Role of Ni-tolerant Bacillus spp. and Althea rosea L. in the phytoremediation of Ni-contaminated soils

In our current study, four nickel-tolerant (Ni-tolerant) bacterial species viz, Bacillus thuringiensis 002, Bacillus fortis 162, Bacillus subtilis 174, and Bacillus farraginis 354, were screened using Ni-contaminated media. The screened microbes exhibited positive results for synthesis of indole acetic acid (IAA), siderophore production, and phosphate solubilization. The effects of these screened microbes on Ni mobility in the soil, root elongation, plant biomass, and Ni uptake in Althea rosea plants grown in Ni-contaminated soil (200 mg Ni kg^?1) were evaluated. Significantly higher value for water-extractable Ni (38 mg kg^?1) was observed in case of Ni-amended soils inoculated with B. subtilis 174. Similarly, B. thuringiensis 002, B. fortis 162, and B. subtilis 174 significantly enhanced growth and Ni uptake in A. rosea. The Ni uptake in the shoots and roots of B. subtilis 174-inoculated plants enhanced up to 1.7 and 1.6-fold, respectively, as compared to that in the un-inoculated control. Bacterial inoculation also significantly improved the root and shoot biomass of treated plants. The current study presents a novel approach for bacteria-assisted phytoremediation of Ni-contaminated areas.     

Role of Burkholderia cepacia CS8 in Cd-stress alleviation and phytoremediation by Catharanthus roseus

The current study was performed to assess the effect of Burkholderia cepacia CS8 on the phytoremediation of cadmium (Cd) by Catharanthus roseus grown in Cd-contaminated soil. The plants cultivated in Cd amended soil showed reduced growth, dry mass, gas-exchange capacity, and chlorophyll contents. Furthermore, the plants exhibited elevated levels of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) under Cd stress. The bacterized plants showed higher shoot length, root length; fresh and dry weight. The improved stress tolerance in inoculated plants was attributed to the reduced quantity of MDA and H₂O₂, enhanced synthesis of protein, proline, phenols, flavonoids, and improved activity of antioxidant enzymes including peroxidase, superoxide dismutase, ascorbate peroxidase, and catalase. Similarly, the 1-aminocyclopropane-1-carboxylate deaminase activity, phosphate solubilization, auxin, and siderophore production capability of B. cepacia CS8 improved growth and stress alleviation in treated plants. The bacterial inoculation enhanced the amount of water extractable Cd from soil. Furthermore, the inoculated plants showed higher bioconcentration factor and translocation factor. The current study exhibits that B. cepacia CS8 improves stress alleviation and phytoextraction potential of C. roseus plants growing under Cd stress.     

The beneficial role of potassium in Cd-induced stress alleviation and growth improvement in Gladiolus grandiflora L.

Heavy metal contaminated agricultural soils are one of the most important constraints for successful cultivation of crops. The current research was conducted to evaluate the role of potassium (K) on plant growth and amelioration of cadmium (Cd) stress in Gladiolus grandiflora under greenhouse conditions. G. grandiflora corms were sown in media contaminated with 0 (C), 50 (Cd50) and 100 (Cd100) mg Cd kg^?1 soil. The plants growing in Cd-contaminated media exhibited reduced gas exchange attributes, chlorophyll (Chl) contents, vegetative and reproductive growth as compared to control. The plants raised in Cd contaminated media showed reduced nutrition yet higher Cd contents. However, supplementation of 60 mg Kg^?1 K in treated plants (C+K, Cd50+K and Cd100+K) improved quantity of total soluble protein and proline (Pro) along with activity of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX) under Cd stress. Similarly, K supplementation reduced the level of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) in treated plants. Potassium supplemented plants exhibited better vegetative and reproductive growth. The improved stress tolerance in K supplemented plants was attributed to the reduced quantity of MDA and H₂O₂, enhanced synthesis of protein, proline, phenols, flavonides and improved activity of antioxidant enzymes. The present research supports the application of K for alleviation of Cd stress in G. grandiflora.     

Phyto-Extraction of Nickel by Linum usitatissimum in Association with Glomus intraradices

Plants show enhanced phytoremediation of heavy metal contaminated soils particularly in response to fungal inoculation. Present study was conducted to find out the influence of Nickel (Ni) toxicity on plant biomass, growth, chlorophyll content, proline production and metal accumulation by L. usitatissimum (flax) in the presence of Glomus intraradices. Flax seedlings of both inoculated with G. intraradices and non-inoculated were exposed to different concentrations i.e., 250, 350 and 500 ppm of Ni at different time intervals. Analysis of physiological parameters revealed that Ni depressed the growth and photosynthetic activity of plants. However, the inoculation of plants with arbuscular mycorrhizae (G. intraradices) partially helped in the alleviation of Ni toxicity as indicated by improved plant growth under Ni stress. Ni uptake of non- mycorrhizal flax plants was increased by 98% as compared to control conditions whereas inoculated plants showed 19% more uptake when compared with the non-inoculated plants. Mycorrhizal plants exhibited increasing capacity to remediate contaminated soils along with improved growth. Thus, AM assisted phytoremediation helps in the accumulation of Ni in plants to reclaim Ni toxic soils. Based on our findings, it can be concluded that the role of flax plants and mycorrhizal fungi is extremely important in phytoremediation.     

Interactive effect of calcium and gibberellin on nickel tolerance in relation to antioxidant systems in <Emphasis Type="Italic">Triticum aestivum</Emphasis> L.

Nickel toxicity affects many metabolic facets of plants and induces anatomical and morphological changes resulting in reduced growth and productivity. To overcome the damaging effects of nickel (Ni) stress, different strategies of the application of nutrients with plant hormones are being adopted. The present experiment was carried out to assess the growth and physiological response of wheat plant (Triticum aestivum L.) cv. Samma to pre-sowing seed treatment with GA₃ alone as well as in combination with Ca²⁺ and/or Ni stress. The pre-sowing seed treatment of Ni decreased all the growth characteristics (plant height, root length, fresh, and dry weight) as well as chlorophyll (Chl) content and enzyme carbonic anhydrase (CA: E.C. 4.2.1.1) activity. However, an escalation was recorded in malondialdehyde content and electrolyte leakage in plants raised from seed soaked with Ni alone. Moreover, all the growth parameters and physiological attributes (Chl content, proline (Pro) content, CA, peroxidase (E.C.1.11.1.7), catalase (E.C. 1.11.1.6), superoxide dismutase (E.C. 1.15.1.1), ascorbate peroxidase (E.C. 1.11.1.11), and glutathione reductase (E.C. 1.6.4.2) were enhanced in the plants developed from the seeds soaked with the combination of GA₃ (10⁻⁶ M), Ca^2+, and Ni. The present study showed that pre-sowing seed treatment of GA₃ with Ca²⁺ was more capable in mitigation of adverse effect of Ni toxicity by improving the antioxidant system and Pro accumulation.     

Ethephon mitigates nickel stress by modulating antioxidant system,glyoxalase system and proline metabolism in Indian mustard

The role of ethylene (through application of ethephon) in the regulation of nickel (Ni) stress tolerance was investigated in this study. Ethephon at concentration of 200 µl l^?1 was applied to mustard (Brassica juncea) plants grown without and with 200 mg kg^?1 soil Ni to study the increased growth traits, biochemical attributes, photosynthetic efficiency, nutrients content, activities of antioxidants such as superoxide dismutase, ascorbate peroxidase, glutathione reductase, and glutathione peroxidase, glyoxalase systems and enhanced the proline metabolism. In the absence of ethephon, Ni increased oxidative stress with a concomitant decrease in photosynthesis, growth and nutrients content. However, application of ethephon positively increased growth traits, photosynthetic parameters, nutrients content and also elevated the generation of antioxidants enzymes and glyoxalase systems, proline production to combat oxidative stress. Plants water relations and cellular homeostasis were maintained through increased photosynthetic efficiency and proline production. This signifies the role of ethylene in mediating Ni tolerance via regulating proline production and photosynthetic capacity. Ethephon can be used as an exogenous supplement on plants to confer Ni tolerance. The results can be exploited to develop tolerance in plants via gene editing technology encoding enzymes responsible for proline synthesis, antioxidant defence, glyoxalase systems and photosynthetic effectiveness.

     

Differential Effects of a <Emphasis Type="Italic">Bacillus megaterium</Emphasis> Strain on <Emphasis Type="Italic">Lactuca sativa</Emphasis> Plant Growth Depending on the Origin of the Arbuscular Mycorrhizal Fungus Coinoculated: Physiologic and Biochemical Traits

Coinoculation with plant growth–promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) has been proposed as an efficient method to increase plant growth. In this article we investigate how the interaction between three different AMF isolates (Glomus constrictum autochthonous, GcA; G. constrictum from collection, GcC; and commercial Glomus intraradices, Gi) and a Bacillus megaterium strain isolated from a Mediterranean calcareous soil affects Lactuca sativa L. plant growth. Inoculation with B. megaterium increased plant growth when in combination with two of the AMF isolates (GcA and Gi), but decreased it when in combination with GcC. At the same time, plants inoculated with the GcC fungus alone or in combination with B. megaterium (GcC+Bm) showed leaf symptoms of stress injury by accumulating proline and reducing the amount of photosynthetic pigments, whereas the opposite occurred in plants coinoculated with Gi fungus and B. megaterium (Gi+Bm). GcC+Bm leaves also presented the highest glucose-6-phosphate dehydrogenase (G6PDH) and the lowest glutamine synthetase (GS) enzymatic activities, whereas Gi+Bm leaves showed the highest GS activity. Results on these enzymatic activities are further discussed in relation to plant growth and performance.     

Effect of 28-homobrassinolide treatment on nickel toxicity in <Emphasis Type="Italic">Brassica juncea</Emphasis>

Plants of Brassica juncea L. cv. T-59 were supplied with 50 or 100 μM nickel (Ni₅₀, Ni₁₀₀) at 10 d after sowing (DAS), and sprayed with 28-homobrassinolide (HBR) at 20 DAS. The plants treated with Ni alone exhibited reduced growth, net photosynthetic rate, content of chlorophyll, and the activities of nitrate reductase (E.C.1.6.6.1) and carbonic anhydrase (E.C. 4.2.1.1) at observed 40 DAS, whereas, the contents of peroxidase (PER), catalase (CAT), and proline were increased. However, the spray of HBR partially neutralized the toxic effect of Ni on most of the parameters. Moreover, the treatment of HBR in association with either of the Ni concentration boosted the contents of PER and CAT in leaves and that of proline both in leaves and roots.     

Responses to drought stress in two poplar species originating from different altitudes

Cuttings of Populus kangdingensis and Populus cathayana, originating from high and low altitudes in the eastern Himalaya, respectively, were examined during one growing season in a greenhouse to determine their responses to drought stress (soil moisture decreased from 100 to 55 or 25 % field capacity). Compared to control plants grown under 100 % field capacity, those poplars grown under 55 and 25 % field capacity possessed lower increases in height and stem diameter, and higher contents of soluble sugars, free proline, malondialdehyde (MDA) and hydrogen peroxide, and higher activities of catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX) and glutathione reductase (GR). Compared with P. cathayana with greater leaf area, P. kangdingensis with greater root/shoot ratio exhibited lower MDA and H₂O₂ contents, higher soluble sugar and free proline contents, and higher activities of CAT, SOD, POD, APX and GR. These results suggested that P. kangdingensis was more drought tolerant than P. cathayana.     

Triacontanol Protects Mentha arvensis L. from Nickel-Instigated Repercussions by Escalating Antioxidant Machinery,Photosynthetic Efficiency and Maintaining Leaf Ultrastructure and Root Morphology

Nickel (Ni), an essential micronutrient and a prime component of the plant enzyme urease, has an indispensable role in plants. Triacontanol (TRIA) is a conspicuous plant growth regulator in agriculture, which proved advantageous in enhancing the overall production of plants. Therefore, an experiment was laid down to understand the effects of Ni toxicity on the menthol mint (Mentha arvensis L.) and its mitigation by exogenously applied TRIA. The different treatments applied to the plants were 0 (control), TRIA (10⁻⁶ M), Ni (60 mg kg⁻¹), Ni (80 mg kg⁻¹), TRIA (10⁻⁶ M) + Ni (60 mg kg⁻¹), and TRIA (10⁻⁶ M) + Ni (80 mg kg⁻¹). This work was evaluated on the basis of various growth, biochemical, physiological, yield and quality parameters. Nickel applied at 80 mg kg⁻¹ of soil exhibited maximum inhibition in the parameters studied. Application of TRIA improved all the growth parameters such as plant height, fresh and dry weights as well as herbage yield under non stress and stressed conditions. The levels of carbonic anhydrase (CA) activity, photosynthetic parameters (chlorophyll and carotenoids), and chlorophyll fluorescence of the plants were also stimulated by TRIA under Ni stress. Exogenous TRIA also displayed positive effects on the cellular antioxidant defense mechanism of Ni-affected plants as it increased the levels of proline (PRO), electrolytic leakage (EL), and activities of antioxidant enzymes, viz. superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX), therefore, restrained the triggering of the oxidative burst (reactive oxygen species) in the plant cells. Moreover, TRIA improved the overall production (in terms of yield and content) of EO in the plants and maintained the leaf ultrastructure and root morphology under Ni treatment. GC–MS analysis revealed that TRIA upregulated the level of menthone and menthyl acetate over their respective controls and under Ni-stressed condition.

     

Effects of exogenously applied salicylic acid and putrescine alone and in combination with rhizobacteria on the phytoremediation of heavy metals and chickpea growth in sandy soil

The present attempt was made to study the role of exogenously applied salicylic acid (SA) and putrescine (Put) on the phytoremediation of heavy metals and on the growth parameters of chickpea grown in sandy soil. The SA and Put were applied alone as well as in combination with plant growth promoting rhizobacteria (PGPR). The PGPRs, isolated from the rhizosphere of chickpea, were characterized on the basis of colony morphology and biochemical traits through gram staining, catalase and oxidase tests, and identified by 16S-rRNA gene sequencing as Bacillus subtilis, Bacillus thuringiensis and Bacillus megaterium. The chickpea seeds were soaked in 24 h old fresh cultures of isolates for 2–3 h prior to sowing. The growth regulators (PGRs), SA and Put (150 mg/L), were applied to the seedlings as foliar spray at three-leaf stage. The result revealed that plants treated with SA and Put alone or in combination with PGPRs, significantly enhanced the accumulation of heavy metals in plant shoot. PGPR induces Ni accumulation in sensitive variety and Pb in both the varieties, the PGR in combination augment the bioremediation effects of PGPR and both sensitive and tolerant variety showed significant accumulation of Ni, Cd, and Pb. SA was more effective in accumulating Ni and Cd whereas, significant accumulation of Pb was recorded in Put. PGPRs further augmented the PGRs induced accumulation of heavy metals and macronutrients in chickpea shoot and in rhizosphere. SA increased the proline content of tolerant variety while decreasing the lipid peroxidation and proline content of the sensitive variety but decreased the stimulating effect of PGPR in proline production. Interactive effects of PGPR and PGRs are recommended for inducing phytoremediation in chickpea plants under drought stress.     

Stress tolerance in flax plants inoculated with Bacillus and Azotobacter species under deficit irrigation

Drought stress affects not only crop growth but also its morpho-physiological and biochemical traits to reduce crop productivity. The study reported in this article was designed and implemented to determine the effects of deficit irrigation and bacterial inoculation on flax plants. For this purpose, seeds were inoculated with Bacillus amyloliquefaciens (B₁), Bacillus sp. Strain1 (B₂), and Azotobacter chroococcum (A) as plant growth promoting rhizobacteria (PGPR). The individual inoculated plants were then grown under field conditions in 2015, while individually and in combination in pots in 2016. The irrigation regimes in either experiments included 50, 75 and 100% crop water requirement. Bacterial cultures were observed to produce ammonia (except B₂), indole acetic acid and siderophores. Results showed that the PGPRs significantly mitigated the effects of water deficit. Compared with the control plants, the bacterially-inoculated plants had an enhanced relative water content, plant height, water-soluble carbohydrate and proline contents and antioxidant enzyme activities, but a decreased malondialdehyde content. B₁ exhibited greater effects on most of the traits investigated under the field conditions rather than those with moderate and severe drought stress, while application of the triple bacteria in pots had greater effects on relative water content, carbohydrate and proline contents as well as malondialdehyde. The significant differences in abiotic stress indicators in PGPR-treated plants suggest that these bacteria could be used as biofertilizers to assist plant growth and to reduce the adverse effects of deficit irrigation.     

Stimulation of Plant Growth and Drought Tolerance by Native Microorganisms (AM Fungi and Bacteria) from Dry Environments: Mechanisms Related to Bacterial Effectiveness

In this study we tested whether rhizosphere microorganisms can increase drought tolerance to plants growing under water-limitation conditions. Three indigenous bacterial strains isolated from droughted soil and identified as Pseudomonas putida, Pseudomonas sp., and Bacillus megaterium were able to stimulate plant growth under dry conditions. When the bacteria were grown in axenic culture at increasing osmotic stress caused by polyethylene glycol (PEG) levels (from 0 to 60%) they showed osmotic tolerance and only Pseudomonas sp. decreased indol acetic acid (IAA) production concomitantly with an increase of osmotic stress (PEG) in the medium. P. putida and B. megaterium exhibited the highest osmotic tolerance and both strains also showed increased proline content, involved in osmotic cellular adaptation, as much as increased osmotic stress caused by NaCl supply. These bacteria seem to have developed mechanisms to cope with drought stress. The increase in IAA production by P. putida and B. megaterium at a PEG concentration of 60% is an indication of bacterial resistance to drought. Their inoculation increased shoot and root biomass and water content under drought conditions. Bacterial IAA production under stressed conditions may explain their effectiveness in promoting plant growth and shoot water content increasing plant drought tolerance. B. megaterium was the most efficient bacteria under drought (in successive harvests) either applied alone or associated with the autochthonous arbuscular mycorrhizal fungi Glomus coronatum, Glomus constrictum or Glomus claroideum. B. megaterium colonized the rhizosphere and endorhizosphere zone. We can say, therefore, that microbial activities of adapted strains represent a positive effect on plant development under drought conditions.     

The Effect of Long-Term Cd and Ni Exposure on Seed Endophytes of Agrostis capillaris and Their Potential Application in Phytoremediation of Metal-Contaminated Soils

We examined whether long-term Cd exposure leads to beneficial changes in the cultivable endophytic bacteria present in the seeds of Agrostis capillaris. Therefore the cultivable seed endophytes of Agrostis capillaris growing on a long-term Cd/Ni-contaminated plot (Cd/Ni seeds) were compared with those originating from a non-contaminated plot (control seeds). We observed plant- and contaminant-dependent effects on the population composition between control and Cd/Ni seeds. Also differences in phenotypic characteristics were found: endophytes from Cd/Ni seeds exhibited more ACC deaminase activity and production of siderophores and IAA, while endophytes from control seeds, very surprisingly, showed more metal tolerance.

Finally, the 3 most promising seed endophytes were selected based on their metal tolerance and plant growth promoting potential, and inoculated in Agrostis capillaris seedlings. In case of non-exposed plants, inoculation resulted in a significantly improved plant growth; after inoculation of Cd-exposed plants an increased Cd uptake was achieved without affecting plant growth. This indicates that inoculation of Agrostis with its seed endophytes might be beneficial for its establishment during phytoextraction and phytostabilisation of Cd-contaminated soils.     

Changes of antioxidative enzymes,lipid peroxidation and chlorophyll content in chickpea types colonized by different <Emphasis Type="Italic">Glomus</Emphasis> species under drought stress

In order to investigate the effects of Glomus species on some physiological characteristics of two chickpea types (Pirouz cultivar of Desi type and ILC-482 of Kabuli type) under non-stress (NS) and drought stress, an experiment was conducted using a factorial arrangement based on completely randomized design with three replications. Drought stress decreased shoot and total dry weight in plants. However inoculation of plants with mycorrhiza improved these traits. Leaf chlorophyll content was decreased, but leaf proline content and guaiacol peroxidases (EC 1.11.1.7) (POD), catalase (EC 1.11.1.6) (CAT), and ascorbate peroxidase (EC 1.11.1.11) (APX) activities were increased as a result of drought stress. Drought stress had no significant effect on soluble protein content and polyphenol oxidase (EC 1.10.3.1) (PPO) enzymatic activity in chickpea plants. In general, drought stress and especially severe drought stress increased membrane lipid peroxidation (MDA) in chickpea plants, which was more evident in non-inoculated than in inoculated plants. Inoculation of chickpea by AM significantly increased POD and PPO activities compared with non-inoculated chickpea, but had no effect on CAT activity and proline content of leaves. The reaction of chickpea cultivars to inoculation by AM species and irrigation levels were different. ILC-482 showed that antioxidant enzymes activities were more and thus less MDA compared with Pirouz cultivar. In general, the most POD and PPO activities were recorded for inoculated plants with G. etunicatum and G. versiform species, and the most APX activity was observed in plants inoculated with G. intraradices.     

Effect of plant growth promoting <Emphasis Type="Italic">Pseudomonas</Emphasis> spp. on compatible solutes,antioxidant status and plant growth of maize under drought stress

Drought is one of the major abiotic stresses affecting yield of dryland crops. Rhizobacterial populations of stressed soils are adapted and tolerant to stress and can be screened for isolation of efficient stress adaptive/tolerant, plant growth promoting rhizobacterial (PGPR) strains that can be used as inoculants for crops grown in stressed ecosystems. The effect of inoculation of five drought tolerant plant growth promoting Pseudomonas spp. strains namely P. entomophila strain BV-P13, P. stutzeri strain GRFHAP-P14, P. putida strain GAP-P45, P. syringae strain GRFHYTP52, and P. monteilli strain WAPP53 on growth, osmoregulation and antioxidant status of maize seedlings under drought stress conditions was investigated. Drought stress induced by withholding irrigation had drastic effects on growth of maize seedlings. However seed bacterization of maize with Pseudomonas spp. strains improved plant biomass, relative water content, leaf water potential, root adhering soil/root tissue ratio, aggregate stability and mean weight diameter and decreased leaf water loss. The inoculated plants showed higher levels of proline, sugars, free amino acids under drought stress. However protein and starch content was reduced under drought stress conditions. Inoculation decreased electrolyte leakage compared to uninoculated seedlings under drought stress. As compared to uninoculated seedlings, inoculated seedlings showed significantly lower activities of antioxidant enzymes, ascorbate peroxidase (APX), catalase (CAT), glutathione peroxidase (GPX) under drought stress, indicating that inoculated seedlings felt less stress as compared to uninoculated seedlings. The strain GAP-P45 was found to be the best in terms of influencing growth and biochemical and physiological status of the seedlings under drought stress. The study reports the potential of rhizobacteria in alleviating drought stress effects in maize.     

Salicylic Acid Application Mitigates Oxidative Damage and Improves the Growth Performance of Barley under Drought Stress

Drought is a severe environmental constraint, causing a significant reduction in crop productivity across the world. Salicylic acid (SA) is an important plant growth regulator that helps plants cope with the adverse effects induced by various abiotic stresses. The current study investigated the potential effects of SA on drought tolerance efficacy in two barley (Hordeum vulgare) genotypes, namely BARI barley 5 and BARI barley 7. Ten-day-old barley seedlings were exposed to drought stress by maintaining 7.5% soil moisture content in the absence or presence of 0.5, 1.0 and 1.5 mM SA. Drought exposure led to severe damage to both genotypes, as indicated by phenotypic aberrations and reduction of dry biomass. On the other hand, the application of SA to drought-stressed plants protected both barley genotypes from the adverse effects of drought, which was reflected in the improvement of phenotypes and biomass production. SA supplementation improved relative water content and proline levels in drought-stressed barley genotypes, indicating the osmotic adjustment functions of SA under water-deficit conditions. Drought stress induced the accumulation of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂) and superoxide (O₂ ^•− ), and the lipid peroxidation product malondialdehyde (MDA) in the leaves of barley plants. Exogenous supply of SA reduced oxidative damage by restricting the accumulation of ROS through the stimulation of the activities of key antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GPX). Among the three-applied concentrations of SA, 0.5 mM SA exhibited better mitigating effects against drought stress considering the phenotypic performance and biochemical data. Furthermore, BARI barley 5 showed better performance under drought stress than BARI barley 7 in the presence of SA application. Collectively, our results suggest that SA played a crucial role in improving water status and antioxidant defense strategy to protect barley plants from the deleterious effects of water deficiency.     

Antioxidant activity in the leaves of <Emphasis Type="Italic">Melilotus albus</Emphasis> and <Emphasis Type="Italic">Trifolium medium</Emphasis> from man-made disturbed habitats in the Middle Urals under the influence of copper

Physiological mechanisms of adaptation to copper-induced stress in two widespread legume plants, white sweet clover (Melilotus albus Merik.) and zigzag clover (Trifolium medium L.), growing in habitats differing in the man-made pollution. An antioxidant plant defense system was activated in response to 10 mM CuSO₄, which is a stress factor. Specific biochemical features related to adaptation to soil contamination with copper were observed in tested plant species. Superoxide dismutase was activated in response to stress in both species from various habitats. M. albus from the impact zone manifested the better capacity of proline accumulation as compared with plants from less polluted habitats. T. medium plants from the impact zone contained more active peroxidase. It was suggested that plants growing for a long time under stressful conditions manifest the greater tolerance to copper ions than plants, which did not experience stress or were subjected to the milder stress.     

Co-inoculation Effects of Rhizobium sullae and Pseudomonas sp. on Growth,Antioxidant Status,and Expression Pattern of Genes Associated with Heavy Metal Tolerance and Accumulation of Cadmium in Sulla coronaria

Recently, phytoremediation assisted by soil bacteria has emerged as a potential tool to clean up the metal-contaminated/polluted environment. Three plant-growth-promoting bacteria (PGPBs): Rhizobium sullae, Pseudomonas fluorescens, and Pseudomonas sp. were found to tolerate cadmium (Cd) stress. Sulla coronaria inoculated with these PGPBs, and grown under different Cd concentrations (0, 100, and 200 µM), showed increases in dry biomass and proline content. Notable increases in different gas-exchange characteristics such as photosynthesis rate (A), transpiration rate (E), and water-use efficiency (WUE), as well as increases in nitrogen (N) and Cd accumulations were also recorded in inoculated plants compared to non-inoculated Cd stressed plants. The activities of antioxidant enzymes superoxide dismutase (SOD), guaiacol peroxidase (GPOX), catalase (CAT), and ascorbate peroxidase (APX) in S. coronaria roots increased under Cd stress after PGPB co-inoculation, suggesting that these PGPB species could be used for amelioration of stress tolerance in S. coronaria. The expression patterns of ScPCS, ScMT, ScF-box, ScGR, and ScGST in roots of S. coronaria indicated that these genes are differentially expressed under Cd treatments, suggesting their possible roles in Cd and heavy metal stress responses. The results indicate that co-inoculation with R. sullae and Pseudomonas sp. could alleviate Cd toxicity in S. coronaria. In the present study, the obtained data suggest that the application of PGPBs could be a promising strategy for enhancing the phytostabilization efficiency of Cd-contaminated soils.

     

Pongamia pinnata inoculated with Bradyrhizobium liaoningense PZHK1 shows potential for phytoremediation of mine tailings

Mine tailings contain high concentrations of metal contaminants and only little nutrients, making the tailings barren for decades after the mining has been terminated. Effective phytoremediation of mine tailings calls for deep-rooted, metal accumulating, and soil fertility increasing plants with tolerance against harsh environmental conditions. We assessed the potential of the biofuel leguminous tree Pongamia pinnata inoculated with plant growth promoting rhizobia to remediate iron–vanadium–titanium oxide (V–Ti magnetite) mine tailing soil by pot experiment and in situ remediation test. A metal tolerant rhizobia strain PZHK1 was isolated from the tailing soil and identified as Bradyrhizobium liaoningense by phylogenetic analysis. Inoculation with PZHK1 increased the growth of P. pinnata both in V–Ti magnetite mine tailings and in Ni-contaminated soil. Furthermore, inoculation increased the metal accumulation capacity and superoxide dismutase activity of P. pinnata. The concentrations of Ni accumulated by inoculated plants were higher than the hyperaccumulator threshold. Inoculated P. pinnata accumulated high concentration of Fe, far exceeding the upper limit (1000 mg kg⁻¹) of Fe in plant tissue. In summary, P. pinnata–B. liaoningense PZHK1 symbiosis showed potential to be applied as an effective phytoremediation technology for mine tailings and to produce biofuel feedstock on the marginal land.