Comparative investigation on chemical and green synthesized titanium dioxide nanoparticles against chromium (VI) stress eliciting differential physiological, biochemical, and cellular attributes in Helianthus annuus L

Nanotechnology is a new scientific area that promotes unique concepts to comprehend the optimal mechanics of nanoparticles (NPs) in plants under heavy metal stress. The present investigation focuses on effects of synthetic and green synthesized titanium dioxide nanoparticles (TiO2 NPs and gTiO2 NPs) against Cr(VI). Green TiO2 NPs have been produced from plant leaf extract (Ricinus communis L.). Synthesis was confirmed employing an array of optical spectroscopic and electron microscopic techniques. Chromium strongly accelerated H2O2 and MDA productions by 227 % and 266 % at highest chromium concentration (60 mg/kg of soil), respectively, and also caused DNA damage, and decline in photosynthesis. Additionally, anomalies were observed in stomatal cells with gradual increment in chromium concentrations. Conversely, foliar applications of TiO2 NPs and gTiO2 NPs considerably mitigated chromium stress. Sunflower plants treated with modest amounts of green TiO2 NPs had significantly better growth index compared to chemically synthesized ones. Principal component analysis highlighted the variations among photosynthetic attributes, oxidative stress markers, and antioxidant defense systems. Notably, gTiO2 supplementation to the Cr(VI) strained plants minimized PC3 production which is a rare report so far. Conclusively, gTiO2 NPs have been identified to be promising nano-based nutrition resource for farming applications.

Nitric oxide, energy and redox-dependent responses to hypoxia

Hypoxia occurs when the oxygen levels fall below the levels required for mitochondria to support respiration. Regulated hypoxia is associated with quiescence, particularly in storage organs (seeds) and stem cell niches. In contrast, environmentally-induced hypoxia poses significant challenges for metabolically-active cells that are adapted to aerobic respiration. The perception of oxygen availability through cysteine oxidases, which function as oxygen-sensing enzymes in plants that control the N-degron pathway, and the regulation of hypoxia-responsive genes and processes is essential to survival. Functioning together with reactive oxygen species (ROS), particularly hydrogen peroxide and reactive nitrogen species (RNS), such as nitric oxide (•NO), nitrogen dioxide (•NO2), S-nitrosothiols (SNOs), and peroxynitrite (ONOO-), hypoxia signaling pathways trigger anatomical adaptations such as formation of aerenchyma, mobilization of sugar reserves for anaerobic germination, formation of aerial adventitious roots and hyponastic response. NO and hydrogen peroxide (H2O2) participate in local and systemic signaling pathways that facilitate acclimation to changing energetic requirements, controlling glycolytic fermentation, the GABA shunt and amino acid synthesis. NO enhances antioxidant capacity and contributes to the recycling of redox equivalents energy metabolism through the phytoglobin (Pgb)-NO cycle. Here, we summarize current knowledge, highlighting the central role of NO and redox regulation in adaptive responses that prevent hypoxia-induced death in challenging conditions such as flooding.

Co-overexpression of SWEET sucrose transporters modulates sucrose synthesis and defence responses to enhance immunity against bacterial blight in rice

Enhancing carbohydrate export from source to sink tissues is considered to be a realistic approach for improving photosynthetic efficiency and crop yield. The rice sucrose transporters OsSUT1, OsSWEET11a and OsSWEET14 contribute to sucrose phloem loading and seed filling. Crucially, Xanthomonas oryzae pv. oryzae (Xoo) infection in rice enhances the expression of OsSWEET11a and OsSWEET14 genes, and causes leaf blight. Here we show that co-overexpression of OsSUT1, OsSWEET11a and OsSWEET14 in rice reduced sucrose synthesis and transport leading to lower growth and yield but reduced susceptibility to Xoo relative to controls. The immunity-related hypersensitive response (HR) was enhanced in the transformed lines as indicated by the increased expression of defence genes, higher salicylic acid content and presence of HR lesions on the leaves. The results suggest that the increased expression of OsSWEET11a and OsSWEET14 in rice is perceived as a pathogen (Xoo) attack that triggers HR and results in constitutive activation of plant defences that are related to the signalling pathways of pathogen starvation. These findings provide a mechanistic basis for the trade-off between plant growth and immunity because decreased susceptibility against Xoo compromised plant growth and yield.

Unearthing the secrets of ERFVIIs: new insights into hypoxia signaling

Group VII ethylene-responsive factor (ERFVII) transcription factors are crucial for the adaption of plants to conditions that limit oxygen availability. A recent study by Zubrycka et al. reveals new aspects of ERFVII stabilization through the PLANT CYSTEINE OXIDASE (PCO)-N degron pathway and non-autonomous regulation in response to different endogenous and exogenous cues.

The emerging roles of nitric oxide and its associated scavengers-phytoglobins-in plant symbiotic interactions

A key feature in the establishment of symbiosis between plants and microbes is the maintenance of the balance between the production of the small redox-related molecule, nitric oxide (NO), and its cognate scavenging pathways. During the establishment of symbiosis, a transition from a normoxic to a microoxic environment often takes place, triggering the production of NO from nitrite via a reductive production pathway. Plant hemoglobins [phytoglobins (Phytogbs)] are a central tenant of NO scavenging, with NO homeostasis maintained via the Phytogb-NO cycle. While the first plant hemoglobin (leghemoglobin), associated with the symbiotic relationship between leguminous plants and bacterial Rhizobium species, was discovered in 1939, most other plant hemoglobins, identified only in the 1990s, were considered as non-symbiotic. From recent studies, it is becoming evident that the role of Phytogbs1 in the establishment and maintenance of plant-bacterial and plant-fungal symbiosis is also essential in roots. Consequently, the division of plant hemoglobins into symbiotic and non-symbiotic groups becomes less justified. While the main function of Phytogbs1 is related to the regulation of NO levels, participation of these proteins in the establishment of symbiotic relationships between plants and microorganisms represents another important dimension among the other processes in which these key redox-regulatory proteins play a central role.

Moderate modulation by S-nitrosoglutathione of photorespiratory enzymes in pea (Pisum sativum) leaves, compared to the strong effects of high light

When plants are exposed to water stress, photosynthesis is downregulated due to enhanced reactive oxygen species (ROS) and nitric oxide (NO). In contrast, photorespiratory metabolism protected photosynthesis and sustained yield. Modulation of photorespiration by ROS was established, but the effect of NO on photorespiratory metabolism was unclear. We, therefore, examined the impact of externally added NO by using S-nitrosoglutathione (GSNO), a natural NO donor, in leaf discs of pea (Pisum sativum) under dark or light: moderate or high light (HL). Maximum NO accumulation with GSNO was under high light. The presence of 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a NO scavenger, prevented the increase in NO, confirming the release of NO in leaves. The increase in S-nitrosothiols and tyrosine-nitrated proteins on exposure to GSNO confirmed the nitrosative stress in leaves. However, the changes by GSNO in the activities and transcripts of five photorespiratory enzymes: glycolate oxidase, hydroxypyruvate reductase, catalase, glycerate kinase, and phosphoglycolate phosphatase activities were marginal. The changes in photorespiratory enzymes caused by GSNO were much less than those with HL. Since GSNO caused only mild oxidative stress, we felt that the key modulator of photorespiration might be ROS, but not NO.

Metabolomic response to high light from pgrl1 and pgr5 mutants of Chlamydomonas reinhardtii

Chlamydomonas (C.) reinhardtii metabolomic changes in cyclic electron flow-dependent mutants are still unknown. Here, we used mass spectrometric analysis to monitor the changes in metabolite levels in wild-type, cyclic electron-deficient mutants pgrl1 and pgr5 grown under high-light stress. A total of 55 metabolites were detected using GC-MS analysis. High-light stress-induced selective anaplerotic amino acids in pgr5. In addition, pgr5 showed enhancement in carbohydrate, polyamine, and polyol metabolism by 2.5-fold under high light. In response to high light, pgr5 triggers an increase in several metabolites involved in regulating osmotic pressure. Among these metabolites are glycerol pathway compounds such as glycerol-3-phosphate and glyceryl-glycoside, which increase significantly by 1.55 and 3.07 times, respectively. In addition, pgr5 also enhanced proline and putrescine levels by 2.6- and 1.36-fold under high light. On the other hand, pgrl1-induced metabolites, such as alanine and serine, are crucial for photorespiration when subjected to high-light stress. We also observed a significant increase in levels of polyols and glycerol by 1.37- and 2.97-fold in pgrl1 under high-light stress. Both correlation network studies and KEGG pathway enrichment analysis revealed that metabolites related to several biological pathways, such as amino acid, carbohydrate, TCA cycle, and fatty acid metabolism, were positively correlated in pgrl1 and pgr5 under high-light stress conditions. The relative mRNA expression levels of genes related to the TCA cycle, including PDC3, ACH1, OGD2, OGD3, IDH3, and MDH4, were significantly upregulated in pgrl1 and pgr5 under HL. In pgr5, the MDH1 level was significantly increased, while ACS1, ACS3, IDH2, and IDH3 levels were reduced considerably in pgrl1 under high-light stress. The current study demonstrates both pgr5 and prgl1 showed a differential defense response to high-light stress at the primary metabolites and mRNA expression level, which can be added to the existing knowledge to explore molecular regulatory responses of prg5 and pgrl1 to high-light stress.

Nitric oxide regulates mitochondrial biogenesis in plants

The site of nitric oxide (NO) production in mitochondrial cytochrome c oxidase and the role of NO in mitochondrial biogenesis are not known in plants. By imposing osmotic stress and recovery on Arabidopsis seedlings we investigated the site of NO production and its role in mitochondrial biogenesis. Osmotic stress reduced growth and mitochondrial number while increasing NO production. During the recovery phase the mitochondrial number increased and this increase was higher in wild type and the high NO-producing Pgb1 silencing line in comparison to the NO-deficient nitrate reductase double mutant (nia1/nia2). Application of nitrite stimulated NO production and mitochondrial number in the nia1/nia2 mutant. Osmotic stress induced COX6b^- 3 and COA6-L genes encoding subunits of COX. The mutants cox6b-3 and coa6-l were impaired both in NO production and mitochondrial number during stress to recovery suggesting the involvement of these subunits in nitrite-dependent NO production. Transcripts encoding the mitochondrial protein import machinery showed reduced expression in cox6b-3 and coa6-l mutants. Finally, COX6b-3 and COA6-L interacted with the VQ27 motif-containing protein in the presence of NO. The vq27 mutant was impaired in mitochondrial biogenesis. Our results suggest the involvement of COX derived NO in mitochondrial biogenesis.

Lactate Dehydrogenase Superfamily in Rice and Arabidopsis: Understanding the Molecular Evolution and Structural Diversity

Lactate/malate dehydrogenases (Ldh/Maldh) are ubiquitous enzymes involved in the central metabolic pathway of plants and animals. The role of malate dehydrogenases in the plant system is very well documented. However, the role of its homolog L-lactate dehydrogenases still remains elusive. Though its occurrence is experimentally proven in a few plant species, not much is known about its role in rice. Therefore, a comprehensive genome-wide in silico investigation was carried out to identify all Ldh genes in model plants, rice and Arabidopsis, which revealed Ldh to be a multigene family encoding multiple proteins. Publicly available data suggest its role in a wide range of abiotic stresses such as anoxia, salinity, heat, submergence, cold and heavy metal stress, as also confirmed by our qRT-PCR analysis, especially in salinity and heavy metal mediated stresses. A detailed protein modelling and docking analysis using Schrodinger Suite reveals the presence of three putatively functional L-lactate dehydrogenases in rice, namely OsLdh3, OsLdh7 and OsLdh9. The analysis also highlights the important role of Ser-219, Gly-220 and His-251 in the active site geometry of OsLdh3, OsLdh7 and OsLdh9, respectively. In fact, these three genes have also been found to be highly upregulated under salinity, hypoxia and heavy metal mediated stresses in rice.

Physiological implications of SWEETs in plants and their potential applications in improving source-sink relationships for enhanced yield

The sugars will eventually be exported transporters (SWEET) family of transporters in plants is identified as a novel class of sugar carriers capable of transporting sugars, sugar alcohols and hormones. Functioning in intercellular sugar transport, SWEETs influence a wide range of physiologically important processes. SWEETs regulate the development of sink organs by providing nutritional support from source leaves, responses to abiotic stresses by maintaining intracellular sugar concentrations, and host-pathogen interactions through the modulation of apoplastic sugar levels. Many bacterial and fungal pathogens activate the expression of SWEET genes in species such as rice and Arabidopsis to gain access to the nutrients that support virulence. The genetic manipulation of SWEETs has led to the generation of bacterial blight (BB)-resistant rice varieties. Similarly, while the overexpression of the SWEETs involved in sucrose export from leaves and pathogenesis led to growth retardation and yield penalties, plants overexpressing SWEETs show improved disease resistance. Such findings demonstrate the complex functions of SWEETs in growth and stress tolerance. Here, we review the importance of SWEETs in plant-pathogen and source-sink interactions and abiotic stress resistance. We highlight the possible applications of SWEETs in crop improvement programmes aimed at improving sink and source strengths important for enhancing the sustainability of yield. We discuss how the adverse effects of the overexpression of SWEETs on plant growth may be overcome.

Detection of Nitric Oxide from Chickpea Using DAF Fluorescence and Chemiluminescence Methods

The free radical nitric oxide (NO) has emerged as an important signal molecule in plants, due to its involvement in various plant growth, development, and stress responses. For elucidating the role of NO, it is very important to precisely determine, localize, and quantify NO levels. Due to a relatively short half-life and its rapid, complex reactivity with other radicals, together with its capacity to diffuse from the source of production, the quantification of NO in whole plants, tissues, organelles, and extracts is notoriously difficult. Hence, it is essential to employ sensitive procedures for precise detection of NO. Currently available methods can fulfill many requirements to precisely determine NO, but each method has several advantages and pitfalls. In this article, we describe a detailed procedure for the measurement of NO by diaminofluorescein (DAF) in cell-permeable forms (DAF-FM-DA). In this method, the tissues are immersed in DAF-FM DA, leading to their diffusion from the plasma membrane to the inside of the cell, where intracellular esterases cleave the ester bonds, leading to DAF-FM release. The resulting DAF-FM reacts with intracellularly generated NO and forms highly fluorescent triazolofluorescein (DAF-FMT), which can be localized and monitored by fluorescence or confocal microscopy, and can also be detected via fluorimetry and flow cytometry. DAF dyes are very popular as they are non-invasive, relatively easy to handle, and commercially available. Another precise and very sensitive method is chemiluminescence detection of NO, where NO reacts with ozone (O₃ ), leading to emission of a quantum of light from which NO can be calculated. Using chickpea seedlings, we describe in detail the measurement of NO using DAF-FM-DA and chemiluminescence methods. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Measurement of nitric oxide from chickpea seedlings using DAF-FM DA fluorescence with fluorescence and confocal microscopy Basic Protocol 2: Chemiluminescence detection of nitric oxide from chickpea seedlings.

Alternative oxidase plays a role in minimizing ROS and RNS produced under salinity stress in Arabidopsis thaliana

Under stress conditions, the overproduction of different reactive oxygen species (ROS) and reactive nitrogen species (RNS) causes imbalance in the redox homeostasis of the cell leading to nitro-oxidative stress in plants. Alternative oxidase (AOX) is a conserving terminal oxidase of the mitochondrial electron transport chain, which can minimize the ROS. Still, the role of AOX in the regulation of RNS during nitro-oxidative stress imposed by salinity stress is not known. Here, we investigated the role of AOX in minimizing ROS and RNS induced by 150 mM NaCl in Arabidopsis using transgenic plants overexpressing (AOX OE) and antisense lines (AOX AS) of AOX. Imposing NaCl treatment leads to a 4-fold enhanced expression of AOX accompanied by enhanced AOX capacity in WT Col-0. Further AOX-OE seedlings displayed enhanced growth compared with the AOX-AS line under stress. Examination of NO levels by DAF-FM fluorescence and chemiluminescence revealed that AOX overexpression leads to reduced levels of NO. The total NR activity was elevated under NaCl, but no significant change was observed in wild-type (WT), AOX OE, and AS lines. The total ROS, superoxide, H₂ O₂ levels, and lipid peroxidation were higher in the AOX-AS line than in WT and AOX-OE lines. The peroxynitrite levels were also higher in the AOX-AS line than in WT and AOX-OE lines; further, the expression of antioxidant genes was elevated in AOX-AS. Taken together, our results suggest that AOX plays an important role in the mitigation of ROS and RNS levels and enhances plant growth, thus providing tolerance against nitro-oxidative stress exerted by NaCl.

Seedling-stage salinity tolerance in rice: Decoding the role of transcription factors

Rice is an important staple food crop that feeds over half of the human population, particularly in developing countries. Increasing salinity is a major challenge for continuing rice production. Though rice is affected by salinity at all the developmental stages, it is most sensitive at the early seedling stage. The yield thus depends on how many seedlings can withstand saline water at the stage of transplantation, especially in coastal farms. The rapid development of "omics" approaches has assisted researchers in identifying biological molecules that are responsive to salt stress. Several salinity-responsive quantitative trait loci (QTL) contributing to salinity tolerance have been identified and validated, making it essential to narrow down the search for the key genes within QTLs. Owing to the impressive progress of molecular tools, it is now clear that the response of plants toward salinity is highly complex, involving multiple genes, with a specific role assigned to the repertoire of transcription factors (TF). Targeting the TFs for improving salinity tolerance can have an inbuilt advantage of influencing multiple downstream genes, which in turn can contribute toward tolerance to multiple stresses. This is the first comparative study for TF-driven salinity tolerance in contrasting rice cultivars at the seedling stage that shows how tolerant genotypes behave differently than sensitive ones in terms of stress tolerance. Understanding the complexity of salt-responsive TF networks at the seedling stage will be helpful to alleviate crop resilience and prevent crop damage at an early growth stage in rice.

Nitric oxide regulation of plant metabolism

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

Phytoglobin-NO cycle and AOX pathway play a role in anaerobic germination and growth of deepwater rice

An important and interesting feature of rice is that it can germinate under anoxic conditions. Though several biochemical adaptive mechanisms play an important role in the anaerobic germination of rice but the role of phytoglobin-nitric oxide cycle and alternative oxidase pathway is not known, therefore in this study we investigated the role of these pathways in anaerobic germination. Under anoxic conditions, deepwater rice germinated much higher and rapidly than aerobic condition and the anaerobic germination and growth were much higher in the presence of nitrite. The addition of nitrite stimulated NR activity and NO production. Important components of phytoglobin-NO cycle such as methaemoglobin reductase activity, expression of Phytoglobin1, NIA1 were elevated under anaerobic conditions in the presence of nitrite. The operation of phytoglobin-NO cycle also enhanced anaerobic ATP generation, LDH, ADH activities and in parallel ethylene levels were also enhanced. Interestingly nitrite suppressed the ROS production and lipid peroxidation. The reduction of ROS was accompanied by enhanced expression of mitochondrial alternative oxidase protein and its capacity. Application of AOX inhibitor SHAM inhibited the anoxic growth mediated by nitrite. In addition, nitrite improved the submergence tolerance of seedlings. Our study revealed that nitrite driven phytoglobin-NO cycle and AOX are crucial players in anaerobic germination and growth of deepwater rice.

Isolation and Measurement of Respiration and Structural Studies of Purified Mitochondria from Heterotrophic Plant Tissues

Mitochondria are the power houses of eukaryotic cells. These organelles contain various oxidoreductase complexes. Electron transfer from different reducing equivalents channeled via these complexes drives proton translocation across the inner mitochondrial membrane, leading to ATP generation. Plant mitochondria contain alternative NAD(P)H dehydrogenases, alternative oxidase, and uncoupling protein, and TCA cycle enzymes are located in their matrix. Apart from ATP production, mitochondria are also involved in synthesis of vitamins and cofactors and participate in fatty acid, nucleotide, photorespiratory, and antioxidant metabolism. Recent emerging evidence suggests that mitochondria play a role in redox signaling and generation of reactive oxygen and nitrogen species. For mitochondrial studies, it is essential to isolate physiologically active mitochondria with good structural integrity. In this article, we explain a detailed procedure for isolation of mitochondria from various heterotrophic tissues, such as germinating chickpea seeds, potato tubers, and cauliflower florets. This procedure requires discontinuous Percoll gradient centrifugation and can give a good yield of mitochondria, in the range of 4 to 8 mg per 50 g tissue with active respiratory capacity. After MitoTracker staining, isolated mitochondria can be visualized by using a confocal microscope. The structure of mitochondria can be monitored by scanning electron microscopy. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Isolation of mitochondria from germinating chickpea seeds, potato tubers, and cauliflower florets Basic Protocol 2: Quantification of mitochondrial protein concentration by Bradford assay Basic Protocol 3: Quantification of mitochondrial respiration using single-channel free-radical analyzer Basic Protocol 4: Staining of mitochondria and confocal imaging Basic Protocol 5: Visualization of isolated mitochondria under scanning electron microscope.

Adaptive Reprogramming During Early Seed Germination Requires Temporarily Enhanced Fermentation-A Critical Role for Alternative Oxidase Regulation That Concerns Also Microbiota Effectiveness

Plants respond to environmental cues via adaptive cell reprogramming that can affect whole plant and ecosystem functionality. Microbiota constitutes part of the inner and outer environment of the plant. This Umwelt underlies steady dynamics, due to complex local and global biotic and abiotic changes. Hence, adaptive plant holobiont responses are crucial for continuous metabolic adjustment at the systems level. Plants require oxygen-dependent respiration for energy-dependent adaptive morphology, such as germination, root and shoot growth, and formation of adventitious, clonal, and reproductive organs, fruits, and seeds. Fermentative paths can help in acclimation and, to our view, the role of alternative oxidase (AOX) in coordinating complex metabolic and physiological adjustments is underestimated. Cellular levels of sucrose are an important sensor of environmental stress. We explored the role of exogenous sucrose and its interplay with AOX during early seed germination. We found that sucrose-dependent initiation of fermentation during the first 12 h after imbibition (HAI) was beneficial to germination. However, parallel upregulated AOX expression was essential to control negative effects by prolonged sucrose treatment. Early downregulated AOX activity until 12 HAI improved germination efficiency in the absence of sucrose but suppressed early germination in its presence. The results also suggest that seeds inoculated with arbuscular mycorrhizal fungi (AMF) can buffer sucrose stress during germination to restore normal respiration more efficiently. Following this approach, we propose a simple method to identify organic seeds and low-cost on-farm perspectives for early identifying disease tolerance, predicting plant holobiont behavior, and improving germination. Furthermore, the research strengthens the view that AOX can serve as a powerful functional marker source for seed hologenomes.

ROS/RNS Balancing, Aerobic Fermentation Regulation and Cell Cycle Control - a Complex Early Trait ('CoV-MAC-TED') for Combating SARS-CoV-2-Induced Cell Reprogramming

In a perspective entitled 'From plant survival under severe stress to anti-viral human defense' we raised and justified the hypothesis that transcript level profiles of justified target genes established from in vitro somatic embryogenesis (SE) induction in plants as a reference compared to virus-induced profiles can identify differential virus signatures that link to harmful reprogramming. A standard profile of selected genes named 'ReprogVirus' was proposed for in vitro-scanning of early virus-induced reprogramming in critical primary infected cells/tissues as target trait. For data collection, the 'ReprogVirus platform' was initiated. This initiative aims to identify in a common effort across scientific boundaries critical virus footprints from diverse virus origins and variants as a basis for anti-viral strategy design. This approach is open for validation and extension. In the present study, we initiated validation by experimental transcriptome data available in public domain combined with advancing plant wet lab research. We compared plant-adapted transcriptomes according to 'RegroVirus' complemented by alternative oxidase (AOX) genes during de novo programming under SE-inducing conditions with in vitro corona virus-induced transcriptome profiles. This approach enabled identifying a major complex trait for early de novo programming during SARS-CoV-2 infection, called 'CoV-MAC-TED'. It consists of unbalanced ROS/RNS levels, which are connected to increased aerobic fermentation that links to alpha-tubulin-based cell restructuration and progression of cell cycle. We conclude that anti-viral/anti-SARS-CoV-2 strategies need to rigorously target 'CoV-MAC-TED' in primary infected nose and mouth cells through prophylactic and very early therapeutic strategies. We also discuss potential strategies in the view of the beneficial role of AOX for resilient behavior in plants. Furthermore, following the general observation that ROS/RNS equilibration/redox homeostasis is of utmost importance at the very beginning of viral infection, we highlight that 'de-stressing' disease and social handling should be seen as essential part of anti-viral/anti-SARS-CoV-2 strategies.

From Plant Survival Under Severe Stress to Anti-Viral Human Defense - A Perspective That Calls for Common Efforts

Reprogramming of primary virus-infected cells is the critical step that turns viral attacks harmful to humans by initiating super-spreading at cell, organism and population levels. To develop early anti-viral therapies and proactive administration, it is important to understand the very first steps of this process. Plant somatic embryogenesis (SE) is the earliest and most studied model for de novo programming upon severe stress that, in contrast to virus attacks, promotes individual cell and organism survival. We argued that transcript level profiles of target genes established from in vitro SE induction as reference compared to virus-induced profiles can identify differential virus traits that link to harmful reprogramming. To validate this hypothesis, we selected a standard set of genes named 'ReprogVirus'. This approach was recently applied and published. It resulted in identifying 'CoV-MAC-TED', a complex trait that is promising to support combating SARS-CoV-2-induced cell reprogramming in primary infected nose and mouth cells. In this perspective, we aim to explain the rationale of our scientific approach. We are highlighting relevant background knowledge on SE, emphasize the role of alternative oxidase in plant reprogramming and resilience as a learning tool for designing human virus-defense strategies and, present the list of selected genes. As an outlook, we announce wider data collection in a 'ReprogVirus Platform' to support anti-viral strategy design through common efforts.

Gaining Acceptance of Novel Plant Breeding Technologies

Ensuring the sustainability of agriculture under climate change has led to a surge in alternative strategies for crop improvement. Advances in integrated crop breeding, social acceptance, and farm-level adoption are crucial to address future challenges to food security. Societal acceptance can be slow when consumers do not see the need for innovation or immediate benefits. We consider how best to address the issue of social licence and harmonised governance for novel gene technologies in plant breeding. In addition, we highlight optimised breeding strategies that will enable long-term genetic gains to be achieved. Promoted by harmonised global policy change, innovative plant breeding can realise high and sustainable productivity together with enhanced nutritional traits.

The uncoupling of respiration in plant mitochondria: keeping reactive oxygen and nitrogen species under control

Plant mitochondrial respiration involves the operation of various alternative pathways. These pathways participate, both directly and indirectly, in the maintenance of mitochondrial functions though they do not contribute to energy production, being uncoupled from the generation of an electrochemical gradient across the mitochondrial membrane and thus from ATP production. Recent findings suggest that uncoupled respiration is involved in reactive oxygen species (ROS) and nitric oxide (NO) scavenging, regulation, and homeostasis. Here we discuss specific roles and possible functions of uncoupled mitochondrial respiration in ROS and NO metabolism. The mechanisms of expression and regulation of the NDA-, NDB- and NDC-type non-coupled NADH and NADPH dehydrogenases, the alternative oxidase (AOX), and the uncoupling protein (UCP) are examined in relation to their involvement in the establishment of the stable far-from-equilibrium state of plant metabolism. The role of uncoupled respiration in controlling the levels of ROS and NO as well as inducing signaling events is considered. Secondary functions of uncoupled respiration include its role in protection from stress factors and roles in biosynthesis and catabolism. It is concluded that uncoupled mitochondrial respiration plays an important role in providing rapid adaptation of plants to changing environmental factors via regulation of ROS and NO.

Regulating the regulator: nitric oxide control of post-translational modifications

Nitric oxide (NO) is perfectly suited for the role of a redox signalling molecule. A key route for NO bioactivity occurs via protein S-nitrosation, and involves the addition of a NO moiety to a protein cysteine (Cys) thiol (-SH) to form an S-nitrosothiol (SNO). This process is thought to underpin a myriad of cellular processes in plants that are linked to development, environmental responses and immune function. Here we collate emerging evidence showing that NO bioactivity regulates a growing number of diverse post-translational modifications including SUMOylation, phosphorylation, persulfidation and acetylation. We provide examples of how NO orchestrates these processes to mediate plant adaptation to a variety of cellular cues.

Recommendations on terminology and experimental best practice associated with plant nitric oxide research

Nitric oxide (NO) emerged as a key signal molecule in plants. During the last two decades impressive progress has been made in plant NO research. This small, redox-active molecule is now known to play an important role in plant immunity, stress responses, environmental interactions, plant growth and development. To more accurately and robustly establish the full spectrum of NO bioactivity in plants, it will be essential to apply methodological best practice. In addition, there are some instances of conflicting nomenclature within the field, which would benefit from standardization. In this context, we attempt to provide some helpful guidance for best practice associated with NO research and also suggestions for the cognate terminology.

The role of nitrite and nitric oxide under low oxygen conditions in plants

Plant tissues, particularly roots, can be subjected to periods of hypoxia due to environmental circumstances. Plants have developed various adaptations in response to hypoxic stress and these have been described extensively. Less well-appreciated is the body of evidence demonstrating that scavenging of nitric oxide (NO) and the reduction of nitrate/nitrite regulate important mechanisms that contribute to tolerance to hypoxia. Although ethylene controls hyponasty and aerenchyma formation, NO production apparently regulates hypoxic ethylene biosynthesis. In the hypoxic mitochondrion, cytochrome c oxidase, which is a major source of NO, also is inhibited by NO, thereby reducing the respiratory rate and enhancing local oxygen concentrations. Nitrite can maintain ATP generation under hypoxia by coupling its reduction to the translocation of protons from the inner side of mitochondria and generating an electrochemical gradient. This reaction can be further coupled to a reaction whereby nonsymbiotic haemoglobin oxidizes NO to nitrate. In addition to these functions, nitrite has been reported to influence mitochondrial structure and supercomplex formation, as well as playing a role in oxygen sensing via the N-end rule pathway. These studies establish that nitrite and NO perform multiple functions during plant hypoxia and suggest that further research into the underlying mechanisms is warranted.

Does the alternative respiratory pathway offer protection against the adverse effects resulting from climate change?

Elevated greenhouse gases (GHGs) induce adverse conditions directly and indirectly, causing decreases in plant productivity. To deal with climate change effects, plants have developed various mechanisms including the fine-tuning of metabolism. Plant respiratory metabolism is highly flexible due to the presence of various alternative pathways. The mitochondrial alternative oxidase (AOX) respiratory pathway is responsive to these changes, and several lines of evidence suggest it plays a role in reducing excesses of reactive oxygen species (ROS) and reactive nitrogen species (RNS) while providing metabolic flexibility under stress. Here we discuss the importance of the AOX pathway in dealing with elevated carbon dioxide (CO2), nitrogen oxides (NOx), ozone (O3), and the main abiotic stresses induced by climate change.

Using Foldscope to Monitor Superoxide Production and Cell Death During Pathogen Infection in Arabidopsis Under Different Nitrogen Regimes

Nitrogen nutrition plays a role in plant growth development and resistance against biotic and abiotic stress. During pathogen infection various signal molecules such as reactive oxygen species, calcium, reactive nitrogen species, salicylic acid, and ethylene plays an important role. The form of nitrogen nutrition such as nitrate or ammonium plays a role in production of these molecules. Under nitrate nutrition NO is predominant. The produced NO plays a role in reacting with superoxide to generate peroxynitrite to induce cell death during hypersensitive response elicited by avirulent pathogens. Excess of ROS is also detrimental to plants and NO plays a role in regulating ROS. Hence it is important to observe superoxide production during infection. By using an avirulent Pseudomonas syringae and Arabidopsis differential N nutrition we show superoxide production in leaves using a paper microscope called Foldscope, which can be applied as a simple microscope to observe objects. The data also compared with root system infected with pathogenic Fusarium oxysporum. Taken together here we show that Foldscope is a cost-effective and powerful technique to visualize superoxide and cell death in plants during infection.

Methods for Measuring Nitrate Reductase, Nitrite Levels, and Nitric Oxide from Plant Tissues

Nitrogen (N) is one of the most important nutrients which exist in both inorganic and organic forms. Plants assimilate inorganic form of N [nitrate (NO₃^-), nitrite (NO₂^-) or ammonium (NH₄⁺)] and incorporate into amino acids. The metabolism of N involves a series of events such as sensing, uptake, and assimilation. The initial stage is sensing, triggered by nitrate or ammonium signals initiating signal transduction processes in N metabolism. The assimilation pathway initiates with NO₃^-/NH₄⁺ transport to roots via specific high and low affinity (HATs and LATs) nitrate transporters or directly via ammonium transporters (AMTs). In cytosol the NO₃^- is reduced to NO₂^- by cytosolic nitrate reductase (NR) and the produced NO₂^- is further reduced to NH₄⁺ by nitrite reductase (NiR) in plastids. NR has capability to reduce NO₂^- to nitric oxide (NO) under specific conditions such as hypoxia, low pH, and pathogen infection. The produced NO acts as a signal for wide range of processes such as plant growth development and stress. Here, we provide methods to measure NR activity, NO₂^- levels, and NO production in plant tissues.

The PHYTOGLOBIN-NO Cycle Regulates Plant Mycorrhizal Symbiosis

The production of the redox-active signaling molecule, NO, has long been associated with interactions between microbes and their host plants. Emerging evidence now suggests that specific NO signatures and cognate patterns of PHYTOGLOBIN1 (PHYTOGB1) expression, a key regulator of cellular NO homeostasis, may help determine either symbiosis or pathogenicity.

Novel and conserved functions of S-nitrosoglutathione reductase in tomato

Nitric oxide (NO) is emerging as a key signalling molecule in plants. The chief mechanism for the transfer of NO bioactivity is thought to be S-nitrosylation, the addition of an NO moiety to a protein cysteine thiol to form an S-nitrosothiol (SNO). The enzyme S-nitrosoglutathione reductase (GSNOR) indirectly controls the total levels of cellular S-nitrosylation, by depleting S-nitrosoglutathione (GSNO), the major cellular NO donor. Here we show that depletion of GSNOR function impacts tomato (Solanum lycopersicum. L) fruit development. Thus, reduction of GSNOR expression through RNAi modulated both fruit formation and yield, establishing a novel function for GSNOR. Further, depletion of S. lycopersicum GSNOR (SlGSNOR) additionally impacted a number of other developmental processes, including seed development, which also has not been previously linked with GSNOR activity. In contrast to Arabidopsis, depletion of GSNOR function did not influence root development. Further, reduction of GSNOR transcript abundance compromised plant immunity. Surprisingly, this was in contrast to previous data in Arabidopsis that reported that reducing Arabidopsis thaliana GSNOR (AtGSNOR) expression by antisense technology increased disease resistance. We also show that increased SlGSNOR expression enhanced pathogen protection, uncovering a potential strategy to enhance disease resistance in crop plants. Collectively, our findings reveal, at the genetic level, that some but not all GSNOR activities are conserved outside the Arabidopsis reference system. Thus, manipulating the extent of GSNOR expression may control important agricultural traits in tomato and possibly other crop plants.

Polyamine Induction in Postharvest Banana Fruits in Response to NO Donor SNP Occurs via l-Arginine Mediated Pathway and Not via Competitive Diversion of S-Adenosyl-l-Methionine

Nitric oxide (NO) is known to antagonize ethylene by various mechanisms; one of such mechanisms is reducing ethylene levels by competitive action on S-adenosyl-L-methionine (SAM)—a common precursor for both ethylene and polyamines (PAs) biosynthesis. In order to investigate whether this mechanism of SAM pool diversion by NO occur towards PAs biosynthesis in banana, we studied the effect of NO on alterations in the levels of PAs, which in turn modulate ethylene levels during ripening. In response to NO donor sodium nitroprusside (SNP) treatment, all three major PAs viz. putrescine, spermidine and spermine were induced in control as well as ethylene pre-treated banana fruits. However, the gene expression studies in two popular banana varieties of diverse genomes, Nanjanagudu rasabale (NR; AAB genome) and Cavendish (CAV; AAA genome) revealed the downregulation of SAM decarboxylase, an intermediate gene involved in ethylene and PA pathway after the fifth day of NO donor SNP treatment, suggesting that ethylene and PA pathways do not compete for SAM. Interestingly, arginine decarboxylase belonging to arginine-mediated route of PA biosynthesis was upregulated several folds in response to the SNP treatment. These observations revealed that NO induces PAs via l-arginine-mediated route and not via diversion of SAM pool.

Current approaches to measure nitric oxide in plants

Nitric oxide (NO) is now established as an important signalling molecule in plants where it influences growth, development, and responses to stress. Despite extensive research, the most appropriate methods to measure and localize these signalling radicals are debated and still need investigation. Many confounding factors such as the presence of other reactive intermediates, scavenging enzymes, and compartmentation influence how accurately each can be measured. Further, these signalling radicals have short half-lives ranging from seconds to minutes based on the cellular redox condition. Hence, it is necessary to use sensitive and specific methods in order to understand the contribution of each signalling molecule to various biological processes. In this review, we summarize the current knowledge on NO measurement in plant samples, via various methods. We also discuss advantages, limitations, and wider applications of each method.

Nitric oxide accelerates germination via the regulation of respiration in chickpea

Seed germination is crucial for the plant life cycle. We investigated the role of nitric oxide (NO) in two chickpea varieties that differ in germination capacity: Kabuli, which has a low rate of germination and germinates slowly, and Desi, which shows improved germination properties. Desi produced more NO than Kabuli and had lower respiratory rates. As a result of the high respiration rates, Kabuli had higher levels of reactive oxygen species (ROS). Treatment with the NO donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP) reduced respiration in Kabuli and decreased ROS levels, resulting in accelerated germination rates. These findings suggest that NO plays a key role in the germination of Kabuli. SNAP increased the levels of transcripts encoding enzymes involved in carbohydrate metabolism and the cell cycle. Moreover, the levels of amino acids and organic acids were increased in Kabuli as a result of SNAP treatment. 1H-nuclear magnetic resonance analysis revealed that Kabuli has a higher capacity for glucose oxidation than Desi. An observed SNAP-induced increase in 13C incorporation into soluble alanine may result from enhanced oxidation of exogenous [13C]glucose via glycolysis and the pentose phosphate pathway. A homozygous hybrid that originated from a recombinant inbred line population of a cross between Desi and Kabuli germinated faster and had increased NO levels and a reduced accumulation of ROS compared with Kabuli. Taken together, these findings demonstrate the importance of NO in chickpea germination via the control of respiration and ROS accumulation.

Alternative oxidase is an important player in the regulation of nitric oxide levels under normoxic and hypoxic conditions in plants

Plant mitochondria possess two different pathways for electron transport from ubiquinol: the cytochrome pathway and the alternative oxidase (AOX) pathway. The AOX pathway plays an important role in stress tolerance and is induced by various metabolites and signals. Previously, several lines of evidence indicated that the AOX pathway prevents overproduction of superoxide and other reactive oxygen species. More recent evidence suggests that AOX also plays a role in regulation of nitric oxide (NO) production and signalling. The AOX pathway is induced under low phosphate, hypoxia, pathogen infections, and elicitor treatments. The induction of AOX under aerobic conditions in response to various stresses can reduce electron transfer through complexes III and IV and thus prevents the leakage of electrons to nitrite and the subsequent accumulation of NO. Excess NO under various stresses can inhibit complex IV; thus, the AOX pathway minimizes nitrite-dependent NO synthesis that would arise from enhanced electron leakage in the cytochrome pathway. By preventing NO generation, AOX can reduce peroxynitrite formation and tyrosine nitration. In contrast to its function under normoxia, AOX has a specific role under hypoxia, where AOX can facilitate nitrite-dependent NO production. This reaction drives the phytoglobin-NO cycle to increase energy efficiency under hypoxia.

Nitrite and nitric oxide are important in the adjustment of primary metabolism during the hypersensitive response in tobacco

Nitrate and ammonia deferentially modulate primary metabolism during the hypersensitive response in tobacco. In this study, tobacco RNAi lines with low nitrite reductase (NiRr) levels were used to investigate the roles of nitrite and nitric oxide (NO) in this process. The lines accumulate NO2-, with increased NO generation, but allow sufficient reduction to NH4+ to maintain plant viability. For wild-type (WT) and NiRr plants grown with NO3-, inoculation with the non-host biotrophic pathogen Pseudomonas syringae pv. phaseolicola induced an accumulation of nitrite and NO, together with a hypersensitive response (HR) that resulted in decreased bacterial growth, increased electrolyte leakage, and enhanced pathogen resistance gene expression. These responses were greater with increases in NO or NO2- levels in NiRr plants than in the WT under NO3- nutrition. In contrast, WT and NiRr plants grown with NH4+ exhibited compromised resistance. A metabolomic analysis detected 141 metabolites whose abundance was differentially changed as a result of exposure to the pathogen and in response to accumulation of NO or NO2-. Of these, 13 were involved in primary metabolism and most were linked to amino acid and energy metabolism. HR-associated changes in metabolism that are often linked with primary nitrate assimilation may therefore be influenced by nitrite and NO production.

Tomato roots exhibit in vivo glutamate dehydrogenase aminating capacity in response to excess ammonium supply

In higher plants ammonium (NH₄⁺) assimilation occurs mainly through the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway. Nevertheless, when plants are exposed to stress conditions, such as excess of ammonium, the contribution of alternative routes of ammonium assimilation such as glutamate dehydrogenase (GDH) and asparagine synthetase (AS) activities might serve as detoxification mechanisms. In this work, the in vivo functions of these pathways were studied after supplying an excess of ammonium to tomato (Solanum lycopersicum L. cv. Agora Hybrid F1) roots previously adapted to grow under either nitrate or ammonium nutrition. The short-term incorporation of labelled ammonium (¹⁵NH₄⁺) into the main amino acids was determined by GC-MS in the presence or absence of methionine sulphoximine (MSX) and azaserine (AZA), inhibitors of GS and GOGAT activities, respectively. Tomato roots were able to respond rapidly to excess ammonium by enhancing ammonium assimilation regardless of the previous nutritional regime to which the plant was adapted to grow. The assimilation of ¹⁵NH₄⁺ could take place through pathways other than GS/GOGAT, since the inhibition of GS and GOGAT did not completely impede the incorporation of the labelled nitrogen into major amino acids. The in vivo formation of Asn by AS was shown to be exclusively Gln-dependent since the root was unable to incorporate ¹⁵NH₄⁺ directly into Asn. On the other hand, an in vivo aminating capacity was revealed for GDH, since newly labelled Glu synthesis occurred even when GS and/or GOGAT activities were inhibited. The aminating GDH activity in tomato roots responded to an excess ammonium supply independently of the previous nutritional regime to which the plant had been subjected.

Nitrate nutrition influences multiple factors in order to increase energy efficiency under hypoxia in Arabidopsis

BACKGROUND AND AIMS

Nitrogen (N) levels vary between ecosystems, while the form of available N has a substantial impact on growth, development and perception of stress. Plants have the capacity to assimilate N in the form of either nitrate (NO3-) or ammonium (NH4+). Recent studies revealed that NO3- nutrition increases nitric oxide (NO) levels under hypoxia. When oxygen availability changes, plants need to generate energy to protect themselves against hypoxia-induced damage. As the effects of NO3- or NH4+ nutrition on energy production remain unresolved, this study was conducted to investigate the role of N source on group VII transcription factors, fermentative genes, energy metabolism and respiration under normoxic and hypoxic conditions.

METHODS

We used Arabidopsis plants grown on Hoagland medium with either NO3- or NH4+ as a source of N and exposed to 0.8 % oxygen environment. In both roots and seedlings, we investigated the phytoglobin-nitric oxide cycle and the pathways of fermentation and respiration; furthermore, NO levels were tested using a combination of techniques including diaminofluorescein fluorescence, the gas phase Griess reagent assay, respiration by using an oxygen sensor and gene expression analysis by real-time quantitative reverse transcription-PCR methods.

KEY RESULTS

Under NO3- nutrition, hypoxic stress leads to increases in nitrate reductase activity, NO production, class 1 phytoglobin transcript abundance and metphytoglobin reductase activity. In contrast, none of these processes responded to hypoxia under NH4+ nutrition. Under NO3- nutrition, a decreased total respiratory rate and increased alternative oxidase capacity and expression were observed during hypoxia. Data correlated with decreased reactive oxygen species and lipid peroxidation levels. Moreover, increased fermentation and NAD+ recycling as well as increased ATP production concomitant with the increased expression of transcription factor genes HRE1, HRE2, RAP2.2 and RAP2.12 were observed during hypoxia under NO3- nutrition.

CONCLUSIONS

The results of this study collectively indicate that nitrate nutrition influences multiple factors in order to increase energy efficiency under hypoxia.

Alternative Oxidase (AOX) Senses Stress Levels to Coordinate Auxin-Induced Reprogramming From Seed Germination to Somatic Embryogenesis-A Role Relevant for Seed Vigor Prediction and Plant Robustness

Somatic embryogenesis (SE) is the most striking and prominent example of plant plasticity upon severe stress. Inducing immature carrot seeds perform SE as substitute to germination by auxin treatment can be seen as switch between stress levels associated to morphophysiological plasticity. This experimental system is highly powerful to explore stress response factors that mediate the metabolic switch between cell and tissue identities. Developmental plasticity per se is an emerging trait for in vitro systems and crop improvement. It is supposed to underlie multi-stress tolerance. High plasticity can protect plants throughout life cycles against variable abiotic and biotic conditions. We provide proof of concepts for the existing hypothesis that alternative oxidase (AOX) can be relevant for developmental plasticity and be associated to yield stability. Our perspective on AOX as relevant coordinator of cell reprogramming is supported by real-time polymerase chain reaction (PCR) analyses and gross metabolism data from calorespirometry complemented by SHAM-inhibitor studies on primed, elevated partial pressure of oxygen (EPPO)-stressed, and endophyte-treated seeds. In silico studies on public experimental data from diverse species strengthen generality of our insights. Finally, we highlight ready-to-use concepts for plant selection and optimizing in vivo and in vitro propagation that do not require further details on molecular physiology and metabolism. This is demonstrated by applying our research & technology concepts to pea genotypes with differential yield performance in multilocation fields and chickpea types known for differential robustness in the field. By using these concepts and tools appropriately, also other marker candidates than AOX and complex genomics data can be efficiently validated for prebreeding and seed vigor prediction.

Nitrate, NO and ROS Signaling in Stem Cell Homeostasis

Shoot and root growth is facilitated by stem cells in the shoot and root apical meristems (SAM and RAM). Recent reports have demonstrated a close link between nitrogen nutrition, nitric oxide (NO), and reactive oxygen species (ROS) in the regulation of SAM and RAM functions in response to nitrogen availability.

A discrete role for alternative oxidase under hypoxia to increase nitric oxide and drive energy production

Alternative oxidase (AOX) is an integral part of the mitochondrial electron transport and can prevent reactive oxygen species (ROS) and nitric oxide (NO) production under non-stressed, normoxic conditions. Here we assessed the roles of AOX by imposing stress under normoxia in comparison to hypoxic conditions using AOX over expressing (AOX OE) and anti-sense (AOX AS) transgenic Arabidopsis seedlings and roots. Under normoxic conditions stress was induced with the defence elicitor flagellin (flg22). AOX OE reduced NO production whilst this was increased in AOX AS. Moreover AOX AS also exhibited an increase in superoxide and therefore peroxynitrite, tyrosine nitration suggesting that scavenging of NO by AOX can prevent toxic peroxynitrite formation under normoxia. In contrast, during hypoxia interestingly we found that AOX is a generator of NO. Thus, the NO produced during hypoxia, was enhanced in AOX OE and suppressed in AOX AS. Additionally, treatment of WT or AOX OE with the AOX inhibitor SHAM inhibited hypoxic NO production. The enhanced levels of NO correlated with expression of non-symbiotic haemoglobin, increased NR activity and ATP production. The ATP generation was suppressed in nia1,2 mutant and non symbiotic haemoglobin antisense line treated with SHAM. Taken together these results suggest that hypoxic NO generation mediated by AOX has a discrete role by feeding into the haemoglobin-NO cycle to drive energy efficiency under conditions of low oxygen tension.

Interaction of nitric oxide with the components of the plant mitochondrial electron transport chain

Mitochondria are not only major sites for energy production but also participate in several alternative functions, among these generation of nitric oxide (NO), and its different impacts on this organelle, is receiving increasing attention. The inner mitochondrial membrane contains the chain of protein complexes, and electron transfer via oxidation of various organic acids and reducing equivalents leads to generation of a proton gradient that results in energy production. Recent evidence suggests that these complexes are sources and targets for NO. Complex I and rotenone-insensitive NAD(P)H dehydrogenases regulate hypoxic NO production, while complex I also participates in the formation of a supercomplex with complex III under hypoxia. Complex II is a target for NO which, by inhibiting Fe-S centres, regulates reactive oxygen species (ROS) generation. Complex III is one of the major sites for NO production, and the produced NO participates in the phytoglobin-NO cycle that leads to the maintenance of the redox level and limited energy production under hypoxia. Expression of the alternative oxidase (AOX) is induced by NO under various stress conditions, and evidence exists that AOX can regulate mitochondrial NO production. Complex IV is another major site for NO production, which can also be linked to ATP generation via the phytoglobin-NO cycle. Inhibition of complex IV by NO can prevent oxygen depletion at the frontier of anoxia. The NO production and action on various complexes play a major role in NO signalling and energy metabolism.

Isolation of Physiologically Active and Intact Mitochondria from Chickpea

Chickpea is an important leguminous crop that belongs to Fabaceae family, highly valued for its nutritious seeds. Seeds contain reserve food for the developing embryos. Mitochondria are crucial organelle for generation of chemical energy in the form of ATP which is required for achieving metabolically active state; therefore, investigating mitochondrial function and respiration rate is crucial for exploring various metabolic and physio-biochemical changes that occur during seed germination. Here we describe a method for isolation of mitochondria from germinating seeds of two chickpea varieties, i.e., Desi and Kabuli. Structure of Mitotracker-stained isolated mitochondria was observed by confocal microscopy and respiration rate was measured using an oxygen microsensor.

Reactive oxygen species, nitric oxide production and antioxidant gene expression during development of aerenchyma formation in wheat

In response to hypoxia, plant roots produce very high levels of nitric oxide. Recently, it was demonstrated that NO and ethylene both are essential for development of aerenchyma in wheat roots under hypoxia. Increased NO under hypoxia correlated with induction of NADPH oxidase gene expression, ROS production and lipid peroxidation in cortical cells. Tyrosine nitration was prominent in cells developing aerenchyma suggesting that NO and ROS play a key role in development of aerenchyma. However, the role of antioxidant genes during development of aerenchyma is not known, therefore, we checked gene expression of various antioxidants such as SOD1, AOX1A, APX and MnSOD at different time points after hypoxia treatment and found that expression of these genes elevated in 2 h but downregulated in 24 h where development of aerenchyma is prominent. Further, we found that plants growing under ammonium nutrition displayed delayed aerenchyma development. Taken together, new insights presented in this short communication highlighted additional regulatory role of antioxidants gene expression during aerenchyma development.

Pathways of nitric oxide metabolism and operation of phytoglobins in legume nodules: missing links and future directions

The interaction between legumes and rhizobia leads to the establishment of a beneficial symbiotic relationship. Recent advances in legume - rhizobium symbiosis revealed that various reactive oxygen and nitrogen species including nitric oxide (NO) play important roles during this process. Nodule development occurs with a transition from a normoxic environment during the establishment of symbiosis to a microoxic environment in functional nodules. Such oxygen dynamics are required for activation and repression of various NO production and scavenging pathways. Both the plant and bacterial partners participate in the synthesis and degradation of NO. However, the pathways of NO production and degradation as well as their cross-talk and involvement in the metabolism are still a matter of debate. The plant-originated reductive pathways are known to contribute to the NO production in nodules under hypoxic conditions. Non-symbiotic hemoglobin (phytoglobin) (Pgb) possesses high NO oxygenation capacity, buffers and scavenges NO. Its operation, through a respiratory cycle called Pgb-NO cycle, leads to the maintenance of redox and energy balance in nodules. The role of Pgb/NO cycle under fluctuating NO production from soil needs further investigation for complete understanding of NO regulatory mechanism governing nodule development to attain optimal food security under changing environment.

Nitric oxide is essential for the development of aerenchyma in wheat roots under hypoxic stress

In response to flooding/waterlogging, plants develop various anatomical changes including the formation of lysigenous aerenchyma for the delivery of oxygen to roots. Under hypoxia, plants produce high levels of nitric oxide (NO) but the role of this molecule in plant-adaptive response to hypoxia is not known. Here, we investigated whether ethylene-induced aerenchyma requires hypoxia-induced NO. Under hypoxic conditions, wheat roots produced NO apparently via nitrate reductase and scavenging of NO led to a marked reduction in aerenchyma formation. Interestingly, we found that hypoxically induced NO is important for induction of the ethylene biosynthetic genes encoding ACC synthase and ACC oxidase. Hypoxia-induced NO accelerated production of reactive oxygen species, lipid peroxidation, and protein tyrosine nitration. Other events related to cell death such as increased conductivity, increased cellulase activity, DNA fragmentation, and cytoplasmic streaming occurred under hypoxia, and opposing effects were observed by scavenging NO. The NO scavenger cPTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt) and ethylene biosynthetic inhibitor CoCl₂ both led to reduced induction of genes involved in signal transduction such as phospholipase C, G protein alpha subunit, calcium-dependent protein kinase family genes CDPK, CDPK2, CDPK 4, Ca-CAMK, inositol 1,4,5-trisphosphate 5-phosphatase 1, and protein kinase suggesting that hypoxically induced NO is essential for the development of aerenchyma.

Moving nitrogen to the centre of plant defence against pathogens

BACKGROUND

Plants require nitrogen (N) for growth, development and defence against abiotic and biotic stresses. The extensive use of artificial N fertilizers has played an important role in the Green Revolution. N assimilation can involve a reductase series ( NO3- → NO2- → NH4+ ) followed by transamination to form amino acids. Given its widespread use, the agricultural impact of N nutrition on disease development has been extensively examined.

SCOPE

When a pathogen first comes into contact with a host, it is usually nutrient starved such that rapid assimilation of host nutrients is essential for successful pathogenesis. Equally, the host may reallocate its nutrients to defence responses or away from the site of attempted infection. Exogenous application of N fertilizer can, therefore, shift the balance in favour of the host or pathogen. In line with this, increasing N has been reported either to increase or to decrease plant resistance to pathogens, which reflects differences in the infection strategies of discrete pathogens. Beyond considering only N content, the use of NO3- or NH4+ fertilizers affects the outcome of plant-pathogen interactions. NO3- feeding augments hypersensitive response- (HR) mediated resistance, while ammonium nutrition can compromise defence. Metabolically, NO3- enhances production of polyamines such as spermine and spermidine, which are established defence signals, with NH4+ nutrition leading to increased γ-aminobutyric acid (GABA) levels which may be a nutrient source for the pathogen. Within the defensive N economy, the roles of nitric oxide must also be considered. This is mostly generated from NO2- by nitrate reductase and is elicited by both pathogen-associated microbial patterns and gene-for-gene-mediated defences. Nitric oxide (NO) production and associated defences are therefore NO3- dependent and are compromised by NH4+ .

CONCLUSION

This review demonstrates how N content and form plays an essential role in defensive primary and secondary metabolism and NO-mediated events.