Exogenous silicon alters ascorbate-glutathione cycle in two salt-stressed indica rice cultivars (MTU 1010 and Nonabokra)

Silicon uptake and transport mechanisms in plants: processes, applications and challenges in sustainable plant management

Silicon (Si) is an abundant element in the earth's crust essential for plant growth and development. Recent studies silicon's potential for improving plant resilience to numerous biotic stressors, notably fungal diseases. This review seeks to offer a comprehensive understanding of the processes and advantages of silicon-induced systemic resistance in plants, with a special focus on its interactions with fungal pathogens. Furthermore, we investigate the effect of silicon on plant physiological and biochemical changes, such as enhanced lignification, strengthening of physical barriers, and activation of antioxidant systems. Additionally, we examine the influence of silicon on microbial populations within the rhizosphere and its effects on mycorrhizal associations. Lastly, we discuss the potential applications and challenges of integrating silicon-based strategies in sustainable plant disease management. This review provides valuable insights into using silicon as a novel approach to enhance plant systemic resistance against fungal pathogens, offering prospects for developing eco-friendly and efficient agricultural practices.

The Physiological and Molecular Mechanisms of Silicon Action in Salt Stress Amelioration

Salinity is one of the most common abiotic stress factors affecting different biochemical and physiological processes in plants, inhibiting plant growth, and greatly reducing productivity. During the last decade, silicon (Si) supplementation was intensively studied and now is proposed as one of the most convincing methods to improve plant tolerance to salt stress. In this review, we discuss recent papers investigating the role of Si in modulating molecular, biochemical, and physiological processes that are negatively affected by high salinity. Although multiple reports have demonstrated the beneficial effects of Si application in mitigating salt stress, the exact molecular mechanism underlying these effects is not yet well understood. In this review, we focus on the localisation of Si transporters and the mechanism of Si uptake, accumulation, and deposition to understand the role of Si in various relevant physiological processes. Further, we discuss the role of Si supplementation in antioxidant response, maintenance of photosynthesis efficiency, and production of osmoprotectants. Additionally, we highlight crosstalk of Si with other ions, lignin, and phytohormones. Finally, we suggest some directions for future work, which could improve our understanding of the role of Si in plants under salt stress.

Silicon inhibits cadmium uptake by regulating the genes associated with the lignin biosynthetic pathway and plant hormone signal transduction in maize plants

Cadmium (Cd) contamination in soil poses a severe threat to plant growth and development. In contrast, silicon (Si) has shown promise in enhancing plant resilience under Cd-induced stress. In this study, we conducted an integrated investigation employing morphological studies, gene expression analysis, and metabolomics to unravel the molecular mechanisms underlying Cd tolerance in maize plants. Our results demonstrate that Si biofortification significantly mitigated Cd stress by reducing Cd accumulation in plant tissues, increasing Si content, and enhancing maize biomass in Cd-stressed plants resulted in a substantial enhancement in shoot dry weight (+ 75%) and root dry weight (+ 30%). Notably, Si treatment upregulated key lignin-related genes (TaPAL, TaCAD, Ta4CL, and TaCOMT) and promoted the accumulation of metabolites (sinapyl alcohol, phenylalanine, p-coumaryl alcohol, cafeyl alcohol, and coniferaldehyde) essential for cell wall strength, particularly under Cd stress conditions. Si application enriched the signal transduction by hormones and increased resistance by induction of biosynthesis genes (TaBZR1, TaLOX3, and TaNCDE1) and metabolites (brassinolide, abscisic acid, and jasmonate) in the roots and leaves under Cd stress. Furthermore, our study provides a comprehensive view of the intricate molecular crosstalk between Si, Cd stress, and plant hormonal responses. We unveil a network of genetic and metabolic interactions that culminate in a multifaceted defense system, enabling maize plants to thrive even in the presence of Cd-contaminated soil. This knowledge not only advances our understanding of the protective role of Si but also highlights the broader implications for sustainable agricultural practices. By harnessing the insights gained from this research, we may pave the way for innovative strategies to fortify crops against environmental stressors, ultimately contributing to the goal of food security in an ever-changing world. In summary, our research offers valuable insights into the protective mechanisms facilitated by Si, which enhance plants' ability to withstand environmental stress, and holds promise for future applications in sustainable agriculture.

Bio-priming with a Novel Plant Growth-Promoting Acinetobacter indicus Strain Alleviates Arsenic-Fluoride Co-toxicity in Rice by Modulating the Physiome and Micronutrient Homeostasis

Sustainable remediation of arsenic-fluoride from rice fields through efficient bio-extraction is the need of the hour, since these toxicants severely challenge safe cultivation of rice and food biosafety. In the present study, we screened an arsenic-fluoride tolerant strain AB-ARC of Acinetobacter indicus from the soil of a severely polluted region of West Bengal, India, which was capable of efficiently removing extremely high doses of arsenate and fluoride from the media. The strain also behaved as a plant growth-promoting rhizobacterium, since it could produce indole-3-acetic acid and solubilize phosphate, zinc, and starch. Due to these properties of the identified strain, it was used for bio-priming the seeds of the arsenic-fluoride susceptible rice cultivar, Khitish for testing the efficacy of the AB-ARC strain to promote combined arsenic-fluoride tolerance in the rice genotype. Bio-priming with AB-ARC led to accelerated uptake of crucial elements like iron, copper, and nickel which behave as co-factors of physiological and antioxidative enzymes. Thus, the activation of superoxide dismutase, catalase, guaiacol peroxidase, glutathione peroxidase, and glutathione-S-transferase enabled detoxification of reactive oxygen species (ROS) and reduction of the oxidative injuries like malondialdehyde and methylglyoxal generation. Overall, due to ameliorated molecular damages and low uptake of the toxic xenobiotics, the plants were able to maintain improved growth vigor and photosynthesis, as evident from the elevated levels of Hill activity and chlorophyll content. Hence, bio-priming with the A. indicus AB-ARC strain may be advocated for sustainable rice cultivation in arsenic-fluoride co-polluted fields.

The impact of engineered nickel oxide nanoparticles on ascorbate glutathione cycle in Allium cepa L

Engineered nickel oxide nanoparticle (NiO-NP) can inflict significant damages on exposed plants, even though very little is known about the modus operandi. The present study investigated effects of NiO-NP on the crucial stress alleviation mechanism Ascorbate-Glutathione Cycle (Asa-GSH cycle) in the model plant Allium cepa. Cellular contents of reduced glutathione (GSH) and oxidised glutathione (GSSG), was disturbed upon NiO-NP exposure. The ratio of GSH to GSSG changed from 20:1 in NC to 4:1 in roots exposed to 125 mg L-1 NiO-NP. Even the lowest treatments of NiO-NP (10 mg L-1) increased ascorbic acid (2.9-folds) and cysteine contents (1.6-folds). Enzymes like glutathione reductase, ascorbate peroxidase, glutathione peroxidase and glutathione-S-transferase also showed altered activities in the affected tissues. Further, intracellular methylglyoxal, a harbinger of ROS (Reactive oxygen species), increased significantly (~ 26 to 65-fold) across different concentrations NiO-NP. Intracellular H2O2 (hydrogen peroxide) and ROS levels increased with NiO-NP doses, as did electrolytic leakage from damaged cells. The present work indicated that multiple pathways were compromised in NiO-NP affected plants and this information can bolster our general understanding of the actual mechanism of its toxicity on living cells, and help formulate strategies to thwart ecological pollution.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01314-8.

Application of RNA sequencing to understand the response of rice seedlings to salt-alkali stress

BACKGROUND

Salt-alkali stress represents one of the most stressful events with deleterious consequences for plant growth and crop productivity. Despite studies focusing on the effects of salt-alkali stress on morphology and physiology, its molecular mechanisms remain unclear. Here, we employed RNA-sequencing (RNA-seq) to understand how Na₂CO₃ stress inhibits rice seedling growth.

RESULTS

Na₂CO₃ stress significantly inhibited the growth of rice seedlings. Through RNA-seq, many differentially expressed genes (DEGs) were shown to be potentially involved in the rice seedling response to salt-alkali stress. After 1-day and 5-day treatments, RNA-seq identified 1780 and 2315 DEGs in the Na₂CO₃-treated versus -untreated rice seedling shoots, respectively. According to the gene ontology enrichment and the Kyoto Encylopedia of Genes and Genomes annotation of DEGs, the growth-inhibition processes associated with salt-alkali stress involve a myriad of molecular events, including biosynthesis and metabolism, enzyme activity, and binding, etc. CONCLUSION: Collectively, the transcriptome analyses in the present work revealed several potential key regulators of plant response to salt-alkali stress, and might pave a way to improve salt-alkali stress tolerance in rice.

Mapping QTLs for yield and photosynthesis-related traits in three consecutive backcross populations of Oryza sativa cultivar Cottondora Sannalu (MTU1010) and Oryza rufipogon

Identification of trait enhancing QTLs for yield and photosynthesis-related traits in rice using interspecific mapping population and chromosome segment substitution lines derived from a cross between Oryza sativa and Oryza rufipogon. Wild rice contains novel genes which can help in improving rice yield. Common wild rice Oryza rufipogon is a known source for enhanced photosynthesis and yield-related traits. We developed BC₂F_2:3:4 mapping populations using O. rufipogon IC309814 with high photosynthetic rate as donor, and elite cultivar MTU1010 as recurrent parent. Evaluation of 238 BC₂F₂ families for 13 yield-related traits and 208 BC₂F₂ families for seven photosynthesis-related physiological traits resulted in identification of significantly different lines which performed better than MTU1010 for various yield contributing traits. 49 QTLs were identified for 13 yield traits and 7 QTLs for photosynthesis-related traits in BC₂F_2. In addition, 34 QTLs in BC₂F₃ and 26 QTLs in BC₂F₄ were also detected for yield traits.11 common QTLs were identified in three consecutive generations and their trait-increasing alleles were derived from O. rufipogon. Significantly, one major effect common QTL qTGW3.1 for thousand grain weight with average phenotypic variance 8.1% and one novel QTL qBM7.1 for biomass were identified. Photosynthesis-related QTLs qP_N9.1, qP_N12.1, qP_N12.2 qSPAD1.1 and qSPAD6.1 showed additive effect from O. rufipogon. A set of 145 CSSLs were identified in BC₂F₂ which together represented 87% of O. rufipogon genome. In addition, 87 of the 145 CSSLs were significantly different than MTU1010 for at least one trait. The major effect QTLs can be fine mapped for gene discovery. CSSLs developed in this study are a good source of novel alleles from O. rufipogon in the background of Cottondora Sannalu for rapid improvement of any trait in rice.

Prospective Evaluation of an Amide-Based Zinc Scaffold as an Anti-Alzheimer Agent: In Vitro, In Vivo, and Computational Studies

Alzheimer's disease is the most common progressive neurodegenerative mental disorder associated with loss of memory, decline in cognitive function, and dysfunction of language. The prominent pathogenic causes of this disease involve deposition of amyloid-β plaques, acetylcholine neurotransmitter deficiency, and accumulation of neurofibrillary tangles. There are multiple pathways that have been targeted to treat this disease. The inhibition of the intracellular cyclic AMP regulator phosphodiesterase IV causes the increase in CAMP levels that play an important role in the memory formation process. Organometallic chemistry works in a different way in treating pharmacological disorders. In the field of medicinal chemistry and pharmaceuticals, zinc-based amide carboxylates have been shown to be a preferred pharmacophore. The purpose of this research work was to investigate the potential of zinc amide carboxylates in inhibition of phosphodiesterase IV for the Alzheimer's disease management. Swiss Albino mice under controlled conditions were divided into seven groups with 10 mice each. Group I was injected with carboxymethylcellulose (CMC) at 1 mL/100 g dose, group II was injected with Streptozotocin (STZ) at 3 mg/kg dose, group III was injected with Piracetam acting as a standard drug at 200 mg/kg dosage, while groups IV-VII were injected with a zinc scaffold at the dose regimen of 10, 20, 40, and 80 mg/kg through intraperitoneal injection. All groups except group I were injected with Streptozotocin on the first day and third day of treatment at the dose of 3 mg/kg through an intracerebroventricular route to induce Alzheimer's disease. Afterward, respective treatment was continued for all groups for 23 days. In between the treatment regimen, groups were analyzed for memory and learning improvement through various behavioral tests such as open field, elevated plus maze, Morris water maze, and passive avoidance tests. At the end of the study, different biochemical markers in the brain were estimated like neurotransmitters (dopamine, serotonin and adrenaline), oxidative stress markers (superoxide dismutase, glutathione, and catalase), acetylcholinesterase (AchE), tau proteins, and amyloid-β levels. A PCR study was also performed. Results showed that the LD₅₀ of the zinc scaffold is greater than 2000 mg/kg. Research indicated that the zinc scaffold has the potential to improve the memory impairment and learning behavior in Alzheimer's disease animal models in a dose-dependent manner. At the dose of 80 mg/kg, a maximum response was observed for the zinc scaffold. Maximum reduction in the acetylcholinesterase enzyme was observed at 80 mg/kg dose, which was further strengthened and verified by the PCR study. Oxidative stress was restored by the zinc scaffold due to the significant activation of the endogenous antioxidant enzymes. This research ended up with the conclusion that the zinc-based amide carboxylate scaffold has the potential to improve behavioral disturbances and vary the biochemical markers in the brain.

Forms of application of silicon in quinoa and benefits involved in the association between productivity with grain biofortification

Multiple aspects of the physiological and nutritional mechanisms involved with silicon (Si) absorption by quinoa plants remain poorly investigated, as well as the best way of supplying this element to crops. Thus, this study aimed at evaluating whether the application of Si increases its uptake by quinoa plants and consequently the use efficiency of N and P, as well as the levels of phenolic compounds in the leaves, crop productivity and the biofortification of grains. For this purpose, the concentration of 3 mmol L⁻¹ of Si was tested, according to the following procedures: foliar application (F), root application in the nutrient solution (R), combined Si application via nutrient solution and foliar spraying (F + R), and no Si application (0). The provision of Si through the leaves and roots promoted the highest uptake of the element by the plant, which resulted in an increased use efficiency of N and P. Consequently, such a higher uptake favored the productivity of grains. The optimal adoption of the application of Si through leaves and roots promoted the highest Si concentration and ascorbic acid content in quinoa grains.

Alleviative mechanisms of silicon solubilizing Bacillus amyloliquefaciens mediated diminution of arsenic toxicity in rice

Silicon (Si) has gained considerable attention for its utility in improved plant health under biotic and abiotic stresses through alteration of physiological and metabolic processes. Its interaction with arsenic (As) has been the compelling area of research amidst heavy metal toxicity. However, microbe mediated Si solubilization and their role for reduced As uptake is still an unexplored domain. Foremost role of Bacillus amyloliquefaciens (NBRISN13) in impediment of arsenite (AsIII) translocation signifies our work. Reduced grain As content (52-72%) during SN13 inoculation under feldspar supplementation (Si+SN+As) highlight the novel outcome of our study. Upregulation of Lsi1, Lsi2 and Lsi3genes in Si+SN+As treated rice plants associated with restricted As translocation, frames new propositions for future research on microbemediated reduced As uptake through increased Si transport. In addition to low As accumulation, alleviation of oxidative stress markers by modulation of defense enzyme activities and differential accumulation of plant hormones was found to be associated with improved growth and yield. Thus, our findings confer the potential role of microbe mediated Si solubilization in mitigation of As stress to restore plant growth and yield.

The Ability of Silicon Fertilisation to Alleviate Salinity Stress in Rice is Critically Dependent on Cultivar

Silicon (Si) fertiliser can improve rice (Oryza sativa) tolerance to salinity. The rate of Si uptake and its associated benefits are known to differ between plant genotypes, but, to date, little research has been done on how the benefits, and hence the economic feasibility, of Si fertilisation varies between cultivars. In this study, a range of rice cultivars was grown both hydroponically and in soil, at different levels of Si and NaCl, to determine cultivar variation in the response to Si. There was significant variation in the effect of Si, such that Si alleviated salt-induced growth inhibition in some cultivars, while others were unaffected, or even negatively impacted. Thus, when assessing the benefits of Si supplementation in alleviating salt stress, it is essential to collect cultivar-specific data, including yield, since changes in biomass were not always correlated with those seen for yield. Root Si content was found to be more important than shoot Si in protecting rice against salinity stress, with a root Si level of 0.5-0.9% determined as having maximum stress alleviation by Si. A cost-benefit analysis indicated that Si fertilisation is beneficial in mild stress, high-yield conditions but is not cost-effective in low-yield production systems.

Exogenous silicon enhances the systemic defense of cucumber leaves and roots against CA-induced autotoxicity stress by regulating the ascorbate-glutathione cycle and photosystem II

Cinnamic acid (CA), one of the main autotoxins secreted by cucumber roots during continuous cropping, inhibits plant growth and reduces yield. Silicon (Si) is an environmentally friendly element that alleviates abiotic stresses in plants, but the mechanism underlying its resistance to autotoxicity remain unclear. Here, we used 0.8 mmol L^-1 CA to study the effects of Si application on the growth, chlorophyll fluorescence, and ascorbate-glutathione (AsA-GSH) cycle of cucumber seedlings under CA inducing conditions. Our results indicated that CA significantly induced photoinhibition and overaccumulation of reactive oxygen species (ROS), thereby inhibiting cucumber growth. Treatment with 1.0 mmol L^-1 Si improved plant height, stem diameter and biomass accumulation, and protected the photosynthetic electron transport function of photosystem II in the presence of CA. Similarly, Si application maintained the ROS status by increasing ascorbate (AsA) and glutathione (GSH) production, as well as the ratios of AsA/DHA and GSH/GSSG in both leaves and roots during CA stress. In addition, Si application in CA-treated seedlings enhanced the activity of key enzymes such as ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione S-transferase (GST), and the transcription of several enzyme genes (CsAPX, CsMDHAR and CsGR） from the AsA-GSH cycle. These results suggest that exogenous Si enhances CA tolerance in cucumber seedlings by protecting photosystem II activity, upregulating AsA-GSH pathway, and reducing ROS levels.

Multi-omics analyses on the response mechanisms of 'Shine Muscat' grapevine to low degree of excess copper stress (Low-ECS)

Copper stress is one of the most severe heavy metal stresses in plants. Grapevine has a relatively higher copper tolerance than other fruit crops. However, there are no reports regarding the tolerance mechanisms of the 'Shine Muscat' ('SM') grape to a low degree of excess copper stress (Low-ECS). Based on the physiological indicators and multi-omics (transcriptome, proteome, metabolome, and microRNAome) data, 8 h (h) after copper treatment was the most severe stress time point. Nonetheless, copper stress was alleviated 64 h after treatment. Cu ion transportation, photosynthesis pathway, antioxidant system, hormone metabolism, and autophagy were the primary response systems in 'SM' grapevine under Low-ECS. Numerous genes and proteins, such as HMA5, ABC transporters, PMM, GME, DHAR, MDHAR, ARGs, and ARPs, played essential roles in the 'SM' grapevine's response to Low-ECS. This work was carried out to gain insights into the multi-omics responses of 'SM' grapevine to Low-ECS. This study provides genetic and agronomic information that will guide better vinery management and breeding copper-resistant grape cultivars.

Exploring the significant contribution of silicon in regulation of cellular redox homeostasis for conferring stress tolerance in plants

Silicon (Si), a bioactive metalloid is beneficial for plant growth and development. It also plays a key role in the amelioration of different abiotic and biotic stresses. Extensive studies have elucidated the morpho-physiological, biochemical and molecular background of Si-mediated stress tolerance in plants. However, the mechanism acquired by Si to enhance stress tolerance in plants is still unheeded. Present review summarized the prospective mechanisms of Si in acquisition of stress tolerance with emphasis on its interactions with secondary messengers. Silicon usually modulates the different gene expressions in plants under stress conditions rather than acting as a direct signal or secondary messengers. Silicon regulates the production and accumulation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in plants under stress conditions. Furthermore, Si also activates the antioxidant defence system in plants; thereby, maintaining the cellular redox homeostasis and preventing the oxidative damage of cells. Silicon also up-regulates the synthesis of hydrogen sulfide (H₂S) or acts synergistically with nitric oxide (NO), consequently conferring stress tolerance in plants. Overall, the review may provide a progressive understanding of the role of Si in conservation of the redox homeostasis in plants.

Silicon augments salt tolerance through modulation of polyamine and GABA metabolism in two indica rice (Oryza sativa L.) cultivars

Polyamines (PA) have multifarious roles in plant-environment interaction and stress responses. In conjunction with GABA shunt, they regulate induction of tolerance under salinity stress in plants. Here, we tested the hypothesis that silicon improves salt tolerance through mediating vital metabolic pathways rather than acting as a mere mechanical barrier. Seedlings of two rice (Oryza sativa L.) cultivars MTU 1010 (salt-sensitive) & Nonabokra (salt-tolerant) growing in hydroponic culture were treated with NaCl (0, 25, 50 & 100 mM) combined with or without Si (2 mM). NaCl stress enhanced PA synthesizing enzymes activity and PA production in salt tolerant cultivar Nonabokra, whereas in the sensitive cultivar, MTU 1010 both declined. Enhanced activities of GABA synthesizing enzymes along with a decline in the activities of GABA degrading enzymes under NaCl exposure led to GABA accumulation in both the cultivars. The interactive effects of silicon and NaCl also induced the activities of the enzymes related to polyamine biosynthesis and inhibited polyamine degrading enzymes that enhanced PA contents in the cultivars. Supplemental Si decreased endogenous GABA levels by modulating GABA metabolising enzymes under NaCl stress. On the basis of all tested parameters cv. MTU 1010 was proven to be more responsive towards silicon application than cv. Nonabokra. Such study of silicon-induced polyamine accretion and reduced GABA accumulation may lower oxidative damage in rice cultivars under NaCl stress and thereby form a successful strategy to boost tolerance.

Silicon in mitigation of abiotic stress-induced oxidative damage in plants

Accumulation of reactive oxygen species (ROS), and their destructive effects on cellular organelles are the hallmark features of plants exposed to abiotic stresses. Plants are well-equipped with defensive mechanisms like antioxidant systems to deal with ROS-induced oxidative stress. Silicon has been emerged as an important regulator of plant protective mechanisms under environmental stresses, which can be up-taken from soil through a system of various silicon-transporters. In plants, silicon is deposited underneath of cuticles and in the cell wall, and help plant cells reduce deleterious effects of stresses. Furthermore, silicon can provide resistance to ROS-toxicity, which often accounts for silicon-mediated improvement of plant tolerance to different abiotic constraints, including salinity, drought, and metal toxicity. Silicon enhances the ROS-detoxification ability of treated plants by modulating the antioxidant defense systems, and the expression of key genes associated with oxidative stress mitigation and hormone metabolism. Silicon also displays additive roles in ROS-elimination when supplied with other external stimuli. Here, we discuss recent findings on how silicon is able to modulate antioxidant defense of plants in response to oxidative stress triggered by different abiotic constraints. We also review interactions of silicon with other signaling molecules, including nitric oxide, ROS, polyamines, and phytohormones in the mediation of plant protection against abiotic stress-induced oxidative damage.

Effect of Engineered Nickel Oxide Nanoparticle on Reactive Oxygen Species-Nitric Oxide Interplay in the Roots of Allium cepa L

Scientists anxiously follow instances of heavy metals augmenting in the environment and undergoing bioaccumulation and trace their biomagnification across food webs, wary of their potent toxicity on biological entities. Engineered nanoparticles supplement natural pools of respective heavy metals and can mimic their effects, exerting toxicity at higher concentrations. Thus, a thorough understanding of the underlying mechanism of this precarious interaction is mandatory. Most urban and industrial environments contain considerable quantities of nickel oxide nanoparticles. These in excess can cause considerable damage to plant metabolism through a significant increase in cellular reactive oxygen species and perturbation of its cross-talk with the reactive nitrogen species. In the present work, the authors have demonstrated how the intrusion of nickel oxide nanoparticles (NiO-NP) affected the exposed roots of Allium cepa: starting with disruption of cell membranes, before being interiorized within cell organelles, effectively disrupting cellular homeostasis and survival. A major shift in the reactive oxygen species (ROS) and nitric oxide (NO) equanimity was also observed, unleashing major altercations in several crucial biochemical profiles. Altered antioxidant contents and upregulation of stress-responsive genes, namely, Catalase, Ascorbate peroxidase, Superoxide dismutase, and Rubisco activase, showing on average 50-250% rise across NiO-NP concentrations tested, also entailed increased cellular hydrogen peroxide contents, with tandem rise in cellular NO. Increased NO content was evinced from altered concentrations of nitric oxide synthase and nitrate reductase, along with NADPH oxidase, when compared with the negative control. Though initially showing a dose-dependent concomitant rise, a significant decrease of NO was observed at higher concentrations of NiO-NP, while cellular ROS continued to increase. Modified K/Na ratios, with increased proline concentrations and GABA contents, all hallmarks of cellular stress, correlated with ROS-NO perturbations. Detailed studies showed that NiO-NP concentration had a significant role in inducing toxicity, perturbing the fine balance of ROS-NO, which turned lethal for the cell at higher dosages of the ENP precipitating in the accumulation of stress markers and an inevitable shutdown of cellular mechanisms.

Silicon nanoparticle-pulsing mitigates fluoride stress in rice by fine-tuning the ionomic and metabolomic balance and refining agronomic traits

The present manuscript investigates the roles of silicon nanoparticles (SiNPs) in ameliorating fluoride toxicity in the susceptible rice cultivar, IR-64. Fluoride toxicity reduced overall growth and yield by suppressing grain development. Fluoride stress alarmingly increased the accumulation of cobalt, which together with fluoride triggered electrolyte leakage, malondialdehyde, methylglyoxal and hydrogen peroxide accumulation and NADPH oxidase activity. The overall photosynthesis was compromised due to chlorosis and inhibited Hill activity. Nano-Si-priming efficiently ameliorated molecular injuries and restored yield by reducing fluoride bioaccumulation particularly in the grains. The level of non-enzymatic antioxidants like anthocyanins, flavonoids, phenolics and glutathione was stimulated upon SiNP-priming. Nano-Si-pulsing removed fluoride-mediated inhibition of glutathione synthesis by activating glutathione reductase. Glutathione was utilized to activate glyoxalases and associated enzymes like glutathione-S-transferase and glutathione peroxidase. Uptake of nutrients like silicon, potassium, zinc, copper, iron, nickel, manganese, selenium and vanadium improved seedling health even during prolonged fluoride stress. Nano-Si-pulsing produced a nanozymatic effect, since high level of crucial co-factors like copper, zinc and iron stimulated the activity of superoxide dismutase, catalase, ascorbate peroxidase and guaiacol peroxidase, which synergistically with other enzymatic and non-enzymatic antioxidants scavenged reactive oxygen species and promoted fluoride tolerance. Overall, the study supported by statistical modelling using principal component analysis, t-distributed stochastic neighbour embedding and multidimensional scaling, established the potential of SiNP to promote safe rice cultivation and precision farming even in fluoride-infested environments.

Differential lead-fluoride and nickel-fluoride uptake in co-polluted soil variably affects the overall physiome in an aromatic rice cultivar

The present study aimed to show that nickel and fluoride exhibited synchronized co-inhibited uptake in the aromatic rice cultivar, Gobindobhog, since bioaccumulation of the two elements was lower than that during individual stress, so that overall growth under combined stress was similar to control seedlings. On the contrary, lead and fluoride stimulated their co-uptake which triggered oxidative damages, NADPH oxidase activity, methylglyoxal accumulation, photosynthetic inhibition, membrane-protein damages, necrosis and genomic template degradation. Accumulation of proline, anthocyanins, non-protein thiols and phytochelatins was stimulated for systemic protection against reactive oxygen species (ROS) and xenobiotic-mediated injuries during lead-fluoride toxicity. ROS accumulation during nickel-fluoride stress was insignificant due to which enhanced accumulation of most antioxidants was not required. Glutathione depletion during combined lead-fluoride toxicity was due to its utilization in the glyoxalase cycle and also inhibition of glutathione reductase. However, the nickel-fluoride-treated sets maintained glutathione reserves and glyoxalase activity similar to those in control. Presence of fluoride 'safeguarded' the glutathione-utilizing enzymes like glutathione reductase, glutathione peroxidase and glutathione-S-transferase during dual lead-fluoride stress. This was because these enzymes showed higher activity compared to that under lead toxicity alone. Enzymatic antioxidants like superoxide dismutase, ascorbate peroxidase and guaiacol peroxidase were activated during lead-fluoride toxicity due to altered iron and copper homeostasis. Catalase activity was strongly inhibited, resulting in the inability to scavenge H₂O₂ and suppression of the fluoride-adaptable phenotype. However, none of the enzymatic antioxidants were inhibited during nickel-fluoride stress, which cumulatively allowed the seedlings to maintain normal physiology. Overall our findings holistically reveal the physiological plasticity of Gobindobhog in response to two different heavy metals under the influence of fluoride.

The role of nitrate reductase in brassinosteroid-induced endogenous nitric oxide generation to improve cadmium stress tolerance of pepper plants by upregulating the ascorbate-glutathione cycle

A study was performed to assess if nitrate reductase (NR) participated in brassinosteroid (BR)-induced cadmium (Cd) stress tolerance primarily by accelerating the ascorbate-glutathione (AsA-GSH) cycle. Prior to initiating Cd stress (CdS), the pepper plants were sprayed with 0.5 μM 24-epibrassinolide (EBR) every other day for 10 days. Thereafter the seedlings were subjected to control or CdS (0.1 mM CdCl₂) for four weeks. Cadmium stress decreased the plant growth related attributes, water relations as well as the activities of monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR), but enhanced proline content, leaf Cd²⁺ content, oxidative stress-related traits, activities of ascorbate peroxidase (APX) and glutathione reductase (GR), and the activities of antioxidant defence system-related enzymes as well as NR activity and endogenous nitric oxide content. EBR reduced leaf Cd²⁺ content and oxidative stress-related parameters, enhanced plant growth, regulated water relations, and led to further increases in proline content, AsA-GSH cycle-related enzymes' activities, antioxidant defence system-related enzymes as well as NR activity and endogenous nitric oxide content. The EBR and the inhibitor of NR (tungstate) reversed the positive effects of EBR by reducing NO content, showing that NR could be a potential contributor of EBR-induced generation of NO which plays an effective role in tolerance to CdS in pepper plants by accelerating the AsA-GSH cycle and antioxidant enzymes.

Mechanisms of Silicon-Mediated Amelioration of Salt Stress in Plants

Silicon (Si), the second most predominant element in the earth crust consists of numerous benefits to plant. Beneficial effect of Si has been apparently visible under both abiotic and biotic stress conditions in plants. Supplementation of Si improved physiology and yield on several important agricultural and horticultural crops. Salinity is one of the major abiotic stresses that affect growth and yield. The presence of high concentration of salt in growing medium causes oxidative, osmotic, and ionic stresses to plants. In extreme conditions salinity affects soil, ground water, and limits agricultural production. Si ameliorates salt stress in several plants. The Si mediated stress mitigation involves various regulatory mechanisms such as photosynthesis, detoxification of harmful reactive oxygen species using antioxidant and non-antioxidants, and proper nutrient management. In the present review, Si mediated alleviation of salinity stress in plants through the regulation of photosynthesis, root developmental changes, redox homeostasis equilibrium, and regulation of nutrients have been dealt in detail.

Role of Silicon in Mediating Salt Tolerance in Plants: A Review

Salt stress is a major threat for plant growth worldwide. The regulatory mechanisms of silicon in alleviating salt stress have been widely studied using physiological, molecular genetics, and genomic approaches. Recently, progresses have been made in elucidating the alleviative effects of silicon in salt-induced osmotic stress, Na toxicity, and oxidative stress. In this review, we highlight recent development on the impact of silicon application on salt stress responses. Emphasis will be given to the following aspects. (1) Silicon transporters have been experimentally identified in different plant species and their structure feature could be an important molecular basis for silicon permeability. (2) Silicon could mediate salt-induced ion imbalance by (i) regulating Na⁺ uptake, transport, and distribution and (ii) regulating polyamine levels. (3) Si-mediated upregulation of aquaporin gene expression and osmotic adjustment play important roles in alleviating salinity-induced osmotic stress. (4) Silicon application direct/indirectly mitigates oxidative stress via regulating the antioxidant defense and polyamine metabolism. (5) Omics studies reveal that silicon could regulate plants' response to salt stress by modulating the expression of various genes including transcription factors and hormone-related genes. Finally, research areas that require further investigation to provide a deeper understanding of the role of silicon in plants are highlighted.

Modulation of photosynthetic parameters, sugar metabolism, polyamine and ion contents by silicon amendments in wheat (Triticum aestivum L.) seedlings exposed to arsenic

The objective of the present investigation was to consider the effectiveness of exogenous silicate supplementation in reviving the arsenate imposed alterations on pigment content, Hill activity, photosynthetic parameters, sugar metabolism, polyamine, and ion contents in wheat (Triticum aestivum L. cv. PBW-343) seedlings. Experiments were conducted under different levels of arsenate (0, 25 μM, 50 μM, and 100 μM) in combination with silicate (0, 5 mM) in a hydroponic environment with modified Hoagland's solution for 21 days to determine the ameliorative role of silicon (Si). Arsenate exposure led to a decline in chlorophyll content by 28% and Hill activity by 30% on an average along with photosynthetic parameters. Activity of starch phosphorylase increased causing a subsequent decrease in starch contents by 26%. Degradation of starch enhanced sugar contents by 61% in the test cultivar. Dose-dependant increments in the activities of carbohydrate metabolizing enzymes viz., sucrose synthase, sucrose phosphate synthase, and acid invertase were also noted. Putrescine content was significantly enhanced along with a consequent decline in spermidine and spermine contents. The macro- and micronutrient contents declined proportionally with arsenate imposition. Conversely, silicate amendments irrespective of all arsenate concentrations brought about considerable alterations in all parameters tested with respect to arsenate treatment alone. Marked improvement in pigment content and Hill activity also improved the gas exchange parameters. Soluble sugar contents decreased and starch contents were enhanced. Increase in polyamine contents improved the ionic balance in the test cultivar as well. This study highlights the potentiality of silicon in ameliorating the ecotoxicological risks associated with arsenic pollution and the probable ability of silicon to offer an approach in mitigating arsenate-induced stress leading to restoration of growth and metabolism in wheat seedlings.

Exogenous silicon alters organic acid production and enzymatic activity of TCA cycle in two NaCl stressed indica rice cultivars

The activities of TCA cycle enzymes viz., pyruvate dehydrogenase, citrate synthase, isocitrate dehydrogenase, succinate dehydrogenase and malate dehydrogenase as well as levels of different organic acids viz., pyruvic acid, citric acid, succinic acid and malic acid were studied in two rice cultivars viz. cv. Nonabokra and cv. MTU 1010 differing in salt tolerance grown under 25, 50 and 100 mM NaCl salinity levels. A contrasting response to salt stress on enzyme activities of TCA cycle and accumulation of organic acid was observed between two cultivars during twenty-one days period of study. Salinity caused enhanced organic acid production and increase in all five enzyme activities in cv. Nonabokra whereas in cv. MTU 1010 decrease in both organic acid production and enzymes activities were noted. Joint application of exogenous silicon along with NaCl, altered the organic acids levels and activities of enzymes in both cultivars of rice seedlings conferring tolerance against salt induced stress. Rice cv. MTU 1010 showed better response to exogenous silicon on parameters tested compared to cv. Nonabokra.

Modulation of growth, ascorbate-glutathione cycle and thiol metabolism in rice (Oryza sativa L. cv. MTU-1010) seedlings by arsenic and silicon

Arsenic is a carcinogenic metalloid, exists in two important oxidation states-arsenate (As-V) and arsenite (As-III). The influence of arsenate with or without silicate on the growth and thiol metabolism in rice (Oryza sativa L. cv. MTU-1010) seedlings were investigated. Arsenate was more toxic for root growth than shoot growth where the root lengths were short, characteristically fragile and root tips turned brown. The multiple comparison analysis using Tukey's HSD (honest significant difference) tests indicated that the rate of arsenate accumulation and its conversion to arsenite by arsenate reductase were significantly increased in all arsenate treated seedlings while in seedlings treated jointly with arsenate and silicate, arsenate accumulation and its conversion to arsenite decreased. Silicate content was detected in the seedlings treated with silicate alone and under co-application of arsenate with silicate. In the test seedlings arsenic toxicity increased ascorbate and glutathione contents along with the activities of their regulatory enzymes, viz., ascorbate peroxidase, glutathione reductase, glutathione peroxidase and glutathione-s-transferase to reduce the toxicity level induced by arsenic whereas ascorbate oxidase activity was decreased to maintain sufficient ascorbate pool under arsenate treatment. Phytochelatins production were increased in both root and shoot of the test seedlings under arsenate exposure to alter the detrimental effects of arsenic by chelation with arsenite and their subsequent sequestration into vacuole. Thus, joint application of silicate along with arsenate showed significant alterations on all the parameters tested compared to arsenate treatment alone due to less availability of arsenic in the tissue leading to better growth and metabolism in rice seedlings. Thus use of silicon in arsenic contaminated medium may help to grow rice with improved vigour.