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Role of soil associated Exiguobacterium in reducing arsenic toxicity and promoting plant growth in Vigna radiata
... Further, root colonization was determined with electron microscopic examination. Root samples (2 cm) were excised from the inoculated and non-inoculated plants and were processed for Field Emission Gun-Scanning Electron Microscopy (FEG-SEM, JEOL JSM-7600F, Japan), as described by Kim and Kremer (2005) and Pandey and Bhatt (2016). ...
... The As-resistant bacteria have been demonstrated to assist in the removal of As in the early phase of the treatment because aerobic conditions support the growth and survival of these organisms (Titah et al., 2018). Many of the known metal resistant bacterial species which shown successful removal of As from their surrounding environment (Pandey and Bhatt, 2016;Aw et al., 2020). ...
... The light micrographs have uncovered severe damage in the roots as compared to the shoots. This is because accumulation of As occurs more in the roots, owing to its direct contact with the soil and, therefore shows a more inhibitory effect on the roots than that determined on the shoots (Pandey and Bhatt, 2016). Because of the intense toxic behaviour of As(III), the level of impairment were more prominent in As(III) treated plants, despite the fact that the same concentration of both the As oxyanions were used in the study (Coelho et al., 2020). ...
Arsenic (As) is a toxic metalloid which pollutes soil and water, and negatively affects the growth and development of plants at different levels. This study investigated the effects of As-resistant and plant growth promoting (PGP) bacterial consortia on the germination and growth attributes of two cultivars (Swarna and MTU 1010) of rice (Oryza sativa L.) under As-flooded environment. The consortium consisted of five bacterial strains; Bacillus nealsonii strain ARP2, Pseudomonas nitritireducens strain ARP3, Exiguobacterium aestuarii strain ARRP3, Bacillus tequilensis strain ART2 and Microbacterium paraoxydans strain ADT5, which were isolated from different regions of Chhattisgarh, India. Soils inoculated with the bacterial consortia and supplemented with As(V)/As(III) were used to grow rice seeds under in vitro conditions. The results ascertained that the seedlings inoculated with the bacterial consortia grew well even in the presence of As, which was marked by increased shoot and root length, biomass, and total chlorophyll content. Further, inoculation of bacterial consortia reduced the oxidative stress to a significant level by up-regulating the expressions of protective genes encoding antioxidant enzymes. This consortium could decrease the As accumulation in plants upon successful colonization in the rhizosphere, suggesting possible exploitation of it for enhanced growth of plants and in the remediation of As-contaminated soils.
... Biosorption of metal ions is an example of the wide variety of potential and actual applications of the bioremediation technique in polluted water treatments (Pandey and Bhatt 2016b). The use of microorganisms for the biosorption of As ions from water is an extremely efficient process. ...
... At the end of the experiment (168 h), E. indicum successfully removed approximately 74.83 ± 1.2% of As(V) from the medium, while only 61.77 ± 0.8% removal was achieved in the case of As(III) (Fig. 4a). For this genus, as in previous studies, removal percentages of 99% for As(V) and 90-99% for As(III) (Pandey and Bhatt 2016a;2016b) have been reported, depending on the method developed (Table 4). Bacteria of the genus Exiguobacterium are adapted to use multiple strategies to maintain themselves under the stress conditions they face in their environments, namely the high toxicity generated by As (Andreasen et al. 2018). ...
... For the cases of the bacteria belonging to the Actinobacteria phylum, R. gordoniae and M. hydrocarbonoxydans were slightly more efficient in removing As(III), with values of 81.6 ± 1.1 and 79.98 ± 2.2%, whereas for As(V) the values were 77.21 ± 0.9 and 68.93 ± 2.8%, respectively ( Corsini et al. 2014;Pandey and Bhatt 2016b;Prasad et al. 2011), the results obtained fell within the reported range, going from 48.43% to greater than 90% (Table 4). The possible explanation for the removal capacity of As(III) and As(V) by R. gordionae and M. hydrocarbonoxydans, observed in this study, is that they can present systems capable of reducing and oxidizing As. ...
The study of arsenic (As)-resistant microorganisms with high As removal capacity is fundamental for the development of economically sustainable technologies used for the treatment of water contaminated with metalloid. In the current study, four bacterial strains were isolated from As-contaminated water samples of the Xichu region, Mexico. Based on 16S rRNA gene sequencing and phylogenetic analysis of the isolated strains, Rhodococcus gordoniae, Microbacterium hydrocarbonoxydans, Exiguobacterium indicum, and Pseudomonas kribbensis were identified as potential As removal strains. R. gordoniae shows the highest growth capacity in both As(III) and As(V). R. gordoniae, M. hydrocarbonoxydans, and E. indicum removed approximately 81.6, 79.9, and 61.7% of As(III), as well as 77.2, 68.9, and 74.8% of As(V), respectively. P. kribbensis removed only about 80.2% of As(V). This study contributes to the possible detoxification mechanisms employed by these bacteria. Such insight could be crucial in the successful implementation of in situ bioremediation programs using these little-known bacteria.
... Improved root length, shoot length, biomass of roots and shoots + Improved chlorophyll content + Increased SOD, CAT, APX, GR, and GPX activity+ Decreased As(V) and As(III) accumulation in root and shoot Pandey and Bhatt, 2016 Brevundimonas diminuta NBRI012 ...
... However, when seedlings were grown in presence of bioinoculant Exiguobacterium, the enzymatic activities were reduced and the values were found comparable to that of control sets (without stress and bioinoculant). This result is indicative of bacteria-mediated reduction of oxidative damage due to attenuation of ROS production (Pandey and Bhatt, 2016). SOD activity in rice also showed a similar pattern when exposed to As(V) (50 μg mL − 1 ), under influence of bacteria belongs to Bacillus cereus cluster (Pandey et al., 2013). ...
... MDA content in Vigna radiata was found to be increased by 50.2 % and 82.8 % in shoots upon As(V) and As(III) treatment respectively. Whereas it was found to be increased by 42.85 % and 84.52 % in roots upon As(V) and As(III) treatment respectively (Pandey and Bhatt, 2016). As induced MDA production has also been reported in other plants like rice, Brassica napus, chickpea, Vigna mungo, etc. by various authors (Adhikary et al., 2019;Ahmad et al., 2020;Farooq et al., 2016;Jung et al., 2019;Singh et al., 2016). ...
The toxic metalloid arsenic (As), is a major pollutant of soil and water, imposing severe health concerns on human lives. It enters the food chain mainly through As-contaminated crops. The uptake, translocation and accumulation of As in plant tissue are often controlled by certain soil-inhabiting microbial communities. Among them, indigenous, free-living As-resistant plant growth-promoting rhizobacteria (PGPR) plays a pivotal role in As-immobilization. Besides, the plant’s inability to withstand As after a threshold level is actively managed by these PGPR increasing As-tolerance in host plants by a synergistic plant-microbe interaction. The dual functionality of As-resistant PGPR i.e., phytostimulation and minimization of As-induced phytotoxic damages are one of the main focal points of this review article. It is known that such PGPR having the functional arsenic resistant genes (in ars operon) including As-transporters, As-transforming genes contributed to the As accumulation and detoxification/transformation respectively. Apart from assisting in nutrient acquisition and modulating phytohormone levels, As-resistant PGPR also influences the antioxidative defense system in plants by maneuvering multiple enzymatic and non-enzymatic antioxidants. Furthermore, they are effective in reducing membrane damage and electrolyte leakage in plant cells. As-induced photosynthetic damage is also found to be salvaged by As-resistant PGPR. Briefly, the eco-physiological, biochemical and molecular mechanisms of As-resistant PGPR are thus elaborated here with regard to the As-exposed crops.
... Bioremediation technique, which involves the use of microbes to detoxify toxic heavy metals or metalloids, has been receiving increasing attention in recent times as a means of cleaning up a polluted environment. Biosorption of metal ions is an example of a wide variety of potential and actual applications of the bioremediation technique in polluted water treatments (Pandey & Bhatt, 2016b). The use of microorganisms for the biosorption of As ions from water is an extremely e cient process. ...
... for As (III) (Pandey & Bhatt, 2016a, 2016b have been reported, depending on the method developed, Table 5. Bacteria of the genus Exiguobacterium are adapted to use multiple strategies that allow them to maintain themselves under the stress conditions they face in their environments, namely the high toxicity generated by ...
... For both genders, in comparison with previous work (A. Pandey & Bhatt, 2016b;Prasad et al., 2011), the results obtained were within the reported range of 48.43% to greater than 90%, Table 5. The possible explanation between the removal capacity of As(III) and As(V) by R. gordionae and M. hydrocarbonoxydans, observed in this study, is that they can present systems capable of both reducing and oxidizing As. ...
The study of arsenic resistant microorganisms with high arsenic removal capacity is fundamental for the development of economically sustainable technologies for the treatment of water contaminated with this metalloid. In this work, the isolation and identification of 4 native strains was carried out.: Rhodococcus gordoniae, Microbacterium hydrocarbonoxydans, Exiguobacterium indicum and Pseudomonas kribbensis . R.gordoniae was identified as the bacterium with the highest growth capacity in both As(III) and As(V). E.indicum removed about 74.8% of Arsenate, As(V), and 61.7% of Arsenite , As(III), while R.gordoniae removed about 81.6 % of As(III), and 77.2% of As(V), while that M.hydrocarbonoxydans was able to remove up to 79.9% of As(III) and 68.9% of As(V). Finally, it was observed that P. kribbensis removed about 80.2% of As(V). This study also contributes to the possible detoxification mechanisms employed by these bacteria, the knowledge of which could be crucial in the successful implementation of in situ bioremediation programs.
... Identification reveals the AS18 strain to be a species of Pantoea which has sustained IAA production under As-stress and As(V) reducing potential Table 2), siderophore production, ammonia production, and HCN production (Supplementary Table 3; Fig. 1c-e). As per previous reports, many rhizospheric soil bacteria that possess heavy metal-resistant property were also found to promote plant growth (Pandey and Bhatt 2016;Pramanik et al. 2017;Das and Sarkar 2018;Ghosh et al. 2018;Pramanik et al. 2018;Rahman et al. 2020;Xiao et al. 2020;Pandey et al. 2020;Vezza et al. 2020) as found in this study. Therefore, the strain Pantoea dispersa isolated during this work should be further exploited for bioremediation and plant growth promotion in salt-and metalloid-contaminated agricultural fields. ...
... Here, PGPR act as a phytostimulator and assist plants to draw nutrition from contaminated soils thereby promoting their overall growth and development. This result is positively correlated with other works also (Das et al. 2016;Pandey and Bhatt 2016;Ghosh et al. 2018;Mallick et al. 2018;and Xiao et al. 2020). ...
... The reduction in lipid peroxidation levels further helps to maintain cellular integrity. As-induced membrane damage due to chain-like peroxidation of unsaturated fatty acids in the membrane and its reclamination using bacterial inoculant has also been reported by various authors (Pandey and Bhatt 2016;Singh et al. 2016;Das and Sarkar 2018;Ghosh et al. 2018). Besides, catalase activity was found significantly (p < 0.05) increased with the comparison to both EC50 and control (Fig. 6g). ...
Population detonation and rapid industrialization are the major factors behind the reduction in cultivable land that affects crop production seriously. This situation is further being deteriorated due to the negative effects of abiotic stresses. Under such conditions, plant growth-promoting rhizobacteria (PGPR) are found to improve crop production which is essential for sustainable agriculture. This study is focused on the isolation of potent arsenic (As)-resistant PGPR from the agricultural land of West Bengal, India, and its application to reduce As translocation in rice seedlings. After screening, an As-resistant PGPR strain AS18 was identified by phenotypic characters and 16S rDNA sequence-based homology as Pantoea dispersa. This strain displayed nitrogen fixation, phosphate solubilization, 1-aminocyclopropane-1-carboxylic acid deaminase (ACCD) activity, indole-3-acetic acid (IAA) production, in addition to As (III) resistance up to 3750 μg/mL. The As removal efficiency of this strain was up to 93.12% from the culture medium as evidenced by AAS. The bioaccumulation property of AS18 strain was further validated by TEM-EDAX-XRD-XRF-FTIR studies. This strain showed significant morpho-biochemical improvements including antioxidant enzymatic activities and As-minimization in plant (rice) cells. Thus, being an As-resistant potent PGPR, AS18 strain is expected to be applied in As-spiked agricultural fields for bioremediation and phytostimulation.
... By colonising the root surface, it also safeguarded the plants from the harmful effects of arsenic by limiting its uptake and transfer. Additionally, the presence of bacteria significantly reduced arsenic-induced oxidative stress (Pandey & Bhatt, 2016) [62] . Irrigation water polluted with arsenic and related agricultural soil included bacteria that promoted plant growth and were resistant to arsenic. ...
... By colonising the root surface, it also safeguarded the plants from the harmful effects of arsenic by limiting its uptake and transfer. Additionally, the presence of bacteria significantly reduced arsenic-induced oxidative stress (Pandey & Bhatt, 2016) [62] . Irrigation water polluted with arsenic and related agricultural soil included bacteria that promoted plant growth and were resistant to arsenic. ...
... In the study conducted by Armendariz et al. [31], As (25-200 µM)-treated soybean roots were characterized by broken cells in the outer layer, a reduction in the cortex area (due to decreased cell size but not the number of cell layers) and dark deposits in cortex cells. Pandey and Bhatt [66] reported that As (200-1000 mgL −1 ) exposure caused structural alterations in mungbean roots, such as changes in the size, shape, color, and arrangement of cortical parenchyma cells, breakdown of cortex and endodermal cells, damaged vascular bundle, and distorted pericycle. In an earlier study, Sresty and Rao [67] noted that Ni (0.5 and 1.5 mM) and Zn (2.5 and 7.5 mM) toxicity cause disintegration of cell organelles, disruption of membranes, condensation of chromatin material and increase in the number of nucleoli in the main root of pigeon pea. ...
... Increased Zn concentrations (1, 200, 600 and 1200 mM) have been found to decrease stem cortex width of common bean plants stepwise, as well as the number of cortical cell rows and vascular bundles [30]. According to Pandey and Bhatt [66], As (200-1000 mg L −1 )treated mungbean stems exhibited distorted epidermis, blackening of cortex cell walls, large pith, decreased and damaged vascular bundles, as well as decreased intercellular spaces. Conversely, Talukdar [60] recorded intact vascular bundle and cortex in stems, leaves, and petioles of As (50 µM) treated bean plants, whereas Tripathi et al. [70] noted that stem cortex region in chickpea was most severely affected by As (100 mg kg −1 ) stress. ...
Rapid industrialization, urbanization, and mine tailings runoff are the main sources of heavy metal contamination of agricultural land, which has become one of the major constraints to crop growth and productivity. Finding appropriate solutions to protect plants and agricultural land from heavy metal pollution/harmful effects is important for sustainable development. Phytoremediation and plant growth-promoting rhizobacteria (PGPR) are promising methods for this purpose, which both heavily rely on an appropriate understanding of the anatomical structure of plants. Specialized anatomical features, such as those of epidermis and endodermis and changes in the root vascular tissue, are often associated with heavy metal tolerance in legumes. This review emphasizes the uptake and transport of heavy metals by legume plants that can be used to enhance soil detoxification by phytoremediation processes. Moreover, the review also focuses on the role of rhizospheric organisms in the facilitation of heavy metal uptake, the various mechanisms of enhancing the availability of heavy metals in the rhizosphere, the genetic diversity, and the microbial genera involved in these processes. The information presented here can be exploited for improving the growth and productivity of legume plants in metal-prone soils.
... In addition, protein's activity is affected following amino acids oxidation and free carbonyl groups formation (Abbas et al., 2018). Unlike antioxidant enzymes like superoxide dismutase (SOD) or catalase (CAT), whose concentration increases as a response to trace element stress, MDA concentration decreases when the toxicity caused by trace element induced ROS is decreased (Pandey and Bhatt, 2016;Pramanik et al., 2017Pramanik et al., , 2018. For that reason, quantifying MDA or oxidized proteins can be considered as a useful As toxicity biomarker (Dave et al., 2013). ...
... The affected proteins are normally chaperons, Krebs cycle enzymes and ROS detoxifiers (Srivastava et al., 2014). In this sense, antioxidant enzymes like SOD and CAT play a role in ROS scavenging and inactivation and, therefore, their formation is normally promoted in the presence of trace element induced oxidative stress (Pandey and Bhatt, 2016;Pramanik et al., 2017Pramanik et al., , 2018Sharma et al., 2012). Nevertheless, thiol rich peptides are major targets of ROS attack. ...
Arsenic (As) uptake by plants is mainly carried out as arsenate (As(V)), whose chemical analogy with phosphate is largely responsible for its elevated toxicity. Arsenate is known to stimulate reactive oxygen species (ROS) formation in plants that provoke oxidative stress. This manuscript reports the results of a hydroponics study using rice (Oryza sativa L.) seedlings as a test plant, where the effects of increasing arsenate concentrations (0–10 mg L⁻¹) on both lipid and protein oxidation, as well as As accumulation and speciation in plant roots and shoots were examined. Plant yield was negatively affected by increasing As concentration. Accumulation in plant roots was higher than in shoots at low arsenate doses (0.5–2.5 mg L⁻¹), while root to shoot transport was drastically enhanced at the highest doses (5 and 10 mg L⁻¹). Moreover, As(V) was the dominating species in the shoots and As(III) in the roots. Rice leaves in the 10 mg As L⁻¹ treatment showed the highest lipid peroxidation damage (malondialdehyde concentration), whilst protein oxidation was not remarkably influenced by As dose. Lipid peroxidation seems to be therefore conditioned by As accumulation in rice plants, particularly by the presence of high As(V) concentrations in the aerial part of the plants as a consequence of unregulated translocation from roots to shoots above a threshold concentration (1.25–2.5 mg L⁻¹) in the growing media. These results provide relevant information regarding As(V) toxic concentrations for rice plants, highlight the importance of major As species analysis in plant tissues regarding As toxicity and contribute to better understand plants response to elevated As concentrations in the growing media.
... Those include a decrease in the diameter of xylem vessels in the leaves of soybean exposed to both AsIII and AsV (Vezza et al. 2018) which agrees with the reduction in the width of midrib and the adjoining minor veins under different AsV concentrations studied through X-ray phase contrast images (Fatima et al. 2021). Other legume species, Vigna radiata root and stem, showed blackening of the cortex cell walls, decreased and damaged vascular bundles together with a decrease in the number of root hairs (Pandey et al. 2016), which also happened in Phaseolus vulgaris plants with more pronounced effect on the roots than the shoots (Talukdar et al. 2013). All these findings may be responsible for the inhibition of the apoplastic water movement through conducting vessels as well as in the tissue that surrounds them while the protoplastic water flow occurs predominantly through aquaporins and Table 3 As-induced structural modifications related to water status in legumes Content courtesy of Springer Nature, terms of use apply. ...
Grain and forage legumes have the potential to be a key component of climate-smart agriculture, even under arsenic (As) stress. Recent publications seem to indicate that agricultural practices with legumes can help in climate change mitigation and simultaneously in remediation of As contaminated areas. The legume-rhizobia symbiosis is one of the most beneficial interactions in the agricultural environment. First, since this symbiosis can improve soil structure, fertility and moisture retention through the incorporation of nitrogen, utilizing the biological nitrogen fixation (BNF) process, legumes have unique advantages to be used in crop rotation and intercropping schemes in pursuit of sustainability. On the other hand, legumes are able to grow in unfavorable environments, like those contaminated with toxic metal(loids). Among them, As is one of the most worldwide distributed non-essential metalloids, that induces different phytotoxic effects on plants and bacteria depending on its speciation, redox state, concentration, plant species, exposure time and repeated exposition. Recently, an interesting link between As and plant water status has been established and this aspect is particularly deepened here. Furthermore, inoculation of legumes with As-resistant rhizobia and/or other As-resistant plant growth promoting bacteria (PGPB) is increasingly being exploited as an As mitigation strategy. In this review, we provide a comprehensive description of the main effects of As on grain and forage legumes with a focus on water status changes and recent advances related with the potential of legume-rhizobia symbiosis under As stress in the climate-smart agriculture.
... The use of the metal-tolerant strain B. megaterium reduced the translocation of the heavy metal Ni [104]. Arsenic-resistant Exiguobacterium by colonization of root surfaces caused decreased arsenic uptake in V. radiata plants [105]. According to a previous study [106], the production of indole-3-acetic acid and siderophores, as well as biofilm production, are adaptation strategies of bacteria to cope with heavy metal stress. ...
The presence of toxic heavy metals and dyes in textile wastewater is a serious problem contaminating vegetables by irrigation. This contaminated food upon consumption undermines human health and is lethal for human life. The endophytic bacteria have the ability to degrade textile dyes and remediate heavy metals. The purpose of the present investigation was to evaluate useful concentration levels of textile wastewater (TWW) for irrigation in combination with the endophytic bacterium Enterobacter cloacae ZA14 to remediate heavy metals for improving growth of the tomato (Solanum lycopersicum) plant. The tomato seedlings showed inhibited germination (52%); suppressed root length (55%) and shoot length (53%); declined RWC (47%); lowest CSI (34%); reduced MSI (32%); increased accumulation of heavy metals Cr, Pb, and Cd in roots and shoots; with decreased metal tolerance index; and rise in production of total thiols (57%) at use of 100% TWW without bacterial application. On the contrary, the supplementation of endophytic bacterium ZA14 showed improved germination (100%), a decline of 3 and 5% in root and shoot length respectively, increased CSI (13%), decrease in MSI (6%), reduced bioaccumulation of Cr (root 30 and shoot 56%), Pb (root 58 and shoot 65%), and Cd (root 21 and shoot 58%), total thiols (76%), when irrigated with 25% TWW. Hence, it is concluded that the irrigation with 25% TWW, along with the application of Enterobacter cloacae ZA14, may improve the growth of tomato by mitigating the phytotoxicity of dyes and heavy metals from textile wastewater.
... Number of As resistant microbes (∼180) have been characterized for different As biotransformation potential based on their ability to change the colour of KMnO 4 (Table S2). Arsenic resistant microbes are known to detoxify different forms of As (Mallick et al., 2018;Pandey and Bhatt, 2016). Similarly, in present study As biotransformation was evident with retain of pink colour by Citrobacter sp. ...
Arsenic (As) has become natural health hazard for millions of people across the world due to its distribution in the food chain. Naturally, it is present in different oxidative states of inorganic [As(V) and As(III)] and organic (DMA, MMA and TMA) forms. Among different mitigation approaches, microbe mediated mitigation of As toxicity is an effective and eco-friendly approach. The present study involves the characterization of bacterial strains containing arsenite methyltransferase (Pseudomonas oleovorans, B4.10); arsenate reductase (Sphingobacterium puteale, B4.22) and arsenite oxidase (Citrobacter sp., B5.12) activity with plant growth promoting (PGP) traits. Efficient reduction of grain As content by 61 % was observed due to inoculation of methyltransferase containing B4.10 as compared to B4.22 (47 %) and B5.12 (49 %). Reduced bioaccumulation of As in root (0.339) and shoot (0.166) in presence of B4.10 was found to be inversely related with translocation factor for Mn (3.28), Fe (0.073), and Se (1.82). Bioaccumulation of these micro elements was found to be associated with the modulated expression of different mineral transporters (OsIRT2, OsFRO2, OsTOM1, OsSultr4;1, and OsZIP2) in rice shoot. Improved dehydrogenase (407 %), and β-glucosidase (97 %) activity in presence of P. oleovorans (B4.10) as compared to arsenate reductase (198 and 50 %), and arsenite oxidase (134 and 69 %) containing bacteria was also observed. Our finding confers the potential of methyltransferase positive P. oleovorans (B4.10) for As stress amelioration. Reduced grain As uptake was found to be mediated by improved plant growth and nutrient uptake associated with enhanced soil microbial activity.
... Similarly, there is evidence that organic pollutants and excessive amounts of nutrients can significantly alter the structure of the rhizosphere microbial community (Gałązka et al., 2018;Parajuli et al., 2017). Accordingly, the physical characteristics of aged refuse and its enriched microbial communities that can contribute to the degradation of pollutants or reduction of pollutant toxicity make it a potential green resource that can be applied in the restoration of degraded soil and water Madanan et al., 2021;Pandey & Bhatt, 2016;Sathya et al., 2020). Moreover, aged refuse has potential utility as a source of cultivable soil, with different grades being used to enhance the quality of nutrient-poor soils (G. ...
Aged refuse, also named mineralized refuse, refers to the garbage that has been buried in landfills for many years, has basically stabilized and can be mined and utilized. In this study, we examined the effects of different mass ratios of aged refuse on Tagetes patula and rhizosphere microbes. Compared to growth in ordinary soil, growth in aged refuse or mixed soil with aged refuse significantly increased chlorophyll content, activities of superoxide dismutase, catalase, and peroxidase, while decreased malondialdehyde levels and protein carbonyl content in leaf tissue of Tagetes patula. Aged refuse was enriched in a variety of rhizosphere microbes that contribute to pollutant degradation, although microbial diversity was found to be relatively low. Bacterial genera such as Ferruginibacter, Hymenobacter, unclassified_Gemmataceae, Longimicrobium, Tychonema CCAP 1459‐11B, Gemmatirosa, and Rubellimicrobium were depleted in soil:aged refuse groups compared with ordinary soil. Correspondingly, bacterial genera such as Emticicia, Caedibacter, Anaerosalibacter, Tumebacillus, Patulibacter, Oceanotoga, Dyadobacter, Chloroflexus, and Acidobacteria bacterium SCN 69‐37, Polycyclovorans, were enriched in soil:aged refuse groups compared with ordinary soil. Furthermore, rhizosphere microbe functions changed markedly following the addition of aged refuse. These findings indicate that aged refuse may represent a source of environmental stress for plants and modifies the dominant bacterial composition of rhizosphere microbes. There were underlying close associations among pollutants, plant physiological stress responses, and rhizosphere microbial community composition. With respect to identifying potential approaches to recycling aged refuse, it will be necessary to focus on selecting optimal mass ratios of aged refuse and ordinary soil to control contaminant exposure.
... Meanwhile, some bacteria candidates are used as plant growth-promoting rhizobacteria based on their abilities of salt stress alleviation, production of indole acetic acid and acetoin, and siderophore [43]. Exiguobacterium spp can promote rhizosphere growth and decompose organic pollution [44]. Thus, the water flow increases the bacteria community diversity of the V. natans epiphytic biofilm conducive the increased DO concentration, decreased turbidity and the good growth of V. natans. ...
This paper investigates the effects of water flow on the growth and physiological indicators of the submerged macrophyte, Vallisneria natans, and the bacteria and algae community composition on its epiphytic biofilm-covered leaves. The authors set up a simulated flowing water laboratory experiment testing high nitrogen (N) and phosphorus (P) concentrations. Total chlorophyll and dissolved oxygen (DO) was significantly enhanced, and turbidity was reduced, thereby accelerating the growth of V. natans. These experiments were compared to another set of observations on a static group. The accumulation of malonaldehyde (MDA) in the dynamic groups was significantly higher than that in the static group. As an antioxidant stress response, the total superoxide dismutase (T-SOD) was also induced in plants exposed to nutrient-rich flowing water. The results of 16S rRNA high-throughput sequencing analyses showed that the water flow increased the bacteria community diversity of biofilm-producing bacteria with N and P removing bacteria, carbon cycle bacteria, and plant growth-promoting rhizobacteria on the epiphytic biofilm. This research determined that water flow alleviates the adverse effects of eutrophication when V. natans grows in water containing high N and P concentrations. Water flow also inhibits the growth of cyanobacteria (also referred to as blue-green algae) in epiphytic biofilm. The ecological factor of water flow, such as water disturbance and aeration measures, could alleviate the adverse effect of eutrophic water by providing a new way to restore submerged macrophytes, such as V. natans, in eutrophic water.
... The ex situ method is, however, cost-effective, exhaustive, and dangerous for the environment and hence is generally not considered for remediation of polluted environment. Among naturally abundant resources, biosensor soil microbiomes, especially bacteria (Esertas et al., 2020;Afzal et al., 2017), fungi (Liaquat et al., 2020;Kumar et al., 2019), algae (Bordoloi et al., 2020;Salama et al., 2019), and yeasts (Sun et al., 2020), have been considered as the potential organisms for detoxification of pollutants and promoting the crop production under stressed soils Pandey and Bhatt, 2016). Despite the fact that numerous bacterial communities with metal resistance abilities have been reported, yet there is a growing need to discover novel biofertilizer that can facilitate the growth and yield of plants in contaminated soils. ...
Bioremediation among many cleanup technologies, considered an attractive and affordable remediation technology with no side effects on agroenvironment, is adopted to circumvent polluted soils making ecologically disturbed soils cultivable again. Soil microbes spanning different genera and groups evade the toxicity of various environmental contaminants like heavy metals, pesticides, and hydrocarbons. Biofertilizer organisms, among soil microbiomes, have been used in the management of abiotic and biotic stresses and as a formulation to optimize yields and quality of food crops. The exploitation of soil microbes both as pollution alleviating agents (bioremediation) and as crop stimulants (biofertilizers) has provided solutions to both the challenges of environmental stresses and expensive chemical fertilizers. Considering the impactful role of biofertilizers in soil amelioration and plant growth promotion, the priority of current research has been directed toward finding unexplored microbes with dual features as bioremediating materials and as biofertilizers. Despite the growing interest in biofertilizer-based remediation technology, the full potential of this technology has not yet been realized. Recent developments in bacterial biofertilizer (especially nitrogen and phosphate biofertilizers)-based bioremediation of polluted soils and sustainable crop production are reviewed. Here, the microbial formulations, biofertilizer-based strategies for stress management, and their prospects for sustainable crop production are surveyed and presented. Collectively, the information provided herein is desirable to fully explore the bioremediation potential of bacterial biofertilizers and is likely to generate interest for adoption and application of this microbiological technology for remediation of contaminated soils vis-à-vis crop production under both conventional and stressful conditions to satisfy global “food and feed” demands.
... However, persistent exposure to Cd and other abiotic stresses can transmute the efficient endogenous mechanisms plants use for reducing oxidative damage . Several prior studies have reported multiple oxidative stress-reducing PGPR strains that have reduced oxidative damage in plants (Pandey & Bhatt, 2016). In the present study, all the antioxidant enzymes showed a significant increase in activity in Cd resistant PGPB inoculated plants as compared to Cd stress thus suggesting that PGPB inoculation reduced the oxidative stress of the rice plants. ...
Heavy metal resistant bacteria are of great importance because they play a crucial role in bioremediation. In the present study, 11 bacterial strains isolated from industrial waste were screened under different concentrations of cadmium (Cd) (100 µM and 200 µM). Among 11 strains, the Cd tolerant Bacillus cereus (S 6 D 1–105 ) strain was selected for in vitro and in vivo studies. B. cereus was able to solubilize potassium, and phosphate as well as produce protease and siderophores during plate essays. Moreover, we observed the response of hydroponically grown rice plants, inoculated with B. cereus which was able to promote plant growth, by increasing plant biomass, chlorophyll contents, relative water content, different antioxidant enzymatic activity such as catalase, superoxide dismutase, ascorbate peroxidase, polyphenol oxidase and phenylalanine ammonia-lyase and reducing malondialdehyde content in both roots and leaves of rice plants under Cd stress. Our results showed that the B. cereus can be used as a biofertilizer which might be beneficial for rice cultivation in Cd contaminated soils.
... Previous studies suggested that Z. mays when contaminated with A. chroococcum bacterium lowered Cu and Pb accumulation in plant parts which is most likely due to the synthesis of various metabolites, protons and exudates that act as metal chelators and immobilize [64]. Moreover, it was suggested that Bacillus megaterium, a metal tolerant strain, decreases the Ni translocation [65], and As-resistance Exiguobacterium decrease as translocation in Vigna radiata plants by accumulation at the root parts [66]. It has been observed that Acinetobacter lwofi promotes growth and reduces uptake in V. radiata [67]. ...
... Previously, some researchers reported about plant growth-promoting rhizobacteria (PGPR) significantly regulated plant antioxidants enzymes activity. About 100 mg kg −1 of both As(III) and As(V) treatments-induced higher enzymatic antioxidants responses were reduced significantly in V. radiata [174]. Incubation of Asstressed V. radiata with Exiguobacterium showed lower activity of SOD, CAT, APX and GPX near to control treatment thus resulted bacteria-mediated suppression of oxidative stress (as lessening ROS generation. ...
The non-essential metalloid arsenic (As) is widely distributed in soil and underground water of many countries. Arsenic contamination is a concern because it creates threat to food security in terms of crop productivity and food safety. Plants exposed to As show morpho-physiological, growth and developmental disorder which altogether result in loss of productivity. At physiological level, As-induced altered biochemistry in chloroplast, mitochondria, peroxisome, endoplasmic reticulum, cell wall, plasma membrane causes reactive oxygen species (ROS) overgeneration which damage cell through disintegrating the structure of lipids, proteins, and DNA. Therefore, plants tolerance to ROS-induced oxidative stress is a vital strategy for enhancing As tolerance in plants. Plants having enhanced antioxidant defense system show greater tolerance to As toxicity. Depending upon plant diversity (As hyperaccumulator/non-hyperaccumulator or As tolerant/susceptible) the mechanisms of As accumulation, absorption or toxicity response may differ. There can be various crop management practices such as exogenous application of nutrients, hormones, antioxidants, osmolytes, signaling molecules, different chelating agents, microbial inoculants, organic amendments etc. can be effective against As toxicity in plants. There is information gap in understanding the mechanism of As-induced response (damage or tolerance response) in plants. This review presents the mechanism of As uptake and accumulation in plants, physiological responses under As stress, As-induced ROS generation and antioxidant defense system response, various approaches for enhancing As tolerance in plants from the available literatures which will make understanding the to date knowledge, knowledge gap and future guideline to be worked out for the development of As tolerant plant cultivars.
... Exiguobacterium sp. strain As-9 is reported to solubilize phosphorus and produce IAA; this promotes plant growth and lowers oxidative stress in green gram under As stressed in vitro sterile soil (Pandey and Bhatt, 2016). In a pot trial with sterile soil, Pseudomonas citronellolis was found to well tolerate As, and enhanced plant growth and dry biomass in chickpea grown in As affected soil (Adhikary et al., 2019). ...
Arsenic is a toxic metalloid categorized under class 1 carcinogen and is detrimental to both plants and animals. Agricultural land in several countries is contaminated with arsenic, resulting in its accumulation in food grains. Increasing global food demand has made it essential to explore neglected lands like arsenic-contaminated lands for crop production. This has posed a severe threat to both food safety and security. Exploration of arsenic-resistant plant growth-promoting rhizobacteria (PGPR) is an environment-friendly approach that holds promise for both plant growth promotion and arsenic amelioration in food grains. However, their real-time performance is dependent upon several biotic and abiotic factors. Therefore, a detailed analysis of associated mechanisms and constraints becomes inevitable to explore the full potential of available arsenic-resistant PGPR germplasm. Authors in this review have highlighted the role and constraints of arsenic-resistant PGPR in reducing the arsenic toxicity in food crops, besides providing the details of arsenic transport in food grains.
https://authors.elsevier.com/a/1eCtu15DSlK5VW
... Although the GPX activity has been recorded to generally rise in plants under different stressful cues (Erofeeva, 2015), particularly under metal stress such as As, a differential response has been noticed in different plant species. For example, Pandey and Bhatt (2016) reported a rise in GPX activity in the roots and shoot of Vigna radiata under As stress. Similarly, increased GPX activity has also been testified in other plants such as spinach (Natasha et al., 2021), Calotropis procera (Singh and Fulzele, 2021), Dahlia pinnata (Raza et al., 2019), and quinoa (Parvez et al., 2020). ...
Arsenic (As) is a highly toxic contaminant in the environment. Although both inorganic and organic types of arsenic exist in the environment, the most common inorganic forms of As that adversely affect plants are arsenite (As III) and arsenate (As V). Despite no evidence for As being essential for plant growth, exposure of roots to this element can cause its uptake primarily via transporters responsible for the transport of essential mineral nutrients. Arsenic exposure even at low concentrations disturbs the plant normal functioning via excessive generation of reactive oxygen species, a condition known as oxidative stress leading to an imbalance in the redox system of the plant. This is associated with considerable damage to the cell components thereby impairing normal cellular functions and activation of several cell survival and cell death pathways. To counteract this oxidative disorder, plants possess natural defense mechanisms such as chemical species and enzymatic antioxidants. This review considers how different types of antioxidants participate in the oxidative defense mechanism to alleviate As stress in plants. Since the underlying phenomena of oxidative stress tolerance are not yet fully elucidated, the potential for “Omics” technologies to uncover molecular mechanisms are discussed. Variousstrategies to improve As-induced oxidative tolerance in plants such as exogenous supplementation of effective growth regulators, protectant chemicals, transgenic approaches, and genome editing are also discussed thoroughly in this review.
... Although the GPX activity has been recorded to generally rise in plants under different stressful cues (Erofeeva, 2015), particularly under metal stress such as As, a differential response has been noticed in different plant species. For example, Pandey and Bhatt (2016) reported a rise in GPX activity in the roots and shoot of Vigna radiata under As stress. Similarly, increased GPX activity has also been testified in other plants such as spinach (Natasha et al., 2021), Calotropis procera (Singh and Fulzele, 2021), Dahlia pinnata (Raza et al., 2019), and quinoa (Parvez et al., 2020). ...
The potential uses of nanotechnology in agricultural and horticultural production are significant. There is potentially greater production with lower inputs; however, an understanding of the risks related to plant and soil fauna health is currently lacking. Understandably, this causes great concern among researchers. Recent literature related to the interaction of nanoparticles with both plant growth and soil fauna has shown impacts from the use of nanoparticles. Before expanding or commercializing the use of nanoparticles agrochemicals, it is critical to consider potential negative impacts related to plant growth and the soil ecosystem. This chapter reviews currently recognized negative impacts of silver nanoparticles (AgNPs) on plant growth and rhizosphere microbes. The effects of AgNPs on crop plants vary with plant species, plant age, plant health, growth stage, rate, and duration of AgNPs exposure along with other factors.
... The biogeochemistry and bioavailability of As in Chernozem soils are still insufficiently studied; thus, analysis of As uptake in Chernozem soils is an important research objective because of the effect of As on the morphological, biometric, and ultrastructural parameters of plants grown in contaminated soil (Fitz and Wenzel 2002;Otones et al. 2011;Pandey and Bhatt 2016;Williams et al. 2007). Speciation of As in Haplic Chernozem was investigated for the first time using a sequential extraction procedure (Motuzova et al. 2006). ...
Aims
Arsenic is a nonessential element for plants; however, high levels of As can inhibit plant growth. The toxicity of As is influenced mainly by its speciation in soil. The objectives of the present study were to determine the fractional composition of As in soil, its accumulation in plants, and its potentially toxic effects on the morphological, anatomical, and ultrastructural levels.
Methods
In a model experiment, barley (Hordeum sativum L.) was planted in Haplic Chernozem spiked with three different concentrations of As (20, 50 and 100 mg/kg). The fraction composition of As in the experimental soil was analysed using a method of sequential fractionation. The phytotoxic effects of As were analysed microscopically at the tissue, cellular, and intracellular levels.
Results
Analysis of the fraction composition of As revealed a higher amount of mobile forms of As that contaminated the generative organs of plants. Oxides of Fe, Al, and Mn became the main soil components to retain As when contamination of As increased. Arsenic toxicity inhibited plant growth by affecting morphological parameters (shape, size, and colour). It caused impairment in the root cells and a reduction in the size of the chlorophyllic parenchyma in the leaves. The ultrastructural analysis found changes in the main cellular organelles (chloroplasts, mitochondria, and peroxisomes).
Conclusions
The bioconcentration factor (BCF), bioaccumulation factor (BF-soluble), and translocation factor (TF) allowed evaluation of plant protection mechanisms and determination of hazardous concentrations of As in soil. Despite the high buffering capacity of the soil, high As concentration affected morphological and ultrastructural parameters of the H. sativum.
... Members of the different genera of Firmicutes are known for their diverse functions. For instance, Exiguobacterim is a phosphate solubilizing bacteria and known for its role in plant growth promotion (Pandey & Bhatt, 2016). Fusibacter degrades complex organic matter and also reduces volatile solids . ...
With the global concern on the role of wastewater treatment technologies in manifesting the emergence and dissemination of antibiotic-resistant bacteria (ARB), it has now become imperative to analyze the emerging technologies for handling them. This study assesses the efficiency of two deep constructed wetlands (CWs) receiving partially treated sewage from a residential complex and a hospital designed for the removal of organics and pathogens. These systems were further analyzed for the presence of major ARB to identify the role of CWs in mitigating antibiotic resistance among microbial communities. The bacterial community responsible for metabolic conversions was analyzed by metagenomic sequencing. Finally, the efficiencies of deep CWs were analyzed for the removal of specific bacteria resistant to three antibiotics—piperacillin, colistin (polymixin E), and cefoperazone. The overall removal of extended spectrum beta lactamase producers and carbapenemase producers was also studied. Our results indicate that CWs offer decent BOD and COD removal efficiencies of 74.04–78.71% and 53.85–64.37% respectively. However, a zero-order reaction between loading rate and removal rate was obtained after loading rate of 170 g/m2.day indicating the organic loading capacity of the system. Metagenomic analysis revealed the presence of bacteria with diverse metabolic potentials for substrate conversion. Removal of fecal coliforms was high in the CWs, but the most interesting observation was the attenuation in ARB, which was found to be comparable to, or even better, than the reported values from conventional moving bed bioreactor. This observation may be attributed to the high retention times offered by CWs compared to that of conventional systems making them an attractive future alternative for treating domestic as well as hospital sewage for emerging pollutants.
... To date, there is no reliable data on soil components that x As and contribute to its bioavailability in soils in natural landscapes. The biogeochemistry and bioavailability of As in Chernozem soils is still insu ciently studied; thus, analysis of As uptake in Chernozem soils is an important research objective because of the effect of As on the morphological, biometric, and ultrastructural parameters of plants grown in contaminated soil (Fitz and Wenzel 2002;Otones et al. 2011;Pandey and Bhatt 2016). Speciation of As in Haplic Chernozem was investigated for the rst time using an sequential extraction procedure (Motuzova et al. 2006). ...
Aims Arsenic is a nonessential element for plants, however, high levels of As can inhibit plant growth. Toxicity of As is largely influenced by its speciation in soil. The objectives of the present study were to determine fractional composition of As in soil, its accumulation in plants, and toxic effects on the morphological, anatomical, and ultrastructural levels.
Methods In a model experiment, barley (Hordeum sativum) was planted in Haplic Chernozem spiked with three different concentrations of As (20, 50 and 100 mg/kg). The fraction composition of As in the experimental soil was analysed using a method of sequential fractionation. The effect of As on plants was analysed microscopically at tissue, cellular, and intracellular levels.
Results Analysis of the fraction composition of As revealed a higher amount of mobile forms of As that contaminated the generative organs of plants. Oxides of Fe, Al, and Mn became the main soil components to retain As when contamination of As increased. Arsenic toxicity inhibited plant growth by affecting morphological parameters (shape, size, and colour). It was shown impairment in the root cells and a reduction in the size of the chlorophyllic parenchyma in the leaves. Ultrastructural analysis found changes in the main cellular organelles (chloroplasts, mitochondria, and peroxisomes).
Conclusions The bioconcentration factor (BCF), bioaccumulation factor (BF-soluble), and translocation factor (TF) allowed evaluation of plant protection mechanisms and determination of hazardous concentrations of As in soil. Despite high buffering capacity of soil, high As concentration affected morphological and ultrastructural parameters of the H. sativum.
... Moreover, phytohormone-producing microbes have the ability to reduce and translocate heavy metals by plant roots and thus help them to adapt to stressful environments (Das and Sarkar 2018). The reduced Ni and As accumulation roots of Vigna radiata plant roots and as a result less translocation to shoots due to inoculation with Bacillus megaterium and Exiguobacterium has been reported in previous studies (Rajkumar et al. 2013;Pandey and Bhatt 2016). They attributed these features with the production of plant hormones such as siderophores and indole-3-acetic acid by these heavy metal-tolerant bacterial strains which helped in metal stabilization in soil. ...
Microbial population of soils irrigated with ind ustr i al was t ewate r may con tain cert ain exopolysaccharides (EPS) and indole-3-acetic acid (IAA) producing bacterial strains having the ability to tolerate heavy metals along with plant growth-promoting (PGP) traits. As cadmium is one of the most toxic heavy metals for soils, plants, animals, and human beings, the present study was planned to isolate and characterize EPS-and IAA-producing, Cd-tolerant bacterial strains having tolerance against heavy metals along with plant growth-promoting traits. A total of 30 rhizobacterial strains (FN1-FN30) were isolated from rhizosphere soil collected from fields around industrial areas and roadsides irrigated with industrial wastewater. Out of these, eight isolates with the combined ability of IAA production and EPS production were characterized for PGP traits. On the basis of multifarious PGP traits and the results of root colonization assay, three most efficient EPS-and IAA-producing, Cd-tolerant plant growth-promoting strains, i.e., FN13, FN14, and FN16, were selected for multiple metal (Cd, Pb, Ni, and Cu) tolerance test along with quantification of growth, and IAA and EPS production abilities under Cd stress. Increasing levels of Cd stress negatively affected the tested characteristics of these strains, but FN13 showed more stability in growth, IAA production (18.24 μg mL −1), and EPS production (148.99 μg mL −1) compared to other strains under Cd stress. The morphological and biochemical analysis confirmed FN13 as Gram-positive, rod-shaped bacteria with smooth colonies of yellow appearance. The strain FN13 has strong root colonization (3.36 × 10 6 CFU g −1) ability for mustard seedlings and can solubilize Zn and phosphate along with the production of HCN, ammonia, and siderophores. The 16S rRNA sequencing confirmed it as the Bacillus safensis strain FN13. It can be explored as potential phytostabilizing biofertilizer for heavy metal-contaminated soils.
... Moreover, phytohormone-producing microbes have the ability to reduce and translocate heavy metals by plant roots and thus help them to adapt to stressful environments (Das and Sarkar 2018). The reduced Ni and As accumulation roots of Vigna radiata plant roots and as a result less translocation to shoots due to inoculation with Bacillus megaterium and Exiguobacterium has been reported in previous studies (Rajkumar et al. 2013;Pandey and Bhatt 2016). They attributed these features with the production of plant hormones such as siderophores and indole-3-acetic acid by these heavy metal-tolerant bacterial strains which helped in metal stabilization in soil. ...
Microbial population of soils irrigated with industrial wastewater may contain certain exopolysaccharides (EPS) and indole-3-acetic acid (IAA) producing bacterial strains having the ability to tolerate heavy metals along with plant growth–promoting (PGP) traits. As cadmium is one of the most toxic heavy metals for soils, plants, animals, and human beings, the present study was planned to isolate and characterize EPS- and IAA-producing, Cd-tolerant bacterial strains having tolerance against heavy metals along with plant growth–promoting traits. A total of 30 rhizobacterial strains (FN1–FN30) were isolated from rhizosphere soil collected from fields around industrial areas and roadsides irrigated with industrial wastewater. Out of these, eight isolates with the combined ability of IAA production and EPS production were characterized for PGP traits. On the basis of multifarious PGP traits and the results of root colonization assay, three most efficient EPS- and IAA-producing, Cd-tolerant plant growth–promoting strains, i.e., FN13, FN14, and FN16, were selected for multiple metal (Cd, Pb, Ni, and Cu) tolerance test along with quantification of growth, and IAA and EPS production abilities under Cd stress. Increasing levels of Cd stress negatively affected the tested characteristics of these strains, but FN13 showed more stability in growth, IAA production (18.24 μg mL⁻¹), and EPS production (148.99 μg mL⁻¹) compared to other strains under Cd stress. The morphological and biochemical analysis confirmed FN13 as Gram-positive, rod-shaped bacteria with smooth colonies of yellow appearance. The strain FN13 has strong root colonization (3.36 × 10⁶ CFU g⁻¹) ability for mustard seedlings and can solubilize Zn and phosphate along with the production of HCN, ammonia, and siderophores. The 16S rRNA sequencing confirmed it as the Bacillus safensis strain FN13. It can be explored as potential phytostabilizing biofertilizer for heavy metal–contaminated soils.
... Several reports suggested that reduction of arsenic concentration in soil was carried out by several microbes through various mechanisms such as sorption, biomethylation, complex action, and redox reaction [9]. 50ppb arsenic tolerant rhizobacteria were screened for plant growth-promoting activity such as IAA, Phosphate solubilization, Siderophore production, ACC deaminase activity shown in figure 2. Rhizobacterial isolates including several microbes can promote plant growth under varied environmental conditions/stress [15,16,21,29,30,31]. Maximum IAA activity was observed in rhizobacterial isolates, isolated from different media ( Fig: 2A, B, C, and D). ...
In the present study, the maximum rhizobacterial population was observed in Nutrient Agar (NA) (average; Cfu=135×10 6) followed by King's-B (average; Cfu=57×10 6), Soil extract agar (SEA) (average; Cfu=11×10 6), and Trypticase soy agar (TSA) (average; Cfu=9×10 6). Screening of arsenic tolerant rhizobacterial isolate revealed that about 1% of the bacterial isolate was from Nutrient Agar and King's-B survived at 20ppb arsenic concentration, while 0.8% and 0.7% survived from TSA and SEA media respectively. 50ppb arsenic tolerant rhizobacteria were screened for plant growth-promoting activity such as IAA, Phosphate solubilization, Siderophore production, ACC deaminase activity. Maximum IAA activity was observed in rhizobacterial isolates, isolated from all different media. P-solubilizer, Siderophore producer, ACC deaminase, proline, and TSS activities were observed in the isolates of NA media followed by King's-B media. 50ppb tolerate best suitable PGP traits producing isolates were inoculated to observe carrot plant growth in the pot experiment, Interesting and significant (p<0.05) result were observed in King's-B media producer isolates; (Pseudomonas) induces plant length, chlorophyll-a and chlorophyll-b content of the plant after 60 days followed by 30 days.
Arsenic is a ubiquitous toxic metalloid, the concentration of which is beyond WHO safe drinking water standards in many areas of the world, owing to many natural and anthropogenic activities. Long-term exposure to arsenic proves lethal for plants, humans, animals, and even microbial communities in the environment. Various sustainable strategies have been developed to mitigate the harmful effects of arsenic which include several chemical and physical methods, however, bioremediation has proved to be an eco-friendly and inexpensive technique with promising results. Many microbes and plant species are known for arsenic biotransformation and detoxification. Arsenic bioremediation involves different pathways such as uptake, accumulation, reduction, oxidation, methylation, and demethylation. Each of these pathways has a certain set of genes and proteins to carry out the mechanism of arsenic biotransformation. Based on these mechanisms, various studies have been conducted for arsenic detoxification and removal. Genes specific for these pathways have also been cloned in several microorganisms to enhance arsenic bioremediation. This review discusses different biochemical pathways and the associated genes which play important roles in arsenic redox reactions, resistance, methylation/demethylation, and accumulation. Based on these mechanisms, new methods can be developed for effective arsenic bioremediation.
The current work is an extensive review addressing the effects of heavy metals in major pulse crops such as Chickpea (Cicer arietinum L.), Pea (Pisum sativum L.), Pigeonpea (Cajanus cajan L.), Mung bean (Vigna radiata L.), Black gram (Vigna mungo L.) and Lentil (Lens culinaris Medik.). Pulses are important contributors to the global food supply in the world, due to their vast beneficial properties in providing protein, nutritional value and health benefits to the human population. Several studies have reported that heavy metals are injurious to plants causing inhibition in plant germination, a decrease in the root and shoot length, reduction in respiration rate and photosynthesis. Properly disposing of heavy metal wastes has become an increasingly difficult task to solve in developed countries. Heavy metals pose one of the substantial constraints to pulse crops growth and productivity even at low concentrations. This article attempts to present the morphological, biochemical and various physiological changes induced on the pulse crops grown under various heavy metal stress such as As, Cd, Cr, Cu, Pb, and Ni.
Application of plant growth-promoting rhizobacteria plays a vital role in enhancing phytoremediation efficiency. In this study, multiple approaches were employed to investigate the underlying mechanisms of Burkholderia sp. SRB-1 (SRB-1) on elevating Cd uptake and accumulation. Inoculation experiment indicated that SRB-1 could facilitate plant growth and Cd tolerance, as evidenced by the enhanced plant biomass and antioxidative enzymes activities. Cd content in plant shoots and roots increased about 36.56%-39.66% and 25.97%-130.47% assisted with SRB-1 when compared with control. Transcriptomics analysis revealed that SRB-1 upregulated expression of amiE, AAO1-2 and GA2-ox related to auxin and gibberellin biosynthesis in roots. Auxin and gibberellin, as hormone signals, regulated plant Cd tolerance and growth through activating hormone signal transduction pathways, which might also contribute to 67.94% increase of dry weight. The higher expression levels of ATP-binding cassette transporter subfamilies (ABCB, ABCC, ABCD and ABCG) in Chrysopogon zizanioides roots contributed to higher Cd uptake in Cd15 B (323.83 mg kg-1) than Cd15 (136.28 mg kg-1). Further, SRB-1 facilitated Cd migration from roots to shoots via upregulating the expression of Nramp, ZIP and HMA families. Our integrative analysis provided a molecular-scale perspective on Burkholderia sp. SRB-1 contributing to C. zizanioides performance.
Arsenic contamination in aquatic and terrestrial ecosystem is a serious environmental issue. Both natural and anthropogenic processes can introduce it into the environment. The speciation of the As determine the level of its toxicity. Among the four oxidation states of As (-3, 0, +3, and + 5), As(III) and As(V) are the common species found in the environment, As(III) being the more toxic with adverse impact on the plants and animals including human health. Therefore, it is very necessary to remediate arsenic from the polluted water and soil. Different physicochemical as well as biological strategies can be used for the amelioration of arsenic polluted soil. Among the microbial approaches, oxidation of arsenite, methylation of arsenic, biosorption, bioprecipitation and bioaccumulation are the promising transformation activities in arsenic remediation. The purpose of this review is to discuss the significance of the microorganisms in As toxicity amelioration in soil, factors affecting the microbial remediation, interaction of the plants with As resistant bacteria, and the effect of microorganisms on plant arsenic tolerance mechanism. In addition, the exploration of genetic engineering of the bacteria has a huge importance in bioremediation strategies, as the engineered microbes are more potent in terms of remediation activity along with quick adaptively in As polluted sites.
Plants harbor numerous genera of plant growth promoting bacteria (PGPB). PGPB facilitates metal uptake and promotes growth of plants either as free-living bacteria, binds to plant externally for instance, on roots (rhizosphere) or leaves (phyllosphere), or colonize as endophytic bacteria within internal plant tissues. Plant growth promoting rhizospheric bacteria (PGPR) and plant growth-promoting endophytic bacteria (PGPE) are major categories of PGPB. PGPR on the basis of PGPR host relationship are categorized as intracellular-plant growth promoting rhizobacteria (iPGPR) which live in structural and nonstructural nodules of root cells or extracellular-plant growth promoting rhizobacteria (ePGPR) which colonize rhizosphere over the rhizoplane or superficial intercellular spaces of root cells. The tagline “everything is everywhere, but the environment selects” stands very practical for PGPBs. PGPB produces siderophores, multiply easily, wide action spectrum, harmless to environment, stress-tolerant, and promotes plant growth and health by facilitating the improved nutrient availability, mobilization of nutrients, and assimilation and production of several metabolites. Due to the growing human population and critical world food production challenge of high and stable successful crop yields, the need for smart crop production technologies will increase for sustainable next-generation agriculture. The commercialization of plant-associated microbes is one of the most dynamic fields with a market value of 946.6 million dollars (2015), which will upsurge with 14.08% till end of 2022.
Arsenic (As) is one of the most common toxic metalloids as its intake affects various forms of life viz. human, animals and crops. The major discharge of As occurs due to natural and anthropogenic sources that leads to contamination of soil and groundwater. The long-term exposure of contaminated water and food adversely affects the human health. Therefore, it is important to minimize the level of arsenic in environment through different approaches. Several conventional and plant-based remediation methods have been reported but these techniques are not cost effective and has not been longer used due to their generic limitations. However, in present scenario microbe-based bioremediation occurs through different mechanisms such as biotransformation, degradation, detoxification and immobilization. Another microbial strategy involves the utilization of oxy-anion arsenic as an electron acceptor or donor to sustain their growth, known as arsenotrophy. This review discusses about the different aspects of (i) As contamination and its ill-effects on human health, (ii) microbial role in As-geocycling, (iii) modulation of microbial system for As resistance and detoxification and (iv) detailed prospects of arsenotrophy, its mechanisms, and plant-microbe interaction for As bioremediation.
Heavy metal pollution has caused serious environmental crises worldwide and has drawn a great deal of attention from ecologists. Owing to their high toxicity, non-biodegradability, and long residence times, heavy metals have a great impact on both the ecological systems and human health. The United States Environmental Protection Agency recognizes Pb, Cu, Zn, As, Cr, Cd, and Ni as priority control heavy metal pollutants. Bioremediation, an approach to clean up the contamination in the environment, can be carried out using microorganisms, plants, and animals. In this review, the development and studies of microorganisms-based bioremediation are summarized. In addition, the mechanism of the microbial resistance system and its putative remediation ability to remove toxic metals are discussed from the viewpoints of metagenomics and metatranscriptomics.
Heavy metals can be found in soil as natural components or as product of contaminations events; plants growing in soils are prone to bioaccumulate heavy metals on their biomass. Theobroma cacao L. can bioaccumulate cadmium (Cd) in the seed and could be in derived food products, it considered a human health risk; therefore, removal of Cd is desirable but not vet technically and economically feasible; only to avoid Cd in cocoa is by selecting lands plots exhibiting lower Cd concentrations in soils, imposing a serious limitation to farmers and regulators. The study of bacterial communities and isolation bacteria with tolerance and mechanisms to counteract the translocation of Cd to the parts of cocoa plant exhibits high relevance in Colombia economy and especially to companies producing chocolate and derivatives. Here, we explore bacterial communities associated with soils having relatively high natural Cd concentrations in a large agricultural cocoa plot located in the Santander region. We characterized the bacterial communities’ compositions by amplicon 16S rRNA sequencing from metagenomics soil DNA and by culturing-based enumeration and isolation approaches.
Culture-dependent techniques allowed the isolation of bacteria tolerant to Cd concentration, complement the information for Colombia, and expand the number of strains characterized with adaptive capacity against Cd with tolerance in a concentration of 120 mg/L, which represents the first capacity for Exiguobacterium sp., Ralstonia sp., Serratia sp., Dermacoccus sp., Klebsiella sp., Lactococcus sp. and Staphylococcus sp. In addition to confirming that there is a greater diversity of Cd-tolerant bacteria present in soils of farms cultivated with cocoa in Colombia. As for the results of new generation sequencing, they revealed that, the alpha-diversity in bacterial composition, according to the ANOVA, there are statistically significant differences of the bacterial communities present in the samples. Regarding Pearson correlation analysis, it was found the Shannon Simpson indices, have a positive correlation against OM, C, pH, Mn, C.E.C.I., Ca, P and negatively correlated with S; respect to bacterial community structure, a principal component analysis, which revealed that independent of the concentration of Cd present in soil samples, separates them according to pH value. Phyla to high abundance relative in all samples were Proteobacteria, Acidobacteriota, Actinobacteriota, Verrucomicrobiota, Myxococcota, Chloroflexi, Plactomycetota, Bacteroidota, Gemmatimonadota, Nitrospirota, Firmicutes and NB1_J; the bacteria genera with higher relative abundance (>0.5%) Nitrospira, candidatus Udaeobacter, Haliangium, Cupriavidus, MND1, Bacillus, Kitasatospora, Niveibacterium, Acidothermus, Burkholderia, Acidibacter, Terrimonas, Gaiella, candidatus Solibacter, Kitasatospora, Sphingomonas, Streptomyces, this genus with a relationship with the Cd tolerance process. After it, redundancy analysis was performed between the variation of the bacterial communities identified by dependent and independent techniques and edaphic soil variables, where their positive correlation was found against K, OM, C, Ca, pH (p<0.01) and P, C.E.C.I (p<0.05). For soil samples, the bacterial genera that make up the core community were identified, which are present in all samples as Nitrospira sp., Cupriavidus sp., Burkholderia sp., Haliangium sp., candidatus Udaeobacter, MND1, Kitasatospora, Acidothermus, Acidibacter, Streptomyces, Gaiella, candidatus Solibacter and Terramonas; the genera identified has a different and fundamental role in ecosystem functioning. The combination of different approaches offers new clues regarding the assessment of bacterial communities in soils cultivated with cocoa in soils with elevated Cd content in Colombia, and the ecological role and interplay of soil components and bacterial communities that contribute to modulate the effect of bioaccumulation in products.
At the present time, cadmium (Cd) and arsenic (As) contamination has become an outstanding agronomic concern. Bacterial-induced mitigation of Cd and As toxicity in plants- and bacteria-mediated immobilization of Cd and As to prevent their uptake by the crops (with no or minimum accumulation of As and Cd in edible parts) has attracted an increased attention worldwide. This chapter presents information about the features and mechanisms by which some Cd- and As-resistant plant growth-promoting bacteria (PGPB) could ameliorate Cd and As stress in heavy metal-affected plants and it provides some examples of using the PGPB in Cd- and As-contaminated agronomic soils. A centric perspective of this chapter deals with feasible mechanisms how Cd and As accumulation may be diminished by Cd- and As-resistant PGPB. This chapter suggests an effective technique for decreased uptake of Cd and As by plants and improved plant biomass in the soils contaminated with Cd and As.
Heavy metals, metaloids and persistent organic contaminants are released in the environment from industry, agriculture, urban zone, waste deposits and accidental spills presenting a serious threat for the ecosystems and health of human beings. Integrated phytobial remediation by bacteria and fungi has become a great challenge for successful removal of contaminants from polluted sites that is, combined plant/bacteria/fungi cross-talk offers effective tools to assist phytoremediation. Plant growth-promoting bacteria and mycorrhizal fungi obtain food for their metabolism whereas they promote plant growth, decrease metal(loid)s toxicity and degrade persistent organic compounds. This chapter focuses on joint action of plant/bacteria and fungi in removal of metal(loid)s, radionuclides, and chlorinated compounds, such as trichlorethylene, chlorinated pesticides, and polychlorinated biphenyls. Therefore, opportunities in exploitation of plant – bacteria-fungi interactions should be tailored in a framework platform and sustainable phytomanagement of polluted sites in the future around the globe.
Biofertilizers are the substances containing variety of microbes having the capacity to enhance plant nutrient uptake by colonizing the rhizosphere and make the nutrients easily accessible to plant root hairs. Biofertilizers are well known for their cost effectiveness, environment-friendly nature, and composition. These are effective alternatives to the hazardous synthetic fertilizers. This chapter covers various types of microbial biofertilizers pronouncing symbiotic and free-living nitrogen-fixers, phosphorus-solubilizer and mobilizers, their formulations, applications of few commercially available biofertilizers toward sustainable agriculture, and recent approaches to develop next-generation biofertilizers.
The traditional anoxic/aerobic process (A/O) process is widely used for treating digested piggery wastewater, but the lack of carbon sources leads to poor efficiency. Therefore, the process needs optimization to achieve high-efficiency and low-cost operation mode. In this study, an improved A/O system with bionic aquatic weed and Myriophyllum sp. was established to decontaminate digested piggery wastewater. The average removal efficiencies of chemical oxygen demand (COD), NH4⁺-N, and total nitrogen (TN) by the improved A/O system was satisfactory. The average removal efficiencies of COD, NH4⁺-N, and TN were 62.1%, 87.5%, and 61.9%, respectively. High-throughput sequencing identified a number of dominant microorganisms. The relative abundance of Nitrosomonas (ammonia-oxidizing bacteria) and Nitrospira (nitrite-oxidizing bacteria) was 0.07%–3.52% and 0.32%–1.30%, respectively. Combining bionic aquatic weed and Myriophyllum sp. altered the microbial community structure and metabolic pathways. The results demonstrate a cost-effective method for treating digested piggery wastewater.
Due to the increased industrialization, use of agrochemicals and various anthropogenic activities, heavy metal contamination is spreading rapidly, which is responsible for serious threats for crop production. In this context, plant growth-promoting rhizobacteria (PGPR)-mediated bioremediation is an ecofriendly, inexpensive, and sustainable approach toward the effective reclamation of metal-contaminated agricultural field. PGPR are naturally dwelling microflora that colonizes around the plant roots to fulfill their nutritional requirement acquired from the root exudates. This consortium results in a mutualistic benefit for both PGPR and plants by an effective plant-microbe interaction.
Three major heavy metals viz., Cd, Cr, Pb, and one metalloid viz., As have been focused on in this chapter revealing their sources, impact on living beings, causes of toxicity, possible remedies, the role of PGPR in the alleviation of metal toxicity and their role in plant growth promotion, and the mechanisms involved to confront such metals.
Purple nonsulfur bacteria (PNSB), a diverse group of photosynthetic microorganisms, inhabit in a wide variety of aquatic habitats where sunlight penetrates. These microorganisms show pigmentation ranging from deep red to brown and propagate under anoxic conditions. Depending on the presence of nutrients, oxygen concentration, and light intensity, they can shift their modes of metabolism between photoautotrophy, photoheterotrophy, and chemoheterotrophy. Most promisingly, many of the PNSB are known to thrive in the presence of various toxicants such as heavy metals and thus play an important role in the remediation of contaminated sites. PNSB are one of the most potential candidates for the production of biohydrogen, taking us a step further towards the era of “green technology.” They also serve as a potential source of single-cell proteins, enzymes, biofertilizers, carotenoids, plant growth-producing hormones, and several precursor molecules. This chapter discusses the versatile and important role of PNSB in biotechnology.
In this investigation, arsenic (As) and nickel (Ni) removal ability of fungus Trichoderma lixii CR700 have been explored. During the batch study, it showed efficient removal ability for As and Ni which was 94.7 and 68% respectively at 10 mgL⁻¹ and 144 h after incubation, however, increasing concentration from 10 to 500 mgL⁻¹ of As/Ni affected the removal ability of the fungus. The optimum pH, temperature and time for removal of As and Ni were 8.0, 28 °C and 240 h after inoculation. T. lixii CR700 also exhibited significant removal ability under the presence of salts, anions, EDTA and heavy metals. As and Ni removal was mainly happened by accumulation, where glutathione and thiol content might be played an important role in addition to surface sorption mechanism as confirmed from FTIR. T. lixii CR700 showed multifarious morphological and biochemical responses to deal As and Ni toxicity. Toxicity potential of fungal treated solution of As/Ni was diminished as confirmed from superior growth and high germination rate of Vigna radiata seeds in fungus treated As/Ni solution. Therefore, T. lixii CR700 can be potentially exploited for the management of As/Ni polluted aqueous system in safe and sustainable way.
Thermal springs have been the most resourceful ecological niches to understand the intricacies of the microbial community structure building. In the present study, the microbial community structure was investigated in five ecologically different hot springs. The highest number of OTUs was observed at low temperatures (42 C) whereas an increasing number of unclassified bacteria was observed with a temperature rise. The statistical correlation predicted that temperature, total dissolved solids and ions were the primary environmental factors in controlling the community composition and diversity. The relative abundance of Proteobacteria, Cyanobacteria showed a positive correlation with moderate temperature whereas growth of Chloroflexi and Nitrospira was unstable at 65 C. The observed LCBD was negatively correlated to the bacteria richness. A high relative abundance of Planctomycetes was restricted to Odisha hot springs (AT, TP, and DJ). We further hypothesize that abundance of most common cellulose-degrading bacteria such as Acinetobacter, Pseudomonas and Aeromonas only in DJ hot spring is possibly due to the high carbon content of the runoff water received from dense pandanus forest around it and could be a prospective source of industrially relevant cellulase after detailed characterization. The present study concludes that the association of physicochemical components with key species drive the microbial community structure. ARTICLE HISTORY
Arsenic (As) is a widespread toxic heavy metal which lowers plant growth and development. Salicylic acid (SA) is a prominent signaling molecule in plants and is involved in response to environmental stresses. In this study, the possible role of SA in alleviating As toxicity was investigated on mung bean. Seeds after treatment with SA (0, 0.25, 0.5 and 1 mM) were sown in pots and were exposed to As stress (0 or 50 mg kg− 1 soil). It was found that As toxicity markedly lowered chlorophyll value, relative water content (RWC), shoot length, shoot and root biomass, leaf area and seed yield, whereas it enhanced As accumulation in roots, shoots and seeds, malondialdehyde (MDA) and proline contents and also activities of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR). Nonetheless, pre-sowing of seeds in SA (especially 1 mM) significantly reduced As contents in roots, shoots, and seeds, but further increased proline content and antioxidant enzyme activities, led to decreased MDA content. In SA-treated plants, also chlorophyll value, RWC, shoot length, shoot and root biomass, leaf area and seed yield were enhanced under As toxicity conditions. These results suggest that exogenous application of SA could improve As toxicity tolerance in mung bean plants through reducing As uptake and accumulation in different plant parts, enhancing activities of antioxidant enzymes and proline content, a decline of lipid peroxidation, improving chlorophyll value and plant water status.
The contamination of terrestrial and aquatic ecosystems by arsenic (As) is a very sensitive environmental issue due to its adverse impact on organisms. Although arsenic contamination is not only of anthropogenic origin, the problem of arsenic contamination in water sources in many areas has been considered calamitous because of its significant risk to different organisms. Many of the organisms are already suffering from the irreversible effects of arsenic poisoning. The disposal of industrial and mining waste has led to extensive contamination of land and water resources. It also causes a potential problem for food chain contamination. Awareness of arsenic poisoning to the mass majority of people has led to the development of efficient remediation technologies for its mitigation. There are many strategies for remediation such as coagulation‐flocculation, membrane techniques, nanoparticles, and many more. In this chapter, different sources of arsenic contamination, health effects, and important management strategies currently being practiced for arsenic‐contaminated areas (surface and groundwater) are shown. The chapter concludes with different remediation techniques for the removal of arsenic contaminants from water systems, and some are evolving as alternative green techniques.
Remediating soils contaminated with heavy metals due to urbanization and industrialization is very important not only for human health but also for ecosystem sustainability. Of the available remediation technologies for heavy metal-contaminated soils, phytoremediation is a relatively low-cost environmentfriendly technology which preserves biodiversity and soil fertility. The application of plant growth-promoting bacteria (PGPB) during the phytoremediation of heavy metal-contaminated soils can enhance plant growth against heavy metal toxicity and increase heavy metal removal efficiency. In this study, the sources of heavy metals that have adverse effects on microorganisms, plants, and humans, and the plant growthpromoting traits of PGPB are addressed and the research trends of PGPB-assisted phytoremediation over the last 10 years are summarized. In addition, the effects of environmental factors and PGPB inoculation methods on the performance of PGPB-assisted phytoremediation are discussed. For the innovation of PGPB-assisted phytoremediation, it is necessary to understand the behavior of PGPB and the interactions among plant, PGPB, and indigenous microorganisms in the field.
At the dawn of the industrial revolution, the exorbitant use of heavy metals and toxic elements by mankind unfurls a powerful and complex web of hazard all around the world that significantly contributed to unprecedented trends in environmental degradation. Plants as sessile organisms, that cannot escape from the stress directly, have adapted to this environment via concurrent configurations of several traits. Among them the anatomy has been identified as much more advanced field of research that brought the explosion of interest among the expertise and its prodigious importance in stress physiology is unavoidable. In conjunction with various other disciplines, like physiology, biochemistry, genomics and metabolomics, the plant anatomy provides a large data sets that are paving the way towards a comprehensive and holistic understanding of plant growth, development, defense and productivity under heavy metal and toxic element stress. Present paper advances our recent knowledge about structural alterations of plant tissues induced by metals and metalloids, like antimony (Sb), arsenic (As), aluminium (Al), copper (Cu), cadmium (Cd), chromium (Cr), lead (Pb), manganese (Mn), mercury (Hg), nickel (Ni) and zinc (Zn) and points on essential role of plant anatomy and its understanding for plant growth and development in changing environment. Understanding of anatomical adaptations of various plant organs and tissues to heavy metals and metalloids could greatly contribute to integral and modern approach for investigation of plants in changing environmental conditions. These findings are necessary for understanding of the whole spectra of physiological and biochemical reactions in plants and to maintain the crop productivity worldwide. Moreover, our holistic perception regarding the processes underlying the plant responses to metal(loids) at anatomical level are needed for improving crop management and breeding techniques.
Water scarcity and high input costs have compelled farmers to use untreated wastewater and industrial effluents to increase profitability of their farms. Normally, these effluents improve crop productivity by serving as carbon source for microbes, providing nutrients to plants and microbes, and improving soil physicochemical and biological properties. They, however, may also contain significant concentrations of potential heavy metals, the main inorganic pollutants affecting plant systems, in addition to soil deterioration. The continuous use of untreated industrial wastes and agrochemicals may lead to accumulation of phytotoxic concentration of heavy metals in soils. Phytotoxic concentration of heavy metals in soils has been reported in Pakistan along the road sides and around metropolitan areas, which may cause its higher accumulation in edible plant parts. A number of bacterial that can induce heavy metal tolerance in plants due to their ability to produce phytohormones strains have been reported. Inoculation of crop plants with these microbes can help to improve their growth and productivity under normal, as well as stressed, conditions. This review reports the recent developments in heavy metal pollution as one of the major inorganic sources, the response of plants to these contaminants, and heavy metal stress mitigation strategies. We have also summarized the exogenous application of phytohormones and, more importantly, the use of phytohormone-producing, heavy metal-tolerant rhizobacteria as one of the recent tools to deal with heavy metal contamination and improvement in productivity of agricultural systems.
Water scarcity and high input costs have compelled farmers to use untreated wastewater and industrial effluents to increase profitability of their farms. Normally, these effluents improve crop productivity by serving as carbon source for microbes, providing nutrients to plants and microbes, and improving soil physicochemical and biological properties. They, however, may also contain significant concentrations of potential heavy metals, the main inorganic pollutants affecting plant systems, in addition to soil deterioration. The continuous use of untreated industrial wastes and agrochemicals may lead to accumulation of phytotoxic concentration of heavy metals in soils. Phytotoxic concentration of heavy metals in soils has been reported in Pakistan along the road sides and around metropolitan areas, which may cause its higher accumulation in edible plant parts. A number of bacterial that can induce heavy metal tolerance in plants due to their ability to produce phytohormones strains have been reported. Inoculation of crop plants with these microbes can help to improve their growth and productivity under normal, as well as stressed, conditions. This review reports the recent developments in heavy metal pollution as one of the major inorganic sources, the response of plants to these contaminants, and heavy metal stress mitigation strategies. We have also summarized the exogenous application of phytohormones and, more importantly, the use of phytohormone-producing, heavy metal-tolerant rhizobacteria as one of the recent tools to deal with heavy metal contamination and improvement in productivity of agricultural systems.
Hydroponically-grown ivyleaf morningglory (Ipomoea hederacea) seedlings inoculated with deleterious rhizobacteria (DRB) were studied to observe colonization of roots using scanning and transmission electron microscopy. The DRB, Bradyrhizobium japonicum isolate GD3, previously isolated as a DRB producing high concentrations of indole-3-acetic acid (IAA), and Pseudomonas putida isolate GD4, were compared with a plant growth promoting rhizobacterium (PGPR), Bacillus megaterium isolate GP4. Scanning electron microscopy revealed that the colonization of isolates GP4 and GD4 were consistently distributed on the surface of roots; however, isolate GD3 was deeply localized into surface furrows of roots. Transmission electron microscopy showed considerable alterations of root cells including vesiculation, partial cell wall degradation, and cytoplasm disorganization. The average population density of isolate GD4 on the root surface was about 10 and 100 times greater than GP4 and GD3, respectively. Root elongation of seedlings inoculated with isolates GD3 and GD4 after 7 d of growth was significantly inhibited by ca. 26% and 90%, respectively, compared to the control. This study showed that inhibition of morningglory root growth by isolate GD3 might be related to production of high concentrations of IAA although other phytotoxins likely contributed to inhibiting root elongation of morningglory inoculated with isolate GD4. Rhizobacteria able to suppress morningglory growth may be effective as biological control agents to supplement herbicide weed management in crops where morningglory is difficult to control.
Forty-three indigenous arsenic resistant bacteria were isolated from arsenic rich soil of Rajnandgaon district in the state of Chhattisgarh, India by enrichment culture technique. Among the isolates, two of the bacteria (As-9 and As-14) exhibited high resistance to As(V) [MIC ≥ 700 mM] and As(III) [MIC ≥ 10 mM] and were selected for further studies. Both these bacteria grew well in the presence of arsenic [20 mM As(V) and 5 mM As(III)], but the isolate As-14 strictly required arsenic for its survival and growth and was characterized as a novel arsenic dependent bacterium. The isolates contributed to 99% removal of arsenic from the growth medium which was efficiently accumulated in the cell. Quantitative estimation of arsenic through Atomic Absorption Spectrophotometer revealed that there was >60% accumulation of both As(V) and As(III) by the two isolates. Scanning Electron Microscopic analysis showed a fourfold increase in bacterial cell volume when grown in the presence of arsenic and the results of Transmission Electron Microscopy and energy-dispersive X-ray spectroscopy proved that such an alteration was due to arsenic accumulation. Such arsenic resistant bacteria with efficient accumulating property could be effectively applied in the treatment of arsenic contaminated water.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
The main objective of this study was to identify and isolate arsenic resistant bacteria that can be used for removing arsenic from the contaminated environment. Here we report a soil borne bacterium, B1-CDA that can serve this purpose. B1-CDA was isolated from the soil of a cultivated land in Chuadanga district located in the southwest region of Bangladesh. The morphological, biochemical and 16S rRNA analysis suggested that the isolate belongs to Lysinibacillus sphaericus. The minimum inhibitory concentration (MIC) value of the isolate is 500 mM (As) as arsenate. TOF-SIMS and ICP-MS analysis confirmed intracellular accumulation and removal of arsenics. Arsenic accumulation in cells amounted to 5.0 mg g(-1) of the cells dry biomass and thus reduced the arsenic concentration in the contaminated liquid medium by as much as 50%. These results indicate that B1-CDA has the potential for remediation of arsenic from the contaminated water. We believe the benefits of implementing this bacterium to efficiently reduce arsenic exposure will not only help to remove one aspect of human arsenic poisoning but will also benefit livestock and native animal species. Therefore, the outcome of this research will be highly significant for people in the affected area and also for human populations in other countries that have credible health concerns as a consequence of arsenic-contaminated water.
O processo de fixação biológica de nitrogênio (FBN), realizado por bactérias fixadoras de N2 simbióticas de leguminosas, comumente denominados α e β rizóbios, proporciona alta sustentabilidade aos ecossistemas. Seu manejo como uma biotecnologia é bem sucedido para aumentar a produtividade das culturas. Um exemplo notável desse sucesso é a inoculação da soja com estirpes de Bradyrhizobium.Os rizóbios produzem grande diversidade de estruturas químicas dos exopolissacarídeos (EPS). Embora o papel dos EPS seja relativamente bem estudado no processo de FBN, o seu potencial econômico e ambiental ainda não é explorado. Esses EPS são principalmente heteropolissacarídeos espécie-específicos, que podem variar de acordo com a composição dos açúcares, as suas ligações em uma única subunidade, o tamanho da unidade repetitiva e o grau de polimerização. Estudos mostram que os EPS produzidos por essas bactérias exercem importante papel no processo de invasão, formação do cordão de infecção, desenvolvimento do bacteroide e do nódulo e resposta de defesa da planta. Esses EPS também conferem proteção a essas bactérias quando submetidas a diversos estresses ambientais. Em geral, estirpes de rizóbios que produzem maior quantidade de EPS são mais tolerantes às condições adversas, quando comparadas com estirpes que produzem menor quantidade. Além disso, sabe-se que os EPS produzidos por microrganismos são amplamente utilizados em vários segmentos industriais. Esses compostos, também denominados biopolímeros, fornecem uma alternativa válida para a substituição das gomas comumente usadas na indústria de alimentos, por meio do desenvolvimento de produtos com propriedades praticamente idênticas ou com melhores características reológicas, que podem ser usados para novas aplicações. Os EPS microbianos também são capazes de aumentar a adesão de partículas do solo, favorecendo a estabilidade mecânica dos agregados, além de aumentarem os níveis de retenção de água e fluxo de ar nesse ambiente. Diante da importância dos EPS, na presente revisão discute-se o papel desses compostos no processo de fixação biológica de N2, na adaptação dos rizóbios a estresses ambientais, bem como no processo de agregação do solo. As possíveis aplicações desses biopolímeros na indústria também são discutidas.
Wedelia chinensis Merrill (Asteraceae) is a medicinally important herb, grown abundantly in soils contaminated with heavy metals, including toxic metalloid arsenic (As). The leaves have immense significance in treatment of various ailments.
The present study was undertaken to ascertain whether the edible/usable parts experience oxidative stress in the form of membrane damage during As exposure or not.
Responses of seven antioxidant enzymes were studied in leaves under 20 mg/L of As treatment in pot experiment.
When compared to control, activities of superoxide dismutase, monodehydroascorbatereductase, dehydroascorbatereductase, glutathione reductase, and gluathione peroxidase had increased, while the catalase level reduced and ascorbate peroxidase activity changed non-significantly in As-treated seedlings. This suggested overall positive response of antioxidant enzymes to As-induced oxidative stress. Although hydrogen peroxide content increased, level of lipid peroxidation and magnitude of membrane damage was quite normal, leading to normal growth (dry weight of shoot) of plant under Astreatment.
W.chinensis is tolerant of As-toxicity, and thus, can be grown in As-contaminated zones.
Bioassays show that rhizobitoxine-producing strains of Bradyrhizobium japonicum excreted another phytotoxic compound into their culture fluid. This compound was purified and identifi- ed by HPLC and mass spectrometry as indole-3-acetic acid (IAA). The levels of IAA produced by the different strains of B. japonicum, for which the genotype groups have been determined with respect to the degree of base substitution in and around nifDKE, were quantified using gas chromatography/mass spectrometry and a deuterated internal standard. Genotype II strains, which produce rhizobitoxine, excreted more than 20fits of IAA into their culture fluid. How- ever, no IAA was detected in the culture supernatants of genotype I strains, which do not produce rhizobitoxine. This was true even when tryptophan was added to the medium. Moreover, cells of genotypes I and II strains, which were grown under our culture conditions, did not show IAA degradation activity. These results suggest that, in wild-type B. japonicum strains, complete IAA biosynthesis is confined exclusively to genotype II strains that produce rhizobitoxine.
Phytoextraction makes use of trace element-accumulating plants that concentrate the pollutants in their tissues. Pollutants can be then removed by harvesting plants. The success of phytoextraction depends on trace element availability to the roots and the ability of the plant to intercept, take up, and accumulate trace elements in shoots. Current phytoextraction practises either employ hyperaccumulators or fast-growing high biomass plants; the phytoextraction process may be enhanced by soil amendments that increase trace element availability in the soil. This review will focus on the role of plant-associated bacteria to enhance trace element availability in the rhizosphere. We report on the kind of bacteria typically found in association with trace element - tolerating or - accumulating plants and discuss how they can contribute to improve trace element uptake by plants and thus the efficiency and rate of phytoextraction. This enhanced trace element uptake can be attributed to a microbial modification of the absorptive properties of the roots such as increasing the root length and surface area and numbers of root hairs, or by increasing the plant availability of trace elements in the rhizosphere and the subsequent translocation to shoots via beneficial effects on plant growth, trace element complexation and alleviation of phytotoxicity. An analysis of data from literature shows that effects of bacterial inoculation on phytoextraction efficiency are currently inconsistent. Some key processes in plant-bacteria interactions and colonization by inoculated strains still need to be unravelled more in detail to allow full-scale application of bacteria assisted phytoremediation of trace element contaminated soils.
In this review, we brie fl y describe the biological application of PGPR for purposes of phytoremediating heavy metals. We address the agronomic practices that can be used to maximize the remediation potential of plants. Plant roots have limited ability to absorb metals from soil, mainly because metals have low solubility in the soil solution. The phytoavailability of metal is closely tied to the soil properties and the metabolites that are released by PGPR (e.g., siderophores, organic acids, and plant growth regulators). The role played by PGPR may be accomplished by their direct effect on plant growth dynamics, or indirectly by acidification, chelation, precipitation, or immobilization of heavy metals in the rhizosphere.
Extracellular polymeric substances (EPS) of microbial origin are a complex mixture of biopolymers comprising polysaccharides, proteins, nucleic acids, uronic acids, humic substances, lipids, etc. Bacterial secretions, shedding of cell surface materials, cell lysates and adsorption of organic constituents from the environment result in EPS formation in a wide variety of free-living bacteria as well as microbial aggregates like biofilms, bioflocs and biogranules. Irrespective of origin, EPS may be loosely attached to the cell surface or bacteria may be embedded in EPS. Compositional variation exists amongst EPS extracted from pure bacterial cultures and heterogeneous microbial communities which are regulated by the organic and inorganic constituents of the microenvironment. Functionally, EPS aid in cell-to-cell aggregation, adhesion to substratum, formation of flocs, protection from dessication and resistance to harmful exogenous materials. In addition, exopolymers serve as biosorbing agents by accumulating nutrients from the surrounding environment and also play a crucial role in biosorption of heavy metals. Being polyanionic in nature, EPS forms complexes with metal cations resulting in metal immobilization within the exopolymeric matrix. These complexes generally result from electrostatic interactions between the metal ligands and negatively charged components of biopolymers. Moreover, enzymatic activities in EPS also assist detoxification of heavy metals by transformation and subsequent precipitation in the polymeric mass. Although the core mechanism for metal binding and / or transformation using microbial exopolymer remains identical, the existence and complexity of EPS from pure bacterial cultures, biofilms, biogranules and activated sludge systems differ significantly, which in turn affects the EPS-metal interactions. This paper presents the features of EPS from various sources with a view to establish their role as central elements in bioremediation of heavy metals.
Six phosphate solubilizing bacteria (PSB) were isolated from paddy fields of Eastern Uttar Pradesh, India harboring low available phosphorus. Taxonomic delineation employing morphological, biochemical, 16S rRNA gene sequences and phylogenetic affiliations suggests that they are members of Enterobacter and Exiguobacterium genera. Of the six isolates, Enterobacter sp. LCR1 and LCR2 exhibited high level (568 -642 g/ml) of phosphate solubilization in NBRIP liquid medium. Exiguobacterium sp. LCR4 and LCR5 showed increased phosphate solubilization efficiency under alkaline pH while Enterobacter sp. LCR3 remained unaffected. At high salt and temperature, Enterobacter sp. LCR1 and LCR2 produced 1.6 fold soluble phosphorus in comparison with earlier studies. Thus, these isolates may be useful for the development of potential bio-inoculants for soils having alkaline pH, high salt, temperature and insoluble phosphorus.
Extensive use of chromium (Cr) and arsenic (As) based preservatives from the leather tanning industry in Pakistan has had a deleterious effect on the soils surrounding production facilities. Bacteria have been shown to be an active component in the geochemical cycling of both Cr and As, but it is unknown how these compounds affect microbial community composition or the prevalence and form of metal resistance. Therefore, we sought to understand the effects that long-term exposure to As and Cr had on the diversity and structure of soil microbial communities. Soils from three spatially isolated tanning facilities in the Punjab province of Pakistan were analyzed. The structure, diversity and abundance of microbial 16S rRNA genes were highly influenced by the concentration and presence of hexavalent chromium (Cr (VI)) and arsenic. When compared to control soils, contaminated soils were dominated by Proteobacteria while Actinobacteria and Acidobacteria (which are generally abundant in pristine soils) were minor components of the bacterial community. Shifts in community composition were significant and revealed that Cr (VI)-containing soils were more similar to each other than to As contaminated soils lacking Cr (VI). Diversity of the arsenic resistance genes, arsB and ACR3 were also determined. Results showed that ACR3 becomes less diverse as arsenic concentrations increase with a single OTU dominating at the highest concentration. Chronic exposure to either Cr or As not only alters the composition of the soil bacterial community in general, but affects the arsenic resistant individuals in different ways.
The two forms of inorganic arsenic, arsenate (AsV) and arsenite (AsIII), are easily taken up by the cells of the plant root. Once in the cell, AsV can be readily converted to AsIII, the more toxic of the two forms. AsV and AsIII both disrupt plant metabolism, but through distinct mechanisms. AsV is a chemical analog of phosphate that can disrupt at least some phosphate-dependent aspects of metabolism. AsV can be translocated across cellular membranes by phosphate transport proteins, leading to imbalances in phosphate supply. It can compete with phosphate during phosphorylation reactions, leading to the formation of AsV adducts that are often unstable and short-lived. As an example, the formation and rapid autohydrolysis of AsV-ADP sets in place a futile cycle that uncouples photophosphorylation and oxidative phosphorylation, decreasing the ability of cells to produce ATP and carry out normal metabolism. AsIII is a dithiol reactive compound that binds to and potentially inactivates enzymes containing closely spaced cysteine residues or dithiol co-factors. Arsenic exposure generally induces the production of reactive oxygen species that can lead to the production of antioxidant metabolites and numerous enzymes involved in antioxidant defense. Oxidative carbon metabolism, amino acid and protein relationships, and nitrogen and sulfur assimilation pathways are also impacted by As exposure. Readjustment of several metabolic pathways, such as glutathione production, has been shown to lead to increased arsenic tolerance in plants. Species- and cultivar-dependent variation in arsenic sensitivity and the remodeling of metabolite pools that occurs in response to As exposure gives hope that additional metabolic pathways associated with As tolerance will be identified.
"Phytoremediation" know-how to do-how is rapidly expanding and is being commercialized by harnessing the phyto-microbial diversity. This technology employs biodiversity to remove/contain pollutants from the air, soil and water. In recent years, there has been a considerable knowledge explosion in understanding plant-microbes-heavy metals interactions. Novel applications of plant-associated microbes have opened up promising areas of research in the field of phytoremediation technology. Various metabolites (e.g., 1-aminocyclopropane-1-carboxylic acid deaminase, indole-3-acetic acid, siderophores, organic acids, etc.) produced by plant-associated microbes (e.g., plant growth promoting bacteria, mycorrhizae) have been proposed to be involved in many biogeochemical processes operating in the rhizosphere. The salient functions include nutrient acquisition, cell elongation, metal detoxification and alleviation of biotic/abiotic stress in plants. Rhizosphere microbes accelerate metal mobility, or immobilization. Plants and associated microbes release inorganic and organic compounds possessing acidifying, chelating and/or reductive power. These functions are implicated to play an essential role in plant metal uptake. Overall the plant-associated beneficial microbes enhance the efficiency of phytoremediation process directly by altering the metal accumulation in plant tissues and indirectly by promoting the shoot and root biomass production. The present work aims to provide a comprehensive review of some of the promising processes mediated by plant-associated microbes and to illustrate how such processes influence heavy metal uptake through various biogeochemical processes including translocation, transformation, chelation, immobilization, solubilization, precipitation, volatilization and complexation of heavy metals ultimately facilitating phytoremediation.
Widespread contamination of arsenic (As) is recognized as a global problem due to its well-known accumulation by edible and medicinal plants and associated health risks for the humans. In this study, phytotoxicity imposed upon exposure to arsenate [As(V); 0-250 μM for 1-7 days] and ensuing biochemical responses were investigated in a medicinal herb Bacopa monnieri L. vis-à-vis As accumulation. Plants accumulated substantial amount of As (total 768 μg g(-1) dw at 250 μM As(V) after 7 days) with the maximum As retention being in roots (60%) followed by stem (23%) and leaves (17%). The level of cysteine and total nonprotein thiols (NP-SH) increased significantly at all exposure concentrations and durations. Besides, the level of metalloid binding ligands viz., glutathione (GSH) and phytochelatins (PCs) increased significantly at the studied concentrations [50 and 250 μM As(V)] in both roots and leaves. The activities of various enzymes viz., arsenate reductase (AR), glutathione reductase (GR), superoxide dismutase (SOD), guaiacol peroxidase (GPX), ascorbate peroxidase (APX), and catalase (CAT) showed differential but coordinated stimulation in leaves and roots to help plants combat As toxicity up to moderate exposure concentrations (50 μM). However, beyond 50 μM, biomass production was found to decrease along with photosynthetic pigments and total soluble proteins, whereas lipid peroxidation increased. In conclusion, As accumulation potential of Bacopa may warrant its use as a phytoremediator but if Bacopa growing in contaminated areas is consumed by humans, it may prove to be toxic for health. © 2011 Wiley Periodicals, Inc. Environ Toxicol, 2011.
The disposal of toxic heavy metals such as arsenic posed high risk to the environment. Arsenite [As(III)], a reduced form of arsenic, is more toxic and mobile than arsenate [As(V)]. The aim of this work was to isolate arsenic-tolerant bacteria from contaminated soil collected in Ronphibun District, Nakorn Srithammarat Province, followed by screening these bacteria for their ability to adsorb arsenite. Twenty-four bacterial isolates were obtained from samples cultivated in basal salts medium plus 0.1% yeast extract and up to 40 mM sodium-arsenite at 30oC under aerobic condition. From these, isolates B-2, B-3, B-4, B-21, B-25 and B-27 produced extracellular polymeric-like substances into the culture medium, which may potentially be used in the bioremediation of arsenic and other contaminants. All isolates displayed arsenite adsorbing activities in the ranges of 36.87-96.93% adsorption from initial concentration of 40 mM sodium-arsenite, without any arsenic transforming activity. Five isolates with the highest arsenite adsorbing capacity include B-4, B-7, B-8, B-10 and B-13 which adsorbed 80.90, 86.72, 87.08, 84.36 and 96.93% arsenite, respectively. Identification of their 16S rDNA sequences showed B -7, B-8, and B-10 to have 97%, 99% and 97% identities to Microbacterium oxydans, Achromobacter sp. and Ochrobactrum anthropi, respectively. Isolates B-4 and B-13, which did not show sequence similarity to any bacterial species, may be assigned based on their morphological and biochemical characteristics to the genus Streptococcus and Xanthomonas, respectively. Thus, both isolates B-4 and B-13 appear to be novel arsenite adsorbing bacteria within these genuses.
Withania somnifera has been an important herb in the Ayurvedic and indigenous medical systems for centuries in India. However, these grow as weeds mostly in the wastelands, which receive contaminated water from municipal and industrial sources. In the present investigation, plants of Withania somnifera were exposed to various concentrations of arsenate (AsV) and arsenite (AsIII) (0, 10, 25, 50, 100 μM) for 10 days and analysed for accumulation of arsenic (As) and physiological and biochemical changes. Plants showed more As accumulation upon exposure to AsIII (320 μg g−1 DW in roots and 161 μg g−1 DW in leaves) than to AsV (173 μg g−1 DW in roots and 100 μg g−1 DW in leaves) after 10 days of treatment. Consequently, AsIII exposure caused more toxicity to plants as compared to that AsV, as evaluated in terms of the level of photosynthetic pigments and oxidative stress parameters (superoxide, hydrogen peroxide and lipid peroxidation), particularly at higher concentrations and on longer durations. Plants could tolerate low concentrations (variable for AsIII and AsV) until longer durations (10 days) and high concentrations for shorter durations (1–5 days) through increase in antioxidant enzymes and by augmented synthesis of thiols. In conclusion, As tolerance potential of Withania plants on one hand advocates its prospective use for remediation under proper supervision and on the other demonstrates possible threat of As entry into humans due to medicinal uses.
Arsenic is a metalloid that is toxic to living organisms. It is known that high concentration of arsenic causes toxic damage to cells and tissues of plants. While the toxic effect of arsenic is known, limited efforts have been made to study its genotoxic effect on the crops. In the present study, effects of arsenic trioxide (As2O3) on seed germination, root length, reactive oxygen species (ROS), lipid peroxidation (malondialdehyde (MDA)), and activities of antioxidant enzymes, as well as DNA in wheat seedlings were investigated. Seedlings were exposed to different (10 to 40 mg/L) As2O3 concentrations for 7 days. Seed germination and root elongation decreased with increase of As2O3 concentration. The values of hydrogen peroxide (H2O2), superoxide anion (O2(·-)), and MDA contents significantly increased by As2O3 concentrations. The highest values for H2O2, O2(·-), and MDA were obtained in 40 mg/L treated wheat seedling. A significant increase of peroxidase (POX) and catalase (CAT) activity in seedlings were observed with increased concentration of As2O3, then decreased when reaching a value of 40 mg/L, whereas the activities of superoxide dismutase (SOD) were gradually enhanced with increasing As2O3 concentration. Alterations of DNA in wheat seedlings were detected using randomly amplified polymorphic DNA (RAPD) technique. The changes occurring in RAPD profiles of seedlings following As2O3 treatment included loss of normal bands and appearance of new bands in comparison to that of control seedlings. The results of our study showed that As2O3 induced DNA damage in a dose-dependent meaner, and the root cells of wheat studied showed a defense against As2O3-induced oxidative stress by enhancing their antioxidant activities.
Arsenic (As) contamination of soil and water has been considered as a major global environmental issue during last few decades. Among the various methods so far reported for reclamation of As contaminated rhizosphere soil, bioremediation using bacteria has been found to be most promising. An As resistant bacterial isolate Brevibacillus sp. KUMAs2 was obtained from As contaminated soil of Nadia, West Bengal, India, which could resist As(V) and As(III) a maximum of 265mM and 17mM, respectively. The strain could remove ~40 percent As under aerobic culture conditions. As resistant property in KUMAs2 was found to be plasmid-borne, which carried both As oxidizing and reducing genes. The strain could promote chilli plant growth under As contaminated soil environment by decreasing As accumulation in plant upon successful colonization in the rhizosphere, which suggests the possibility of using this isolate for successful bioremediation of As in the crop field.
This study was conducted to investigate the uptake of lead (Pb) and arsenic (As) from contaminated soil using Melastoma malabathricum L. species. The cultivated plants were exposed to As and Pb in separate soils for an observation period of 70 days. From the results of the analysis, M. malabathricum accumulated relatively high range of As concentration in its roots, up to a maximum of 2800 mg/kg. The highest accumulation of As in stems and leaves was 570 mg/kg of plant. For Pb treatment, the highest concentration (13,800 mg/kg) was accumulated in the roots of plants. The maximum accumulation in stems was 880 mg/kg while maximum accumulation in leaves was 2,200 mg/kg. Only small amounts of Pb were translocated from roots to above ground plant parts (TF 1) is indicative this plants is a good bioaccumulator for these metals. Therefore, phytostabilisation is the mechanism at work in M. malabathricum's uptake of Pb, while phytoextraction is the dominant mechanism with As.
The application of plant growth promoting rhizobacteria (PGPR) at field scale has been hindered by inadequate understand of mechanisms that enhance plant growth, rhizosphere incompetence and inability of bacterial strains in thriving different soil types and environmental conditions. Actinobacteria with their sporulation, nutrient cycling, root colonizing, bio-control and other plant growth promoting activities could be potential field bio-inoculants. We report isolation of 5 rhizospheric and 2 root endophytic actinobacteria from Triticum aestivum (wheat) plants. The cultures exhibited plant growth promoting activities namely phosphate solubilization (1916 mg l-1), phytase (0.68 U ml-1), chitinase (6.2 U ml-1), IAA (136.5 mg l-1) and siderophore (47.4 mg l-1) production as well utilized all rhizospheric sugars under test. Malate (50-55 mmol l-1) was estimated in culture supernatant of highest phosphate solublizer Streptomyces mhcr0816. The mechanism of malate overproduction was studied by gene expression and assays of key glyoxalate cycle enzymes - isocitrate dehydrogenase (IDH), isocitrate lyase (ICL) and malate synthase (MS). The significant increase in gene expression (ICL 4, MS 6 fold) and enzyme activity (ICL 4, MS 10 fold) of ICL and MS during stationary phase resulted in malate production as indicated by lowered pH (2.9) and HPLC analysis (RT 13.1 min). Similarly, the secondary metabolites for chitinase independent biocontrol activity of Streptomyces mhcr0817 as identified by GC-MS and 1H-NMR spectra were isoforms of pyrrole derivatives. The inoculation of actinobacterial isolate mhce0811 in Triticum aestivum (wheat) significantly improved plant growth, biomass (33%) and mineral (Fe, Mn, P) content in non-axenic conditions. Thus actinobacterial isolates reported were efficient PGPR possessing significant antifungal activity and may have potential field applications.
The adverse effects of arsenic (As) toxicity on seedling growth, root and shoot anatomy, chlorophyll and carotenoid contents, root oxidizability (RO), antioxidant enzyme activities, H2O2 content, lipid peroxidation and electrolyte leakage (EL%) in common bean (Phaseolus vulgaris L.) were investigated. The role of exogenous nitric oxide (NO) in amelioration of As-induced inhibitory effect was also evaluated using sodium nitroprusside (100 μM SNP) as NO donor and 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (200 μM PTIO) as NO scavenger in different combinations with 50 μM As. As-induced growth inhibition was associated with marked anomalies in anatomical features, reduction in pigment composition, increased RO and severe perturbations in antioxidant enzyme activities. While activity of superoxide dismutase and catalase increased, levels of ascorbate peroxidase, dehydroascorbate reductase and glutathione reductase decreased significantly and guaiacol peroxidase remained normal. The over-accumulation of H2O2 content along with high level of lipid peroxidation and electrolyte leakage indicates As-induced oxidative damage in P. vulgaris seedlings with more pronounced effect on the roots than the shoots. Exogenous addition of NO significantly reversed the As-induced oxidative stress, maintaining H2O2 in a certain level through balanced alterations of antioxidant enzyme activities. The role of NO in the process of amelioration has ultimately been manifested by significant reduction of membrane damage and improvement of growth performance in plants grown on As + SNP media. Onset of oxidative stress was more severe after addition of PTIO, which confirms the protective role of NO against As-induced oxidative damage in P. vulgaris seedlings.
The use of metal tolerant plants for the phytostabilization of metal contaminated soil is an area of extensive research and development. In this study the effects of inoculation of Ni-resistant bacterial strains on phytostabilization potential of various plants, including Brassica juncea, Luffa cylindrica and Sorghum halepense, were studied. A Ni-resistant bacterial strain SR28C was isolated from a nickel rich serpentine soil and identified as Bacillus megaterium based on the morphological features, biochemical characteristics and partial 16S rDNA sequence analysis. The strain SR28C tolerated concentrations up to 1200 mg Ni L(-1) on a Luria-Bertani (LB) agar medium. Besides, it showed high degree of resistance to various metals (Cu, Zn, Cd, Pb and Cr) and antibiotics (ampicillin, tetracycline, streptomycin, chloramphenicol, penicillin and kanamycin) tested. In addition, the strain bound considerable amounts of Ni in their resting cells. Besides, the strain exhibited the plant growth promoting traits, such as solubilization of phosphate and production of indole-3-acetic acid (IAA) in modified Pikovskayas medium and LB medium, respectively in the absence and presence of Ni. Considering such potential, the effects of SR28C on the growth and Ni accumulation of B. juncea, L. cylindrica and S. halepense, were assessed with different concentrations of Ni in soil. Inoculation of SR28C stimulated the biomass of the test plants grown in both Ni contaminated and non-contaminated soils. Further, SR28C alleviated the detrimental effects of Ni by reducing its uptake and translocation to the plants. This study suggested that the PGPB inoculant due to its intrinsic abilities of growth promotion and attenuation of the toxic effects of Ni could be exploited for phytostabilization of Ni contaminated site.
The mechanism of phosphate and arsenate uptake was investigated in arsenate-tolerant and non-tolerant genotypes of Holcus lanatus L. It appeared that arsenate and phosphate were taken up by the roots by the same uptake system in both genotypes, although the uptake system had a much greater affinity for phosphate than arsenate. In non-tolerant plants the uptake of both anions was much greater than for tolerant plants at low phosphate and arsenate concentrations. High-affinity uptake appeared to be absent in the tolerant plants for both arsenate and phosphate. The absence of this high-affinity uptake is discussed in terms of arsenate tolerance in this species. The identification of a mutant with only one phosphate uptake system provides further evidence that ‘normal’ uptake of ions occurs via two distinct transport systems. The theoretical and ecological implications of this altered phosphate uptake mechanism are considered.
Plant-growth-promoting (PGP) bacteria especially with the resistance to multiple heavy metals are helpful to phytoremediation. Further development of PGP bacteria is very necessary because of the extreme diversity of plants, soils, and heavy metal pollution. A Burkholderia sp. strain, numbered LD-11, was isolated, which showed resistances to multiple heavy metals and antibiotics. It can produce indole-3-acetic acid, 1-aminocyclopropane-1-carboxylic acid deaminase and siderophores. Inoculation with the LD-11 improved germination of seeds of the investigated vegetable plants in the presence of Cu, promoted elongation of roots and hypocotyledonary axes, enhanced the dry weights of the plants grown in the soils polluted with Cu and/or Pb, and increased activity of the soil urease and the rhizobacteria diversity. Inoculation with the LD-11 significantly enhanced Cu and/or Pb accumulation especially in the roots of the plants grown in the polluted soils. Notably, LD-11 could produce siderophores in the presence of Cu. Conclusively, the PGP effects and concurrent heavy metal accumulation in the plant tissues results from combined effects of the above-mentioned multiple factors. Cu is an important element that represses production of the siderophore by the bacteria. Phytoremediation by synergistic use of the investigated plants and the bacterial strain LD-11 is a phytoextraction process.
Pollution of the biosphere by heavy metals is a global threat that has accelerated dramatically since the beginning of industrial revolution. The aim of the study is to check the resistance of RL9 towards the metals and to observe the effect of Rhizobium species on growth, pigment content, protein and nickel uptake by lentil in the presence and absence of nickel. The multi metal tolerant and plant growth promoting Rhizobium strain RL9 was isolated from the nodules of lentil. The strain not only tolerated nickel but was also tolerant o cadmium, chromium, nickel, lead, zinc and copper. The strain tolerated nickel 500 μg/mL, cadmium 300 μg/mL, chromium 400 μg/mL, lead 1,400 μg/mL, zinc 1,000 μg/mL and copper 300 μg/mL, produced good amount of indole acetic acid and was also positive for siderophore, hydrogen cyanide and ammonia. The strain RL9 was further assessed with increasing concentrations of nickel when lentil was used as a test crop. The strain RL9 significantly increased growth, nodulation, chlorophyll, leghaemoglobin, nitrogen content, seed protein and seed yield compared to plants grown in the absence of bioinoculant but amended with nickel The strain RL9 decreased uptake of nickel in lentil compared to plants grown in the absence of bio-inoculant. Due to these intrinsic abilities strain RL9 could be utilized for growth promotion as well as for the remediation of nickel in nickel contaminated soil.
Pseudomonas aeruginosa strain OSG41, isolated from the heavy metal contaminated water irrigated to rhizospheric soil of mustard crop, tolerated chromium up to the concentration of 1800 μg ml−1 and reduced it by 100% at pH 6–8 after 120 h incubation at 30–40 °C. P. aeruginosa produced plant growth-promoting substances, both in the presence and absence of chromium; it produced 32 μg ml−1 indole acetic acid ml−1, in Luria Bertani broth with 100 mg tryptophan ml−1, solubilized tri-calcium phosphate (417 μg ml−1) and secreted 20.8 μg ml−1 exopolysaccharides (EPS) which decreased with increasing concentration of chromium added to growth medium. While investigating the impact of hexavalent chromium on chickpea, chromium application to soil had a phytotoxic effect. The application of P. aeruginosa strain OSG41 even with three times concentration of chromium increased the dry matter accumulation, symbiotic attributes (like nodule formation), grain yield and protein of chickpea compared to non-inoculated plants. The bio-inoculant decreased the uptake of chromium by 36, 38 and 40% in roots, shoots and grains, respectively. The present finding suggests that the bioinoculant effectively reduced the toxicity of hexavalent chromium to chickpea plants and concurrently enhanced the biological and chemical characteristics of chickpea, when grown in chromium treated soils.