Consequences of photoinhibition of photosystem I on photosynthetic electron transport and carbon metabolism

Photosynthesis allows plants to store light energy in organic compounds. Plants have an efficient apparatus to harvest photons from sunlight and use the energy to split water and transport electrons to specific high-energy electron acceptors. A proper balance between light reactions and electron consumption is important to maintain fluent photosynthetic activity during environmental conditions that are constantly changing. At the same time, photosynthetic components are protected through several regulatory mechanisms. The avoidance of damage to photosystem I (PSI) is particularly important because its recovery occurs extremely slowly as compared to that of photosystem II (PSII). Studies on damage, photoinhibition and recovery of PSI are scarcer than those of PSII. In this thesis, the occurrence of photoinhibition of PSI and some of its consequences to the plant metabolism were investigated. Arabidopsis thaliana L. plants lacking the PROTON GRADIENT REGULATION 5 protein (pgr5 mutants) that were treated with excess light were used as a model system for controlled PSI-photoinhibition. This experimental model was validated, and the impact of PSI photoinhibition and recovery on photosynthetic electron transport, primary metabolism, reactive oxygen species (ROS) production and chloroplast retrograde signalling were thoroughly characterised. The results highlight that PSI photoinhibition induces impairment of CO2 fixation, starch accumulation, and dark respiration. The recovery of PSI function after photoinhibition proved to be dependent on light conditions, being especially deleterious for CO2 fixation under low irradiances, and supporting the idea that a pool of surplus PSI can be recruited to support photosynthesis under demanding conditions. High light-treated pgr5 mutants also displayed low occurrence of lipid oxidation associated with attenuated enzymatic oxylipin synthesis and consequent chloroplast regulation of nuclear gene expression. This model also showed that PSI photoinhibition prevents oxidative stress and accumulation of ROS, evidencing a role of PSI inactivation in avoiding over-reduction of downstream redox components

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

Peroxisomal APX knockdown triggers antioxidant mechanisms favourable for coping with high photorespiratory H2O2 induced by CAT deficiency in rice

The physiological role of peroxisomal ascorbate peroxidases (pAPX) is unknown; therefore, we utilized pAPX4 knockdown rice and catalase (CAT) inhibition to assess its role in CAT compensation under high photorespiration. pAPX4 knockdown induced co-suppression in the expression of pAPX3. The rice mutants exhibited metabolic changes such as lower CAT and glycolate oxidase (GO) activities and reduced glyoxylate content; however, APX activity was not altered. CAT inhibition triggered different changes in the expression of CAT, APX and glutathione peroxidase (GPX) isoforms between non-transformed (NT) and silenced plants. These responses were associated with alterations in APX, GPX and GO activities, suggesting redox homeostasis differences. The glutathione oxidation-reduction states were modulated differently in mutants, and the ascorbate redox state was greatly affected in both genotypes. The pAPX suffered less oxidative stress and photosystem II (PSII) damage and displayed higher photosynthesis than the NT plants. The improved acclimation exhibited by the pAPX plants was indicated by lower H2O2 accumulation, which was associated with lower GO activity and glyoxylate content. The suppression of both pAPXs and/or its downstream metabolic and molecular effects may trigger favourable antioxidant and compensatory mechanisms to cope with CAT deficiency. This physiological acclimation may involve signalling by peroxisomal H2O2, which minimized the photorespiration.</p

Impairment of peroxisomal APX and CAT activities increases protection of photosynthesis under oxidative stress

Retrograde signalling pathways that are triggered by changes in cellular redox homeostasis remain poorly understood. Transformed rice plants that are deficient in peroxisomal ascorbate peroxidase APX4 (OsAPX4-RNAi) are known to exhibit more effective protection of photosynthesis against oxidative stress than controls when catalase (CAT) is inhibited, but the mechanisms involved have not been characterized. An in-depth physiological and proteomics analysis was therefore performed on OsAPX4-RNAi CAT-inhibited rice plants. Loss of APX4 function led to an increased abundance of several proteins that are involved in essential metabolic pathways, possibly as a result of increased tissue H2O2 levels. Higher photosynthetic activities observed in the OsAPX4-RNAi plants under CAT inhibition were accompanied by higher levels of Rubisco, higher maximum rates of Rubisco carboxylation, and increased photochemical efficiencies, together with large increases in photosynthesis-related proteins. Large increases were also observed in the levels of proteins involved in the ascorbate/glutathione cycle and in other antioxidant-related pathways, and these changes may be important in the protection of photosynthesis in the OsAPX4-RNAi plants. Large increases in the abundance of proteins localized in the nuclei and mitochondria were also observed, together with increased levels of proteins involved in important cellular pathways, particularly protein translation. Taken together, the results show that OsAPX4-RNAi plants exhibit significant metabolic reprogramming, which incorporates a more effective antioxidant response to protect photosynthesis under conditions of impaired CAT activity.</p

Overexpression of the Rieske FeS protein of the Cytochrome b 6 f complex increases C4 photosynthesis in Setaria viridis.

C4 photosynthesis is characterised by a CO2 concentrating mechanism that operates between mesophyll and bundle sheath cells increasing CO2 partial pressure at the site of Rubisco and photosynthetic efficiency. Electron transport chains in both cell types supply ATP and NADPH for C4 photosynthesis. Cytochrome b 6 f is a key control point of electron transport in C3 plants. To study whether C4 photosynthesis is limited by electron transport we constitutively overexpressed the Rieske FeS subunit in Setaria viridis. This resulted in a higher Cytochrome b 6 f content in mesophyll and bundle sheath cells without marked changes in the abundances of other photosynthetic proteins. Rieske overexpression plants showed better light conversion efficiency in both Photosystems and could generate higher proton-motive force across the thylakoid membrane underpinning an increase in CO2 assimilation rate at ambient and saturating CO2 and high light. Our results demonstrate that removing electron transport limitations can increase C4 photosynthesis

Mitochondrial glutathione peroxidase of rice is crucial for growth by favoring photosynthesis

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel SuperiorO papel fisiolÃgico das peroxidases de glutationa (GPX) da mitocÃndria em plantas à muito pouco conhecido. Suas relaÃÃes com a fotossÃntese sÃo desconhecidas, ainda mais na presenÃa de estresse salino. Essa enzima possui grande importÃncia na remoÃÃo de H2O2 e hidroperÃxidos orgÃnicos, contribuindo na proteÃÃo oxidativa e na homeostase redox. Neste estudo, mutantes de arroz silenciados nos genes OsGPX1 ou OsGPX3, das proteÃnas mitocondriais, foram utilizados para entender os mecanismos fisiolÃgicos do papel dessa enzima no crescimento e fotossÃntese. Adicionalmente, estes processos foram estudados tambÃm em condiÃÃes de estresse salino para as plantas silenciadas em OsGPX1. Os resultados mostram, pela primeira vez, que a deficiÃncia de uma GPX mitocondrial à capaz de restringir o crescimento vegetal por deficiÃncia na fotossÃntese. Este efeito deve ser causado indiretamente por mudanÃas nas redes genÃticas e metabÃlicas desencadeadas por alteraÃÃes nos nÃveis de H2O2 (aumentado) e/ou glutationa reduzida (diminuÃda). à provÃvel que o estado redox alterado em mitocÃndrias pelo efeito da GPX possa aumentar a fotossÃntese atravÃs da comunicaÃÃo entre esta organela e cloroplastos por mecanismos ainda nÃo estabelecidos. AlÃm disso, o gene OsGPX1 mostrou ter papel significativo no controle do movimento estomÃtico, que à crucial para a eficiÃncia do uso da Ãgua sob condiÃÃo de estresse salino. As GPX mitocondriais tambÃm parecem estar envolvidas com a dissipaÃÃo do excesso de energia luminosa na forma de calor (NPQ) no aparato do fotossistema II e na rota da fotorrespiraÃÃo. Em conclusÃo, o gene OsGPX1, associado com seu produto proteico e mudanÃas desencadeadas nas redes metabÃlicas e gÃnicas, sÃo essenciais para o crescimento de arroz pelo aumento da fotossÃntese, especialmente a nÃvel de eficiÃncia de uso da luz envolvendo atividade do fotossistema II e eficiÃncia quÃntica do CO2 em condiÃÃes normais de crescimento. Adicionalmente, a GPX1 aparenta mostrar uma importÃncia menor para a resistÃncia ao estresse salino.The physiological role of glutathione peroxidases (GPX) in plant mitochondria is little known. Their relations with the photosynthesis are unknown, even more in presence of salt stress. This enzyme have great importance in H2O2 and organic hydroperoxides scavenging, contributing in oxidative protection and redox homeostasis. In this study, silenced rice mutants in OsGPX1 and OsGPX3 genes, coding for the mitochondrial proteins, were used to better understand the physiological mechanisms of the role of this protein in growth and photosynthesis. Additionally, these processes were also studied in salt stress conditions with the plants silenced in OsGPX1. The results show, for the first time, that the lacking of a mitochondrial GPX is capable of restricting plant growth by impairment in photosynthesis. This response might be an indirect consequence of changes in gene and metabolic networks trigged by alterations in H2O2 (raised) and/or reduced glutathione (diminished). It is likely that the altered redox state in mitochondria by GPX effects can improve photosynthesis through cross-talk between this organelle and chloroplasts by yet unknown mechanisms. Furthermore, the OsGPX1 gene showed significant role in stomatal control, which is crucial to the water use efficiency under salt stress conditions. The mitochondrial GPX also seems to be involved with dissipation of excess light energy as heat (NPQ) in photosystem II apparatus and in photorespiratory pathway. In conclusion, the OsGPX1 gene, associated with it protein product and changes in gene and metabolic networks, are essential to rice growth by improvement of photosynthesis, especially at light use efficiency level involving photosystem II activity and CO2 quantum efficiency in normal and growth conditions. Additionally, GPX1 seems to be less important to salt stress tolerance

Consequences of photosystem‐I damage and repair on photosynthesis and carbon use in Arabidopsis thaliana

Natural growth environments commonly include fluctuating conditions that can disrupt the photosynthetic energy balance and induce photoinhibition through inactivation of the photosynthetic apparatus. Photosystem II (PSII) photoinhibition is efficiently reversed by the PSII repair cycle, whereas photoinhibited photosystem I (PSI) recovers much more slowly. In the current study, treatment of the Arabidopsis thaliana mutant proton gradient regulation 5 (pgr5) with excess light was used to compromise PSI functionality in order to investigate the impact of photoinhibition and subsequent recovery on photosynthesis and carbon metabolism. The negative impact of PSI photoinhibition on CO2 fixation was especially deleterious under low irradiance. Impaired starch accumulation after PSI photoinhibition was reflected in reduced respiration in the dark, but this was not attributed to impaired sugar synthesis. Normal chloroplast and mitochondrial metabolisms were shown to recover despite the persistence of substantial PSI photoinhibition for several days. The results of this study indicate that the recovery of PSI function involves the reorganization of the light‐harvesting antennae, and suggest a pool of surplus PSI that can be recruited to support photosynthesis under demanding conditions.</p

Function and Compensatory Mechanisms Among the Components of the Chloroplastic Redox Network

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Expression in public datasets of genes found to be down-regulated in pgr5 from Interaction between photosynthetic electron transport and chloroplast sinks triggers protection and signalling important for plant productivity

The expression fold change of a set of 130 genes found to be most strongly down-regulated in pgr5 / gl1 WT after 1 hr HL treatment and 1 hr recovery in GL was analysed in selected experimental treatments using Genevestigator (see file for additional details