Spermine ameliorates prolonged fluoride toxicity in soil-grown rice seedlings by activating the antioxidant machinery and glyoxalase system

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 role of plant growth promoting rhizobacteria in strengthening plant resistance to fluoride toxicity: a review

A wide variety of bacteria are present in soil but in rhizospheric area, the majority of microbes helps plant in defending diseases and facilitate nutrient uptake. These microorganisms are supported by plants and they are known as plant growth-promoting rhizobacteria (PGPR). The PGPRs have the potential to replace chemical fertilizers in a way that is more advantageous for the environment. Fluoride (F) is one of the highly escalating, naturally present contaminants that can be hazardous for PGPRs because of its antibacterial capacity. The interactions of F with different bacterial species in groundwater systems are still not well understood. However, the interaction of PGPR with plants in the rhizosphere region reduces the detrimental effects of pollutants and increases plants' ability to endure abiotic stress. Many studies reveal that PGPRs have developed F defense mechanisms, which include efflux pumps, Intracellular sequestration, enzyme modifications, enhanced DNA repair mechanism, detoxification enzymes, ion transporter/antiporters, F riboswitches, and genetic mutations. These resistance characteristics are frequently discovered by isolating PGPRs from high F-contaminated areas or by exposing cells to fluoride in laboratory conditions. Numerous studies have identified F-resistant microorganisms that possess additional F transporters and duplicates of the well-known targets of F. Plants are prone to F accumulation despite the soil's low F content, which may negatively affect their growth and development. PGPRs can be used as efficient F bioremediators for the soil environment. Environmental biotechnology focuses on creating genetically modified rhizobacteria that can degrade F contaminants over time. The present review focuses on a thorough systemic analysis of contemporary biotechnological techniques, such as gene editing and manipulation methods, for improving plant-microbe interactions for F remediation and suggests the importance of PGPRs in improving soil health and reducing the detrimental effects of F toxicity. The most recent developments in the realm of microbial assistance in the treatment of F-contaminated environments are also highlighted.

Seed pretreatment with melatonin confers cadmium tolerance to chickpea seedlings through cellular redox homeostasis and antioxidant gene expression improvement

Several phytoremediation strategies have been undertaken to alleviate cadmium (Cd)-mediated injury to crop yield resulting from agricultural land pollution. In the present study, the potentially beneficial effect of melatonin (Me) was appraised. Therefore, chickpea (Cicer arietinum L.) seeds were imbibed for 12 H in distilled water or Me (10 µM) solution. Then, the seeds germinated in the presence or the absence of 200 µM CdCl₂ for 6 days. Seedlings obtained from Me-pretreated seeds exhibited enhanced growth traits, reflected by fresh biomass and length increase. This beneficial effect was associated with a decreased Cd accumulation in seedling tissues (by 46 and 89% in roots and shoots, respectively). Besides, Me efficiently protected the cell membrane integrity of Cd-subjected seedlings. This protective effect was manifested by the decreased lipoxygenase activity and the subsequently reduced accumulation of 4-hydroxy-2-nonenal. Melatonin counteracted the Cd-mediated stimulation of the pro-oxidant NADPH-oxidase (90 and 45% decrease compared to non-pretreated Cd-stressed roots and shoots, respectively) and NADH-oxidase activities (almost 40% decrease compared to non-pretreated roots and shoots), preventing, thus, hydrogen peroxide overaccumulation (50 and 35% lesser than non-pretreated roots and shoots, respectively). Furthermore, Me enhanced the cellular content of pyridine nicotinamide reduced forms [NAD(P)H] and their redox state. This effect was associated with the Me-mediated stimulation of the glucose-6-phosphate dehydrogenase (G6PDH) and malate dehydrogenase activities, concomitantly with the inhibition of NAD(P)H-consuming activities. These effects were accompanied by the up-regulation of G6PDH gene expression (45% increase in roots) and the down-regulation of the respiratory burst oxidase homolog protein F (RBOHF) gene expression (53% decrease in roots and shoots). Likewise, Me induced an increased activity and gene transcription of the Asada-Halliwell cycle, namely ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase, concomitantly with a reduction of the glutathione peroxidase activity. This modulating effect led to the restoration of the redox homeostasis of the ascorbate and the glutathione pools. Overall, current results attest that seed pretreatment with Me is effective in Cd stress relief and can be a beneficial crop-protective approach.

Alleviation of arsenic toxicity in pepper plants by aminolevulinic acid and heme through modulating its sequestration and distribution within cell organelles

Aminolevulinic acid (ALA) is essential for chlorophyll and heme synthesis. However, whether heme interacts with ALA to elicit antioxidants in arsenic (As)-exposed plants is still unknown. ALA was applied daily to pepper plants for 3 days prior to beginning As stress (As-S). Then, As-S was initiated for 14 days by employing sodium hydrogen arsenate heptahydrate (0.1 mM AsV). Arsenic treatment decreased photosynthetic pigments (chl a by 38% and chl b by 28%), biomass by 24%, and heme by 47% content, but it elevated contents of malondialdehyde (MDA) by 3.3-fold, hydrogen peroxide (H₂O₂) by 2.3-fold, glutathione (GSH), methylglyoxal (MG), and phytochelatins (PCs) and electrolyte leakage (EL) by 2.3-fold along with enhanced subcellular As concentration in the pepper plant's roots and leaves. The supplementation of ALA to the As-S-pepper seedlings enhanced the amount of chlorophyll, heme content, and antioxidant enzyme activity as well as plant growth, while it reduced the levels of H₂O₂, MDA, and EL. ALA boosted GSH and phytochelates (PCs) in the As-S-seedlings by controlling As sequestration and rendering it harmless. The addition of ALA enhanced the amount of As that accumulated in the root vacuoles and reduced the poisonousness of the soluble As in the vacuoles. The ALA treatment facilitated the deposition and fixation of As in the vacuoles and cell walls, thereby reducing the transport of As to other cell organelles. This mechanism may have contributed to the observed decrease in As accumulation in the leaves. The administration of 0.5 mM hemin (H) (a source of heme) significantly enhanced ALA-induced arsenic stress tolerance. Hemopexin (Hx, 0.4 μg L^-1), a heme scavenger, was treated with the As-S plants along with ALA and ALA + H to observe if heme was a factor in ALA's increased As-S tolerance. Heme synthesis/accumulation in the pepper plants was reduced by Hx, which counteracted the positive effects of ALA. Supplementation of H along with ALA + Hx reversed the negative effects of Hx, demonstrating that heme is required for ALA-induced seedling As-S tolerance.

Extensive cross-talk among stress-regulated protective metabolites, biogenic-amines and phytohormone-signalling, co-ordinated by dopamine-mediated seed-priming, governs tolerance against fluoride stress in rice

Seed priming with dopamine reduced fluoride bioaccumulation, induced endogenous dopamine level, thereby orchestrating phytohormone homeostasis and biogenic amine metabolism, and modulating osmolyte and antioxidant machinery to enhance fluoride tolerance in rice. The aim of this study was to decipher the efficacy of seed priming with dopamine in curtailing the adverse impacts of fluoride toxicity in rice seedlings. Fluoride-stressed seedlings exhibited severe growth retardation, high fluoride bioaccumulation, electrolyte leakage and marked cellular injuries. Dopamine priming stimulated the overall physiological growth parameters during stress, via reduced formation of H₂O₂, malondialdehyde and methylglyoxal, due to lesser fluoride-accumulation. Fluoride stress-induced endogenous dopamine level was further induced upon dopamine priming, marked by the up regulated DOPA decarboxylase expression. Additionally, dopamine treatment led to escalated activity of catalase, superoxide dismutase and glutathione peroxidase in the stressed seedlings, concomitant with altered CAT, SOD and GPX expression. The higher accumulation of protective osmolytes (proline and total amino acids) and non-enzymatic antioxidants (phenolics, flavonoids, anthocyanins, glutathione and carotenoids), upon dopamine priming, during fluoride stress, could be linked with the altered expression pattern of the respective genes. Dopamine promoted active utilization of the biogenic amine (polyamines and ϒ-amino butyric acid) pools for toxicity mitigation, correlated with the modulation of the concerned enzyme activity and gene expression. Dopamine stimulated the accumulation of phytohormones like gibberellin and salicylic acid, via inducing the biosynthetic genes like gibberellin-3-oxidase (GA3ox) and isochorismate synthase (ICS), respectively, while depreciating the abscisic acid and melatonin level during fluoride stress. To our knowledge, this is the first documented report for the remedial role of dopamine priming against fluoride stress in any plant species. This study will open new arenas in sustainable agriculture for the exploitation of this pulsating biomolecule against fluoride stress.

Physiological and Differential Proteomic Analysis at Seedling Stage by Induction of Heavy-Ion Beam Radiation in Wheat Seeds

Novel genetic variations can be obtained by inducing mutations in the plant which help to achieve novel traits. The useful mutant can be obtained through radiation mutation in a short period which can be used as a new material to produce new varieties with high yield and good quality wheat. In this paper, the proteomic analysis of wheat treated with different doses of ¹²C and ⁷Li ion beam radiation at the seedling stage was carried out through a Tandem Mass Tag (TMT) tagging quantitative proteomic analysis platform based on high-resolution liquid chromatography-mass spectrometry, and the traditional ⁶⁰Co-γ-ray radiation treatment for reference. A total of 4,764 up-regulated and 5,542 down-regulated differentially expressed proteins were identified. These proteins were mainly enriched in the KEGG pathway associated with amino acid metabolism, fatty acid metabolism, carbon metabolism, photosynthesis, signal transduction, protein synthesis, and DNA replication. Functional analysis of the differentially expressed proteins showed that the oxidative defense system in the plant defense system was fully involved in the defense response after ¹²C ion beam and ⁷Li ion beam radiation treatments. Photosynthesis and photorespiration were inhibited after ¹²C ion beam and ⁶⁰Co-γ-ray irradiation treatments, while there was no effect on the plant with ⁷Li ion beam treatment. In addition, the synthesis of biomolecules such as proteins, as well as multiple signal transduction pathways also respond to radiations. Some selected differentially expressed proteins were verified by Parallel Reaction Monitoring (PRM) and qPCR, and the experimental results were consistent with the quantitative results of TMT. The present study shows that the physiological effect of ¹²C ion beam radiation treatment is different as compared to the ⁷Li ion beam, but its similar to the ⁶⁰Co-γ ray depicting a significant effect on the plant by using the same dose. The results of this study will provide a theoretical basis for the application of ¹²C and ⁷Li ion beam radiation in the mutation breeding of wheat and other major crops and promote the development of heavy ion beam radiation mutation breeding technology.

Rhizofiltration of combined arsenic-fluoride or lead-fluoride polluted water using common aquatic plants and use of the 'clean' water for alleviating combined xenobiotic toxicity in a sensitive rice variety

Groundwater co-contamination with toxic pollutants like arsenic-fluoride or lead-fluoride is a serious threat for safe rice cultivation, since major stretches of land, involved in cultivation of this staple food crop are presently experiencing severe endemic pollution from these xenobiotic combinations. Preliminary investigations established that the combined pollutants together exerted more phytotoxicity in the widely cultivated indica rice variety Khitish, compared with that exerted by the individual contaminants. Thus, an ecologically sustainable and economically viable phytoremediative strategy was designed where three aquatic plants, viz., Azolla (water fern), Pistia (water lettuce) and Eichhornia (water hyacinth) (commonly located across the co-polluted regions) were tested for their ability to rhizofiltrate the water samples that had been polluted with arsenic-fluoride or lead-fluoride. Water lettuce exhibited the highest ability to 'clean' both arsenic-fluoride and lead-fluoride polluted water due to its capacity of efficient phytoextraction and phytostabilization. Irrigation of Khitish seedlings with this de-polluted water appreciably reduced malondialdehyde formation, electrolyte leakage and irreversible protein carbonylation due to suppression in NADPH oxidase activity and reactive oxygen species production, compared with those in sets grown with non-treated, arsenic-fluoride or lead-fluoride contaminated water. Oxidative injuries, cytotoxic methylglyoxal synthesis and inhibition of biomass growth were ameliorated, and chlorophyll synthesis and Hill activity were increased due to reduced bioaccumulation of xenobiotics, along with the improved uptake of vital micronutrients like iron, copper and nickel. Overall, the current investigation illustrated a cheap, farmer-friendly blueprint which could be easily promulgated to ensure safe rice cultivation even across territories that are severely co-polluted with the mixed contaminants.

Exogenously applied Spd and Spm enhance drought tolerance in tea plants by increasing fatty acid desaturation and plasma membrane H+-ATPase activity

Polyamines, due to their positive charges, bind to ROS Reactive oxygen species (ROS) thereby stabilizing the plasma membrane (PM). Drought is one of the main limiting factors affecting tea plant yield and quality. However, the effect of Spermidine (Spd) or Spermine (Spm) on membrane stability and fluidity in tea plants under drought stress is poorly understood. In this investigation, an exogenous supply of 1 mM Spd or Spm did not mitigate drought stress-induced damage, however, an exogenous supply of 0.2 mM Spd or Spm application significantly alleviated drought-induced damage in tea plants. To further illustrate the role of 0.2 mM Spd or Spm in maintaining membrane integrity and fluidity, the fatty acid percentage and PM H⁺-ATPase activity were analyzed. Spd and Spm application significantly increased PM H⁺-ATPase activity by 43.79% compared with that without the addition of polyamine under drought stress. In addition, exogenous application of Spd and Spm also significantly increased C18:3 by approximately 10%, hence alleviating drought-reduced fatty acid unsaturation. In contrast, Spd and Spm metabolic inhibitors dicyclohexylamine (DCHA) further impaired PM H⁺-ATPase activity and fatty acid desaturation under the drought + DCHA treatment compared with the drought treatment, respectively. Taken together, 0.2 mM Spd and Spm application significantly enhanced drought tolerance by increasing fatty acid unsaturation and maintaining PM H⁺-ATPase activity in tea plants. Therefore, foliar application of 0.2 mM Spd or Spm can be a potential foliar-spraying substances for improving tea drought tolerance.

Explicating the cross-talks between nanoparticles, signaling pathways and nutrient homeostasis during environmental stresses and xenobiotic toxicity for sustainable cultivation of cereals

Precision farming using nanoparticles is a cutting-edge technology for safe cultivation of crop plants in marginal areas afflicted with environmental/climatic stresses like salinity, drought, extremes of temperature, ultraviolet B stress or polluted with xenobiotics like toxic heavy metals and fluoride. Major cereal crops like rice, wheat, maize, barley, sorghum and millets which provide the staple food for the entire global population are mainly glycophytes and are extremely susceptible to abiotic stress-induced oxidative injuries. Nanofertilization/exogenous spraying of beneficial nanoparticles alleviates the oxidative damages in cereals by altering the homeostasis of phytohormones like abscisic acid, gibberellins, cytokinins, auxins, salicylic acid, jasmonic acid and melatonin and by triggering the synthesis of gasotransmitter nitric oxide. Signaling cross-talks of nanoparticles with plant growth regulators enable activation of the defence machinery, comprising of antioxidants, thiol-rich compounds and glyoxalases and restrict xenobiotic mobilization by suppressing the expression of associated transporters. Accelerated nutrient uptake and grain biofortification under the influence of nanoparticles result in optimum crop productivity under sub-optimal conditions. However, over-dosing of even beneficial nanoparticles promotes severe phytotoxicity. Hence, the concentration of nanoparticles and mode of administering need to be thoroughly standardized before large-scale field applications, to ensure sustainable cereal cultivation with minimum ecological imbalance.

Bioaccumulation of Fluoride in Plants and Its Microbially Assisted Remediation: A Review of Biological Processes and Technological Performance

Fluoride is widely found in soil–water systems due to anthropogenic and geogenic activities that affect millions worldwide. Fluoride ingestion results in chronic and acute toxicity, including skeletal and dental fluorosis, neurological damage, and bone softening in humans. Therefore, this review paper summarizes biological processes for fluoride remediation, i.e., bioaccumulation in plants and microbially assisted systems. Bioremediation approaches for fluoride removal have recently gained prominence in removing fluoride ions. Plants are vulnerable to fluoride accumulation in soil, and their growth and development can be negatively affected, even with low fluoride content in the soil. The microbial bioremediation processes involve bioaccumulation, biotransformation, and biosorption. Bacterial, fungal, and algal biomass are ecologically efficient bioremediators. Most bioremediation techniques are laboratory-scale based on contaminated solutions; however, treatment of fluoride-contaminated wastewater at an industrial scale is yet to be investigated. Therefore, this review recommends the practical applicability and sustainability of microbial bioremediation of fluoride in different environments.

Role of exogenously applied putrescine in amelioration of cadmium stress in Coriandrum sativum by modulating antioxidant system

Abiotic stress reduces the plant growth and biomass production. Putrescine (Put) may be applied to alleviate numerous types of abiotic stresses in plants. The present research was intended to evaluate the role of exogenously applied Put in extenuation of cadmium (Cd) stress in coriander plants. Coriander seeds primed with 0.25, 0.5, and1 mM Put were allowed to grow in 50 mg kg^-1 Cd contaminated soil for one month. Put treatment improved seed germination, gas exchange attributes, root growth and shoot growth of coriander. The improved activity of stress-responsive enzymes such as superoxide dismutase, catalase and peroxidase, besides amplification of proline was observed in Put treated seedlings under Cd stress. In addition, a reduced amount of total soluble protein and sugars content were noticed in Cd stressed seedlings. Nevertheless, Put reduced MDA level in treated plants. Our results demonstrated that Put mitigated Cd induced stress by modulating antioxidants and photosynthetic activity of coriander plants.Novelty statement Most of the researchers have studied the role of endogenous putrescine in alleviation of plant stress. However, during current study, we primed coriander seeds with putrescine. Our results elucidated very promising role of exogenously applied putrescine in stress mitigation and growth improvement of coriander seedlings under Cd stress. The findings of current study advocate the application of putrescine for stress alleviation in crop plants.

What signals the glyoxalase pathway in plants?

Glyoxalase (GLY) system, comprising of GLYI and GLYII enzymes, has emerged as one of the primary methylglyoxal (MG) detoxification pathways with an indispensable role during abiotic and biotic stresses. MG homeostasis is indeed very closely guarded by the cell as its higher levels are cytotoxic for the organism. The dynamic responsiveness of MG-metabolizing GLY pathway to both endogenous cues such as, phytohormones, nutrient status, etc., as well as external environmental fluctuations (abiotic and biotic stresses) indicates that a tight regulation occurs in the cell to maintain physiological levels of MG in the system. Interestingly, GLY pathway is also manipulated by its substrates and reaction products. Hence, an investigation of signalling and regulatory aspects of GLY pathway would be worthwhile. Herein, we have attempted to converge all known factors acting as signals or directly regulating GLYI/II enzymes in plants. Further, we also discuss how crosstalk between these different signal molecules might facilitate the regulation of glyoxalase pathway. We believe that MG detoxification is controlled by intricate mechanisms involving a plethora of signal molecules.

Fluoride toxicity variably affects overall physiology and grain development in three contrasting rice genotypes, representing a potential biohazard

Ingestion of fluoride through consumption of contaminated food grains has been regarded to be hazardous for consumer health. The current study indicated the possible occurrence of such biohazard due to fluoride bioaccumulation in rice grains and straw (cattle feed). The effects of fluoride toxicity at three stages of grain development in three rice genotypes, viz., IR-64, Gobindobhog (aromatic), and Khitish, were also studied. Irrigation with fluoride-infested water inhibited grain formation in IR-64 and reduced grain yield in Gobindobhog. Fluoride toxicity promoted seed sterility in IR-64 by triggering reactive oxygen species (ROS) production and cellular necrosis, suppressing genes like GIF1, DEP1, and SPL14 (positively controlling seed formation) and inducing GW2 (negatively mediating grain development). Gobindobhog showed intermediate fluoride sensitivity and accumulated high levels of proline, anthocyanins, flavonoids, and phenolics due to the induction of genes like P5CS, ANS, and PAL in developing grains. The agronomic attributes in Khitish were unaffected by fluoride stress due to regulated fluoride uptake and high expression of GIF1, DEP1, and SPL14 along with an increased synthesis of anthocyanins, flavonoids, and phenolics. Khitish also accumulated low ROS as a result of which lowest lipoxygenase expression (among selected cultivars) was observed in developing grains. Fluoride entry was accelerated in the straw of Khitish, possibly due to the absence of regulated uptake mechanism in dead seedlings. Furthermore, the ecological concerns regarding fluoride bioaccumulation and reduced grain yield at the varietal level were also established, based on statistical modelling.

Exogenous melatonin regulates endogenous phytohormone homeostasis and thiol-mediated detoxification in two indica rice cultivars under arsenic stress

Melatonin enhanced arsenic (As) tolerance by inhibiting As bioaccumulation, modulating the expression of As transporters and phytohormone homeostasis, leading to efficient utilization of thiol machinery for sequestration and detoxification of this toxic metalloid. The present study was aimed at investigating the influence of exogenous melatonin on the regulation of endogenous plant growth regulators and their cumulative effects on metal(loid)-binding ligands in two contrasting indica rice cultivars, viz., Khitish (arsenic sensitive) and Muktashri (arsenic tolerant) under arsenic stress. Melatonin supplementation ameliorated arsenic-induced perturbations by triggering endogenous levels of gibberellic acid and melatonin, via up-regulating the expression of key biosynthetic genes like GA3ox, TDC, SNAT and ASMT. The endogenous abscisic acid content was also enhanced upon melatonin treatment by induced expression of the key anabolic gene, NCED3 and concomitant suppression of ABA8ox1. Enhanced melatonin content induced accumulation of higher polyamines (spermidine and spermine), together with up-regulation of SPDS and SPMS in Khitish, thereby modulating stress condition. On the contrary, melatonin escalated putrescine and spermidine levels in Muktashri, via enhanced expression of ADC and SAMDC. The role of melatonin appeared to be more prominent in Khitish, as evident from better utilization of thiol components like cysteine, GSH, non-protein thiols and phytochelatins, with higher GSH/GSSG ratio, despite down-regulated expression of corresponding thiol-metabolic genes (OsMT2 and OsPCS1) to deal with arsenic toxicity. The extent of arsenic bioaccumulation, which was magnified several folds, particularly in Khitish, was decreased upon melatonin application. Overall, our observation highlighted the fact that melatonin enhanced arsenic tolerance by inhibiting arsenic bioaccumulation, via modulating the expression levels of selected arsenic transporters (OsNramp1, OsPT2, OsPT8, OsLsi1) and controlling endogenous phytohormone homeostasis, leading to efficient utilization of thiol machinery for sequestration and detoxification of this toxic metalloid.

Maghemite nano-fertilization promotes fluoride tolerance in rice by restoring grain yield and modulating the ionome and physiome

The present manuscript elucidated the ameliorative potential of nano-maghemite (FeNPs) against the hazardous effects of fluoride toxicity in the sensitive rice cultivar, IR-64. Fluoride pollution triggered bioaccumulation in root, shoot and spikelets which inhibited reproduction, agronomic development and mineral uptake. Suppressed activity of enzymatic antioxidants and excessive cobalt translocation manifested severe ROS-induced oxidative injuries. Seedling priming with FeNPs reduced fluoride bioaccumulation and promoted efficient uptake of macroelements and micronutrients like potassium, calcium, iron, zinc, copper, nickel, manganese, selenium and vanadium and reduced the translocation of cobalt in mature seedlings during stress. This altogether triggered growth and activated the enzymes like SOD, CAT, APX and GPOX. High accumulation of non-enzymatic antioxidants like proline, anthocyanins, flavonoids, phenolics along with stimulated GSH synthesis (determined from high GR, GST and GPX activity) and glyoxalase activity enabled FeNP-pulsed plants to efficiently scavenge ROS, O₂^-, H₂O₂ and methylglyoxal, and mitigate oxidative injuries. The ROS production was also lowered due to suppressed NADPH oxidase activity. This ensured subsequent revitalization of Hill activity and the level of photosynthetic pigments. Due to reduced fluoride partitioning and improved nutritional sink, the grain and panicle development in FeNP-primed, stressed seedlings were more stimulated than even control sets. Overall, our findings supported by statistical modelling established the potential of iron-nanotechnology in promoting safe rice cultivation even in fluoride-polluted environments.

Melatonin application differentially modulates the enzymes associated with antioxidative machinery and ascorbate-glutathione cycle during arsenate exposure in indica rice varieties

Arsenic (As) contamination and accumulation in rice is a serious concern causing severe oxidative damage. Melatonin acts as a protective agent in plant defence against multiple abiotic stresses. The mechanism of antioxidant function of melatonin during As stress in rice genotypes is less studied. In this study, hydroponically-grown As-susceptible (Khitish) and As-tolerant (Muktashri) rice cultivars, subjected to 150 µm arsenate stress, were supplemented with exogenously applied melatonin (20 µm) to examine the plant defence mechanism. Melatonin (Mel) increased root and shoot length, fresh and dry weight, chlorophyll a and b content and activated reducing power and free radical scavenging capacity in both rice cultivars. The role of Mel in the sensitive variety appeared to be more prominent with respect to reduced water saturation deficit by reducing endogenous As and H₂ O₂ accumulation, and enhancing overall antioxidant capacity by imposing reduced requirement of catalase for ROS detoxification, and restoring As-inhibited activity of glutathione-S-transferase, glutathione peroxidase and dehydroascorbate reductase. In contrast, melatonin treatment in the tolerant cultivar required reduced involvement of ascorbate peroxidase to deal with As toxicity, and complemented the stress-mediated inhibition of guaiacol peroxidase activity. Isozyme profiling also established extensive varietal differences with regard to induction of new isoform(s) by Mel during As treatment. This study provides clear insights into mechanistic details of the regulation of antioxidative enzymes by melatonin in contrasting rice genotypes, which may prove helpful in generating As tolerance in susceptible rice varieties grown in marginalized soils, thereby improving crop yield and productivity.

Exogenous melatonin and salicylic acid alleviates cadmium toxicity in safflower (Carthamus tinctorius L.) seedlings

The co-application of exogenous 100 µM melatonin (MT) and 100 µM salicylic acid (SA) on 21-day-old safflower seedlings grown in the presence of cadmium (Cd, 100 µM) toxicity was investigated. The application of MT, SA, or MT + SA efficiently improved toxicity symptoms and declined Cd toxicity as shown by a considerable rise in plant biomass production and chlorophyll content accompanied by decreased level of oxidative stress markers. In Cd stressed plants, the simultaneous application of MT and SA led to sharp decreases in MDA and H2O2 amounts (61.04 and 49.11%, respectively), related to plants treated with Cd alone. With respect to the control, a 41 and 48% increment in reduced glutathione (GSH) and ascorbate (ASC) content was recorded in Cd-treated seedlings. Though, with the addition of MT, SA, or MT + SA, the content of GSH and ASC increased more. The application of MT, SA, or MT + SA caused a sharp induction in phytochelatin content of the leaves of Cd-treated seedlings, while in roots, the highest PC content was recorded only in the presence of MT, which was about 1.8-fold greater than in plant treated with Cd alone. The activity of enzymes responsible for the ascorbate-glutathione cycle and glyoxalase system considerably improved by using MT, SA, or the combination of MT and SA. Our findings suggest a possible synergic interaction between MT and SA in tolerating Cd toxicity by reducing Cd uptake, improving chlorophyll biosynthesis and accelerating ascorbate-glutathione cycle as well as the modulation of glyoxalase system.

Differential Antioxidant Compounds and Activities in Seedlings of Two Rice Cultivars Under Chilling Treatment

Variations in antioxidant compounds were examined in seedlings of two rice cultivars (Qiutianxiaoting and 93-11) exposed to low temperature (4°C) for 0, 12, 36, and 48 h. Antioxidant activity was identified by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) assays. The concentrations of total phenols, flavonoids, chlorophyll, and anthocyanins (ACNs) were determined by spectrophotometry. In addition, high-performance liquid chromatography (HPLC) was used to reveal the changes in phenolic compound concentrations in rice seedlings under chilling treatment. Results showed that antioxidant concentrations and antioxidant activity after chilling treatment were higher in 93-11 compared to Qiutianxiaoting, reaching the highest level at 36 h chilling treatment in 93-11. Phenolic compounds in Qiutianxiaoting decreased between 12 and 36 h but then increased at 48 h, whereas the corresponding levels in 93-11 increased as chilling time increased. Moreover, 10 phenolic compounds were detected and quantified by HPLC, of which gallic acid and caffeic acid tended to only exist in 93-11, whereas rutin was observed only in Qiutianxiaoting. The results of this study could be leveraged to optimize the antioxidant potential of rice in the context of healthy food choices.

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.

De novo RNA-Seq analysis in sensitive rice cultivar and comparative transcript profiling in contrasting genotypes reveal genetic biomarkers for fluoride-stress response

The fluoride-sensitive indica rice cultivar, IR-64 was subjected to NaF-treatment for 25 days, following which RNA-Seq analysis identified significant up and down regulation of 1,303 and 93 transcripts respectively. Gene ontology (GO) enrichment analysis classified transcripts into groups related to 'cellular part', 'membrane', 'catalytic activity', 'transporter activity', 'binding', 'metabolic processes' and 'cellular processes'. Analysis of differentially expressed genes (DEGs) revealed fluoride-mediated suppression of abscisic acid (ABA) biosynthesis and signaling. Instead, the gibberellin-dependent pathway and signaling via ABA-independent transcription factors (TFs) was activated. Comparative profiling of selected DEGs in IR-64 and fluoride-tolerant variety, Khitish revealed significant cytoskeletal and nucleosomal remodelling, accompanied with escalated levels of autophagy in stressed IR-64 (unlike that in stressed Khitish). Genes associated with ion, solute and xenobiotic transport were strongly up regulated in stressed IR-64, indicating potential fluoride entry through these channels. On the contrary, genes associated with xenobiotic mobility were suppressed in the tolerant cultivar, which restricted bioaccumulation and translocation of fluoride. Pairwise expression profile analysis between stressed IR-64 and Khitish, supported by extensive statistical modelling predicted that fluoride susceptibility was associated with high expression of genes like amino acid transporter, ABC transporter2, CLCd, MFS monosaccharide transporter, SulfT2.1 and PotT2 while fluoride tolerance with high expression of Sweet11.

Synergistic Effect of Bacillus thuringiensis IAGS 199 and Putrescine on Alleviating Cadmium-Induced Phytotoxicity in Capsicum annum

Plant growth-promoting bacteria (PGPB) and putrescine (Put) have shown a promising role in the mitigation of abiotic stresses in plants. The present study was anticipated to elucidate the potential of Bacillus thuringiensis IAGS 199 and Put in mitigation of cadmium (Cd)-induced toxicity in Capsicum annum. Cadmium toxicity decreased growth, photosynthetic rate, gas exchange attributes and activity of antioxidant enzymes in C. annum seedlings. Moreover, higher levels of protein and non-protein bound thiols besides increased Cd contents were also observed in Cd-stressed plants. B. thuringiensis IAGS 199 and Put, alone or in combination, reduced electrolyte leakage (EL), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) level in treated plants. Synergistic effect of B. thuringiensis IAGS 199 and Put significantly enhanced the activity of stress-responsive enzymes including peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT) and superoxide dismutase (SOD). Furthermore, Put and microbial interaction enhanced the amount of proline, soluble sugars, and total soluble proteins in C. annum plants grown in Cd-contaminated soil. Data obtained during the current study advocates that application of B. thuringiensis IAGS 199 and Put establish a synergistic role in the mitigation of Cd-induced stress through modulating physiochemical features of C. annum plants.

Enhanced performance of Bacillus megaterium OSR-3 in combination with putrescine ammeliorated hydrocarbon stress in Nicotiana tabacum

Hydrocarbon stress (HS) has been causing decreased plant growth and productivity. Putrescine (Put) and growth promoting microbes are vital for plant growth and development under hydrocarbon stress. Current research work was carried out to evaluate the potential of Bacillus megaterium OSR-3 alone and in combination with Put to alleviate HS in Nicotiana tabacum (L.). The crude petroleum contaminated soil significantly reduced growth attributes of N. tabacum. B. megaterium OSR-3 inoculated plants subjected to HS exhibited improved photosynthetic rate, gas exchange characteristics, poline contents and protein level. Furthermore, bacterial inoculation enhanced the antioxidative activity of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) in tobacco plants subjected to HS. The HS alleviation in B. megaterium OSR-3 inoculated N. tabacum can be credited to the heightened activity of antioxidative enzymes, reduction in hydrogen peroxide (H2O2) and abridged synthesis of malondialdehyde (MDA). The increased synthesis of indole acetic acid (IAA) in HS stressed N. tabacum plants treated with co-application of B. megaterium OSR-3 and Put attenuated toxicity and amplified growth of plants. Additionally, the co-application of B. megaterium OSR-3 and Put also upregulated the activity of antioxidative enzymes and induced augmented level of proline and IAA in plants under HS regimes. Current research provides novel insight into the potential and mechanism of B. megaterium OSR-3 and Put in mitigation of HS in N. tabacum plants.