AtGSNOR1 function is required for multiple developmental programs in Arabidopsis

Nitric oxide and cytokinin cross-talk and their role in plant hypoxia response

Nitric oxide (NO) and cytokinins (CKs) are known for their crucial contributions to plant development, growth, senescence, and stress response. Despite the importance of both signals in stress responses, their interaction remains largely unexplored. The interplay between NO and CKs emerges as particularly significant not only regarding plant growth and development but also in addressing plant stress response, particularly in the context of extreme weather events leading to yield loss. In this review, we summarize NO and CKs metabolism and signaling. Additionally, we emphasize the crosstalk between NO and CKs, underscoring its potential impact on stress response, with a focus on hypoxia tolerance. Finally, we address the most urgent questions that demand answers and offer recommendations for future research endeavors.

Nitric oxide in plants: an insight on redox activity and responses toward abiotic stress signaling

Plants, as sessile organisms, are subjected to diverse abiotic stresses, including salinity, desiccation, metal toxicity, thermal fluctuations, and hypoxia at different phases of plant growth. Plants can activate messenger molecules to initiate a signaling cascade of response toward environmental stresses that results in either cell death or plant acclimation. Nitric oxide (NO) is a small gaseous redox-active molecule that exhibits a plethora of physiological functions in growth, development, flowering, senescence, stomata closure and responses to environmental stresses. It can also facilitate alteration in protein function and reprogram the gene profiling by direct or indirect interaction with different target molecules. The bioactivity of NO can be manifested through different redox-based protein modifications including S-nitrosylation, protein nitration, and metal nitrosylation in plants. Although there has been considerable progress in the role of NO in regulating stress signaling, still the physiological mechanisms regarding the abiotic stress tolerance in plants remain unclear. This review summarizes recent advances in understanding the emerging knowledge regarding NO function in plant tolerance against abiotic stresses. The manuscript also highlighted the importance of NO as an abiotic stress modulator and developed a rational design for crop cultivation under a stress environment.

Role of protein S-nitrosylation in plant growth and development

In plants, nitric oxide (NO) has been widely accepted as a signaling molecule that plays a role in different processes. Among the most relevant pathways by which NO and its derivatives realize their biological functions, post-translational protein modifications are worth mentioning. Protein S-nitrosylation has been the most studied NO-dependent regulatory mechanism; it is emerging as an essential mechanism for transducing NO bioactivity in plants and animals. In recent years, the research of protein S-nitrosylation in plant growth and development has made significant progress, including processes such as seed germination, root development, photosynthetic regulation, flowering regulation, apoptosis, and plant senescence. In this review, we focus on the current state of knowledge on the role of S-nitrosylation in plant growth and development and provide a better understanding of its action mechanisms.

De-nitrosylation Coordinates Appressorium Function for Infection of the Rice Blast Fungus

As a signaling molecule, nitric oxide (NO) regulates the development and stress response in different organisms. The major biological activity of NO is protein S-nitrosylation, whose function in fungi remains largely unclear. Here, it is found in the rice blast fungus Magnaporthe oryzae, de-nitrosylation process is essential for functional appressorium formation during infection. Nitrosative stress caused by excessive accumulation of NO is harmful for fungal infection. While the S-nitrosoglutathione reductase GSNOR-mediated de-nitrosylation removes excess NO toxicity during appressorium formation to promote infection. Through an indoTMT switch labeling proteomics technique, 741 S-nitrosylation sites in 483 proteins are identified. Key appressorial proteins, such as Mgb1, MagB, Sps1, Cdc42, and septins, are activated by GSNOR through de-nitrosylation. Removing S-nitrosylation sites of above proteins is essential for proper protein structure and appressorial function. Therefore, GSNOR-mediated de-nitrosylation is an essential regulator for appressorium formation. It is also shown that breaking NO homeostasis by NO donors, NO scavengers, as well as chemical inhibitor of GSNOR, shall be effective methods for fungal disease control.

Protein S-nitrosylation under abiotic stress: Role and mechanism

Abiotic stress is one of the main threats affecting crop growth and production. Nitric oxide (NO), an important signaling molecule involved in wide range of plant growth and development as well as in response to abiotic stress. NO can exert its biological functions through protein S-nitrosylation, a redox-based posttranslational modification by covalently adding NO moiety to a reactive cysteine thiol of a target protein to form an S-nitrosothiol (SNO). Protein S-nitrosylation is an evolutionarily conserved mechanism regulating multiple aspects of cellular signaling in plant. Recently, emerging evidence have elucidated protein S-nitrosylation as a modulator of plant in responses to abiotic stress, including salt stress, extreme temperature stress, light stress, heavy metal and drought stress. In addition, significant mechanism has been made in functional characterization of protein S-nitrosylated candidates, such as changing protein conformation, and the subcellular localization of proteins, regulating protein activity and influencing protein interactions. In this study, we updated the data related to protein S-nitrosylation in plants in response to adversity and gained a deeper understanding of the functional changes of target proteins after protein S-nitrosylation.

Differential S-nitrosylation and characterization of purified S-nitrosoglutathione reductase (GSNOR) from Brassica juncea shows multiple forms of the enzyme

S-nitrosoglutathione reductase (GSNOR). a master regulator of NO homeostasis, is a single-copy gene in most plants. In Lotus japonicus, two GSNOR isoforms were identified exhibiting similar kinetic properties but differential tissue-specific expressions. Previously, a genome-wide identification in Brassica juncea revealed four copies of GSNOR, each encoding proteins that vary in subunit molecular weights and pI. Here, we report multiple forms of GSNOR using 2D immunoblot which showed 4 immunopositive spots of 41.5 kDa (pl 5.79 and 6.78) and 43 kDa (pl 6.16 and 6.23). To confirm, purification of GSNOR using anion-exchange chromatography yielded 2 distinct pools (GSNOR-A & GSNOR-B) with GSNOR activities. Subsequently, affinity-based purification resulted in 1 polypeptide from GSNOR-A and 2 polypeptides from GSNOR-B. Size exclusion-HPLC confirmed 3 GSNORs with molecular weight of 87.48 ± 2.74 KDa (GSNOR-A); 87.36 ± 3.25 and 82.74 ± 2.75 kDa (GSNOR-B). Kinetic analysis showed Km of 118 ± 11 μM and Vmax of 287 ± 22 nkat/mg for GSNOR-A, whereas Km of 96.4 ± 8 μM and Vmax of 349 ± 15 nkat/mg for GSNOR-B. S-nitrosylation and inhibition by NO showed redox regulation of all BjGSNORs. Both purified GSNORs exhibited variable denitrosylation efficiency as depicted by Biotin Switch assay. To the best of our knowledge, this is the first report confirming multiple isoforms of GSNOR in B. juncea.

Loss of S-nitrosoglutathione reductase disturbs phytohormone homeostasis and regulates shoot side branching and fruit growth in tomato

S-Nitrosoglutathione plays a central role in nitric oxide (NO) homeostasis, and S-nitrosoglutathione reductase (GSNOR) regulates the cellular levels of S-nitrosoglutathione across kingdoms. Here, we investigated the role of endogenous NO in shaping shoot architecture and controlling fruit set and growth in tomato (Solanum lycopersicum). SlGSNOR silencing promoted shoot side branching and led to reduced fruit size, negatively impacting fruit yield. Greatly intensified in slgsnor knockout plants, these phenotypical changes were virtually unaffected by SlGSNOR overexpression. Silencing or knocking out of SlGSNOR intensified protein tyrosine nitration and S-nitrosation and led to aberrant auxin production and signaling in leaf primordia and fruit-setting ovaries, besides restricting the shoot basipetal polar auxin transport stream. SlGSNOR deficiency triggered extensive transcriptional reprogramming at early fruit development, reducing pericarp cell proliferation due to restrictions on auxin, gibberellin, and cytokinin production and signaling. Abnormal chloroplast development and carbon metabolism were also detected in early-developing NO-overaccumulating fruits, possibly limiting energy supply and building blocks for fruit growth. These findings provide new insights into the mechanisms by which endogenous NO fine-tunes the delicate hormonal network controlling shoot architecture, fruit set, and post-anthesis fruit development, emphasizing the relevance of NO-auxin interaction for plant development and productivity.

Fine-tuned nitric oxide and hormone interface in plant root development and regeneration

Plant root growth and developmental capacities reside in a few stem cells of the root apical meristem (RAM). Maintenance of these stem cells requires regenerative divisions of the initial stem cell niche (SCN) cells, self-maintenance, and proliferative divisions of the daughter cells. This ensures sufficient cell diversity to guarantee the development of complex root tissues in the plant. Damage in the root during growth involves the formation of a new post-embryonic root, a process known as regeneration. Post-embryonic root development and organogenesis processes include primary root development and SCN maintenance, plant regeneration, and the development of adventitious and lateral roots. These developmental processes require a fine-tuned balance between cell proliferation and maintenance. An important regulator during root development and regeneration is the gasotransmitter nitric oxide (NO). In this review we have sought to compile how NO regulates cell rate proliferation, cell differentiation, and quiescence of SCNs, usually through interaction with phytohormones, or other molecular mechanisms involved in cellular redox homeostasis. NO exerts a role on molecular components of the auxin and cytokinin signaling pathways in primary roots that affects cell proliferation and maintenance of the RAM. During root regeneration, a peak of auxin and cytokinin triggers specific molecular programs. Moreover, NO participates in adventitious root formation through its interaction with players of the brassinosteroid and cytokinin signaling cascade. Lately, NO has been implicated in root regeneration under hypoxia conditions by regulating stem cell specification through phytoglobins.

NO Is Not the Same as GSNO in the Regulation of Fe Deficiency Responses by Dicot Plants

Iron (Fe) is abundant in soils but with a poor availability for plants, especially in calcareous soils. To favor its acquisition, plants develop morphological and physiological responses, mainly in their roots, known as Fe deficiency responses. In dicot plants, the regulation of these responses is not totally known, but some hormones and signaling molecules, such as auxin, ethylene, glutathione (GSH), nitric oxide (NO) and S-nitrosoglutathione (GSNO), have been involved in their activation. Most of these substances, including auxin, ethylene, GSH and NO, increase their production in Fe-deficient roots while GSNO, derived from GSH and NO, decreases its content. This paradoxical result could be explained with the increased expression and activity in Fe-deficient roots of the GSNO reductase (GSNOR) enzyme, which decomposes GSNO to oxidized glutathione (GSSG) and NH3. The fact that NO content increases while GSNO decreases in Fe-deficient roots suggests that NO and GSNO do not play the same role in the regulation of Fe deficiency responses. This review is an update of the results supporting a role for NO, GSNO and GSNOR in the regulation of Fe deficiency responses. The possible roles of NO and GSNO are discussed by taking into account their mode of action through post-translational modifications, such as S-nitrosylation, and through their interactions with the hormones auxin and ethylene, directly related to the activation of morphological and physiological responses to Fe deficiency in dicot plants.

Nitric oxide-mediated S-nitrosylation of IAA17 protein in intrinsically disordered region represses auxin signaling

The phytohormone auxin plays crucial roles in nearly every aspect of plant growth and development. Auxin signaling is activated through the phytohormone-induced proteasomal degradation of the Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) family of transcriptional repressors. Notably, many auxin-modulated physiological processes are also regulated by nitric oxide (NO) that executes its biological effects predominantly through protein S-nitrosylation at specific cysteine residues. However, little is known about the molecular mechanisms in regulating the interactive NO and auxin networks. Here, we show that NO represses auxin signaling by inhibiting IAA17 protein degradation. NO induces the S-nitrosylation of Cys-70 located in the intrinsically disordered region of IAA17, which inhibits the TIR1-IAA17 interaction and consequently the proteasomal degradation of IAA17. The accumulation of a higher level of IAA17 attenuates auxin response. Moreover, an IAA17C70W nitrosomimetic mutation renders the accumulation of a higher level of the mutated protein, thereby causing partial resistance to auxin and defective lateral root development. Taken together, these results suggest that S-nitrosylation of IAA17 at Cys-70 inhibits its interaction with TIR1, thereby negatively regulating auxin signaling. This study provides unique molecular insights into the redox-based auxin signaling in regulating plant growth and development.

FAT-switch-based quantitative S-nitrosoproteomics reveals a key role of GSNOR1 in regulating ER functions

Reversible protein S-nitrosylation regulates a wide range of biological functions and physiological activities in plants. However, it is challenging to quantitively determine the S-nitrosylation targets and dynamics in vivo. In this study, we develop a highly sensitive and efficient fluorous affinity tag-switch (FAT-switch) chemical proteomics approach for S-nitrosylation peptide enrichment and detection. We quantitatively compare the global S-nitrosylation profiles in wild-type Arabidopsis and gsnor1/hot5/par2 mutant using this approach, and identify 2,121 S-nitrosylation peptides in 1,595 protein groups, including many previously unrevealed S-nitrosylated proteins. These are 408 S-nitrosylated sites in 360 protein groups showing an accumulation in hot5-4 mutant when compared to wild type. Biochemical and genetic validation reveal that S-nitrosylation at Cys337 in ER OXIDOREDUCTASE 1 (ERO1) causes the rearrangement of disulfide, resulting in enhanced ERO1 activity. This study offers a powerful and applicable tool for S-nitrosylation research, which provides valuable resources for studies on S-nitrosylation-regulated ER functions in plants.

Nitric Oxide Regulates Seed Germination by Integrating Multiple Signalling Pathways

Seed germination is of great significance for plant development and crop yield. Recently, nitric oxide (NO) has been shown to not only serve as an important nitrogen source during seed development but also to participate in a variety of stress responses in plants to high salt, drought, and high temperature. In addition, NO can affect the process of seed germination by integrating multiple signaling pathways. However, due to the instability of NO gas activity, the network mechanism for its fine regulation of seed germination remains unclear. Therefore, this review aims to summarize the complex anabolic processes of NO in plants, to analyze the interaction mechanisms between NO-triggered signaling pathways and different plant hormones such as abscisic acid (ABA) and gibberellic acid (GA), ethylene (ET) and reactive oxygen species (ROS) signaling molecules, and to discuss the physiological responses and molecular mechanisms of seeds during the involvement of NO in abiotic stress, so as to provide a reference for solving the problems of seed dormancy release and improving plant stress tolerance.

Enhanced Resistance of atnigr1 against Pseudomonas syringae pv. tomato Suggests Negative Regulation of Plant Basal Defense and Systemic Acquired Resistance by AtNIGR1 Encoding NAD(P)-Binding Rossmann-Fold in Arabidopsis thaliana

Nitric oxide (NO) regulates several biological and physiological processes in plants. This study investigated the role of Arabidopsis thaliana Negative Immune and Growth Regulator 1 (AtNIGR1), encoding an NAD(P)-binding Rossmann-fold superfamily, in the growth and immunity of Arabidopsis thaliana. AtNIGR1 was pooled from the CySNO transcriptome as a NO-responsive gene. Seeds of the knockout (atnigr1) and overexpression plants were evaluated for their response to oxidative [(hydrogen peroxide (H₂O₂) and methyl viologen (MV)] or nitro-oxidative [(S-nitroso-L-cysteine (CySNO) and S-nitroso glutathione (GSNO)] stress. Results showed that the root and shoot growth of atnigr1 (KO) and AtNIGR1 (OE) exhibited differential phenotypic responses under oxidative and nitro-oxidative stress and normal growth conditions. To investigate the role of the target gene in plant immunity, the biotrophic bacterial pathogen Pseudomonas syringae pv. tomato DC3000 virulent (Pst DC3000 vir) was used to assess the basal defense, while the Pst DC3000 avirulent (avrB) strain was used to investigate R-gene-mediated resistance and systemic acquired resistance (SAR). Data revealed that AtNIGR1 negatively regulated basal defense, R-gene-mediated resistance, and SAR. Furthermore, the Arabidopsis eFP browser indicated that the expression of AtNIGR1 is detected in several plant organs, with the highest expression observed in germinating seeds. All results put together suggest that AtNIGR1 could be involved in plant growth, as well as basal defense and SAR, in response to bacterial pathogens in Arabidopsis.

Nitric oxide induces S-nitrosylation of CESA1 and CESA9 and increases cellulose content in Arabidopsis hypocotyls

Nitric oxide (NO), a small signaling gas molecule, participates in several growth and developmental processes in plants. However, how NO regulates cell wall biosynthesis remains unclear. Here, we demonstrate a positive effect of NO on cellulose content that may be related to S-nitrosylation of cellulose synthase 1 (CESA1) and CESA9. Two S-nitrosylated cysteine (Cys) residues, Cys562 and Cys641, which are exposed on the surface of CESA1 and CESA9 and located in the cellulose synthase catalytic domain, were identified to be S-nitrosylated. Meanwhile, Cys641 was located on the binding surface of CESA1 and CESA9, and Cys562 was very close to the binding surface. Cellulose synthase complexes (CSCs) dynamics are closely associated with cellulose content. S-nitrosylation of CESA1 and CESA9 improved particles mobility and thus increased the accumulation of cellulose in Arabidopsis hypocotyl cells. An increase in hemicellulose content as well as an alteration in pectin content facilitated cell wall extension and contributed to cell growth, finally promoting elongation of Arabidopsis hypocotyls. Overall, our work provides a path to investigate the way NO affects the cellulose content of plants.

Regulatory roles of selective autophagy through targeting of native proteins in plant adaptive responses

Selective autophagy functions as a regulatory mechanism by targeting native and functional proteins to ensure their proper levels and activities in plant adaptive responses. Autophagy is a cellular degradation and recycling pathway with a key role in cellular homeostasis and metabolism. Autophagy is initiated with the biogenesis of autophagosomes, which fuse with the lysosomes or vacuoles to release their contents for degradation. Under nutrient starvation or other adverse environmental conditions, autophagy usually targets unwanted or damaged proteins, organelles and other cellular components for degradation and recycling to promote cell survival. Over the past decade, however, a substantial number of studies have reported that autophagy in plants also functions as a regulatory mechanism by targeting enzymes, structural and regulatory proteins that are not necessarily damaged or dysfunctional to ensure their proper abundance and function to facilitate cellular changes required for response to endogenous and environmental conditions. During plant-pathogen interactions in particular, selective autophagy targets specific pathogen components as a defense mechanism and pathogens also utilize autophagy to target functional host factors to suppress defense mechanisms. Autophagy also targets native and functional protein regulators of plant heat stress memory, hormone signaling, and vesicle trafficking associated with plant responses to abiotic and other conditions. In this review, we discuss advances in the regulatory roles of selective autophagy through targeting of native proteins in plant adaptive responses, what questions remain and how further progress in the analysis of these special regulatory roles of autophagy can help understand biological processes important to plants.

A Novel RHS1 Locus in Rice Attributes Seed-Pod Shattering by the Regulation of Endogenous S-Nitrosothiols

Seed or pod shattering in rice (Oryza sativa) is considered to be one of the major factors involved in the domestication of rice as a crop. High seed shattering results in significant yield losses. In this study, we characterize the RICEHIGHSHATTERING 1 (RHS1) that corresponds to the locus LOC_Os04g41250 from a greenhouse screen, involving 145 Ac/Ds transposon mutant rice lines. The knockout mutant line rhs1 exhibited a significantly high shattering of grains in comparison to the wild-type plants. The exogenous application of nitric oxide (NO) resulted in a significant reduction in the expression of RHS1 in wild-type rice plants. The absence of RHS1, which encodes a putative armadillo/beta-catenin repeat family protein, resulted in high sensitivity of the rhs1 plants to nitrosative stress. Interestingly, the basal expression levels of QSH1 and SHAT1 genes (transcription factors that regulate seed-pod shattering in rice) were significantly lower in these plants than in wild-type plants; however, nitrosative stress negatively regulated the expression of QSH1 and SHAT1 in both WT and rhs1 plants, but positively regulated QSH4 expression in rhs1 plants alone. The expression levels of genes responsible for NO production (OsNIA1, OsNIA2, and OsNOA1) were lower in rhs1 plants than in WT plants under normal conditions. However, under nitrosative stress, the expression of OsNIA2 significantly increased in rhs1 plants. The expression of CPL1 (a negative regulator of seed shattering in rice) was significantly lower in rhs1 plants, and we found that CPL1 expression was correlated with S-nitrosothiol (SNO) alteration in rhs1. Interestingly noe1, a rice mutant with high SNO levels, exhibited low seed shattering, whereas rhs1 resulted in low SNO levels with high seed shattering. Therefore, RHS1 is a novel gene that negatively regulates the shattering trait in rice via regulation of endogenous SNO levels. However, the molecular mechanisms involved in the control of RHS1-mediated regulation of seed shattering and its interaction with nitric oxide and involvement in plant defense need to be investigated further.

Focus on Nitric Oxide Homeostasis: Direct and Indirect Enzymatic Regulation of Protein Denitrosation Reactions in Plants

Protein cysteines (Cys) undergo a multitude of different reactive oxygen species (ROS), reactive sulfur species (RSS), and/or reactive nitrogen species (RNS)-derived modifications. S-nitrosation (also referred to as nitrosylation), the addition of a nitric oxide (NO) group to reactive Cys thiols, can alter protein stability and activity and can result in changes of protein subcellular localization. Although it is clear that this nitrosative posttranslational modification (PTM) regulates multiple signal transduction pathways in plants, the enzymatic systems that catalyze the reverse S-denitrosation reaction are poorly understood. This review provides an overview of the biochemistry and regulation of nitro-oxidative modifications of protein Cys residues with a focus on NO production and S-nitrosation. In addition, the importance and recent advances in defining enzymatic systems proposed to be involved in regulating S-denitrosation are addressed, specifically cytosolic thioredoxins (TRX) and the newly identified aldo-keto reductases (AKR).

Transcriptome Analysis of Glutathione Response: RNA-Seq Provides Insights into Balance between Antioxidant Response and Glucosinolate Metabolism

When being stressed, plants require a balance between the resistance pathway and metabolism. Glucosinolates (GS) are secondary metabolics that widely exist in Brassicaceae. Glutathione (GSH) not only participates in plant processing reactive oxygen species (ROS) but also directly participates in GS synthesis as a sulfur donor. Therefore, we used transcriptomic to identify antioxidant and GS metabolism responses in GSH-treated pakchoi. Our study elucidated that GSH can be used as priming to improve oxidative resistance and preferentially stimulate the expression of resistance genes such as CAT1. The reduction in transcription factor expression inhibits the key steps of the GS synthesis pathway. When ROS returned to normal level, the resistance gene decreased and returned to normal level, while GSH restored the gene expression of GS biosynthesis. This work puts forward the mechanism of GSH in regulating the antioxidant system and glucosinolate metabolic pathway, which provides a basis for further study on the relationship between environmental signals and plant metabolism and provides ideas for follow-up research.

High Nitric Oxide Concentration Inhibits Photosynthetic Pigment Biosynthesis by Promoting the Degradation of Transcription Factor HY5 in Tomato

Photosynthetic pigments in higher plants, including chlorophyll and carotenoid, are crucial for photosynthesis and photoprotection. Previous studies have shown that nitric oxide (NO) plays a dual role in plant photosynthesis. However, how pigment biosynthesis is suppressed by NO remains unclear. In this study, we generated NO-accumulated gsnor mutants, applied exogenous NO donors, and used a series of methods, including reverse transcription quantitative PCR, immunoblotting, chromatin immunoprecipitation, electrophoretic mobility shift, dual-luciferase, and NO content assays, to explore the regulation of photosynthetic pigment biosynthesis by NO in tomato. We established that both endogenous and exogenous NO inhibited pigment accumulation and photosynthetic capacities. High levels of NO stimulated the degradation of LONG HYPOCOTYL 5 (HY5) protein and further inactivated the transcription of genes encoding protochlorophyllide oxidoreductase C (PORC) and phytoene synthase 2 (PSY2)—two enzymes that catalyze the rate-limiting steps in chlorophyll and carotenoid biosynthesis. Our findings provide a new insight into the mechanism of NO signaling in modulating HY5-mediated photosynthetic pigment biosynthesis at the transcriptional level in tomato plants.

Role of Nitric Oxide in Plant Senescence

In plants senescence is the final stage of plant growth and development that ultimately leads to death. Plants experience age-related as well as stress-induced developmental ageing. Senescence involves significant changes at the transcriptional, post-translational and metabolomic levels. Furthermore, phytohormones also play a critical role in the programmed senescence of plants. Nitric oxide (NO) is a gaseous signalling molecule that regulates a plethora of physiological processes in plants. Its role in the control of ageing and senescence has just started to be elucidated. Here, we review the role of NO in the regulation of programmed cell death, seed ageing, fruit ripening and senescence. We also discuss the role of NO in the modulation of phytohormones during senescence and the significance of NO-ROS cross-talk during programmed cell death and senescence.

Nitric oxide negatively regulates gibberellin signaling to coordinate growth and salt tolerance in Arabidopsis

In response to dynamically altered environments, plants must finely coordinate the balance between growth and stress responses for their survival. However, the underpinning regulatory mechanisms remain largely elusive. The phytohormone gibberellin promotes growth via a derepression mechanism by proteasomal degradation of the DELLA transcription repressors. Conversely, the stress-induced burst of nitric oxide (NO) enhances stress tolerance, largely relaying on NO-mediated S-nitrosylation, a redox-based posttranslational modification. Here, we show that S-nitrosylation of Cys-374 in the Arabidopsis RGA protein, a key member of DELLAs, inhibits its interaction with the F-box protein SLY1, thereby preventing its proteasomal degradation under salinity condition. The accumulation of RGA consequently retards growth but enhances salt tolerance. We propose that NO negatively regulates gibberellin signaling via S-nitrosylation of RGA to coordinate the balance of growth and stress responses when challenged by adverse environments.

Phytohormonal Regulation Through Protein S-Nitrosylation Under Stress

The liaison between Nitric oxide (NO) and phytohormones regulates a myriad of physiological processes at the cellular level. The interaction between NO and phytohormones is mainly influenced by NO-mediated post-translational modifications (PTMs) under basal as well as induced conditions. Protein S-nitrosylation is the most prominent and widely studied PTM among others. It is the selective but reversible redox-based covalent addition of a NO moiety to the sulfhydryl group of cysteine (Cys) molecule(s) on a target protein to form S-nitrosothiols. This process may involve either direct S-nitrosylation or indirect S-nitrosylation followed by transfer of NO group from one thiol to another (transnitrosylation). During S-nitrosylation, NO can directly target Cys residue (s) of key genes involved in hormone signaling thereby regulating their function. The phytohormones regulated by NO in this manner includes abscisic acid, auxin, gibberellic acid, cytokinin, ethylene, salicylic acid, jasmonic acid, brassinosteroid, and strigolactone during various metabolic and physiological conditions and environmental stress responses. S-nitrosylation of key proteins involved in the phytohormonal network occurs during their synthesis, degradation, or signaling roles depending upon the response required to maintain cellular homeostasis. This review presents the interaction between NO and phytohormones and the role of the canonical NO-mediated post-translational modification particularly, S-nitrosylation of key proteins involved in the phytohormonal networks under biotic and abiotic stresses.

Quantitative Proteome Profiling of a S-Nitrosoglutathione Reductase (GSNOR) Null Mutant Reveals a New Class of Enzymes Involved in Nitric Oxide Homeostasis in Plants

Nitric oxide (NO) is a short-lived radical gas that acts as a signaling molecule in all higher organisms, and that is involved in multiple plant processes, including germination, root growth, and fertility. Regulation of NO-levels is predominantly achieved by reaction of oxidation products of NO with glutathione to form S-nitrosoglutathione (GSNO), the principal bioactive form of NO. The enzyme S-nitrosoglutathione reductase (GSNOR) is a major route of NADH-dependent GSNO catabolism and is critical to NO homeostasis. Here, we performed a proteomic analysis examining changes in the total leaf proteome of an Arabidopsis thaliana GSNOR null mutant (hot5-2/gsnor1-3). Significant increases or decreases in proteins associated with chlorophyll metabolism and with redox and stress metabolism provide insight into phenotypes observed in hot5-2/gsnor1-3 plants. Importantly, we identified a significant increase in proteins that belong to the aldo-keto reductase (AKR) protein superfamily, AKR4C8 and 9. Because specific AKRs have been linked to NO metabolism in mammals, we expressed and purified A. thaliana AKR4C8 and 9 and close homologs AKR4C10 and 11 and determined that they have NADPH-dependent activity in GSNO and S-nitroso-coenzyme A (SNO-CoA) reduction. Further, we found an increase of NADPH-dependent GSNO reduction activity in hot5-2/gsnor1-3 mutant plants. These data uncover a new, NADPH-dependent component of NO metabolism that may be integrated with NADH-dependent GSNOR activity to control NO homeostasis in plants.

Nitric oxide coordinates growth, development, and stress response via histone modification and gene expression

Nitric oxide (NO) is a signaling molecule with multiple regulatory functions in plant physiology and stress response. In addition to direct effects on transcriptional machinery, NO executes its signaling function via epigenetic mechanisms. We report that light intensity-dependent changes in NO correspond to changes in global histone acetylation (H3, H3K9, and H3K9/K14) in Arabidopsis (Arabidopsis thaliana) wild-type leaves, and that this relationship depends on S-nitrosoglutathione reductase and histone deacetylase 6 (HDA6). The activity of HDA6 was sensitive to NO, demonstrating that NO participates in regulation of histone acetylation. Chromatin immunoprecipitation sequencing and RNA-seq analyses revealed that NO participates in the metabolic switch from growth and development to stress response. This coordinating function of NO might be particularly important in plant ability to adapt to a changing environment, and is therefore a promising foundation for mitigating the negative effects of climate change on plant productivity.

Perturbations in nitric oxide homeostasis promote Arabidopsis disease susceptibility towards Phytophthora parasitica

Phytophthora species can infect hundreds of different plants, including many important crops, causing a number of agriculturally relevant diseases. A key feature of attempted pathogen infection is the rapid production of the redox active molecule nitric oxide (NO). However, the potential role(s) of NO in plant resistance against Phytophthora is relatively unexplored. Here we show that the level of NO accumulation is crucial for basal resistance in Arabidopsis against Phytophthora parasitica. Counterintuitively, both relatively low or relatively high NO accumulation leads to reduced resistance against P. parasitica. S-nitrosylation, the addition of a NO group to a protein cysteine thiol to form an S-nitrosothiol, is an important route for NO bioactivity and this process is regulated predominantly by S-nitrosoglutathione reductase 1 (GSNOR1). Loss-of-function mutations in GSNOR1 disable both salicylic acid accumulation and associated signalling, and also the production of reactive oxygen species, leading to susceptibility towards P. parasitica. Significantly, we also demonstrate that secreted proteins from P. parasitica can inhibit Arabidopsis GSNOR1 activity.

Evidence for an arginine-dependent route for the synthesis of NO in the model filamentous fungus Aspergillus nidulans

Nitric oxide (NO) is a signalling molecule in eukaryotic and prokaryotic organisms. NO levels transiently boost upon induction of conidiation in Aspergillus nidulans. Only one pathway for NO synthesis involving nitrate reductase has been reported in filamentous fungi so far, but this does not satisfy all the NO produced in fungal cells. Here we provide evidence for at least one additional biosynthetic pathway in A. nidulans involving l-arginine or an intermediate metabolite as a substrate. Under certain growth conditions, the addition of l-arginine to liquid media elicited a burst of NO that was not dependent on any of the urea cycle genes. The NO levels were controlled by the metabolically available arginine, which was regulated by mobilization from the vacuoles and during development. In vitro assays with protein extracts and amino acid profiling strongly suggested the existence of an arginine-dependent NO pathway analogous to the mammalian NO synthase. Addition of polyamines induced NO synthesis, and mutations in the polyamine synthesis genes puA and spdA reduced the production of NO. In conclusion, here we report an additional pathway for the synthesis of NO in A. nidulans using urea cycle intermediates.

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.

Crosstalk between abscisic acid and nitric oxide under heat stress: exploring new vantage points

Heat stress adversely affects plants growth potential. Global warming is reported to increase in the intensity, frequency, and duration of heatwaves, eventually affecting ecology, agriculture and economy. With an expected increase in average temperature by 2-3 °C over the next 30-50 years, crop production is facing a severe threat to sub-optimum growth conditions. Abscisic acid (ABA) and nitric oxide (NO) are growth regulators that are involved in the adaptation to heat stress by affecting each other and changing the adaptation process. The interaction between these molecules has been discussed in various studies in general or under stress conditions; however, regarding high temperature, their interaction has little been worked out. In the present review, the focus is shifted on the role of these molecules under heat stress emphasizing the different possible interactions between ABA and NO as both regulate stomatal closure and other molecules including hydrogen peroxide (H₂O₂), hydrogen sulfide (H₂S), antioxidants, proline, glycine betaine, calcium (Ca²⁺) and heat shock protein (HSP). Exploring the crosstalk between ABA and NO with other molecules under heat stress will provide us with a comprehensive knowledge of plants mechanism of heat tolerance which could be useful to develop heat stress-resistant varieties.

S-nitrosoglutathione Reductase-Mediated Nitric Oxide Affects Axillary Buds Outgrowth of Solanum lycopersicum L. by Regulating Auxin and Cytokinin Signaling

Auxin and cytokinin are two kinds of important phytohormones that mediate outgrowth of axillary buds in plants. How nitric oxide and its regulator of S-nitrosoglutathione reductase (GSNOR) take part in auxin and cytokinin signaling for controlling axillary buds outgrowth remains elusive. We investigated the roles of GSNOR during tomato axillary bud outgrowth by using physiological, biochemical and genetic approaches. GSNOR negatively regulated NO homeostasis. Suppression of GSNOR promoted axillary bud outgrowth by inhibiting the expression of FZY in both apical and axillary buds. Meanwhile, AUX1 and PIN1 were down-regulated in apical buds but up-regulated in axillary buds in GSNOR-suppressed plants. Thus, reduced IAA accumulation was shown in both apical buds and axillary buds of GSNOR-suppressed plants. GSNOR-mediated changes of NO and auxin affected cytokinin biosynthesis, transport, and signaling. And a decreased ratio of auxin: cytokinin was shown in axillary buds of GSNOR-suppressed plants, leading to bud dormancy breaking. We also found that the original NO signaling was generated by nitrate reductase (NR) catalyzing nitrate as substrate. NR-mediated NO reduced the GSNOR activity through S-nitrosylation of Cys-10, then induced a further NO burst, which played the above roles to promote axillary buds outgrowth. Together, GSNOR-mediated NO played important roles in controlling axillary buds outgrowth by altering the homeostasis and signaling of auxin and cytokinin in tomato plants.

The Role of Nitric Oxide-Induced ATILL6 in Growth and Disease Resistance in Arabidopsis thaliana

Nitric oxide (NO) is a signaling molecule that regulates various processes, including plant growth and development, immunity, and environmental interactions. Using high throughput RNA-seq data, we explored the role of the NO-induced ATILL6 gene in plant growth and defense using functional genomics. The atill6 mutant and wild-types were challenged with either oxidative (H₂O₂, MV) or nitro-oxidative (CySNO, GSNO) stress conditions, and the phenotypic results showed that ATILL6 gene differentially regulates cotyledon development frequency (CDF) as well as the root and shoot lengths of the plants. To investigate whether ATILL6 plays a role in plant basal or resistance (R)-gene-mediated defense, the plants were challenged with either virulent or avirulent strains of Pseudomonas syringae pathovar tomato (Pst) DC3000. The atill6 line showed a susceptible phenotype, higher pathogen growth, and highly reduced transcript accumulation of PR1 and PR2 genes. These results suggested that ATILL6 positively regulates plant basal defense. Furthermore, after the inoculation of atill6 with avirulent Pst (DC3000), the expressions of the PR1 and PR2 genes decreased, suggesting a positive role in R-gene-mediated resistance in protecting the plant from further spread of disease. We also investigated the role of ATILL6 in systemic acquired resistance (SAR), and the results showed that ATILL6 positively regulates SAR, as the mutant line atill6 has significantly (p ≤ 0.05) lower transcript accumulation of PR, G3DPH, and AZI genes. Overall, these results indicate that the NO-induced ATILL6 gene differentially regulates plant growth and positively regulates plant basal defense, R-gene-mediated resistance, and SAR.

Overexpression of SlGSNOR impairs in vitro shoot proliferation and developmental architecture in tomato but confers enhanced disease resistance

The pervasive presence of nitric oxide (NO) in cells and its role in modifying cystein residues through protein S-nitrosylation is a remarkable redox based signalling mechanism regulating a variety of cellular processes. S-NITROSOGLUTATHIONE REDUCTASE (GSNOR) governs NO bioavailability by the breakdown of S-nitrosoglutathione (GSNO), fine-tunes NO signalling and controls total cellular S-nitrosylated proteins. Most of the published data on GSNOR functional analysis is based on the model plant Arabidopsis with no previous report for its effect on in vitro regeneration of tissue cultured plants. Moreover, the effect of GSNOR overexpression (O.E) on tomato growth, development and disease resistance remains enigmatic. Here we show that SlGSNOR O.E in tomato alters multiple developmental programs from in vitro culture establishment to plant growth and fruit set. Moreover, constitutive SlGSNOR O.E in tomato showed enhanced resistance against early blight (EB) disease caused by Alternaria solani and reduction in hypersensitive response (HR)-mediated cell death after Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) infiltrations. High GSNOR transcript levels led to the inhibition of in vitro shoot proliferation in transformed explants as revealed by the fluorescence microscopy after YFP labelling. Transgenic tomato lines overexpressing SlGSNOR showed defective phenotypes exhibiting stunted plant growth and bushy-type plants due to loss of apical dominance, along with reduced seed germination and delayed flowering. Furthermore, SlGSNOR O.E plants exhibited altered leaf arrangement, fruit shape and modified locules number in tomato fruit. These findings give a novel insight into a multifaceted regulatory role of SlGSNOR in tomato plant development, reproduction and response to pathogens.

The Arabidopsis zinc finger proteins SRG2 and SRG3 are positive regulators of plant immunity and are differentially regulated by nitric oxide

Nitric oxide (NO) regulates the deployment of a phalanx of immune responses, chief among which is the activation of a constellation of defence-related genes. However, the underlying molecular mechanisms remain largely unknown. The Arabidopsis thaliana zinc finger transcription factor (ZF-TF), S-nitrosothiol (SNO) Regulated 1 (SRG1), is a central target of NO bioactivity during plant immunity. Here we characterize the remaining members of the SRG gene family. Both SRG2 and, especially, SRG3 were positive regulators of salicylic acid-dependent plant immunity. Analysis of SRG single, double and triple mutants implied that SRG family members have additive functions in plant immunity and, surprisingly, are under reciprocal regulation. SRG2 and SRG3 localized to the nucleus and functioned as ethylene-responsive element binding factor-associated amphiphilic repression (EAR) domain-dependent transcriptional repressors: NO abolished this activity for SRG3 but not for SRG2. Consistently, loss of GSNOR function, resulting in increased (S)NO concentrations, fully suppressed the disease resistance phenotype established from SRG3 but not SRG2 overexpression. Remarkably, SRG3 but not SRG2 was S-nitrosylated in vitro and in vivo. Our findings suggest that the SRG family has separable functions in plant immunity, and, surprisingly, these ZF-TFs exhibit reciprocal regulation. It is remarkable that, through neofunctionalization, the SRG family has evolved to become differentially regulated by the key immune-related redox cue, NO.

Molecular functions of nitric oxide and its potential applications in horticultural crops

Nitric oxide (NO) regulates plant growth, enhances nutrient uptake, and activates disease and stress tolerance mechanisms in most plants, making NO a potential tool for use in improving the yield and quality of horticultural crop species. Although the use of NO in horticulture is still in its infancy, research on NO in model plant species has provided an abundance of valuable information on horticultural crop species. Emerging evidence implies that the bioactivity of NO can occur through many potential mechanisms but occurs mainly through S-nitrosation, the covalent and reversible attachment of NO to cysteine thiol. In this context, NO signaling specifically affects crop development, immunity, and environmental interactions. Moreover, NO can act as a fumigant against a wide range of postharvest diseases and pests. However, for effective use of NO in horticulture, both understanding and exploring the biological significance and potential mechanisms of NO in horticultural crop species are critical. This review provides a picture of our current understanding of how NO is synthesized and transduced in plants, and particular attention is given to the significance of NO in breaking seed dormancy, balancing root growth and development, enhancing nutrient acquisition, mediating stress responses, and guaranteeing food safety for horticultural production.

NO Network for Plant-Microbe Communication Underground: A Review

Mechanisms governing plant-microbe interaction in the rhizosphere attracted a lot of investigative attention in the last decade. The rhizosphere is not simply a source of nutrients and support for the plants; it is rather an ecosystem teeming with diverse flora and fauna including different groups of microbes that are useful as well as harmful for the plants. Plant-microbe interaction occurs via a highly complex communication network that involves sophisticated machinery for the recognition of friend and foe at both sides. On the other hand, nitric oxide (NO) is a key, signaling molecule involved in plant development and defense. Studies on legume-rhizobia symbiosis suggest the involvement of NO during recognition, root hair curling, development of infection threads, nodule development, and nodule senescence. A similar role of NO is also suggested in the case of plant interaction with the mycorrhizal fungi. Another, insight into the plant-microbe interaction in the rhizosphere comes from the recognition of pathogen-associated molecular patterns (PAMPs)/microbe-associated molecular patterns (MAMPs) by the host plant and thereby NO-mediated activation of the defense signaling cascade. Thus, NO plays a major role in mediating the communication between plants and microbes in the rhizosphere. Interestingly, reports suggesting the role of silicon in increasing the number of nodules, enhancing nitrogen fixation, and also the combined effect of silicon and NO may indicate a possibility of their interaction in mediating microbial communication underground. However, the exact role of NO in mediating plant-microbe interaction remains elusive. Therefore, understanding the role of NO in underground plant physiology is very important, especially in relation to the plant's interaction with the rhizospheric microbiome. This will help devise new strategies for protection against phytopathogens and enhancing plant productivity by promoting symbiotic interaction. This review focuses on the role of NO in plant-microbe communication underground.

The light and dark sides of nitric oxide: multifaceted roles of nitric oxide in plant responses to light

Light drives photosynthesis and informs plants about their surroundings. Regarded as a multifunctional signaling molecule in plants, nitric oxide (NO) has been repeatedly demonstrated to interact with light signaling cascades to control plant growth, development and metabolism. During early plant development, light-triggered NO accumulation counteracts negative regulators of photomorphogenesis and modulates the abundance of, and sensitivity to, plant hormones to promote seed germination and de-etiolation. In photosynthetically active tissues, NO is generated at distinct rates under light or dark conditions and acts at multiple target sites within chloroplasts to regulate photosynthetic reactions. Moreover, changes in NO concentrations in response to light stress promote plant defenses against oxidative stress under high light or ultraviolet-B radiation. Here we review the literature on the interaction of NO with the complicated light and hormonal signaling cascades controlling plant photomorphogenesis and light stress responses, focusing on the recently identified molecular partners and action mechanisms of NO in these events. We also discuss the versatile role of NO in regulating both photosynthesis and light-dependent stomatal movements, two key determinants of plant carbon gain. The regulation of nitrate reductase (NR) by light is highlighted as vital to adjust NO production in plants living under natural light conditions.

Nitric oxide affects seed oil accumulation and fatty acid composition through protein S-nitrosation

Nitric oxide (NO) is a key signaling molecule regulating several plant developmental and stress responses. Here, we report that NO plays an important role in seed oil content and fatty acid composition. RNAi silencing of Arabidopsis S-nitrosoglutathione reductase 1 (GSNOR1) led to reduced seed oil content. In contrast, nitrate reductase double mutant nia1nia2 had increased seed oil content, compared with wild-type plants. Moreover, the concentrations of palmitic acid (C16:0), linoleic acid (C18:2), and linolenic acid (C18:3) were higher, whereas those of stearic acid (C18:0), oleic acid (C18:1), and arachidonic acid (C20:1) were lower, in seeds of GSNOR1 RNAi lines. Similar results were obtained with rapeseed embryos cultured in vitro with the NO donor sodium nitroprusside (SNP), and the NO inhibitor NG-Nitro-L-arginine Methyl Ester (L-NAME). Compared with non-treated embryos, the oil content decreased in SNP-treated embryos, and increased in L-NAME-treated embryos. Relative concentrations of C16:0, C18:2 and C18:3 were higher, whereas C18:1 concentration decreased in rapeseed embryos treated with SNP. Proteomics and transcriptome analysis revealed that three S-nitrosated proteins and some key genes involved in oil synthesis, were differentially regulated in SNP-treated embryos. Therefore, regulating NO content could be a novel approach to increasing seed oil content in cultivated oil crops.

The Emerging Role of GSNOR in Oxidative Stress Regulation

Oxidative stress is a common event in aerobic organisms and a fundamental and unavoidable cost of the aerobic lifestyle. Reactive oxygen and nitrogen species (ROS/RNS) and iron (Fe) are the most common agents that trigger oxidative stress. A conserved enzyme in the S-nitrosoglutathione (GSNO) metabolism, GSNO reductase (GSNOR), modulates a multitude of abiotic and biotic stress responses. In this review, we focus on the emerging role of GSNOR as a master regulator in oxidative stress through its regulation of the interaction of ROS, RNS, and Fe, and highlight recent discoveries in post-translational modifications of GSNOR and functional variations of natural GSNOR variants during oxidative stress. Recent advances in understanding GSNOR regulation show promise for the modulation of oxidative stress in plants.

Protein S-nitrosation differentially modulates tomato responses to infection by hemi-biotrophic oomycetes of Phytophthora spp

Regulation of protein function by reversible S-nitrosation, a post-translational modification based on the attachment of nitroso group to cysteine thiols, has emerged among key mechanisms of NO signalling in plant development and stress responses. S-nitrosoglutathione is regarded as the most abundant low-molecular-weight S-nitrosothiol in plants, where its intracellular concentrations are modulated by S-nitrosoglutathione reductase. We analysed modulations of S-nitrosothiols and protein S-nitrosation mediated by S-nitrosoglutathione reductase in cultivated Solanum lycopersicum (susceptible) and wild Solanum habrochaites (resistant genotype) up to 96 h post inoculation (hpi) by two hemibiotrophic oomycetes, Phytophthora infestans and Phytophthora parasitica. S-nitrosoglutathione reductase activity and protein level were decreased by P. infestans and P. parasitica infection in both genotypes, whereas protein S-nitrosothiols were increased by P. infestans infection, particularly at 72 hpi related to pathogen biotrophy-necrotrophy transition. Increased levels of S-nitrosothiols localised in both proximal and distal parts to the infection site, which suggests together with their localisation to vascular bundles a signalling role in systemic responses. S-nitrosation targets in plants infected with P. infestans identified by a proteomic analysis include namely antioxidant and defence proteins, together with important proteins of metabolic, regulatory and structural functions. Ascorbate peroxidase S-nitrosation was observed in both genotypes in parallel to increased enzyme activity and protein level during P. infestans pathogenesis, namely in the susceptible genotype. These results show important regulatory functions of protein S-nitrosation in concerting molecular mechanisms of plant resistance to hemibiotrophic pathogens.

Enhanced Nitric Oxide Synthesis Through Nitrate Supply Improves Drought Tolerance of Sugarcane Plants

Nitric oxide (NO) is an important signaling molecule associated with many biochemical and physiological processes in plants under stressful conditions. Nitrate reductase (NR) not only mediates the reduction of NO3 - to NO2 - but also reduces NO2 - to NO, a relevant pathway for NO production in higher plants. Herein, we hypothesized that sugarcane plants supplied with more NO3 - as a source of N would produce more NO under water deficit. Such NO would reduce oxidative damage and favor photosynthetic metabolism and growth under water limiting conditions. Sugarcane plants were grown in nutrient solution and received the same amount of nitrogen, with varying nitrate:ammonium ratios (100:0 and 70:30). Plants were then grown under well-watered or water deficit conditions. Under water deficit, plants exhibited higher root [NO3 -] and [NO2 -] when supplied with 100% NO3 -. Accordingly, the same plants also showed higher root NR activity and root NO production. We also found higher photosynthetic rates and stomatal conductance in plants supplied with more NO3 -, which was associated with increased root growth. ROS accumulation was reduced due to increases in the activity of catalase in leaves and superoxide dismutase and ascorbate peroxidase in roots of plants supplied with 100% NO3 - and facing water deficit. Such positive responses to water deficit were offset when a NO scavenger was supplied to the plants, thus confirming that increases in leaf gas exchange and plant growth were induced by NO. Concluding, NO3 - supply is an interesting strategy for alleviating the negative effects of water deficit on sugarcane plants, increasing drought tolerance through enhanced NO production. Our data also provide insights on how plant nutrition could improve crop tolerance against abiotic stresses, such as drought.

Exogenous Nitric Oxide Enhances Disease Resistance by Nitrosylation and Inhibition of S-Nitrosoglutathione Reductase in Peach Fruit

Nitric oxide (NO), a signaling molecule, participates in defense responses during plant-pathogen interactions. S-Nitrosoglutathione (GSNO) is found to be an active intracellular NO storage center and regulated by S-nitrosoglutathione reductase (GSNOR) in plants. However, the role of GSNOR in NO-induced disease resistance is not clear. In this research, the effects of NO and GSNOR inhibitor (N6022) on the defense response of harvested peach fruit to Monilinia fructicola infection were investigated. It was found that the disease incidence and lesion diameter of peach fruits were markedly (P < 0.05) reduced by NO and GSNOR inhibitor. However, the expression of GSNOR was significantly inhibited (P < 0.05) by NO only during 2-6 h. Analyses using iodo-TMT tags to detect the nitrosylation sites of GSNOR revealed that the sulfhydryl group of the 85-cysteine site was nitrosylated after NO treatment in peach fruit at 6 and 12 h, suggesting that exogenous NO enhances disease resistance via initial inhibition of gene expression and the S-nitrosylation of GSNOR, thereby inhibiting GSNOR activity. Moreover, NO and GSNOR inhibitor enhanced the expression of systemic acquired resistance (SAR)-related genes, such as pathogenesis-related gene 1 (PR1), nonexpressor of PR1 (NPR1), and TGACG-binding factor 1 (TGA1). These results demonstrated that S-nitrosylation of GSNOR protein and inhibition of GSNOR activity contributed to the enhanced disease resistance in fruit.

Transcriptional and Metabolic Changes Associated with Phytoglobin Expression during Germination of Barley Seeds

To understand how the class 1 phytoglobin is involved in germination process via the modulation of the nitric oxide (NO) metabolism, we performed the analysis of physiological and molecular parameters in the embryos of transgenic barley (Hordeum vulgare L. cv Golden Promise) plants differing in expression levels of the phytoglobin (Pgb1) gene during the first 48 h of germination. Overexpression of Pgb1 resulted in a higher rate of germination, higher protein content and higher ATP/ADP ratios. This was accompanied by a lower rate of NO emission after radicle protrusion, as compared to the wild type and downregulating line, and a lower rate of S-nitrosylation of proteins in the first hours postimbibition. The rate of fermentation estimated by the expression and activity of alcohol dehydrogenase was significantly higher in the Pgb1 downregulating line, the same tendency was observed for nitrate reductase expression. The genes encoding succinate dehydrogenase and pyruvate dehydrogenase complex subunits were more actively expressed in embryos of the seeds overexpressing Pgb1. It is concluded that Pgb1 expression in embryo is essential for the maintenance of redox and energy balance before radicle protrusion, when seeds experience low internal oxygen concentration and exerts the effect on metabolism during the initial development of seedlings.

Silicon Confers Soybean Resistance to Salinity Stress Through Regulation of Reactive Oxygen and Reactive Nitrogen Species

Salt stress is one of the major abiotic stressors that causes huge losses to the agricultural industry worldwide. Different strategies have been adopted over time to mitigate the negative impact of salt stress on plants and reclaim salt-affected lands. In the current study, we used silicon (Si) as a tool for salinity alleviation in soybean and investigated the influence of exogenous Si application on the regulation of reactive oxygen and reactive nitrogen species and other salt stress-related parameters of the treated plants. Our results revealed that the canopy temperature was much higher in sole NaCl-treated plants but declined in Si + NaCl-treated plants. Furthermore, the chlorophyll contents decreased with sole NaCl treatment, whereas Si + NaCl-treated plants showed improved chlorophyll contents. In addition, Si application normalized the photosynthetic responses, such as transpiration rate (E) and net photosynthesis rate (P_N ) in salt-treated plants, and reduced the activity of ascorbate peroxidase and glutathione under salt stress. The expression levels of antioxidant-related genes GmCAT1, GmCAT2, and GmAPX1 started to decline at 12 h after addition of Si to NaCl-treated plants. Similarly, the S-nitrosothiol and nitric oxide (NO)-related genes were upregulated in the salt stress condition but reduced after Si supplementation. Si application downregulated genes associated with reactive oxygen species and reactive nitrogen species and reduced enzymatic and non-enzymatic antioxidants of the treated plants. Results of the current study conclude that Si mitigated the adverse effects of NaCl-induced stress by modulating the crosstalk between antioxidants and NO scavengers. It is suggested that Si may be used in agricultural systems for alleviating salt stress.

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.

Nitric oxide homeostasis is required for light-dependent regulation of conidiation in Aspergillus

Nitric oxide (NO) can be biologically synthesized from nitrite or from arginine. Although NO is involved as a signal in many biological processes in bacteria, plants, and mammals, still little is known about the role of NO in fungi. Here we show that NO levels are regulated by light as an environmental signal in Aspergillus nidulans. The flavohaemoglobin-encoding fhbB gene involved in NO oxidation to nitrate, and the arginine-regulated arginase encoded by agaA, which controls the intracellular concentration of arginine, are both up-regulated by light. The phytochrome fphA is required for the light-dependent induction of fhbB and agaA, while the white-collar gene lreA acts as a repressor when arginine is present in the media. The intracellular arginine pools increase upon induction of both developmental programs (conidiation and sexual development), and the increase is higher under conditions promoting sexual development. The presence of low concentrations of arginine does not affect the light-dependent regulation of conidiation, but high concentrations of arginine overrun the light signal. Deletion of fhbB results in the partial loss of the light regulation of conidiation on arginine and on nitrate media, while deletion of fhbA only affects the light regulation of conidiation on nitrate media. Our working model considers a cross-talk between environmental cues and intracellular signals to regulate fungal reproduction.

Altered Plant and Nodule Development and Protein S-Nitrosylation in Lotus japonicus Mutants Deficient in S-Nitrosoglutathione Reductases

Nitric oxide (NO) is a crucial signaling molecule that conveys its bioactivity mainly through protein S-nitrosylation. This is a reversible post-translational modification (PTM) that may affect protein function. S-nitrosoglutathione (GSNO) is a cellular NO reservoir and NO donor in protein S-nitrosylation. The enzyme S-nitrosoglutathione reductase (GSNOR) degrades GSNO, thereby regulating indirectly signaling cascades associated with this PTM. Here, the two GSNORs of the legume Lotus japonicus, LjGSNOR1 and LjGSNOR2, have been functionally characterized. The LjGSNOR1 gene is very active in leaves and roots, whereas LjGSNOR2 is highly expressed in nodules. The enzyme activities are regulated in vitro by redox-based PTMs. Reducing conditions and hydrogen sulfide-mediated cysteine persulfidation induced both activities, whereas cysteine oxidation or glutathionylation inhibited them. Ljgsnor1 knockout mutants contained higher levels of S-nitrosothiols. Affinity chromatography and subsequent shotgun proteomics allowed us to identify 19 proteins that are differentially S-nitrosylated in the mutant and the wild-type. These include proteins involved in biotic stress, protein degradation, antioxidant protection and photosynthesis. We propose that, in the mutant plants, deregulated protein S-nitrosylation contributes to developmental alterations, such as growth inhibition, impaired nodulation and delayed flowering and fruiting. Our results highlight the importance of GSNOR function in legume biology.

Present knowledge and controversies, deficiencies, and misconceptions on nitric oxide synthesis, sensing, and signaling in plants

After 30 years of intensive work, nitric oxide (NO) has just started to be characterized as a relevant regulatory molecule on plant development and responses to stress. Its reactivity as a free radical determines its mode of action as an inducer of posttranslational modifications of key target proteins through cysteine S-nitrosylation and tyrosine nitration. Many of the NO-triggered regulatory actions are exerted in tight coordination with phytohormone signaling. This review not only summarizes and updates the information accumulated on how NO is synthesized, sensed, and transduced in plants but also makes emphasis on controversies, deficiencies, and misconceptions that are hampering our present knowledge on the biology of NO in plants. The development of noninvasive accurate tools for the endogenous NO quantitation as well as the implementation of genetic approaches that overcome misleading pharmacological experiments will be critical for getting significant advances in better knowledge of NO homeostasis and regulatory actions in plants.

Strigolactones Interact With Nitric Oxide in Regulating Root System Architecture of Arabidopsis thaliana

Both nitric oxide (NO) and strigolactone (SL) are growth regulating signal components in plants; however, regarding their possible interplay our knowledge is limited. Therefore, this study aims to provide new evidence for the signal interplay between NO and SL in the formation of root system architecture using complementary pharmacological and molecular biological approaches in the model Arabidopsis thaliana grown under stress-free conditions. Deficiency of SL synthesis or signaling (max1-1 and max2-1) resulted in elevated NO and S-nitrosothiol (SNO) levels due to decreased S-nitrosoglutathione (GSNO) reductase (GSNOR) protein abundance and activity indicating that there is a signal interaction between SLs and GSNOR-regulated levels of NO/SNO. This was further supported by the down-regulation of SL biosynthetic genes (CCD7, CCD8 and MAX1) in GSNOR-deficient gsnor1-3. Based on the more pronounced sensitivity of gsnor1-3 to exogenous SL (rac-GR24, 2 µM), we suspected that functional GSNOR is needed to control NO/SNO levels during SL-induced primary root (PR) elongation. Additionally, SLs may be involved in GSNO-regulated PR shortening as suggested by the relative insensitivity of max1-1 and max2-1 mutants to exogenous GSNO (250 µM). Collectively, our results indicate a connection between SL and GSNOR-regulated NO/SNO signals in roots of A. thaliana grown in stress-free environment. As this work used max2-1 mutant and rac-GR24 exerting unspecific effects to both SL and karrikin signaling, it cannot be ruled out that karrikins are partly responsible for the observed effects, and this issue needs further clarification in the future.

Protein S-Nitrosylation in plants: Current progresses and challenges

Nitric oxide (NO) is an important signaling molecule regulating diverse biological processes in all living organisms. A major physiological function of NO is executed via protein S-nitrosylation, a redox-based posttranslational modification by covalently adding a NO molecule to a reactive cysteine thiol of a target protein. S-nitrosylation is an evolutionarily conserved mechanism modulating multiple aspects of cellular signaling. During the past decade, significant progress has been made in functional characterization of S-nitrosylated proteins in plants. Emerging evidence indicates that protein S-nitrosylation is ubiquitously involved in the regulation of plant development and stress responses. Here we review current understanding on the regulatory mechanisms of protein S-nitrosylation in various biological processes in plants and highlight key challenges in this field.