Protein Tyrosine Nitration during Development and Abiotic Stress Response in Plants

Functions of nitric oxide-mediated post-translational modifications under abiotic stress

Environmental conditions greatly impact plant growth and development. In the current context of both global climate change and land degradation, abiotic stresses usually lead to growth restriction limiting crop production. Plants have evolved to sense and respond to maximize adaptation and survival; therefore, understanding the mechanisms involved in the different converging signaling networks becomes critical for improving plant tolerance. In the last few years, several studies have shown the plant responses against drought and salinity, high and low temperatures, mechanical wounding, heavy metals, hypoxia, UV radiation, or ozone stresses. These threats lead the plant to coordinate a crosstalk among different pathways, highlighting the role of phytohormones and reactive oxygen and nitrogen species (RONS). In particular, plants sense these reactive species through post-translational modification (PTM) of macromolecules such as nucleic acids, proteins, and fatty acids, hence triggering antioxidant responses with molecular implications in the plant welfare. Here, this review compiles the state of the art about how plant systems sense and transduce this crosstalk through PTMs of biological molecules, highlighting the S-nitrosylation of protein targets. These molecular mechanisms finally impact at a physiological level facing the abiotic stressful traits that could lead to establishing molecular patterns underlying stress responses and adaptation strategies.

Recent progress in molecular genetics and omics-driven research in seed biology

Elucidating the mechanisms that control seed development, metabolism, and physiology is a fundamental issue in biology. Michel Caboche had long been a catalyst for seed biology research in France up until his untimely passing away last year. To honour his memory, we have updated a review written under his coordination in 2010 entitled "Arabidopsis seed secrets unravelled after a decade of genetic and omics-driven research". This review encompassed different molecular aspects of seed development, reserve accumulation, dormancy and germination, that are studied in the lab created by M. Caboche. We have extended the scope of this review to highlight original experimental approaches implemented in the field over the past decade such as omics approaches aimed at investigating the control of gene expression, protein modifications, primary and specialized metabolites at the tissue or even cellular level, as well as seed biodiversity and the impact of the environment on seed quality.

Interactions of melatonin, reactive oxygen species, and nitric oxide during fruit ripening: an update and prospective view

Fruit ripening is a physiological process that involves a complex network of signaling molecules that act as switches to activate or deactivate certain metabolic pathways at different levels, not only by regulating gene and protein expression but also through post-translational modifications of the involved proteins. Ethylene is the distinctive molecule that regulates the ripening of fruits, which can be classified as climacteric or non-climacteric according to whether or not, respectively, they are dependent on this phytohormone. However, in recent years it has been found that other molecules with signaling potential also exert regulatory roles, not only individually but also as a result of interactions among them. These observations imply the existence of mutual and hierarchical regulations that sometimes make it difficult to identify the initial triggering event. Among these 'new' molecules, hydrogen peroxide, nitric oxide, and melatonin have been highlighted as prominent. This review provides a comprehensive outline of the relevance of these molecules in the fruit ripening process and the complex network of the known interactions among them.

Nitric Oxide Implication in Potato Immunity to Phytophthora infestans via Modifications of Histone H3/H4 Methylation Patterns on Defense Genes

Nitric oxide (NO) is an essential redox-signaling molecule operating in many physiological and pathophysiological processes. However, evidence on putative NO engagement in plant immunity by affecting defense gene expressions, including histone modifications, is poorly recognized. Exploring the effect of biphasic NO generation regulated by S-nitrosoglutathione reductase (GNSOR) activity after avr Phytophthora infestans inoculation, we showed that the phase of NO decline at 6 h post-inoculation (hpi) was correlated with the rise of defense gene expressions enriched in the TrxG-mediated H3K4me3 active mark in their promoter regions. Here, we report that arginine methyltransferase PRMT5 catalyzing histone H4R3 symmetric dimethylation (H4R3sme2) is necessary to ensure potato resistance to avr P. infestans. Both the pathogen and S-nitrosoglutathione (GSNO) altered the methylation status of H4R3sme2 by transient reduction in the repressive mark in the promoter of defense genes, R3a and HSR203J (a resistance marker), thereby elevating their transcription. In turn, the PRMT5-selective inhibitor repressed R3a expression and attenuated the hypersensitive response to the pathogen. In conclusion, we postulate that lowering the NO level (at 6 hpi) might be decisive for facilitating the pathogen-induced upregulation of stress genes via histone lysine methylation and PRMT5 controlling potato immunity to late blight.

Nitric oxide regulation of plant metabolism

Nitric oxide (NO) has emerged as an important signal molecule in plants, having myriad roles in plant development. In addition, NO also orchestrates both biotic and abiotic stress responses, during which intensive cellular metabolic reprogramming occurs. Integral to these responses is the location of NO biosynthetic and scavenging pathways in diverse cellular compartments, enabling plants to effectively organize signal transduction pathways. NO regulates plant metabolism and, in turn, metabolic pathways reciprocally regulate NO accumulation and function. Thus, these diverse cellular processes are inextricably linked. This review addresses the numerous redox pathways, located in the various subcellular compartments that produce NO, in addition to the mechanisms underpinning NO scavenging. We focus on how this molecular dance is integrated into the metabolic state of the cell. Within this context, a reciprocal relationship between NO accumulation and metabolite production is often apparent. We also showcase cellular pathways, including those associated with nitrate reduction, that provide evidence for this integration of NO function and metabolism. Finally, we discuss the potential importance of the biochemical reactions governing NO levels in determining plant responses to a changing environment.

Nitric oxide and hydrogen sulfide: an indispensable combination for plant functioning

Nitric oxide (NO) and hydrogen sulfide (H₂S) are gasotransmitters, which are involved in almost all plant physiological and stress-related processes. With its antioxidant regulatory properties, NO on its own ameliorates plant stress, while H₂S, a foul-smelling gas, has differential effects. Recent studies have shown that these signaling molecules are involved in intertwined pathway networks. This is due to the contrasting effects of NO and H₂S depending on cell type, subcellular compartment, and redox status, as well as the flux and dosage of NO and H₂S in different plant species and cellular contexts. Here, we provide a comprehensive review of the complex networks of these molecules, with particular emphasis on root development, stomatal movement, and plant cell death.

Modulation of the Interactions Between α-Synuclein and Lipid Membranes by Post-translational Modifications

Parkinson's disease is characterised by the presence in brain tissue of aberrant inclusions known as Lewy bodies and Lewy neurites, which are deposits composed by α-synuclein and a variety of other cellular components, including in particular lipid membranes. The dysregulation of the balance between lipid homeostasis and α-synuclein homeostasis is therefore likely to be closely involved in the onset and progression of Parkinson's disease and related synucleinopathies. As our understanding of this balance is increasing, we describe recent advances in the characterisation of the role of post-translational modifications in modulating the interactions of α-synuclein with lipid membranes. We then discuss the impact of these advances on the development of novel diagnostic and therapeutic tools for synucleinopathies.

Proteome-wide and lysine crotonylation profiling reveals the importance of crotonylation in chrysanthemum (Dendranthema grandiforum) under low-temperature

BACKGROUND

Low-temperature severely affects the growth and development of chrysanthemum which is one kind of ornamental plant well-known and widely used in the world. Lysine crotonylation is a recently identified post-translational modification (PTM) with multiple cellular functions. However, lysine crotonylation under low-temperature stress has not been studied.

RESULTS

Proteome-wide and lysine crotonylation of chrysanthemum at low-temperature was analyzed using TMT (Tandem Mass Tag) labeling, sensitive immuno-precipitation, and high-resolution LC-MS/MS. The results showed that 2017 crotonylation sites were identified in 1199 proteins. Treatment at 4 °C for 24 h and - 4 °C for 4 h resulted in 393 upregulated proteins and 500 downregulated proteins (1.2-fold threshold and P < 0.05). Analysis of biological information showed that lysine crotonylation was involved in photosynthesis, ribosomes, and antioxidant systems. The crotonylated proteins and motifs in chrysanthemum were compared with other plants to obtain orthologous proteins and conserved motifs. To further understand how lysine crotonylation at K136 affected APX (ascorbate peroxidase), we performed a site-directed mutation at K136 in APX. Site-directed crotonylation showed that lysine decrotonylation at K136 reduced APX activity, and lysine complete crotonylation at K136 increased APX activity.

CONCLUSION

In summary, our study comparatively analyzed proteome-wide and crotonylation in chrysanthemum under low-temperature stress and provided insights into the mechanisms of crotonylation in positively regulated APX activity to reduce the oxidative damage caused by low-temperature stress. These data provided an important basis for studying crotonylation to regulate antioxidant enzyme activity in response to low-temperature stress and a new research ideas for chilling-tolerance and freezing-tolerance chrysanthemum molecular breeding.

Nitric Oxide Overproduction by cue1 Mutants Differs on Developmental Stages and Growth Conditions

The cue1 nitric oxide (NO) overproducer mutants are impaired in a plastid phosphoenolpyruvate/phosphate translocator, mainly expressed in Arabidopsis thaliana roots. cue1 mutants present an increased content of arginine, a precursor of NO in oxidative synthesis processes. However, the pathways of plant NO biosynthesis and signaling have not yet been fully characterized, and the role of CUE1 in these processes is not clear. Here, in an attempt to advance our knowledge regarding NO homeostasis, we performed a deep characterization of the NO production of four different cue1 alleles (cue1-1, cue1-5, cue1-6 and nox1) during seed germination, primary root elongation, and salt stress resistance. Furthermore, we analyzed the production of NO in different carbon sources to improve our understanding of the interplay between carbon metabolism and NO homeostasis. After in vivo NO imaging and spectrofluorometric quantification of the endogenous NO levels of cue1 mutants, we demonstrate that CUE1 does not directly contribute to the rapid NO synthesis during seed imbibition. Although cue1 mutants do not overproduce NO during germination and early plant development, they are able to accumulate NO after the seedling is completely established. Thus, CUE1 regulates NO homeostasis during post-germinative growth to modulate root development in response to carbon metabolism, as different sugars modify root elongation and meristem organization in cue1 mutants. Therefore, cue1 mutants are a useful tool to study the physiological effects of NO in post-germinative growth.

Nitration of protein phosphatase 2A increases via Epac1/PLCε/CaMKII/HDAC5/iNOS cascade in human endometrial stromal cell decidualization

Decidualization of the endometrial stroma is an essential differentiation process for embryo implantation and maintenance of pregnancy. We previously reported that protein phosphatase 2A (PP2A) acts as a key mediator during cAMP-induced decidualization of human endometrial stromal cells (hESCs). However, the mechanism underlying its activation has remained obscure in hESCs. In the present study, we aimed to reveal the mechanism that induces the nitration of PP2A catalytic subunit (PP2Ac) during cAMP-induced decidualization of hESCs. First, cAMP-induced PP2Ac nitration was significantly repressed using L-NAME, an inhibitor of nitric oxide synthase (NOS). Among several NOS isoforms, only inducible NOS (iNOS) was highly expressed in hESCs, indicating that iNOS directly induces the nitration of PP2Ac. Second, cAMP-induced iNOS expression and PP2Ac nitration were decreased by treatment with TSA, an inhibitor of histone deacetylase 5 (HDAC5). cAMP-induced phosphorylation of CaMKII and HDAC5 was suppressed by treatment with U73122 (an inhibitor of phospholipase C) or transfection of PLCε siRNA. Finally, small G protein Rap1 and its guanine nucleotide exchange factor Epac1 were found to be involved in cAMP-induced PP2A activation. Taken together, our results suggest that PP2Ac nitration during cAMP-induced decidualization of hESCs is induced through the Epac1-Rap1-PLCε-CaMKII-HDAC5-iNOS signaling pathway.

The role of NO in plant response to salt stress: interactions with polyamines

Soil salinity is a major abiotic stress that limits plant growth and productivity. High concentrations of sodium chloride can cause osmotic and ionic effects. This stress minimises a plant's ability to uptake water and minerals, and increases Na+ accumulation in the cytosol, thereby disturbing metabolic processes. Prolonged plant exposure to salt stress can lead to oxidative stress and increased production of reactive oxygen species (ROS). Higher plants developed some strategies to cope with salt stress. Among these, mechanisms involving nitric oxide (NO) and polyamines (PAs) are particularly important. NO is a key signalling molecule that mediates a variety of physiological functions and defence responses against abiotic stresses in plants. Under salinity conditions, NO donors increase growth parameters, reduce Na+ toxicity, maintain ionic homeostasis, stimulate osmolyte accumulation and prevent damages caused by ROS. NO enhances salt tolerance of plants via post-translational protein modifications through S-nitrosylation of thiol groups, nitration of tyrosine residues and modulation of multiple gene expression. Several reviews have reported on the role of polyamines in modulating salt stress plant response and the capacity to enhance PA synthesis upon salt stress exposure, and it is known that NO and PAs interact under salinity. In this review, we focus on the role of NO in plant response to salt stress, paying particular attention to the interaction between NO and PAs.

Cyst Nematode Infection Elicits Alteration in the Level of Reactive Nitrogen Species, Protein S-Nitrosylation and Nitration, and Nitrosoglutathione Reductase in Arabidopsis thaliana Roots

Reactive nitrogen species (RNS) are redox molecules important for plant defense against pathogens. The aim of the study was to determine whether the infection by the beet cyst nematode Heterodera schachtii disrupts RNS balance in Arabidopsis thaliana roots. For this purpose, measurements of nitric oxide (NO), peroxynitrite (ONOO⁻), protein S-nitrosylation and nitration, and nitrosoglutathione reductase (GSNOR) in A. thaliana roots from 1 day to 15 days post-inoculation (dpi) were performed. The cyst nematode infection caused generation of NO and ONOO⁻ in the infected roots. These changes were accompanied by an expansion of S-nitrosylated and nitrated proteins. The enzyme activity of GSNOR was decreased at 3 and 15 dpi and increased at 7 dpi in infected roots, whereas the GSNOR1 transcript level was enhanced over the entire examination period. The protein content of GSNOR was increased in infected roots at 3 dpi and 7 dpi, but at 15 dpi, did not differ between uninfected and infected roots. The protein of GSNOR was detected in plastids, mitochondria, cytoplasm, as well as endoplasmic reticulum and cytoplasmic membranes. We postulate that RNS metabolism plays an important role in plant defense against the beet cyst nematode and helps the fine-tuning of the infected plants to stress sparked by phytoparasitic nematodes.

Plant Peroxisomes: A Factory of Reactive Species

Plant peroxisomes are organelles enclosed by a single membrane whose biochemical composition has the capacity to adapt depending on the plant tissue, developmental stage, as well as internal and external cellular stimuli. Apart from the peroxisomal metabolism of reactive oxygen species (ROS), discovered several decades ago, new molecules with signaling potential, including nitric oxide (NO) and hydrogen sulfide (H2S), have been detected in these organelles in recent years. These molecules generate a family of derived molecules, called reactive nitrogen species (RNS) and reactive sulfur species (RSS), whose peroxisomal metabolism is autoregulated through posttranslational modifications (PTMs) such as S-nitrosation, nitration and persulfidation. The peroxisomal metabolism of these reactive species, which can be weaponized against pathogens, is susceptible to modification in response to external stimuli. This review aims to provide up-to-date information on crosstalk between these reactive species families and peroxisomes, as well as on their cellular environment in light of the well-recognized signaling properties of H2O2, NO and H2S.

Tyr-nitration in maize CDKA;1 results in lower affinity for ATP binding

Cyclin-dependent kinase A (CDKA) is a key component for cell cycle progression. The catalytic kinase activity depends on the protein's ability to form an active complex with cyclins and on phosphoregulatory mechanisms. Cell cycle arrest and plant growth impairment under abiotic stress have been linked to different molecular processes triggered by increased levels of reactive oxygen and nitrogen species (ROS and RNS). Among these, posttranslational modifications (PTMs) of key proteins such as CDKA;1 may be of significance. Herein, isolated maize embryo axes were subjected to sodium nitroprusside (SNP) as an inductor of nitrosative conditions to evaluate if CDKA;1 protein was a target for RNS. A high degree of protein nitration was detected; this included the specific Tyr-nitration of CDKA;1. Tyr15 and Tyr19, located at the ATP-binding site, were the selective targets for nitration according to both in silico analysis using the predictive software GPS-YNO₂, and in vitro mass spectrometry studies of recombinant nitrated ZmCDKA;1. Spectrofluorometric measurements demonstrated a reduction of ZmCDKA;1-NO₂ affinity for ATP. From these results, we conclude that Tyr nitration in CDKA;1 could act as an active modulator of cell cycle progression during redox stress.

Peroxynitrite induced signaling pathways in plant response to non-proteinogenic amino acids

Nitro/oxidative modifications of proteins and RNA nitration resulted from altered peroxynitrite generation are elements of the indirect mode of action of canavanine and meta-tyrosine in plants Environmental conditions and stresses, including supplementation with toxic compounds, are known to impair reactive oxygen (ROS) and reactive nitrogen species (RNS) homeostasis, leading to modification in production of oxidized and nitrated derivatives. The role of nitrated and/or oxidized biotargets differs depending on the stress factors and developmental stage of plants. Canavanine (CAN) and meta-tyrosine (m-Tyr) are non-proteinogenic amino acids (NPAAs). CAN, the structural analog of arginine, is found mostly in seeds of Fabaceae species, as a storage form of nitrogen. In mammalian cells, CAN is used as an anticancer agent due to its inhibitory action on nitric oxide synthesis. m-Tyr is a structural analogue of phenylalanine and an allelochemical found in root exudates of fescues. In animals, m-Tyr is recognized as a marker of oxidative stress. Supplementation of plants with CAN or m-Tyr modify ROS and RNS metabolism. Over the last few years of our research, we have collected the complex data on ROS and RNS metabolism in tomato (Solanum lycopersicum L.) plants exposed to CAN or m-Tyr. In addition, we have shown the level of nitrated RNA (8-Nitro-guanine) in roots of seedlings, stressed by the tested NPAAs. In this review, we describe the model of CAN and m-Tyr mode of action in plants based on modifications of signaling pathways induced by ROS/RNS with a special focus on peroxynitrite induced RNA and protein modifications.

The Hypoxic Proteome and Metabolome of Barley (Hordeum vulgare L.) with and without Phytoglobin Priming

Overexpression of phytoglobins (formerly plant hemoglobins) increases the survival rate of plant tissues under hypoxia stress by the following two known mechanisms: (1) scavenging of nitric oxide (NO) in the phytoglobin/NO cycle and (2) mimicking ethylene priming to hypoxia when NO scavenging activates transcription factors that are regulated by levels of NO and O2 in the N-end rule pathway. To map the cellular and metabolic effects of hypoxia in barley (Hordeum vulgare L., cv. Golden Promise), with or without priming to hypoxia, we studied the proteome and metabolome of wild type (WT) and hemoglobin overexpressing (HO) plants in normoxia and after 24 h hypoxia (WT24, HO24). The WT plants were more susceptible to hypoxia than HO plants. The chlorophyll a + b content was lowered by 50% and biomass by 30% in WT24 compared to WT, while HO plants were unaffected. We observed an increase in ROS production during hypoxia treatment in WT seedlings that was not observed in HO seedlings. We identified and quantified 9694 proteins out of which 1107 changed significantly in abundance. Many proteins, such as ion transporters, Ca2+-signal transduction, and proteins related to protein degradation were downregulated in HO plants during hypoxia, but not in WT plants. Changes in the levels of histones indicates that chromatin restructuring plays a role in the priming of hypoxia. We also identified and quantified 1470 metabolites, of which the abundance of >500 changed significantly. In summary the data confirm known mechanisms of hypoxia priming by ethylene priming and N-end rule activation; however, the data also indicate the existence of other mechanisms for hypoxia priming in plants.

Regulation of physiological aspects in plants by hydrogen sulfide and nitric oxide under challenging environment

Plants are exposed to a plethora of abiotic stresses such as drought, salinity, heavy metal and temperature stresses at different stages of their life cycle, from germination to seedling till the reproductive phase. As protective mechanisms, plants release signaling molecules that initiate a cascade of stress-signaling events, leading either to programmed cell death or plant acclimation. Hydrogen sulfide (H₂ S) and nitric oxide (NO) are considered as new 'gasotransmitter' molecules that play key roles in regulating gene expression, posttranslational modification (PTM), as well as cross-talk with other hormones. Although the exact role of NO in plants remains unclear and is species dependent, various studies have suggested a positive correlation between NO accumulation and environmental stress in plants. These molecules are also involved in a large array of stress responses and act synergistically or antagonistically as signaling components, depending on their respective concentration. This study provides a comprehensive update on the signaling interplay between H₂ S and NO in the regulation of various physiological processes under multiple abiotic stresses, modes of action and effects of exogenous application of these two molecules under drought, salt, heat and heavy metal stresses. However, the complete picture of the signaling cascades mediated by H₂ S and NO is still elusive. Recent researches indicate that during certain plant processes, such as stomatal closure, H₂ S could act upstream of NO signaling or downstream of NO in response to abiotic stresses by improving antioxidant activity in most plant species. In addition, PTMs of antioxidative pathways by these two molecules are also discussed.

Oxidation of proline from the cyclin-binding motif in maize CDKA;1 results in lower affinity with its cyclin regulatory subunit

Cyclin dependent kinase A; 1 (CDKA; 1) is essential in G1/S transition of cell cycle and its oxidation has been implicated in cell cycle arrest during plant abiotic stress. In the present study, an evaluation at the molecular level was performed to find possible sites of protein oxidative modifications. In vivo studies demonstrated that carbonylation of maize CDKA,1 is associated with a decrease in complex formation with maize cyclin D (CycD). Control and in vitro oxidized recombinant CDKA; 1 were sequenced by mass spectrometry. Proline at the PSTAIRE cyclin-binding motif was identified as the most susceptible oxidation site by comparative analysis of the resulted peptides. The specific interaction between CDKA; 1 and CycD6; 1, measured by surface plasmon resonance (SPR), demonstrated that the affinity and the kinetic of the interaction depended on the reduced-oxidized state of the CDKA; 1. CDKA; 1 protein oxidative modification would be in part responsible for affecting cell cycle progression, and thus producing plant growth inhibition under oxidative stress.

Short-Term Low Temperature Induces Nitro-Oxidative Stress that Deregulates the NADP-Malic Enzyme Function by Tyrosine Nitration in Arabidopsis thaliana

Low temperature (LT) negatively affects plant growth and development via the alteration of the metabolism of reactive oxygen and nitrogen species (ROS and RNS). Among RNS, tyrosine nitration, the addition of an NO₂ group to a tyrosine residue, can modulate reduced nicotinamide-dinucleotide phosphate (NADPH)-generating systems and, therefore, can alter the levels of NADPH, a key cofactor in cellular redox homeostasis. NADPH also acts as an indispensable electron donor within a wide range of enzymatic reactions, biosynthetic pathways, and detoxification processes, which could affect plant viability. To extend our knowledge about the regulation of this key cofactor by this nitric oxide (NO)-related post-translational modification, we analyzed the effect of tyrosine nitration on another NADPH-generating enzyme, the NADP-malic enzyme (NADP-ME), under LT stress. In Arabidopsis thaliana seedlings exposed to short-term LT (4 °C for 48 h), a 50% growth reduction accompanied by an increase in the content of superoxide, nitric oxide, and peroxynitrite, in addition to diminished cytosolic NADP-ME activity, were found. In vitro assays confirmed that peroxynitrite inhibits cytosolic NADP-ME2 activity due to tyrosine nitration. The mass spectrometric analysis of nitrated NADP-ME2 enabled us to determine that Tyr-73 was exclusively nitrated to 3-nitrotyrosine by peroxynitrite. The in silico analysis of the Arabidopsis NADP-ME2 protein sequence suggests that Tyr73 nitration could disrupt the interactions between the specific amino acids responsible for protein structure stability. In conclusion, the present data show that short-term LT stress affects the metabolism of ROS and RNS, which appears to negatively modulate the activity of cytosolic NADP-ME through the tyrosine nitration process.

Redox Homeostasis in Photosynthetic Organisms: Novel and Established Thiol-Based Molecular Mechanisms

Significance: Redox homeostasis consists of an intricate network of reactions in which reactive molecular species, redox modifications, and redox proteins act in concert to allow both physiological responses and adaptation to stress conditions. Recent Advances: This review highlights established and novel thiol-based regulatory pathways underlying the functional facets and significance of redox biology in photosynthetic organisms. In the last decades, the field of redox regulation has largely expanded and this work is aimed at giving the right credit to the importance of thiol-based regulatory and signaling mechanisms in plants. Critical Issues: This cannot be all-encompassing, but is intended to provide a comprehensive overview on the structural/molecular mechanisms governing the most relevant thiol switching modifications with emphasis on the large genetic and functional diversity of redox controllers (i.e., redoxins). We also summarize the different proteomic-based approaches aimed at investigating the dynamics of redox modifications and the recent evidence that extends the possibility to monitor the cellular redox state in vivo. The physiological relevance of redox transitions is discussed based on reverse genetic studies confirming the importance of redox homeostasis in plant growth, development, and stress responses. Future Directions: In conclusion, we can firmly assume that redox biology has acquired an established significance that virtually infiltrates all aspects of plant physiology.

The Role of the Plant Antioxidant System in Drought Tolerance

Water deficiency compromises plant performance and yield in many habitats and in agriculture. In addition to survival of the acute drought stress period which depends on plant-genotype-specific characteristics, stress intensity and duration, also the speed and efficiency of recovery determine plant performance. Drought-induced deregulation of metabolism enhances generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) which in turn affect the redox regulatory state of the cell. Strong correlative and analytical evidence assigns a major role in drought tolerance to the redox regulatory and antioxidant system. This review compiles current knowledge on the response and function of superoxide, hydrogen peroxide and nitric oxide under drought stress in various species and drought stress regimes. The meta-analysis of reported changes in transcript and protein amounts, and activities of components of the antioxidant and redox network support the tentative conclusion that drought tolerance is more tightly linked to up-regulated ascorbate-dependent antioxidant activity than to the response of the thiol-redox regulatory network. The significance of the antioxidant system in surviving severe phases of dehydration is further supported by the strong antioxidant system usually encountered in resurrection plants.

Nitric Oxide Remodels the Photosynthetic Apparatus upon S-Starvation in Chlamydomonas reinhardtii

Many photosynthetic autotrophs have evolved responses that adjust their metabolism to limitations in nutrient availability. Here we report a detailed characterization of the remodeling of photosynthesis upon sulfur starvation under heterotrophy and photo-autotrophy in the green alga (Chlamydomonas reinhardtii). Photosynthetic inactivation under low light and darkness is achieved through specific degradation of Rubisco and cytochrome b ₆ f and occurs only in the presence of reduced carbon in the medium. The process is likely regulated by nitric oxide (NO), which is produced 24 h after the onset of starvation, as detected with NO-sensitive fluorescence probes visualized by fluorescence microscopy. We provide pharmacological evidence that intracellular NO levels govern this degradation pathway: the addition of a NO scavenger decreases the rate of cytochrome b ₆ f and Rubisco degradation, whereas NO donors accelerate the degradation. Based on our analysis of the relative contribution of the different NO synthesis pathways, we conclude that the NO₂-dependent nitrate reductase-independent pathway is crucial for NO production under sulfur starvation. Our data argue for an active role for NO in the remodeling of thylakoid protein complexes upon sulfur starvation.

Canavanine-Induced Decrease in Nitric Oxide Synthesis Alters Activity of Antioxidant System but Does Not Impact S-Nitrosoglutathione Catabolism in Tomato Roots

Canavanine (CAN) is a nonproteinogenic amino acid synthesized in legumes. In mammalians, as arginine analogue, it is an inhibitor of nitric oxide synthase (NOS) activity. The aim of this study was to investigate the impact of CAN-induced nitric oxide level limitation on the antioxidant system and S-nitrosoglutathione (GSNO) metabolism in roots of tomato seedlings. Treatment with CAN (10 or 50 µM) for 24-72 h led to restriction in root growth. Arginine-dependent NOS-like activity was almost completely inhibited, demonstrating direct effect of CAN action. CAN increased total antioxidant capacity and the level of sulphydryl groups. Catalase (CAT) and superoxide dismutase (SOD) activity decreased in CAN exposed roots. CAN supplementation resulted in the decrease of transcript levels of genes coding CAT (with the exception of CAT1). Genes coding SOD (except MnSOD and CuSOD) were upregulated by CAN short treatment; prolonged exposition to 50-µM CAN resulted in downregulation of FeSOD, CuSOD, and SODP-2. Activity of glutathione reductase dropped down after short-term (10-µM CAN) supplementation, while glutathione peroxidase activity was not affected. Transcript levels of glutathione reductase genes declined in response to CAN. Genes coding glutathione peroxidase were upregulated by 50-µM CAN, while 10-µM CAN downregulated GSHPx1. Inhibition of NOS-like activity by CAN resulted in lower GSNO accumulation in root tips. Activity of GSNO reductase was decreased by short-term supplementation with CAN. In contrast, GSNO reductase protein abundance was higher, while transcript levels were slightly altered in roots exposed to CAN. This is the first report on identification of differentially nitrated proteins in response to supplementation with nonproteinogenic amino acid. Among nitrated proteins differentially modified by CAN, seed storage proteins (after short-term CAN treatment) and components of the cellular redox system (after prolonged CAN supplementation) were identified. The findings demonstrate that due to inhibition of NOS-like activity, CAN leads to modification in antioxidant system. Limitation in GSNO level is due to lower nitric oxide formation, while GSNO catabolism is less affected. We demonstrated that monodehydroascorbate reductase, activity of which is inhibited in roots of CAN-treated plants, is the protein preferentially modified by tyrosine nitration.

UPLC-HRMS-based untargeted metabolic profiling reveals changes in chickpea (Cicer arietinum) metabolome following long-term drought stress

Genetic improvement for drought tolerance in chickpea requires a solid understanding of biochemical processes involved with different physiological mechanisms. The objective of this study is to demonstrate genetic variations in altered metabolic levels in chickpea varieties (tolerant and sensitive) grown under contrasting water regimes through ultrahigh-performance liquid chromatography/high-resolution mass spectrometry-based untargeted metabolomic profiling. Chickpea plants were exposed to drought stress at the 3-leaf stage for 25 days, and the leaves were harvested at 14 and 25 days after the imposition of drought stress. Stress produced significant reduction in chlorophyll content, F_v /F_m , relative water content, and shoot and root dry weight. Twenty known metabolites were identified as most important by 2 different methods including significant analysis of metabolites and partial least squares discriminant analysis. The most pronounced increase in accumulation due to drought stress was demonstrated for allantoin, l-proline, l-arginine, l-histidine, l-isoleucine, and tryptophan. Metabolites that showed a decreased level of accumulation under drought conditions were choline, phenylalanine, gamma-aminobutyric acid, alanine, phenylalanine, tyrosine, glucosamine, guanine, and aspartic acid. Aminoacyl-tRNA and plant secondary metabolite biosynthesis and amino acid metabolism or synthesis pathways were involved in producing genetic variation under drought conditions. Metabolic changes in light of drought conditions highlighted pools of metabolites that affect the metabolic and physiological adjustment in chickpea that reduced drought impacts.

Redox-Dependent Chromatin Remodeling: A New Function of Nitric Oxide as Architect of Chromatin Structure in Plants

Nitric oxide (NO) is a key signaling molecule in all kingdoms. In plants, NO is involved in the regulation of various processes of growth and development as well as biotic and abiotic stress response. It mainly acts by modifying protein cysteine or tyrosine residues or by interacting with protein bound transition metals. Thereby, the modification of cysteine residues known as protein S-nitrosation is the predominant mechanism for transduction of NO bioactivity. Histone acetylation on N-terminal lysine residues is a very important epigenetic regulatory mechanism. The transfer of acetyl groups from acetyl-coenzyme A on histone lysine residues is catalyzed by histone acetyltransferases. This modification neutralizes the positive charge of the lysine residue and results in a loose structure of the chromatin accessible for the transcriptional machinery. Histone deacetylases, in contrast, remove the acetyl group of histone tails resulting in condensed chromatin with reduced gene expression activity. In plants, the histone acetylation level is regulated by S-nitrosation. NO inhibits HDA complexes resulting in enhanced histone acetylation and promoting a supportive chromatin state for expression of genes. Moreover, methylation of histone tails and DNA are important epigenetic modifications, too. Interestingly, methyltransferases and demethylases are described as targets for redox molecules in several biological systems suggesting that these types of chromatin modifications are also regulated by NO. In this review article, we will focus on redox-regulation of histone acetylation/methylation and DNA methylation in plants, discuss the consequences on the structural level and give an overview where NO can act to modulate chromatin structure.

Plant selenium toxicity: Proteome in the crosshairs

The metalloid element, selenium (Se) is in many ways special and perhaps because of this its research in human and plant systems is of great interest. Despite its non-essentiality, higher plants take it up and metabolize it via sulfur pathways, but higher amounts of Se cause toxic symptoms in plants. However, the molecular mechanisms of selenium phytotoxicity have been only partly revealed; the data obtained so far point out that Se toxicity targets the plant proteome. Besides seleno- and oxyproteins, nitroproteins are also formed due to Se stress. In order to minimize proteomic damages induced by Se, certain plants are able to redirect selenocysteine away from protein synthesis thus preventing Se-protein formation. Additionally, the damaged or malformed selenoproteins, oxyproteins and nitroproteins may be removed by proteasomes. Based on the literature this review sets Se toxicity mechanisms into a new concept and it draws attention to the importance of Se-induced protein-level changes.

Short-Term Exposure to Nitrogen Dioxide Provides Basal Pathogen Resistance

Nitrogen dioxide (NO2) forms in plants under stress conditions, but little is known about its physiological functions. Here, we explored the physiological functions of NO2 in plant cells using short-term fumigation of Arabidopsis (Arabidopsis thaliana) for 1 h with 10 µL L-1 NO2. Although leaf symptoms were absent, the expression of genes related to pathogen resistance was induced. Fumigated plants developed basal disease resistance, or pattern-triggered immunity, against the necrotrophic fungus Botrytis cinerea and the hemibiotrophic bacterium Pseudomonas syringae Functional salicylic acid and jasmonic acid (JA) signaling pathways were both required for the full expression of NO2-induced resistance against B. cinerea An early peak of salicylic acid accumulation immediately after NO2 exposure was followed by a transient accumulation of oxophytodienoic acid. The simultaneous NO2-induced expression of genes involved in jasmonate biosynthesis and jasmonate catabolism resulted in the complete suppression of JA and JA-isoleucine (JA-Ile) accumulation, which was accompanied by a rise in the levels of their catabolic intermediates 12-OH-JA, 12-OH-JA-Ile, and 12-COOH-JA-Ile. NO2-treated plants emitted the volatile monoterpene α-pinene and the sesquiterpene longifolene (syn. junipene), which could function in signaling or direct defense against pathogens. NO2-triggered B. cinerea resistance was dependent on enhanced early callose deposition and CYTOCHROME P450 79B2 (CYP79B2), CYP79B3, and PHYTOALEXIN DEFICIENT3 gene functions but independent of camalexin, CYP81F2, and 4-OH-indol-3-ylmethylglucosinolate derivatives. In sum, exogenous NO2 triggers basal pathogen resistance, pointing to a possible role for endogenous NO2 in defense signaling. Additionally, this study revealed the involvement of jasmonate catabolism and volatiles in pathogen immunity.

Crosstalk between reactive oxygen species and nitric oxide in plants: Key role of S-nitrosoglutathione reductase

Nitric oxide (.NO) acts as signaling molecule in plants being involved in diverse physiological processes such as germination, root growth, stomata closing and response to biotic and abiotic stress. S-Nitrosoglutathione (GSNO) is the storage and transport form of.NO and has a very important function in.NO signaling since it can transfer its.NO moiety to other proteins (trans-nitrosylation). The level of GSNO and thus the level of S-nitrosylated proteins are regulated by GSNO-reductase (GSNOR). In this way, this enzyme regulates the S-nitrosothiol levels and plays a balancing role in fine-tuning.NO signaling. Interestingly, oxidative post-translationally modification of GSNOR inhibited the activity of this enzyme suggesting a direct crosstalk between ROS- and RNS-signaling. In this review article the regulatory effects of ROS on GSNOR are highlighted and their physiological function in context of crosstalk between ROS and.NO and species in plants are discussed.

Nitric oxide sensor proteins with revolutionary potential

This article comments on: Calvo-Begueria L, Rubio MC, Martínez JI, Pérez-Rontomé C, Delgado MJ, Bedmar EJ, Becana M. 2018. Redefining nitric oxide production in legume nodules through complementary insights from electron paramagnetic resonance spectroscopy and specific fluorescent probes. Journal of Experimental Botany 69, 3703–3714.

Generation of nitric oxide by olive (Olea europaea L.) pollen during in vitro germination and assessment of the S-nitroso- and nitro-proteomes by computational predictive methods

Nitric oxide is recognized as a signaling molecule involved in a broad range of physiological processes in plants including sexual reproduction. NO has been detected in the pollen grain at high levels and regulates pollen tube growth. Previous studies demonstrated that NO as well as ROS are produced in the olive reproductive tissues in a stage- and tissue-specific manner. The aim of this study was to assess the production of NO throughout the germination of olive (Olea europaea L.) pollen in vitro. The NO fluorescent probe DAF-2DA was used to image NO production in situ, which was correlated to pollen viability. Moreover, by means of a fluorimetric assay we showed that growing pollen tubes release NO. GSNO -a mobile reservoir of NO, formed by the S-nitrosylation of NO with reduced glutathione (GSH) - was for the first time detected and quantified at different stages of pollen tube growth using a LC-ES/MS analysis. Exogenous NO donors inhibited both pollen germination and pollen tube growth and these effects were partially reverted by the specific NO-scavenger c-PTIO. However, little is known about how NO affects the germination process. With the aim of elucidating the putative relevance of protein S-nitrosylation and Tyr-nitration as important post-translational modifications in the development and physiology of the olive pollen, a de novo assembled and annotated reproductive transcriptome from olive was challenged in silico for the putative capability of transcripts to become potentially modified by S-nitrosylation/Tyr-nitration according to well-established criteria. Numerous gene products with these characteristics were identified, and a broad discussion as regards to their potential role in plant reproduction was built after their functional classification. Moreover, the importance of both S-nitrosylation/Tyr-nitrations was experimentally assessed and validated by using Western blotting, immunoprecipitation and proteomic approaches.

Thioredoxin (Trxo1) interacts with proliferating cell nuclear antigen (PCNA) and its overexpression affects the growth of tobacco cell culture

Thioredoxins (Trxs), key components of cellular redox regulation, act by controlling the redox status of many target proteins, and have been shown to play an essential role in cell survival and growth. The presence of a Trx system in the nucleus has received little attention in plants, and the nuclear targets of plant Trxs have not been conclusively identified. Thus, very little is known about the function of Trxs in this cellular compartment. Previously, we studied the intracellular localization of PsTrxo1 and confirmed its presence in mitochondria and, interestingly, in the nucleus under standard growth conditions. In investigating the nuclear function of PsTrxo1 we identified proliferating cellular nuclear antigen (PCNA) as a PsTrxo1 target by means of affinity chromatography techniques using purified nuclei from pea leaves. Such protein-protein interaction was corroborated by dot-blot and bimolecular fluorescence complementation (BiFC) assays, which showed that both proteins interact in the nucleus. Moreover, PsTrxo1 showed disulfide reductase activity on previously oxidized recombinant PCNA protein. In parallel, we studied the effects of PsTrxo1 overexpression on Tobacco Bright Yellow-2 (TBY-2) cell cultures. Microscopy and flow-cytometry analysis showed that PsTrxo1 overexpression increases the rate of cell proliferation in the transformed lines, with a higher percentage of the S phase of the cell cycle at the beginning of the cell culture (days 1 and 3) and at the G2/M phase after longer times of culture (day 9), coinciding with an upregulation of PCNA protein. Furthermore, in PsTrxo1 overexpressed cells there is a decrease in the total cellular glutathione content but maintained nuclear GSH accumulation, especially at the end of the culture, which is accompanied by a higher mitotic index, unlike non-overexpressing cells. These results suggest that Trxo1 is involved in the cell cycle progression of TBY-2 cultures, possibly through its link with cellular PCNA and glutathione.