents carbon cloth-supported hollow polypyrrole fibers) is constructed by combining luminal and abluminal NiCo-LDH nanosheets onto a long polypyrrole tunnel on a carbon cloth. The primary sample shows an excellent specific capacity of 149.16 mAh g-1 at 1.0 mA cm-2, a remarkable rate capability of 80.45%, and comprehensive cyclic stability (93.4%). The improved performance is mainly attributed to the strategic organization of the electrode materials with superior Brunauer-Emmett-Teller (BET) surface area and conductivity. Moreover, an asymmetric supercapacitor device assembled with NiCo-LDH@H-PPy@CC and vanadium phosphate-incorporated carbon nanofiber (VPO@CNFs900) electrodes contributes a specific energy density of 32.42 Wh kg-1 at 3 mA cm-2 with a specific power density of 359.16 W kg-1. When the current density is increased by 6-fold, the specific power density reaches 1999.89 W kg-1 at a specific energy density of 20.06 Wh kg-1. This is a simple, cost-effective, and convenient synthetic strategy for the synthesis of porous nanosheet arrays assimilated into hollow fiber architectures, which can illuminate the ideal approach for the fabrication of novel materials with an immense potential for energy storage.

Characteristics and outcomes of hospitalized adults with COVID-19 in Nepal: a multicenter, prospective cohort study.

INTRODUCTION: There is limited data on clinical course and outcomes of hospitalized adults with COVID-19 in Nepal. Thus, it is imperative to characterize the features of this disease in the domestic context. METHODOLOGY: We identified all adult patients with laboratory-confirmed COVID-19 admitted to five different hospitals in Nepal from June 15 to July 15, 2020. We collected epidemiological, socio-cultural and clinicopathologic data, and stratified the patients based on their symptom status. RESULTS: The study included 220 patients with an overall median age of 31.5 (25-37) years, and 181 (82.3%) were males. 159 (72.3%) were asymptomatic, and 163 (74.1%) were imported cases. Of 217 patients with the available data, 110 (50.7%) reported their annual household income less than 2000 US dollars, and 122 (56.2%) practiced Pranayama (yogic rhythmic breathing techniques) regularly. Eight patients (3.6%) required supplemental oxygen and two patients (0.9%) died. None of the patients who practiced Pranayama regularly required supplemental oxygen. Compared to asymptomatic patients, symptomatic patients had greater proportion of females (31.1% vs. 12.6%, p = 0.001), imported cases (85.2% vs. 69.8%, p = 0.02), illiterates (26.8% vs. 12.1%, p = 0.01), alcohol users (43.3% vs. 24.5%, p = 0.01), and had higher platelet count (253×109/L vs. 185×109/L, p = 0.02). CONCLUSIONS: Most cases were imported, asymptomatic young males, with very few deaths. Pranayama practice was associated with protection against severe COVID-19, but more data is needed to substantiate this. The association of platelets count with symptom status in the Nepalese population needs further exploration.

True oxygen reduction capacity during photosynthetic electron transfer in thylakoids and intact leaves.

Reactive oxygen species (ROS) are generated in electron transport processes of living organisms in oxygenic environments. Chloroplasts are plant bioenergetics hubs where imbalances between photosynthetic inputs and outputs drive ROS generation upon changing environmental conditions. Plants have harnessed various site-specific thylakoid membrane ROS products into environmental sensory signals. Our current understanding of ROS production in thylakoids suggests that oxygen (O2) reduction takes place at numerous components of the photosynthetic electron transfer chain (PETC). To refine models of site-specific O2 reduction capacity of various PETC components in isolated thylakoids of Arabidopsis thaliana, we quantified the stoichiometry of oxygen production and consumption reactions associated with hydrogen peroxide (H2O2) accumulation using membrane inlet mass spectrometry and specific inhibitors. Combined with P700 spectroscopy and electron paramagnetic resonance spin trapping, we demonstrate that electron flow to photosystem I (PSI) is essential for H2O2 accumulation during the photosynthetic linear electron transport process. Further leaf disc measurements provided clues that H2O2 from PETC has a potential of increasing mitochondrial respiration and CO2 release. Based on gas exchange analyses in control, site-specific inhibitor-, methyl viologen-, and catalase-treated thylakoids, we provide compelling evidence of no contribution of plastoquinone pool or cytochrome b6f to chloroplastic H2O2 accumulation. The putative production of H2O2 in any PETC location other than PSI is rapidly quenched and therefore cannot function in H2O2 translocation to another cellular location or in signaling.

Integrating the Essence of a Metal-Organic Framework with Electrospinning: A New Approach for Making a Metal Nanoparticle Confined N-Doped Carbon Nanotubes/Porous Carbon Nanofibrous Membrane for Energy Storage and Conversion.

The fabrication of an economic and efficient multifunctional advanced nanomaterial with a rational composition and configuration by a facile methodology is a crucial challenge. Herein, we are the first to report the growth of Co nanoparticle-integrated nitrogen-doped carbon nanotubes (N-CNTs) on porous carbon nanofibers by simply heating in the situ-developed metal-organic framework (MOF)-based electrospun nanofibrous membrane with no need for an external supply of any additional precursors and reducing gases. The long and entangled N-CNTs originating from highly porous and graphitic carbon nanofibers offer good flexibility, large surface area, high porosity, high conductivity, the homogeneous incorporation of heteroatoms and metallic constituents, and an abundant exposure of active nanocatalytic sites. The as-developed nanoassembly demonstrates attractive characteristics for electrocatalytic hydrogen and oxygen evolution reactions and electrochemical energy storage. This strategy of integrating the essence of an MOF with electrospinning offers a new, direct, and cost-effective approach for making N-doped CNT-based multifunctional membranes.

Temperature-controlled in situ synthesized carbon nanotube-protected vanadium phosphate particle-anchored electrospun carbon nanofibers for high energy density symmetric supercapacitors.

Designing a novel composite material with hierarchical nanostructures as a negative electrode material with high capacitance and outstanding stability is challenging. To this end, we synthesized carbon nanotubes (CNTs)-protected vanadium phosphate (VPO) nanoparticles trapped within an electrospun carbon matrix (CNTs@VPO@CNFs) for potential use in energy storage applications. Temperature was found to be the major controlling factor for the fabrication of composites with CNT decoration. CNTs@VPO@CNFs exhibited the highest capacitance of 576.1F g-1 at a current density of 0.66 A g-1 among other corresponding electrode materials. Furthermore, this electrode exhibited outstanding stability of up to 99% after 5000 cycles, which was attributed to the coating of core-forming VPO@CNFs by the CNTs as the sheath material. Interestingly, the as-fabricated material worked in a wide potential range from -1.2 to 0.6, thereby providing the opportunity to assemble a symmetric supercapacitor device (SSCD). The SSCD showed an exceptionally high energy density of 69.1 W h kg-1 at a power density of 3.2 kW h and ~ 90% stability after 5000 cycles. Thus, this work presents a strategy for fabricating a new composite as a negative electrode material that can be used in a symmetrical supercapacitor device with an ultrahigh energy density.

Development of an Axon-Guiding Aligned Nanofiber-Integrated Compartmentalized Microfluidic Neuron Culture System.

Microfluidic-based neuron cell culture systems have recently gained a lot of attention due to their efficiency in supporting the spatial and temporal control of cellular microenvironments. However, the lack of axon guidance is the key limitation in current culture systems. To combat this, we have developed electrospun aligned nanofiber-integrated compartmentalized microfluidic neuron culture systems (NIMSs), where the nanofibers have enabled axonal guidance and stability. The resulting platform significantly improved axon alignment, length, and stability for both rat primary embryonic motor neurons (MNs) and dorsal root ganglia (DRG) neurons compared to the conventional glass-based microfluidic systems (GMSs). The results showed that axonal growth covered more than two times the area on the axonal chamber of NIMSs compared to the area covered for GMSs. Overall, this platform can be used as a valuable tool for fundamental neuroscience research, drug screening, and biomaterial testing.

EARLY RESPONSE TO DEHYDRATION 7 Remodels Cell Membrane Lipid Composition during Cold Stress in Arabidopsis.

Plants adjust to unfavorable conditions by altering physiological activities, such as gene expression. Although previous studies have identified multiple stress-induced genes, the function of many genes during the stress responses remains unclear. Expression of ERD7 (EARLY RESPONSE TO DEHYDRATION 7) is induced in response to dehydration. Here, we show that ERD7 plays essential roles in both plant stress responses and development. In Arabidopsis, ERD7 protein accumulated under various stress conditions, including exposure to low temperature. A triple mutant of Arabidopsis lacking ERD7 and two closely related homologs had an embryonic lethal phenotype, whereas a mutant lacking the two homologs and one ERD7 allele had relatively round leaves, indicating that the ERD7 gene family has essential roles in development. Moreover, the importance of the ERD7 family in stress responses was evidenced by the susceptibility of the mutant lines to cold stress. ERD7 protein was found to bind to several, but not all, negatively charged phospholipids and was associated with membranes. Lipid components and cold-induced reduction in PIP2 in the mutant line were altered relative to wild type. Furthermore, membranes from the mutant line had reduced fluidity. Taken together, ERD7 and its homologs are important for plant stress responses and development and associated with the modification in membrane lipid composition.

Three dimensional polycaprolactone/cellulose scaffold containing calcium-based particles: a new platform for bone regeneration.

Conventional electrospun membranes are composed of nanofibers arranged packed together with limited porosity akin to a two-dimensional (2D) sheet in which the growth of cells is restricted on the surface. In this study, we report a three-dimensional (3D) hierarchical multilayer scaffold with additional functionality by the modified gas-foaming technique. Calcium hydroxide particles were in-situ deposited on the fiber surface throughout the layers of macroporous 3D scaffold by the sodium borohydride (NaBH4) reduction of calcium salt. Mechanical properties and biocompatibility of the conventional electrospun mat were enhanced by the underlying 3D multilayer structure and incorporation of calcium hydroxide particles. Besides, the expanded 3D nanofiber scaffold with calcium incorporation promoted cellular infiltration, mineralization and osteogenesis. This integrated 3D multilayer structure and additional functionality approach may advance the development of 2D electrospun based scaffolds for biomedical applications.

Integrated design and fabrication strategies for biomechanically and biologically functional PLA/ß-TCP nanofiber reinforced GelMA scaffold for tissue engineering applications.

We present an integrated design and fabrication strategy for the development of hierarchically structured biomechanically and biologically functional tissue scaffold. An integration of ß-TCP incorporated fluffy type nanofibers and biodegradable interpenetrating gelatin-hydrogel networks (IGN) result in biomimetic tissue engineered constructs with fully tunable properties that can match specific tissue requirements. FESEM images showed that nanofibers were efficiently assembled into an orientation of IGN without disturbing its pore architecture. The pore architecture, compressive stiffness and modulus, swelling, and the biological properties of the composite constructs can be tailored by adjusting the composition of nanofiber content with respect to IGN. Experimental results of cell proliferation assay and confocal microscopy imaging showed that the as-fabricated composite constructs exhibit excellent ability for MC3T3-E1 cell proliferation, infiltration and growth. Furthermore, ß-TCP incorporated functionalized nanofiber enhanced the biomimetic mineralization, cell infiltration and cell proliferation. Within two weeks of cell-seeding, the composite construct exhibited enhanced osteogenic performance (Runx2, osterix and ALP gene expression) compared to pristine IGN hydrogel scaffold. Our integrated design and fabrication approach enables the assembly of nanofiber within IGN architecture, laying the foundation for biomimetic scaffold.

Dissecting the interaction of photosynthetic electron transfer with mitochondrial signalling and hypoxic response in the Arabidopsis rcd1 mutant.

The Arabidopsis mutant rcd1 is tolerant to methyl viologen (MV). MV enhances the Mehler reaction, i.e. electron transfer from Photosystem I (PSI) to O2, generating reactive oxygen species (ROS) in the chloroplast. To study the MV tolerance of rcd1, we first addressed chloroplast thiol redox enzymes potentially implicated in ROS scavenging. NADPH-thioredoxin oxidoreductase type C (NTRC) was more reduced in rcd1. NTRC contributed to the photosynthetic and metabolic phenotypes of rcd1, but did not determine its MV tolerance. We next tested rcd1 for alterations in the Mehler reaction. In rcd1, but not in the wild type, the PSI-to-MV electron transfer was abolished by hypoxic atmosphere. A characteristic feature of rcd1 is constitutive expression of mitochondrial dysfunction stimulon (MDS) genes that affect mitochondrial respiration. Similarly to rcd1, in other MDS-overexpressing plants hypoxia also inhibited the PSI-to-MV electron transfer. One possible explanation is that the MDS gene products may affect the Mehler reaction by altering the availability of O2. In green tissues, this putative effect is masked by photosynthetic O2 evolution. However, O2 evolution was rapidly suppressed in MV-treated plants. Transcriptomic meta-analysis indicated that MDS gene expression is linked to hypoxic response not only under MV, but also in standard growth conditions. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

PGR5 and NDH-1 systems do not function as protective electron acceptors but mitigate the consequences of PSI inhibition.

Avoidance of photoinhibition at photosystem (PS)I is based on synchronized function of PSII, PSI, Cytochrome b6f and stromal electron acceptors. Here, we used a special light regime, PSI photoinhibition treatment (PIT), in order to specifically inhibit PSI by accumulating excess electrons at the photosystem (Tikkanen and Grebe, 2018). In the analysis, Arabidopsis thaliana WT was compared to the pgr5 and ndho mutants, deficient in one of the two main cyclic electron transfer pathways described to function as protective alternative electron acceptors of PSI. The aim was to investigate whether the PGR5 (pgr5) and the type I NADH dehydrogenase (NDH-1) (ndho) systems protect PSI from excess electron stress and whether they help plants to cope with the consequences of PSI photoinhibition. First, our data reveals that neither PGR5 nor NDH-1 system protects PSI from a sudden burst of electrons. This strongly suggests that these systems in Arabidopsis thaliana do not function as direct acceptors of electrons delivered from PSII to PSI - contrasting with the flavodiiron proteins that were found to make Physcomitrella patens PSI resistant to the PIT. Second, it is demonstrated that under light-limiting conditions, the electron transfer rate at PSII is linearly dependent on the amount of functional PSI in all genotypes, while under excess light, the PGR5-dependent control of electron flow at the Cytochrome b6f complex overrides the effect of PSI inhibition. Finally, the PIT is shown to increase the amount of PGR5 and NDH-1 as well as of PTOX, suggesting that they mitigate further damage to PSI after photoinhibition rather than protect against it.