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

Activity and gene expression analysis of the NADP-dependent isocitrate dehydrogenase (NADP-ICDH) through pepper fruit ripening and its modulation by nitric oxide (NO). Molecular characterization of the peroxisomal isozyme

NADP-dependent isocitrate dehydrogenase (NADP-ICDH) is one of the main sources of cellular reductant capacity in the form of NADPH. Although there is significant knowledge about the relevance of this enzyme during some physiological and stress processes, the available information about its involvement in fruit ripening is scarce. Using sweet green pepper (Capsicum annuum L.) fruits, a 50-75 % ammonium-sulfate-enriched protein fraction containing the NADP-ICDH activity allowed its biochemical characterization. The enzyme displayed a typical Michaelis-Menten kinetics and exhibited Vmax and Km values of 97 μUnits and 78 µM for isocitrate, and 92 μUnits and 46 µM for NADP+. Three NADP-ICDH isozymes were identified by non-denaturing PAGE designated as NADP-ICDH I to III, each representing 33 %, 24 %, and 43 %, respectively, of the total activity. Based on our previous transcriptome (RNA-Seq), three CaICDH genes (CaNADP-ICDH1, CaNADP-ICDH2, and CaNADP-ICDH3) were identified in sweet pepper fruits encoding isozymes potentially distributed in the cytosol, cytosol/mitochondrion, and peroxisome, according to their percentage of identity with the Arabidopsis isozymes. The time-course expression analysis of these genes during different fruit ripening stages including green immature (G), breaking point (BP), and red ripe (R), and in fruits subjected to nitric oxide (NO) treatments, showed dissimilar expression patterns. During ripening from green to red fruits, CaNADP-ICDH1 and CaNADP-ICDH2 were upregulated but were negatively affected by NO; however, CaNADP-ICDH3 was downregulated during ripening but unaffected by NO treatment. Furthermore, during ripening, the NADP-ICDH activity increased in red ripe fruits whereas the NO gas treatment produced a significant inhibition. These findings provide, to our knowledge, the first characterization of the NADP-ICDH family in this non-climacteric fruit and suggest that NADP-ICDH must play an important role in maintaining the supply of NADPH during pepper fruit ripening and that NO partially modulates this NADPH-generating system.

Redox Control of Seed Germination is Mediated by the Crosstalk of Nitric Oxide and Reactive Oxygen Species

Significance: Seed germination and seedling establishment are characterized by changes in the intracellular redox state modulated by accelerated production of nitric oxide (NO) and reactive oxygen species (ROS). Redox regulation and enhanced accumulation of NO and ROS, approaching excessively high levels during seed imbibition, are critically important for breaking endodormancy and inducing germination. Recent Advances: Upon depletion of oxygen under the seed coat, NO is produced anaerobically in the reductive pathway associated mainly with mitochondria, and it participates in the energy metabolism of the seed until radicle protrusion. NO turnover involves nitrate reduction to nitrite in the cytosol, nitrite reduction to NO in mitochondria, and NO oxygenation in the cytosol in the reaction involving the hypoxically induced class 1 phytoglobin. In postgerminative degradation of seed tissues, NO and ROS are involved in redox signaling via post-translational modification of proteins and mediation of phytohormonal responses. Critical Issues: The crosstalk between the cellular redox potential, NO, ROS, and phytohormones integrates major physiological processes related to seed germination. Intensive accumulation of NO and ROS during imbibition is critically important for breaking seed dormancy. Upon oxygen depletion, NO and other nitrous oxides (NOx) are produced anaerobically and support energy metabolism prior to radicle protrusion. Future Directions: The turnover of NOx and ROS is determined by the intracellular redox balance, and it self-controls redox and energy levels upon germination. The particular details, regulation of this process, and its physiological significance remain to be established. Antioxid. Redox Signal. 00, 000-000.

The Molecular Frequency, Conservation and Role of Reactive Cysteines in Plant Lipid Metabolism

Cysteines (Cys) are chemically reactive amino acids containing sulfur that play diverse roles in plant biology. Recent proteomics investigations in Arabidopsis thaliana have revealed the presence of thiol post-translational modifications (PTMs) in several Cys residues. These PTMs are presumed to impact protein structure and function, yet mechanistic data regarding the specific Cys susceptible to modification and their biochemical relevance remain limited. To help address these limitations, we have conducted a wide-ranging analysis by integrating published datasets encompassing PTM proteomics (comparing S-sulfenylation, persulfidation, S-nitrosylation and S-acylation), genomics and protein structures, with a specific focus on proteins involved in plant lipid metabolism. The prevalence and distribution of modified Cys residues across all analyzed proteins is diverse and multifaceted. Nevertheless, by combining an evaluation of sequence conservation across 100+ plant genomes with AlphaFold-generated protein structures and physicochemical predictions, we have unveiled structural propensities associated with Cys modifications. Furthermore, we have identified discernible patterns in lipid biochemical pathways enriched with Cys PTMs, notably involving beta-oxidation, jasmonic acid biosynthesis, fatty acid biosynthesis and wax biosynthesis. These collective findings provide valuable insights for future investigations targeting the mechanistic foundations of Cys modifications and the regulation of modified proteins in lipid metabolism and other metabolic pathways.

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.

Exogenous Putrescine Modulates Nitrate Reductase-Dependent NO Production in Cucumber Seedlings Subjected to Salt Stress

Polyamines (PAs) are small aliphatic compounds that participate in the plant response to abiotic stresses. They also participate in nitric oxide (NO) production in plants; however, their role in this process remains unknown. Therefore, the study aimed to investigate the role of putrescine (Put) in NO production in the roots of cucumber seedlings subjected to salt stress (120 mM NaCl) for 1 and 24 h. In salinity, exogenous Put can regulate NO levels by managing NO biosynthesis pathways in a time-dependent manner. In cucumber roots exposed to 1 h of salinity, exogenous Put reduced NO level by decreasing nitrate reductase (NR)-dependent NO production and reduced nitric oxide synthase-like (NOS-like) activity. In contrast, during a 24 h salinity exposure, Put treatment boosted NO levels, counteracting the inhibitory effect of salinity on the NR and plasma membrane nitrate reductase (PM-NR) activity in cucumber roots. The role of endogenous Put in salt-induced NO generation was confirmed using Put biosynthesis inhibitors. Furthermore, the application of Put can modulate the NR activity at the genetic and post-translational levels. After 1 h of salt stress, exogenous Put upregulated CsNR1 and CsNR2 expression and downregulated CsNR3 expression. Put also decreased the NR activation state, indicating a reduction in the level of active dephosphorylated NR (dpNR) in the total enzyme pool. Conversely, in the roots of plants subjected to 24 h of salinity, exogenous Put enhanced the NR activation state, indicating an enhancement of the dpNR form in the total NR pool. These changes were accompanied by a modification of endogenous PA content. Application of exogenous Put led to an increase in the amount of Put in the roots and reduced endogenous spermine (Spm) content in cucumber roots under 24 h salinity. The regulatory role of exogenous Put on NO biosynthesis pathways may link with plant mechanisms of response to salt stress.