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Oxidative stress due to hexavalent chromium [Cr(VI)] in Vigna radiata seedlings, and stress amelioration with treatment of methanol extract of Rhododendron arboreum leaves was observed in the present study by analyzing growth parameters, stomatal morphology, malondialdehyde (MDA) content, histological analysis, pigment contents, Cr metal uptake, elemental analysis, and antioxidant analysis. Chromium treatment resulted in the decline of root length, shoot length, fresh weight, and dry weight. Scanning electron microscopic studies revealed that Cr treatment altered the stomatal structure. As compared to control plants, MDA content increased by 80.1% in Cr-treated plants. Histological analysis with confocal microscope confirmed the nuclear damage, membrane damage, enhanced H2O2 accumulation, and decline in pigment concentration. Atomic absorption spectrometry analysis revealed an accumulation of 43.3% Chromium in the plant tissues and decreased concentration of essential elements as consequences of Cr treatment. The methanol extract of R. arboreum leaves (MEL) alleviated Cr stress in Vigna radiata seedlings by restoring normal growth, stomatal structure, and pigment contents, as well as essential elements. Reduction in H2O2 accumulation, reduced MDA content by 29.2%, and decline in Cr accumulation to 32.8% was observed after MEL supplementation to Cr-stressed plants. Decreased nuclear and membrane damage along with increased lipid-soluble as well as water-soluble antioxidants after MEL application in Cr-stressed plants are further symptoms of stress amelioration properties of Rhododendron leaves.

Jasmonic acid (JA) is an important phytohormone associated in defense responses against stress. Crop plants experience heavy metal toxicity and needs to be explored to enhance the crop production. Lead (Pb) is one of the dangerous heavy metal that pollutes soil and water bodies and is released from various sources like discharge from batteries, automobile exhaust, and paints. The present study was designed to evaluate the role of JA (100 nM) on photosynthetic pigments, secondary metabolites, organic acids, and metal ligation compounds in tomato seedlings under different concentrations of Pb (0.25, 0.50, and 0.75 mM). It was observed that Pb treatment declined pigment content, relative water content, and heavy metal tolerance index. Expression of chlorophyllase was also enhanced in Pb-treated seedlings. Seeds primed with JA lowered the expression of chlorophyllase under Pb stress. JA application enhanced the contents of secondary metabolites (total phenols, polyphenols, flavonoids, and anthocyanin) which were confirmed with enhanced expression of chalcone synthase and phenylalanine ammonia lyase in Pb-exposed seedlings. Treatment of JA further elevated the levels of organic acids and metal chelating compounds under Pb toxicity. JA up-regulated the expression of succinate dehydrogenase and fumarate hydratase in Pb-exposed seedlings. Results revealed that seeds primed with JA reduced Pb toxicity by elevating, the levels of photosynthetic pigments, secondary metabolites, osmolytes, metal ligation compounds, organic acids, and polyamine accumulation in tomato seedlings.

In the current investigation, we studied role of castasterone (CS), (a bioactive brassinosteroid) in Brassica juncea grown under imidacloprid (IMI) stress. We observed that CS-seed treatment resulted in the recovery of seedling growth under IMI toxicity. Seed treatment with CS, significantly enhanced the contents of pigments like chlorophylls, carotenoids, anthocyanins and xanthophylls under stress. Oxidative stress generated by the production of reactive oxygen species (ROS) like hydrogen peroxide and superoxide anion, was reduced after CS treatment under IMI toxicity. Antioxidative defense system got activated after CS-seed treatment, resulting in the increased activities of enzymes. Moreover, CS-seed treatment under IMI stress also stimulated the biosynthesis of organic acids of Krebs cycle (citrate, succinate, fumarate and malate) and phenolics. We also noticed that CS is also involved in the regulation of the gene expression of some key enzymes involved in pigment metabolism (CHLASE, PSY, CHS), carbon fixation (RUBISCO), Krebs cycle (CS, SUCLG1, SDH, FH), ROS generation (RBO), antioxidative enzymes (SOD, CAT, POD, DHAR, GR, GST), phenolic biosynthesis (PAL) and pesticide detoxification system (CXE, P450, NADH). This modulated gene expression after CS-treatment activated the insecticide detoxification, leading to the reduction of IMI residues. Data analysis using multivariate statistical technique i.e. multiple linear regression, also supported the fact that CS can efficiently reduce IMI induced phytotoxicity in B. juncea.

The study envisages the stress protective role of selenium (Se) against chromium (Cr) toxicity in Brassica juncea L. Plants were raised in pots with soils containing Cr (0 and 300 μMKg−1) and Se (0, 2, 4 and 6 μMKg−1) in binary combinations and harvested after 30 days of sowing. The harvested plant material was processed for several biochemical and molecular parameters. The observations made on significant results suggested that Se was able to protect the plants by restoring growth, reducing the oxidative damage (low levels of superoxide anions and hydrogen peroxide), and strengthened the enzymatic and non-enzymatic defence systems of B. juncea plants. Quantitative real time PCR studies on gene expression revealed that the expression of genes encoding the antioxidative enzymes (SOD, POD, CAT, GR and GST-1) was enhanced with Se application. Elevated contents of chlorophylls, carotenoids and gas exchange parameters also indicated that Se is instrumental in promoting photosynthetic efficiency, thereby ameliorating Cr toxicity. The expression of genes related to chlorophyll degradation also confirmed the observations. The contents of secondary metabolites (total phenols, flavonoids and anthocyanins) and the relative gene expression of phenylalanine ammonialyase and chalcone synthase, also showed enhanced levels in response to Se. The results thus indicated that Se application improves the overall physio-chemical characteristics of plants and helps them to overcome stress induced by Cr in the soil.

Brassinosteroids (BRs) are group of plant steroidal hormones that modulate developmental processes and also have pivotal role in stress management. Biosynthesis of BRs takes place through established early C-6 and late C-6 oxidation pathways and the C-22 hydroxylation pathway triggered by activation of the DWF4 gene that acts on multiple intermediates. BRs are recognized at the cell surface by the receptor kinases, BRI1 and BAK1, which relay signals to the nucleus through a phosphorylation cascade involving phosphorylation of BSU1 protein and proteasomal degradation of BIN2 proteins. Inactivation of BIN2 allows BES1/BZR1 to enter the nucleus and regulate the expression of target genes. In the whole cascade of signal recognition, transduction and regulation of target genes, BRs crosstalk with other phytohormones that play significant roles. In the current era, plants are continuously exposed to abiotic stresses and heavy metal stress is one of the major stresses. The present study reveals the mechanism of these events from biosynthesis, transport and crosstalk through receptor kinases and transcriptional networks under heavy metal stress.

Rhizosphere comprises inimitable environs, acting as a battleground for PGPR to form an origin of enriched source of unique functionalities for plants. PGPR are closely bound to roots in order to nurture them and protect them from pathogens specifically Root-knot Nematodes (RKNs) through direct and indirect manner. RKNs are mainly endoparasitic to cause dreadful diseases in plants, especially economically important crops. The traditional use of chemical agents for nematode control in agricultural yields has been adversely affecting the soil microbiota as well as physiochemical attributes of soil. Contrastingly, PGPR forms symbiotic associations alongside plants and act as biofertilizers, bioprotectors, and sustainable means for agricultural practices. PGPR are observed to improve nutrient acquisition and release various secondary metabolites like phenols, enzymes, siderophores, hydrogen cyanide (HCN), phytohormones, and hydrolytic enzymes to boost the antioxidant potential of plants damaged by RKNs. Moreover, PGPR also synthesize various antimicrobial agents or toxins in order to directly attack the invading RKNs for activating induced systemic resistance mechanism of plants and enhance antioxidative properties within them. Therefore, all these mechanisms are of paramount importance as they are well-documented biocontrol agents in the field of agronomy. Here, in this chapter, we will discuss the role of PGPR and their interaction along with plant roots within rhizosphere to form RKN resistance. In general, we have elaborated the mechanistic role of PGPR in mediating antioxidant capacity of plants against nematode infestations. Ultimately, we have recapitulated the role of PGPR to be used as alternatives to chemical fertilizers for sustainable agriculture.

Legumes beyond being an imperative source of nutrients have a wide array of antioxidants and other phytochemicals essential for plant growth and development. Melatonin (MEL, 5-acetyl-5-methoxy trptamine) is a ubiquitous phytochemical present in all organisms that plays a multiregulatory role in plant growth and development, senescence, yield, fruit ripening, circadian rhythmes, and response to abiotic stresses. Under stress conditions MEL is accumulates sharply by regulation of its synthesis and varied metabolic pathways. Tryptophan is a precursor for MEL synthesis and four consecutive enzymes mediate synthesis of tryptamine, N-acetylserotonin, and MEL. It is widely documented that production of phytomelatonin is elevated in plants exposed to abiotic stresses including salinity, heavy metal, water, and temperature. It acts as a powerful antioxidant that directly scavenges reactive oxygen species. Moreover, it confers stress tolerance in legumes by activation of antioxidative defense, modulation of transcription factors that are prerequisite for stress resistance and chelation phenomena, and modulation of hormone cross-talk with hormones including salicylic acid, ethylene, and jasmonic acid. Thus the present chapter focuses on biosynthesis of MEL in plants, stressed-induced alteration in endogenous levels of MEL in legumes, the role of exogenously applied MEL in alleviating stress conditions, and underlying mechanisms of MEL-refereed tolerance and detoxification.

Cadmium (Cd) metal toxicity is a crucial ecological matter that requires immediate efforts to mitigate it. Brassica juncea plants were exposed to Cd (0 and 200 &micro, M as CdSO4) and foliar application of 24-Epibrassinolide (EBR) (0, 10&minus, 7and 10&minus, 5 M). The toxic effect of Cd was evident in terms of declined growth and biomass yield, lowered levels of pigment content and chlorophyll fluorescence, and reduction in gas exchange attributes. The levels of proline and glycinebetaine increased in response to Cd treatment. There was an imperative rise in the contents of H2O2 and malondialdehyde as well as electrolyte leakage in the Cd-stressed plants. With the application of EBR, there was a significant replenishment in growth attributes and photosynthetic efficacy. The contents of ROS (reactive oxygen species) and malondialdehyde as well as electrolyte leakage were reduced by the hormone supplementation. Enhancement in the contents of glutathione and ascorbic acid, and the activities of enzymes of the antioxidative defense system and glyoxalase system was recorded in response to Cd as well as hormone treatment. The in-situ levels of Cd in roots and shoot were augmented in response to Cd treatment, but were found to be lowered by the EBR application.

Proline is a proteinogenic amino acid with conformational rigidity and is a pre-requisite for primary metabolism. Proline has been extensively reported for its potential role in osmotic adjustment in plant cells. More the stress regulatory properties of proline have been affirmed by a plethora of researchers. Proline metabolism includes its synthesis and degradation. The synthesis takes place by the glutamate and ornithine pathways. Proline anabolism is induced and its catabolism is suppressed during dehydration, but rehydration stimulates the inverse regulation. Under abiotic stress conditions, the plant synthesizes enormous amounts of various compatible solutes or osmoprotectants including proline. Proline has been found to play a regulatory function in the growth and development of plants by acting as a signaling molecule. Proline is also known to protect photosystem II by protecting the thylakoid membranes from oxidative stress. There is little knowledge about the post-translational modification of enzymes of the proline metabolic pathway.

Root-knot nematodes are major constraints among different pathogens with wide host range and cause severe agricultural loss worldwide. The present study was designed to understand the role of plant growth promoting bacteria (Pseudomonas aeruginosa & Burkholderia gladioli) on growth and antioxidative potential in nematode infected Lycopersicon esculentum seedlings. An experiment was conducted to assess the levels of superoxide anions, H2O2 and MDA contents generated during nematode infection. Moreover, the contribution of antioxidative enzymes, non-enzymatic antioxidants, total antioxidants and gene expression profiling was also carried out in nematode infected Lycopersicon esculentum seedlings. The results of present study revealed that nematode infection reduced the growth of seedlings which upon inoculation of microbes was improved. Moreover, number of galls were reduced upon supplementation of these strains. Nematode infection also caused accumulation of superoxide anion, H2O2, and malondialdehyde contents along with nuclear damage and loss of cell viability which was reduced upon supplementation of microbes. The oxidative burst generated enhanced various antioxidant enzymes such as SOD (30.6%), POD (3.6%), CAT (18.1%), GPOX (65.9%), APOX (24.8%), GST (5.6%), DHAR (13.9%), GR (11%) and PPO (2.5%) which were further elevated upon application of P. aeruginosa (23.9%, 7.2%, 7%, 66%, 28.9%, 71.3%, 14.5%, 10.6% and 38.3%) and B. gladioli (5.1%, 30.6%, 16.2%, 92.1%, 78.5%, 97.5%, 15.5%, 65.7% and 23.2%). The non-enzymatic antioxidants (glutathione, ascorbic acid and tocopherol) and total antioxidants contents (both water soluble and lipid soluble) were also enhanced upon inoculation of microbes. Confocal microscopy revealed the improvement in nuclear damage and cell viability in microbe inoculated roots. Gene expression profiling revealed the enhanced expression levels of SOD, POD, CAT, GR, GPOX, APOX, PPOgenes in P.aeruginosa inoculated nematode infected seedlings by 53%, 2.7%, 64.1%, 10.4%,19.7%, 29.2%, 38.4% and B. gladioli inoculated seedlings by 18.3%,144%, 67%, 43%, 308%, 151% respectively. The results therefore suggest the favourable aspects of micro-organisms in modulating growth characteristics and antioxidative defense expression of Lycopersicon esculentum to encounter oxidative stress generated under nematode infection.

Plants form a substantive portion of human diet that contains a plethora of structurally diverse polyphenols. These polyphenols extend both nutritional as well as disease preventive benefits to the consumer. Many ancient medicinal systems like Ayurveda reiterate the consumption of particular polyphenol rich plant in specific medical condition. Therefore, it is necessary to create a database of the contents of different polyphenols in various plants. This study compiles a bibliographic review of polyphenol distribution in different plants along with their statistical analysis like Pearson's correlation matrix, cluster analysis, principal component and factor analysis, and non-metric multidimensional scaling. It was observed that caffeic acid and quercetin were the most abundant polyphenols in the plants. A significant positive correlation of umbelliferone was found with kaempferol and epicatechin. Species having close proximities on the basis of polyphenols were Parthenium hysterophorus, Rumex dentatus, Achyranthus aspera, Chenopodium ambrosoides, Cannabis sativa, Rhododendron arboreum, Alternanthera philoxeroides, Debregeasia longifolia and C. album. Factor analysis showed four underlying factors for polyphenols. Factor-1 had maximum loadings on epicatechin, umbelliferone and kaempferol. Gallic acid and catechin had maximum loadings on factor-2. Factor-3 had maximum loadings on chlorogenic acid and quercetin, and factor-4 had maximum loading on coumaric acid. Keywords: Polyphenols, Cluster analysis, Principal component analysis, Factor analysis, NMDS, Heatmap analysis

The present study was designed to understand the protective role of Selenium (Se) in Brassica juncea seedlings cultivated under chromium (Cr) stress. The seedlings were raised in Petri dishes containing solutions of Cr (300 μM) and Se (2, 4 and 6 μM) in unary and binary combinations. The 15 days old seedlings were harvested and assessed for pigments, stomatal morphology and stomatal density, total antioxidative capacity, osmolytes and metal chelators. The results showed that the seedlings treated with Cr showed a significant decrease in the contents of chlorophyll (40.95%) and carotenoids (21.38%). Cr application also caused reduction in stomatal density (42.10%) and negatively affected the morphology of stomata. Se supplementation reduced the toxicity of Cr by increasing the pigment contents and improving the morphology of leaves. Relative gene expression of chlorophyllase reduced, while that of phytoene synthase and chalcone synthase enhanced with Se application which supported the above observations. Cr stress increased the levels of lipid- and water-soluble antioxidants by 68.25 and 32.35%, respectively; however, supplementation of Se further increased their contents. Proline, glycine betaine, trehalose, and osmolality increased by 155.19, 49.18, 83.52, and 120.92%, respectively, in seedlings subjected to Cr stress. Further increment in the above parameters was observed by the application of Se to Cr-stressed seedlings. Seedlings treated with 300 μM Cr showed an increase of 27.27% in the content of total thiols, 127.27% in the content of non-protein thiols, and 21.05% in that of protein-bound thiols, and supplementation of Se showed additional an increase by 28.57, 34.00, and 26.08%, respectively, over those in the Cr-treated seedlings. In conclusion, Cr hampered the normal functioning of the B. juncea seedlings and Se application mitigated the negative impact through modulation in the contents of various osmolytes, antioxidants, and other metabolites. Thus, Se application might help the seedlings to withstand stress by strengthening their antioxidant system, osmoregulation, and metal-chelating ability.

The study focuses on potential of combined pre-soaking treatment of 24-Epibrassinolide (EBL) and Salicylic acid (SA) in alleviating Pb phytotoxicity in Brassica juncea L. plants. The seeds after treatment with combination of both the hormones were sown in mixture of soil, sand and manure (3:1:1) and were exposed to Pb concentrations (0.25mM, 0.50mM and 0.75mM). After 30 days of growth, the plants were harvested and processed, for quantification of various metabolites. It was found that pre-sowing of seeds in combination of EBL and SA, mitigated the adverse effects of metal stress by modulating antioxidative defense response and enhanced osmolyte contents. Dry matter content and heavy metal tolerance index were enhanced in response to co-application of EBL and SA. The levels of superoxide anions, hydrogen peroxide and malondialdehyde were lowered by the combined treatment of hormones. Enhancement in activities of guaiacol peroxidase, catalase, glutathione reductase and glutathione-s-transferase were recorded. Contents of glutathione, tocopherol and ascorbic acid were also enhanced in response to co-application of both hormones. Expression of POD, CAT, GR and GST1 genes were up-regulated whereas SOD gene was observed to be down-regulated. Contents of proline, trehalose and glycine betaine were also reported to be elevated as a result of treatment with EBL+SA. The results suggest that co-application of EBL+SA may play an imperative role in improving the antioxidative defense expression of B. juncea plants to combat the oxidative stress generated by Pb toxicity.

Cadmium(II) toxicity is a serious environmental issue warranting effective measures for its mitigation. In the present study, ameliorative effects of a bioactive brassinosteroid, castasterone (CS) and low molecular weight organic acid, citric acid (CA) against the Cd(II) toxicity to Brassica juncea L. were evaluated. Seeds of B. juncea treated with CS (0, 0.01, 1 and 100nM) were sown in cadmium spiked soils (0 and 0.6mmolkg-1 soil). CA (0.6mmolkg-1soil) was added to soil one week after sowing seeds. Plants were harvested 30 days after sowing. Phytotoxicity induced by Cd(II) was evident from stunted growth of the plants, malondialdehyde accumulation, reduction in chlorophyll and carotenoid contents, and leaf gas exchange parameters. Cd(II) toxicity was effectively alleviated by seed soaking with CS (100nM) and/ or soil amendment with CA (0.6mMkg-1 soil). Relative gene expression of genes encoding for some of the key enzymes of pigment metabolism were also analysed. Expression of chlorophyllase (CHLASE) was reduced, while that of phytoene synthase (PSY), and chalcone synthase (CHS) genes were enhanced with CS and/or CA treatments with respect to plants treated with Cd(II) only. Cd also affected the activities of antioxidative enzymes. Plants responded to Cd(II) by accumulation of total sugars. CS (100nM) and CA treatments further enhanced the activities of these parameters and induced the contents of secondary plant pigments (flavonoids and anthocyanins) and proline. The results imply that seed treatment with CS and soil application with CA can effectively alleviate Cd(II) induced toxicity in B. juncea by strengthening its antioxidative defence system and enhancing compatible solute accumulation.

Metal ions are the natural components of earth’s crust. At low concentrations, metal ions are really essential to the plant system, but their excessive amounts lead to toxicity. Industrial revolution and anthropogenic activities are critically disturbing the level of metal ions in the ecosystem, like soil, water, and plant resources. Elemental and chemically combined forms of these metal ions such as Cd, As, Pb, Hg, Cr, and Zn are toxic to plant system, leading to loss in yield. Also, the heavy metal toxicity hinders the efficacy of photosynthetic apparatus, and causes oxidative stress and abnormal plant growth. Plants overcome such toxicity by assimilating these toxic metals into the root system or transforming these complexes into physiologically tolerant forms. Plants have adopted a series of physiological and molecular mechanisms to mitigate these stress responses that might enable them to withstand higher metal ion levels in soil. Scientific community is highly concerned to understand these mechanisms. Researchers are also keen to know the roles of plant growth regulators auxins, gibberellins, brassinosteroids, ethylene, cytokinin, jasmonic acid, salicylic acid, etc. in signaling pathways associated with the heavy metal tolerance in plants. Recent research has possible evidences of PGR’s role in the mitigation of heavy metal toxicity, as well as shows that it can regulate and integrate the factors associated with growth and development. In this chapter, we have discussed the role of PGRs in alleviating heavy metal stress, their mechanism of action, and cross talk.

Lead (Pb) toxicity is a major environmental concern affirming the need of proper mitigation strategies. In the present work, potential of combined treatment of 24-Epibrassinolide (24-EBL) and Salicylic acid (SA) against Pb toxicity to Brassica juncea L. seedlings were evaluated. Seedlings pre-imbibed in EBL (0.1 mM) and SA (1 mM) individually and in combination, were sown in Pb supplemented petri-plates (0.25, 0.50 and 0.75 mM). Various microscopic observations and biochemical analysis were made on 10 days old seedlings of B. juncea. The toxic effects of Pb were evident with enhancement in in-situ accumulation of Pb, hydrogen peroxide (H2O2), malondialdehyde (MDA), nuclear damage, membrane damage, cell death and polyamine. Furthermore, free amino acid were lowered in response to Pb toxicity. The levels of osmoprotectants including total carbohydrate, reducing sugars, trehalose, proline and glycine betaine were elevated in response to Pb treatment. Soaking treatment with combination of 24-EBL and SA led to effective amelioration of toxic effects of Pb. Reduction in Pb accumulation, reactive oxygen content (ROS), cellular damage and GSH levels were noticed in response to treatment with 24-EBL and SA individual and combined levels. The contents of free amino acid, amino acid profiling as well as in-situ localization of polyamine (spermidine) was recorded to be enhanced by co-application of 24-EBLand SA. Binary treatment of 24-EBL and SA, further elevated the content of osmoprotectants. The study revealed that co-application of combined treatment of 24-EBL and SA led to dimination of toxic effects of Pb in B. juncea seedlings.

Lead (Pb) has wide-ranging effects on various essential physiological processes in plants including seed germination, root/ shoot growth, photosynthetic efficiency, water status, and activities of enzymes. The effect of combined treatment of 24-epibrassinolide (10-7 M, EBL) and salicylic acid (1 mM, SA) on growth, photosynthetic attributes, and phenolic compounds in 30-, 60-, and 90-day-old plants of Brassica juncea L. under Pb stress (0.25 mM, 0.50 mM, and 0.75 mM) were studied. It was observed that Pb toxicity resulted in lowered growth and photosynthetic efficiency. The expressions of CHLASE, PSY, CHS, and PAL genes were altered. Presoaking treatment with the combination of EBL and SA for metal-stressed plants mitigated the adverse effects of metal stress by improving growth and levels of pigment and phenolic compounds.

The phytohormone jasmonic acid (JA) plays an imperative role in plants by modulating the activity of their antioxidative defense system under stress conditions. Here, we explored the role of JA-induced alterations in the growth and transcript levels of antioxidative enzymes in tomato seedlings exposed to different Pb concentrations (0.25, 0.50, and 0.75 mM). Pb treatment caused a dose-dependent reduction in their root and shoot lengths. Treatment of 0.75 mM Pb showed an increase in the contents of malondialdehyde (MDA), superoxide anion (O2•−), and hydrogen peroxide (H2O2) as compared to the untreated seedlings. Pb uptake was enhanced with an increase in Pb concentration. The seeds primed with JA showed reduction in Pb uptake and improvement in growth under Pb toxicity. The seedlings treated with both JA (100 nM) and Pb (0.75 mM) showed a decline in the levels of MDA, O2•−, and H2O2 as compared to the seedlings treated with 0.75 mM Pb alone. These results suggested that JA (100 nM) mitigated the oxidative damage by lowering the expression of the RBO and P-type ATPase transporter genes and by modulating antioxidative defense system activity. The biochemical and molecular analyses showed that JA plays a crucial role in plant defense responses against Pb stress.

The present study was designed to determine the role of plant growth-promoting rhizobacteria (Pseudomonas aeruginosa &Burkholderia gladioli) in alleviating Cd stress in Lycopersicon esculentum. Cd concentration of 0.4 mM enhanced superoxide anions, MDA and H2O2 by 136%, 378% and 137% that also caused nuclear and cell viability damage. Cd enhanced the activities of enzymatic antioxidants such as CAT, GST, GPOX, DHAR, and GR by 64%, 126%, 265%, 25% and 93% respectively. However, SOD, POD and PPO was decreased by Cd and enhanced by 119%, 198% and 42% by inoculation of P. aeruginosa and 65%, 119% and 33% by B. gladioli. The contents of non-enzymatic antioxidants and total antioxidants (WSA, LSA) were also enhanced in response to metal stress and reduced by supplementation with PGPR. Confocal microscopy revealed improved cell viability and decreased nuclear damage in Cd-treated L. esculentum roots supplemented with PGPRs. Gene expression studies conducted through qRT-PCR revealed that expression levels of the SOD, POD, and PPO genes were enhanced by 478%, 830% and 253%, while the expression of CAT, GR, GST, GPOX, and APOX genes decreased by 97%, 87%, 75%, 82%, 88% in P. aeruginosa-inoculated Cd-treated seedlings. Also, B. gladioli elevated the expression of SOD, POD and PPO genes and reduced the expression of CAT, GR, GPOX, APOX and GST genes respectively. Therefore, the results suggest that Cd induced oxidative stress in L. esculentum seedlings was reduced by PGPRs through modulation of antioxidative defence expression as demonstrated in terms of antioxidants both quantitatively as well as qualitatively.

In this era of urbanization and environmental pollution, antioxidants and antimutagens derived from plants are promising safeguards for human health. In the current investigation, we analyzed the antioxidant and antimutagenic effects of the hexane, chloroform, and ethyl acetate fractions of Rhododendron arboreum Sm. leaves and determined their chemical composition. The different fractions inhibited lipid peroxidation, repressed the production of nitric oxide radicals, and prevented deoxyribose degradation. The antimutagenic activity of the leaf fractions was analyzed against 4-nitro-O-phenylenediamine, sodium azide and 2-aminofluorene mutagens in two test strains (TA-98 and TA-100) of Salmonella typhimurium. The experiment was conducted using pre- and co-incubation modes. The best results were obtained in the pre-incubation mode, and against indirect acting mutagen. The presence of a number of bioactive constituents was confirmed in the different fractions by GC-MS analysis. The study reveals the strong antioxidant and antimutagenic activity of R. arboreum leaves. We propose that those activities of R. arboreum might correspond to the combined effect of the phytochemicals identified by GC-MS analysis. To the best of our knowledge, this is the first report on the antimutagenic activity of R. arboreum leaves.

The antioxidant defense system of Brassica juncea under Cd stress was examined on supplementation of earthworms in the rhizosphere at different concentrations of Cd (0.50, 0.75, 1.00 and 1.25 mM i.e. 56, 84, 112 and 140 mg kg−1 respectively). Seedlings were raised in small pots containing soil spiked with Cd and earthworms under controlled conditions for 15 days. Improved Cd accumulation, as well as enhanced plant dry weight and metal tolerance were observed following the addition of earthworms. Earthworm supplementation reduced reactive oxygen species (ROS) generation by 7.3% for hydrogen peroxide (H2O2), 7.1% for superoxide anion (O2 −), and 8.4% for malondialdehyde (MDA) in plants treated with 1.25 mM (140 mg kg−1) Cd. Confocal microscopy revealed improved cell viability and reduced H2O2 content due to enhanced antioxidative activity. Activity and expression levels of genes coding for antioxidative enzymes (superoxide dismutase; SOD, catalase; CAT, guaicol peroxidase; POD, glutathione reductase; GR, and glutathione-S-transferase; GST) were higher in plants raised in soils inoculated with earthworms, with expression of SOD increasing by 58.8%, CAT by 75%, POD by 183%, GR by 106.6%, and GST by 11.8%. Moreover, plant pigment (chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids) concentrations increased by 8%, 9.1%, 9.1%, and 7.7% respectively, in plants grown in soils supplemented with earthworms. The results of our study suggest that the addition of earthworms to soil increases antioxidative enzyme activities, gene expression in plants, and ROS inhibition, which enhances tolerance to Cd during the phytoextraction process.

Lead (Pb) is one of most toxic heavy metals that adversely affect growth and developmental in plants. It becomes necessary to explore environment safe strategies to ameliorate its toxic effects. Phytohormones play an imperative role in regulating stress protection in plants. Jasmonic acid (JA) is recognized as a potential phytohormone which mediates immune and growth responses to enhance plant survival under stressful environment. The present study was undertaken to evaluate the effect of JA on the growth, metal uptake, gaseous exchange parameters, and on the contents of pigments, osmolytes, and metal chelating compounds in tomato plants under Pb stress during different stages of growth (in 30-, 45-, and 60-day-old plants). We observed a decrease in shoot and root lengths under Pb stress. Treatment of JA improved the shoot and root lengths in the Pb-treated plants. The Pb uptake was increased with the increasing concentrations of Pb, however, seeds pretreated with JA reduced the Pb uptake by the plants. The chlorophyll and carotenoid contents increased by JA treatment in plants under Pb stress. Pre-soaking of seeds in JA, improved gaseous exchange parameters, such as internal CO2 concentration, net photosynthetic rate, stomatal conductance, and transpiration rate under Pb stress. JA enhanced the enzyme activity of ascorbate-glutathione cycle and reduced H2O2 concentration in Pb-treated plants. The contents of osmolyte and metal chelating compounds (total thiols, and non-protein and protein-bound thiols) were increased with the increase in Pb stress. In seeds primed with JA, the contents of osmolytes and metal chelating compounds were further increased in the Pb-treated plants. Our results suggested that treatment of JA ameliorated the toxic effects of Pb stress by reducing the Pb uptake and improving the growth, photosynthetic attributes, activity of ascorbate-glutathione cycle and increasing the contents of osmolytes and metal chelating compounds in the tomato plants.

The present study aimed at the potential role of selenium in providing protection to plants subjected to chromium toxicity. The study was carried out on 15-day-old seedlings of Brassica juncea raised in the solutions of Cr (300 µM) and Se (2, 4 and 6 µM), both alone and in combinations under controlled laboratory environment. The effects were studied on growth, plant metabolites (involved in osmotic homeostasis and stress protection), and essential elements. The results showed that the exposure of B. juncea seedlings to 300 µM Cr led to an increase in the contents of total sugars, reducing sugars, non-reducing sugars, total phenols and flavonoids. However, a significant decline in growth characteristics, the contents of proteins and free amino acids was observed. The essential elements (Na, K, Ca, Mg, C, H, N) also decreased in response to Cr. Se application in binary combinations, on the other hand, aided in improving seed germination (19%), root (88.3%) and shoot (18.2%) lengths. It also helped to increase the contents of sugars [total (16.3%), reducing (21.6%) and non-reducing (15.2%)], phenols (36.7%) and flavonoids (27.4%), thereby aiding in alleviating the phytotoxicity of Cr. The profiling of polyphenols and amino acids, and histological study of phenols supported the above results. The contents of essential elements also showed a significant increase, while Cr uptake was observed to decline by Se supplementation. The observations from the present study indicate that Se has the ability to influence primary and secondary metabolism, improve mineral nutrition and reduce Cr uptake in B. juncea seedlings to combat the Cr phytotoxicity and enhance the tolerance against stress.

Heavy metal pollution is one of the most perilous pollutions due to the non-degradable nature of heavy metals leading to hazardous stresses in plants. Heavy metal toxicity releases reactive oxygen species (ROS) ultimately causing oxidative stress. ROS have deleterious effects on cellular membranes, vital biomolecules and biochemical processes. Plants possess several mechanisms to combat oxidative stress and one of the mechanisms is the production of compatible solutes. These compounds are primarily involved in osmoregulation in plants. Compatible solutes are organic compounds with low molecular weights which, even at high concentrations, do not disturb cellular metabolism. However, their role in abiotic stress tolerance as an adaptive measure has gathered much interest. Several sugars (sucrose, fructose, trehalose, etc), amino acids (proline, glycine, alanine etc.) and other nitrogen and sulphur containing compounds have been found to play imperative roles in mitigation of free radicals, membrane protection and enzyme protection, apart from maintaining osmotic balance. The present chapter, therefore, aims to put forth the four major categories of compatible solutes viz. sugars and polyhydroxylated sugar alcohols, amino acids and their derivates, quaternary ammonium compounds and tertiary sulphonium compounds and understand their roles in managing heavy metal stress.

Lead (Pb) is considered one the most hazardous pollutant, and its accumulation in soil and plants is of prime concern. To understand the role of plant hormones in combating heavy metal stress, the present study was planned to assess the interactive effects of 24-epibrassinolide (EBL) (10−7 M) and salicylic acid (SA) (1 mM) in regulating growth, pigment contents, antioxidative defense response, and gene expression in Brassica juncea L. seedlings exposed to different concentrations of Pb metal (0.25, 0.50, and 0.75 mM). Reduction in root and shoot lengths, chlorophyll and carotenoid content, and non-enzymatic antioxidants like glutathione, ascorbic acid, and tocopherol in response to Pb toxicity was observed. The enzymatic antioxidants such as guaiacol peroxidase (POD), ascorbate peroxidase (APOX), glutathione peroxidase (GR), dehydroascorbate reductase (DHAR), monodehydroascorbate redductase (MDHAR), glutathione-S-transferease (GST), and glutathione peroxidase (GPOX) were lowered in response to Pb treatments. Other antioxidative enzymes including superoxide dismutase (SOD), catalase (CAT), and polyphenol oxidase (PPO) enhanced under metal stress. Co-application of EBL + SA to 0.75 mM Pb-treated seedlings resulted in improvement of root and shoot lengths, chlorophyll, and carotenoid contents. Similarly, glutathione, ascorbic acid, and tocopherol contents were also elevated. Enzymatic antioxidants were also significantly enhanced in response to pre-sowing combined treatment of both hormones. Gene expression analysis suggested elevation in expression of CAT, POD, GR, DHAR, and GST genes by application of EBL. Our results reveal that Pb metal toxicity caused adverse impact on B. juncea L. seedlings, but pre-soaking treatment with EBL and SA individually and in combination help seedlings to counter the ill effects of Pb by improving growth, contents of pigment, and modulation of antioxidative defense system. The combined application of EBL and SA was found to be more effective in ameliorating Pb stress as compared to their individual treatments.

Plants face a variety of abiotic stresses, which generate reactive oxygen species (ROS), and ultimately obstruct normal growth and development of plants. To prevent cellular damage caused by oxidative stress, plants accumulate certain compatible solutes known as osmolytes to safeguard the cellular machinery. The most common osmolytes that play crucial role in osmoregulation are proline, glycine-betaine, polyamines, and sugars. These compounds stabilize the osmotic differences between surroundings of cell and the cytosol. Besides, they also protect the plant cells from oxidative stress by inhibiting the production of harmful ROS like hydroxyl ions, superoxide ions, hydrogen peroxide, and other free radicals. The accumulation of osmolytes is further modulated by phytohormones like abscisic acid, brassinosteroids, cytokinins, ethylene, jasmonates, and salicylic acid. It is thus important to understand the mechanisms regulating the phytohormone-mediated accumulation of osmolytes in plants during abiotic stresses. In this review, we have discussed the underlying mechanisms of phytohormone-regulated osmolyte accumulation along with their various functions in plants under stress conditions.

Heavy metal (HM) toxicity is one of the leading abiotic stresses, which is very unsafe and risky for plants. With a fleeting enhancement in industrialization and urbanization due to population explosion, heavy metals inclusion into the environment has become a major concern for the environmentalists. These are most deleterious pollutants and cause phytotoxicity in plants. Heavy metal accumulation in plants results in generation of oxidative stress. Salicylic acid (SA) is an imperative endogenous plant hormone. It has a crucial role in regulation of various physiological and metabolic processes in plants. It is considered as one of the most important signaling molecule involved in both abiotic and biotic stress tolerance. Application of optimal concentrations of SA enhances plants’ tolerance to heavy metal stress by modulating levels of several metabolites including components of antioxidative defense cascade, osmolytes, secondary metabolites, and metal-chelating compounds.

Contamination of soil with toxic heavy metals is a major reason for retarded growth of crops and harmful effects on human health. Cultivation of large number of agricultural crops in contaminated soil is a major concern of environmentalist in the present times. Increased level of heavy metals can enter in to the food chain and may available for human consumption. Metal toxicity-induced oxidative stress eventually leads to refrained enzyme activities due to displacement of essential cofactors with other metal ions and blocking of functional groups such as carboxyl, histidyl and thiol, and proteins. Oxidative burst releases large quantities of reactive oxygen species (ROS) such as superoxide anion, hydrogen peroxide, hydroxyl radical, singlet oxygen, etc., which is one of the primary response of plants to heavy metal stress. Production of ROS is an inherent feature of plant cell and contributes to the process of oxidative damage leading to cell death. Its production is restricted to several cellular compartments such as mitochondria, chloroplast, and peroxisomes etc. ROS production leads to alteration of several physiological processes including degradation of enzymes, proteins, and amino acids and change in structure of cells. ROS are well described as secondary messengers in variety of cellular processes including acclimatization of cells to stress conditions. The signaling of ROS as a result of oxidative damage is regulated by several other signaling cascades which are interlinked. Their role has been studied under various stress conditions specifically heavy metals which leads to production of NO, H2O2, synthetic electrophilic compounds, lipid peroxidation molecules, etc.

Salicylic acid (SA) is one of the important plant growth regulators that has garnered focus due to its crucial role in both physiological aspects and plant defense responses. Several studies throw light on its role in regulation of various biochemical pathways that further shows its control on morphology and physiology of plants. It is involved in conferring innate immunity to plants against both biotic and abiotic stresses by influencing multiple processes. It interacts with transcription factors and thus alters gene expression. It is also instrumental in controlling the expression of PR genes through NPR1 proteins. RNA silencing mechanisms in viral infections also become activated with SA and thus provide immunity. Plants also gain systemic acquired resistance through SA that causes stimulation of long-distance mobile signaling and activation of transcriptional coactivators. The studies thus indicate that SA is involved in regulation of mechanisms responsible for plant morphology, physiology, and responses to stress.

Silver nanoparticles have a lot of ways of synthesis like physical and chemical methods; some of these methods use a lot of chemical substances and are very hazardous for humans and environment, so a novel, great, environmental friendly, cheap and easy to use world of green chemistry has been used. A number of characterization techniques such as UV-visible spectroscopy, Fourier transformation infrared spectroscopy, X-ray diffraction study and scanning electron microscopy revealed that silver nanoparticles have been used. Thus the different response of the functional groups and the difference in the peaks and UV-visible data was studied and then compared to understand and know the way these different reducing agents react to the same starting material. The green synthesis had a UV-visible peak at 446 nm while the one with chemical synthesis had a peak at 395 nm. FTIR results of silver nanoparticles synthesis by trisodium citrate (TSC) showed a peak at 1505 cm-1 which shows that the compound has a stretching of the -C=C – bond. In another case, which was done by using Sodium borohydride (NaBH4) a peak at 1695 cm-1 showed a –C=O- bond indicating stretching and a weak absorption intensity. Another peak was present which indicates a –O-H bond formation and presence which is a strong bond are found to exist. A notable peak came for synthesis by orange peel at 1517 cm-1 which represents a –C=C- bond stretching as in aromatic compounds. Another peak at 1732 cm-1 indicates the –C=O- bond. The XRD results on one of the silver sample prepared by green methods showed silver nanomaterials formed which had a average particle size of around 42 nm. FE-SEM results revealed that silver nanomaterials were formed and had a flake like appearance in one of the results. All the overall comparison showed that different modes of synthesis of silver nanomaterials and different reducing agents give same materials but with different peaks and intensities. All this data provided knowledge about the fact that an alternative method can be used to create new nanoparticles if one of the previously considered to tried method fails thus helping in extending the broadways for research.

Heavy metal detoxification in plants is a phenomenon resulting from complex interactions among interconnected physiological pathways and defense shunts leading to reactive oxygen species scavenging and subsequent protection of cellular vitals. These signaling pathways involve cross-talk between a number of antioxidant compounds including two main groups of amino acid rich metal chelators, namely the phytochelatins (PCs) and metallothioneins (MTs). This book chapter traces the mechanism of metal tolerance and detoxification strategies possessed by these biological molecules in addition to their biosynthesis, roles played and genetic aspects involved in their course of action. The isolation, characterization of PC and MT genes involved in metal compartmentalization and their successful induction in other plants is a much more recent application because this is of immense importance to the world of agronomics. Genetic validation and success for the same has been reported widely in this decade and many prominent reports have been included in the text to highlight this. Extending this vast information about the PC and MT gene pool at the proteomic level is gaining a lot of momentum currently and shall remain the future line of investigation for understanding metal resistance pathways at the cellular as well as subcellular level.

Lignin is a major carbon sink in the biosphere accounting for about 30 % of total carbon sequestered in terrestrial plants. Being the second most abundant polymer on earth, it is a complex 3-dimensional polymer which is the principal structural component of plant cell wall. The phenylpropanoid pathway is responsible for biosynthesis of a variety of products that include lignin flavonoids and hydroxycinnamic acid conjugates. The phenylpropanoid metabolism has attracted significant research attention as lignin is a limiting factor in a number of agroindustrial processes like chemical pulping, forage digestibility and the processing of lignocellulosic plant biomass to bioethanol. Further, many functions of lignins and related products make the phenylpropanoid pathway essential to the health and survival of plants by providing resistance from abiotic and biotic stresses. These polymers play crucial role in plethora of ecological and biological functions which include shaping of wood characteristics, mechanical support in plants and most importantly stress management (biotic and abiotic stresses). Since lignins act synergistically in a number of agricultural processes, viz. crop production, vigour and disease resistance, thus insights into both the biosynthetic pathway and biodegradation of lignins are of prime significance. Due to the urgent requirement of upregulation and downregulation of lignin genes, focus has been drawn on the genetic engineering of its biosynthetic pathway. This proposed book chapter lays intensive focus on abiotic stress management through lignins by drawing a comparison between the process of lignification of plants under normal conditions as opposed to plants subjected to a variety of abiotic stresses such as drought, flooding, UV rays, heat, chilling and freezing and heavy metal stress.