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

Nitric oxide (NO) modulates low temperature-stress signaling via S-nitrosation, a NO PTM, inducing ethylene biosynthesis inhibition leading to enhanced post-harvest shelf-life of agricultural produce

Low temperature (cold) stress is one of the major abiotic stress conditions affecting crop productivity worldwide. Nitric oxide (NO) is a dynamic signaling molecule that interacts with various stress regulators and provides abiotic stress tolerance. Stress enhanced NO contributes to S-nitrosothiol accumulation which causes oxidation of the -SH group in proteins leading to S-nitrosation, a post-translational modification. Cold stress induced in vivo S-nitrosation of > 240 proteins majorly belonging to stress/signaling/redox (myrosinase, SOD, GST, CS, DHAR), photosynthesis (RuBisCO, PRK), metabolism (FBA, GAPDH, TPI, SBPase), and cell wall modification (Beta-xylosidases, alpha-l-arabinogalactan) in different crop plants indicated role of NO in these important cellular and metabolic pathways. NO mediated regulation of a transcription factor CBF (C-repeat Binding Factor, a transcription factor) at transcriptional and post-translational level was shown in Solanum lycopersicum seedlings. NO donor priming enhances seed germination, breaks dormancy and provides tolerance to stress in crops. Its role in averting stress, promoting seed germination, and delaying senescence paved the way for use of NO and NO releasing compounds to prevent crop loss and increase the shelf-life of fruits and vegetables. An alternative to energy consuming and expensive cold storage led to development of a storage device called "shelf-life enhancer" that delays senescence and increases shelf-life at ambient temperature (25-27 °C) using NO donor. The present review summarizes NO research in plants and exploration of NO for its translational potential to improve agricultural yield and post-harvest crop loss.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01371-z.

Review on nitric oxide at the forefront of rapid systemic signaling in mitigation of salinity stress in plants: Crosstalk with calcium and hydrogen peroxide

Soil salinity is a global issue that limits plant growth in agricultural fields and contributes to food crisis. Salt stressors impede plant's ionic, osmotic, and oxidative balance, as well as a variety of physiological functions. Exposure to salinity stress manifest considerable ROS clustering, entailing modification in performance of various organelles. To deal with salinity, plants use a variety of coping strategies, such as osmoregulation, ion-homeostasis, increased antioxidant synthesis, and so on. Nitric oxide (NO) is a pivotal signalling molecule that helps facilitate salt stress-induced physiological plant responses. A variety of evidences point to NO being produced under similar stress conditions and with similar kinetics as hydrogen peroxide (H2O2). The interplay between H2O2 and NO has important functional implications for modulating plant transduction processes. Besides, NO and calcium (Ca2+)-dependent pathways also have some connection in salt stress response mechanisms. Extensive crosstalk between NO and Ca2+ signalling pathways is investigated, and it suggests that almost every type of Ca2+ channel is under the tight control of NO, and NO acts as a Ca2+ mobilising compound and aids in signal reliance. The review provides insights into understanding recent advances regarding NO's, Ca2+ and H2O2 role in salt stress reduction with entwine signaling mechanisms.

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.

The Arabidopsis Phytoglobin 2 mediates phytochrome B (phyB) light signaling responses during somatic embryogenesis

During the light induction of somatic embryogenesis, phyB-Pfr suppresses Phytoglobin 2, known to elevate nitric oxide (NO). NO depresses Phytochrome Interacting Factor 4 (PIF4) relieving its inhibition on embryogenesis through auxin. An obligatory step of many in vitro embryogenic systems is the somatic-embryogenic transition culminating with the formation of the embryogenic tissue. In Arabidopsis, this transition requires light and is facilitated by high levels of nitric oxide (NO) generated by either suppression of the NO scavenger Phytoglobin 2 (Pgb2), or its removal from the nucleus. Using a previously characterized induction system regulating the cellular localization of Pgb2, we demonstrated the interplay between phytochrome B (phyB) and Pgb2 during the formation of embryogenic tissue. The deactivation of phyB in the dark coincides with the induction of Pgb2 known to reduce the level of NO; consequently, embryogenesis is inhibited. Under light conditions, the active form of phyB depresses the levels of Pgb2 transcripts, thus expecting an increase in cellular NO. Induction of Pgb2 increases Phytochrome Interacting Factor 4 (PIF4) suggesting that high levels of NO repress PIF4. The PIF4 inhibition is sufficient to induce several auxin biosynthetic (CYP79B2, AMI1, and YUCCA 1, 2, and 6) and response (ARF5, 8, and 16) genes, conducive to the formation of the embryonic tissue and production of somatic embryos. Auxin responses mediated by ARF10 and 17 appear to be regulated by Pgb2, possibly through NO, in a PIF4-independent fashion. Overall, this work provides a new and preliminary model integrating Pgb2 (and NO) with phyB in the light regulation of in vitro embryogenesis.

Light induction of somatic embryogenesis in Arabidopsis is regulated by PHYTOCHROME E

The requirement of light on somatic embryogenesis (SE) has been documented in many species; however, no mechanism of action has been elucidated. Using Arabidopsis SE as a model, the effect of red light (660 nm) during the induction phase corresponding to the formation of the embryogenic tissue was examined. Analyses of several phytochrome mutants revealed that red light signaling, conducive to SE, was mediated by PHYTOCHROME E (PHYE). Both phyE and darkness were sufficient to repress the formation of somatic embryos and reduced the expression of CONSTITUTIVE PHOTOMORPHIC DWARF 3 (CPD3), a rate limiting step in brassinosteroid (BR) biosynthesis, as well as AGAMOUS LIKE 15 (AGL15), a key inducer of many SE genes. We further integrated BR signaling and nitric oxide (NO) with PHYE by demonstrating that applications of both compounds to phyE explants and WT explants cultured in the dark partially restored AGL15 expression. These results demonstrate that SE induction by red light operates via PHYE through BR signaling and NO required to induce AGL15.

Computational prediction of NO-dependent posttranslational modifications in plants: Current status and perspectives

The perception and transduction of nitric oxide (NO) signal is achieved by NO-dependent posttranslational modifications (PTMs) among which S-nitrosation and tyrosine nitration has biological significance. In plants, 100-1000 S-nitrosated and tyrosine nitrated proteins have been identified so far by mass spectrometry. The determination of NO-modified protein targets/amino acid residues is often methodologically challenging. In the past decade, the growing demand for the knowledge of S-nitrosated or tyrosine nitrated sites has motivated the introduction of bioinformatics tools. For predicting S-nitrosation seven computational tools have been developed (GPS-SNO, SNOSite, iSNO-PseACC, iSNO-AAPAir, PSNO, PreSNO, RecSNO). Four predictors have been developed for indicating tyrosine nitration sites (GPS-YNO2, iNitro-Tyr, PredNTS, iNitroY-Deep), and one tool (DeepNitro) predicts both NO-dependent PTMs. The advantage of these computational tools is the fast provision of large amount of information. In this review, the available software tools have been tested on plant proteins in which S-nitrosated or tyrosine nitrated sites have been experimentally identified. The predictors showed distinct performance and there were differences from the experimental results partly due to the fact that the three-dimensional protein structure is not taken into account by the computational tools. Nevertheless, the predictors excellently establish experiments, and it is suggested to apply all available tools on target proteins and compare their results. In the future, computational prediction must be developed further to improve the precision with which S-nitrosation/tyrosine nitration-sites are identified.

Nitric oxide sensing revisited

Nitric oxide (NO) sensing is an ancient trait enabled by hemoproteins harboring a highly conserved Heme-Nitric oxide/OXygen (H-NOX) domain that operates throughout bacteria, fungi, and animal kingdoms including in humans, but that has long thought to be absent in plants. Recently, H-NOX-containing plant hemoproteins mediating crucial NO-dependent responses such as stomatal closure and pollen tube guidance have been reported. There are indications that the detection method that led to these discoveries will uncover many more heme-based NO sensors that operate as regulatory sites in complex proteins. Their characterizations will in turn offer a much more complete picture of plant NO responses at both the molecular and systems level.

Crosstalk between Light- and Temperature-Mediated Processes under Cold and Heat Stress Conditions in Plants

Extreme temperatures are among the most important stressors limiting plant growth and development. Results indicate that light substantially influences the acclimation processes to both low and high temperatures, and it may affect the level of stress injury. The interaction between light and temperature in the regulation of stress acclimation mechanisms is complex, and both light intensity and spectral composition play an important role. Higher light intensities may lead to overexcitation of the photosynthetic electron transport chain; while different wavelengths may act through different photoreceptors. These may induce various stress signalling processes, leading to regulation of stomatal movement, antioxidant and osmoregulation capacities, hormonal actions, and other stress-related pathways. In recent years, we have significantly expanded our knowledge in both light and temperature sensing and signalling. The present review provides a synthesis of results for understanding how light influences the acclimation of plants to extreme low or high temperatures, including the sensing mechanisms and molecular crosstalk processes.