Post-translational modifications of Medicago truncatula glutathione peroxidase 1 induced by nitric oxide

High salt diet accelerates skin aging in wistar rats: an 8-week investigation of cell cycle inhibitors, SASP markers, and oxidative stress

Background

Recent studies have shown that the high salt diet (HSD) is linked to increased dermal pro-inflammatory status and reduced extracellular matrix (ECM) expression in inflamed skin of mice. Decreased ECM content is a known aging phenotype of the skin, and alterations in ECM composition and organization significantly contribute to skin aging. This study aimed to determine whether a high salt diet accelerates skin aging and to identify the time point at which this effect becomes apparent.

Methods

Wistar rats were randomly divided into normal diet and high salt diet groups and fed continuously for 8 weeks. Skin samples were collected at weeks 7 and week 8. Skin pathological sections were evaluated and levels of cell cycle inhibitors, senescence-associated secretory phenotype (SASP), oxidative stress and vascular regulatory factors (VRFs) were examined. Correlation analyses were performed to reveal the effect of a high salt diet as an extrinsic factor on skin aging and to analyse the correlation between a high salt diet and intrinsic aging and blood flow status.

Results

At week 8, HSD rats exhibited thickened epidermis, thinned dermis, and atrophied hair follicles. The expression of cell cycle inhibitors and oxidative stress levels were significantly elevated in the skin of HSD rats at both week 7 and week 8. At week 7, some SASPs, including TGF-β and PAI-1, were elevated, but others (IL-1, IL-6, IL-8, NO) were not significantly changed. By week 8, inflammatory molecules (IL-1, IL-6, TGF-β), chemokines (IL-8), proteases (PAI-1), and non-protein molecules (NO) were significantly increased. Notably, despite elevated PAI-1 levels suggesting possible blood hypercoagulation, the ET-1/NO ratio was reduced in the HSD group at week 8.

Conclusion

The data suggest that a high salt diet causes skin aging by week 8. The effect of a high salt diet on skin aging is related to the level of oxidative stress and the expression of cell cycle inhibitors. Additionally, a potential protective mechanism may be at play, as evidenced by the reduced ET-1/NO ratio, which could help counteract the hypercoagulable state and support nutrient delivery to aging skin.

Plant thiol peroxidases as redox sensors and signal transducers in abiotic stress acclimation

The temporal and spatial patterns of reactive oxygen species (ROS) in cells and tissues decisively determine the plant acclimation response to diverse abiotic and biotic stresses. Recent progress in developing dynamic cell imaging probes provides kinetic information on changes in parameters like H₂O₂, glutathione (GSH/GSSG) and NAD(P)H/NAD(P)⁺, that play a crucial role in tuning the cellular redox state. Central to redox-based regulation is the thiol-redox regulatory network of the cell that integrates reductive information from metabolism and oxidative ROS signals. Sensitive proteomics allow for monitoring changes in redox-related posttranslational modifications. Thiol peroxidases act as sensitive peroxide and redox sensors and play a central role in this signal transduction process. Peroxiredoxins (PRX) and glutathione peroxidases (GPX) are the two main thiol peroxidases and their function in ROS sensing and redox signaling in plants is emerging at present and summarized in this review. Depending on their redox state, PRXs and GPXs act as redox-dependent binding partners, direct oxidants of target proteins and oxidants of thiol redox transmitters that in turn oxidize target proteins. With their versatile functions, the multiple isoforms of plant thiol peroxidases play a central role in plant stress acclimation, e.g. to high light or osmotic stress, but also in ROS-mediated immunity and development.

Signaling by reactive molecules and antioxidants in legume nodules

Legume nodules are symbiotic structures formed as a result of the interaction with rhizobia. Nodules fix atmospheric nitrogen into ammonia that is assimilated by the plant and this process requires strict metabolic regulation and signaling. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are involved as signal molecules at all stages of symbiosis, from rhizobial infection to nodule senescence. Also, reactive sulfur species (RSS) are emerging as important signals for an efficient symbiosis. Homeostasis of reactive molecules is mainly accomplished by antioxidant enzymes and metabolites and is essential to allow redox signaling while preventing oxidative damage. Here, we examine the metabolic pathways of reactive molecules and antioxidants with an emphasis on their functions in signaling and protection of symbiosis. In addition to providing an update of recent findings while paying tribute to original studies, we identify several key questions. These include the need of new methodologies to detect and quantify ROS, RNS, and RSS, avoiding potential artifacts due to their short lifetimes and tissue manipulation; the regulation of redox-active proteins by post-translational modification; the production and exchange of reactive molecules in plastids, peroxisomes, nuclei, and bacteroids; and the unknown but expected crosstalk between ROS, RNS, and RSS in nodules.

Nitrate–Nitrite–Nitric Oxide Pathway: A Mechanism of Hypoxia and Anoxia Tolerance in Plants

Oxygen (O₂) is the most crucial substrate for numerous biochemical processes in plants. Its deprivation is a critical factor that affects plant growth and may lead to death if it lasts for a long time. However, various biotic and abiotic factors cause O₂ deprivation, leading to hypoxia and anoxia in plant tissues. To survive under hypoxia and/or anoxia, plants deploy various mechanisms such as fermentation paths, reactive oxygen species (ROS), reactive nitrogen species (RNS), antioxidant enzymes, aerenchyma, and adventitious root formation, while nitrate (NO₃⁻), nitrite (NO₂⁻), and nitric oxide (NO) have shown numerous beneficial roles through modulating these mechanisms. Therefore, in this review, we highlight the role of reductive pathways of NO formation which lessen the deleterious effects of oxidative damages and increase the adaptation capacity of plants during hypoxia and anoxia. Meanwhile, the overproduction of NO through reductive pathways during hypoxia and anoxia leads to cellular dysfunction and cell death. Thus, its scavenging or inhibition is equally important for plant survival. As plants are also reported to produce a potent greenhouse gas nitrous oxide (N₂O) when supplied with NO₃⁻ and NO₂⁻, resembling bacterial denitrification, its role during hypoxia and anoxia tolerance is discussed here. We point out that NO reduction to N₂O along with the phytoglobin-NO cycle could be the most important NO-scavenging mechanism that would reduce nitro-oxidative stress, thus enhancing plants’ survival during O₂-limited conditions. Hence, understanding the molecular mechanisms involved in reducing NO toxicity would not only provide insight into its role in plant physiology, but also address the uncertainties seen in the global N₂O budget.

Molecular responses of legumes to abiotic stress: post-translational modifications of proteins and redox signaling

Legumes include several major crops that can fix atmospheric nitrogen in symbiotic root nodules, thus reducing the demand for nitrogen fertilizers and contributing to sustainable agriculture. Global change models predict increases in temperature and extreme weather conditions. This scenario might increase plant exposure to abiotic stresses and negatively affect crop production. Regulation of whole plant physiology and nitrogen fixation in legumes during abiotic stress is complex, and only a few mechanisms have been elucidated. Reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS) are key players in the acclimation and stress tolerance mechanisms of plants. However, the specific redox-dependent signaling pathways are far from understood. One mechanism by which ROS, RNS, and RSS fulfil their signaling role is the post-translational modification (PTM) of proteins. Redox-based PTMs occur in the cysteine thiol group (oxidation, S-nitrosylation, S-glutathionylation, persulfidation), and also in methionine (oxidation), tyrosine (nitration), and lysine and arginine (carbonylation/glycation) residues. Unraveling PTM patterns under different types of stress and establishing the functional implications may give insight into the underlying mechanisms by which the plant and nodule respond to adverse conditions. Here, we review current knowledge on redox-based PTMs and their possible consequences in legume and nodule biology.

Plasticity in plastid redox networks: evolution of glutathione-dependent redox cascades and glutathionylation sites

BACKGROUND

Flexibility of plant metabolism is supported by redox regulation of enzymes via posttranslational modification of cysteine residues, especially in plastids. Here, the redox states of cysteine residues are partly coupled to the thioredoxin system and partly to the glutathione pool for reduction. Moreover, several plastid enzymes involved in reactive oxygen species (ROS) scavenging and damage repair draw electrons from glutathione. In addition, cysteine residues can be post-translationally modified by forming a mixed disulfide with glutathione (S-glutathionylation), which protects thiol groups from further oxidation and can influence protein activity. However, the evolution of the plastid glutathione-dependent redox network in land plants and the conservation of cysteine residues undergoing S-glutathionylation is largely unclear.

RESULTS

We analysed the genomes of nine representative model species from streptophyte algae to angiosperms and found that the antioxidant enzymes and redox proteins belonging to the plastid glutathione-dependent redox network are largely conserved, except for lambda- and the closely related iota-glutathione S-transferases. Focussing on glutathione-dependent redox modifications, we screened the literature for target thiols of S-glutathionylation, and found that 151 plastid proteins have been identified as glutathionylation targets, while the exact cysteine residue is only known for 17% (26 proteins), with one or multiple sites per protein, resulting in 37 known S-glutathionylation sites for plastids. However, 38% (14) of the known sites were completely conserved in model species from green algae to flowering plants, with 22% (8) on non-catalytic cysteines. Variable conservation of the remaining sites indicates independent gains and losses of cysteines at the same position during land plant evolution.

CONCLUSIONS

We conclude that the glutathione-dependent redox network in plastids is highly conserved in streptophytes with some variability in scavenging and damage repair enzymes. Our analysis of cysteine conservation suggests that S-glutathionylation in plastids plays an important and yet under-investigated role in redox regulation and stress response.

Nitrogen and Phosphorus Signaling and Transport During Legume-Rhizobium Symbiosis

Nitrogen (N) and phosphorus (P) are the two predominant mineral elements, which are not only essential for plant growth and development in general but also play a key role in symbiotic N fixation in legumes. Legume plants have evolved complex signaling networks to respond to both external and internal levels of these macronutrients to optimize symbiotic N fixation in nodules. Inorganic phosphate (Pi) and nitrate (NO₃ ^-) are the two major forms of P and N elements utilized by plants, respectively. Pi starvation and NO₃ ^- application both reduce symbiotic N fixation via similar changes in the nodule gene expression and invoke local and long-distance, systemic responses, of which N-compound feedback regulation of rhizobial nitrogenase activity appears to operate under both conditions. Most of the N and P signaling and transport processes have been investigated in model organisms, such as Medicago truncatula, Lotus japonicus, Glycine max, Phaseolus vulgaris, Arabidopsis thaliana, Oryza sativa, etc. We attempted to discuss some of these processes wherever appropriate, to serve as references for a better understanding of the N and P signaling and transport during symbiosis.

Identification of client iron-sulfur proteins of the chloroplastic NFU2 transfer protein in Arabidopsis thaliana

Iron-sulfur (Fe-S) proteins have critical functions in plastids, notably participating in photosynthetic electron transfer, sulfur and nitrogen assimilation, chlorophyll metabolism, and vitamin or amino acid biosynthesis. Their maturation relies on the so-called SUF (sulfur mobilization) assembly machinery. Fe-S clusters are synthesized de novo on a scaffold protein complex and then delivered to client proteins via several transfer proteins. However, the maturation pathways of most client proteins and their specificities for transfer proteins are mostly unknown. In order to decipher the proteins interacting with the Fe-S cluster transfer protein NFU2, one of the three plastidial representatives found in Arabidopsis thaliana, we performed a quantitative proteomic analysis of shoots, roots, and seedlings of nfu2 plants, combined with NFU2 co-immunoprecipitation and binary yeast two-hybrid experiments. We identified 14 new targets, among which nine were validated in planta using a binary bimolecular fluorescence complementation assay. These analyses also revealed a possible role for NFU2 in the plant response to desiccation. Altogether, this study better delineates the maturation pathways of many chloroplast Fe-S proteins, considerably extending the number of NFU2 clients. It also helps to clarify the respective roles of the three NFU paralogs NFU1, NFU2, and NFU3.

The Role of Nitric Oxide in Nitrogen Fixation by Legumes

The legume-rhizobia symbiosis is an important process in agriculture because it allows the biological nitrogen fixation (BNF) which contributes to increasing the levels of nitrogen in the soil. Nitric oxide (⋅NO) is a small free radical molecule having diverse signaling roles in plants. Here we present and discuss evidence showing the role of ⋅NO during different stages of the legume-rhizobia interaction such as recognition, infection, nodule development, and nodule senescence. Although the mechanisms by which ⋅NO modulates this interaction are not fully understood, we discuss potential mechanisms including its interaction with cytokinin, auxin, and abscisic acid signaling pathways. In matures nodules, a more active metabolism of ⋅NO has been reported and both the plant and rhizobia participate in ⋅NO production and scavenging. Although ⋅NO has been shown to induce the expression of genes coding for NITROGENASE, controlling the levels of ⋅NO in mature nodules seems to be crucial as ⋅NO was shown to be a potent inhibitor of NITROGENASE activity, to induce nodule senescence, and reduce nitrogen assimilation. In this sense, LEGHEMOGLOBINS (Lbs) were shown to play an important role in the scavenging of ⋅NO and reactive nitrogen species (RNS), potentially more relevant in senescent nodules. Even though ⋅NO can reduce NITROGENASE activity, most reports have linked ⋅NO to positive effects on BNF. This can relate mainly to the regulation of the spatiotemporal distribution of ⋅NO which favors some effects over others. Another plausible explanation for this observation is that the negative effect of ⋅NO requires its direct interaction with NITROGENASE, whereas the positive effect of ⋅NO is related to its signaling function, which results in an amplifier effect. In the near future, it would be interesting to explore the role of environmental stress-induced ⋅NO in BNF.

Plant Nitrate Reductases Regulate Nitric Oxide Production and Nitrogen-Fixing Metabolism During the Medicago truncatula-Sinorhizobium meliloti Symbiosis

Nitrate reductase (NR) is the first enzyme of the nitrogen reduction pathway in plants, leading to the production of ammonia. However, in the nitrogen-fixing symbiosis between legumes and rhizobia, atmospheric nitrogen (N₂) is directly reduced to ammonia by the bacterial nitrogenase, which questions the role of NR in symbiosis. Next to that, NR is the best-characterized source of nitric oxide (NO) in plants, and NO is known to be produced during the symbiosis. In the present study, we first surveyed the three NR genes (MtNR1, MtNR2, and MtNR3) present in the Medicago truncatula genome and addressed their expression, activity, and potential involvement in NO production during the symbiosis between M. truncatula and Sinorhizobium meliloti. Our results show that MtNR1 and MtNR2 gene expression and activity are correlated with NO production throughout the symbiotic process and that MtNR1 is particularly involved in NO production in mature nodules. Moreover, NRs are involved together with the mitochondrial electron transfer chain in NO production throughout the symbiotic process and energy regeneration in N₂-fixing nodules. Using an in vivo NMR spectrometric approach, we show that, in mature nodules, NRs participate also in the regulation of energy state, cytosolic pH, carbon and nitrogen metabolism under both normoxia and hypoxia. These data point to the importance of NR activity for the N₂-fixing symbiosis and provide a first explanation of its role in this process.

Molecular Weapons Contribute to Intracellular Rhizobia Accommodation Within Legume Host Cell

The interaction between legumes and bacteria of rhizobia type results in a beneficial symbiotic relationship characterized by the formation of new root organs, called nodules. Within these nodules the bacteria, released in plant cells, differentiate into bacteroids and fix atmospheric nitrogen through the nitrogenase activity. This mutualistic interaction has evolved sophisticated signaling networks to allow rhizobia entry, colonization, bacteroid differentiation and persistence in nodules. Nodule cysteine rich (NCR) peptides, reactive oxygen species (ROS), reactive nitrogen species (RNS), and toxin-antitoxin (TA) modules produced by the host plants or bacterial microsymbionts have a major role in the control of the symbiotic interaction. These molecules described as weapons in pathogenic interactions have evolved to participate to the intracellular bacteroid accommodation by escaping control of plant innate immunity and adapt the functioning of the nitrogen-fixation to environmental signalling cues.

Nitric oxide signaling, metabolism and toxicity in nitrogen-fixing symbiosis

Interactions between legumes and rhizobia lead to the establishment of a symbiotic relationship characterized by the formation of a new organ, the nodule, which facilitates the fixation of atmospheric nitrogen (N2) by nitrogenase through the creation of a hypoxic environment. Significant amounts of nitric oxide (NO) accumulate at different stages of nodule development, suggesting that NO performs specific signaling and/or metabolic functions during symbiosis. NO, which regulates nodule gene expression, accumulates to high levels in hypoxic nodules. NO accumulation is considered to assist energy metabolism within the hypoxic environment of the nodule via a phytoglobin-NO-mediated respiration process. NO is a potent inhibitor of the activity of nitrogenase and other plant and bacterial enzymes, acting as a developmental signal in the induction of nodule senescence. Hence, key questions concern the relative importance of the signaling and metabolic functions of NO versus its toxic action and how NO levels are regulated to be compatible with nitrogen fixation functions. This review analyses these paradoxical roles of NO at various stages of symbiosis, and highlights the role of plant phytoglobins and bacterial hemoproteins in the control of NO accumulation.

Involvement of Glutaredoxin and Thioredoxin Systems in the Nitrogen-Fixing Symbiosis between Legumes and Rhizobia

Leguminous plants can form a symbiotic relationship with Rhizobium bacteria, during which plants provide bacteria with carbohydrates and an environment appropriate to their metabolism, in return for fixed atmospheric nitrogen. The symbiotic interaction leads to the formation of a new organ, the root nodule, where a coordinated differentiation of plant cells and bacteria occurs. The establishment and functioning of nitrogen-fixing symbiosis involves a redox control important for both the plant-bacteria crosstalk and the regulation of nodule metabolism. In this review, we discuss the involvement of thioredoxin and glutaredoxin systems in the two symbiotic partners during symbiosis. The crucial role of glutathione in redox balance and S-metabolism is presented. We also highlight the specific role of some thioredoxin and glutaredoxin systems in bacterial differentiation. Transcriptomics data concerning genes encoding components and targets of thioredoxin and glutaredoxin systems in connection with the developmental step of the nodule are also considered in the model system Medicago truncatula⁻Sinorhizobium meliloti.