A rhizosphere-associated symbiont, Photobacterium spp. strain MELD1, and its targeted synergistic activity for phytoprotection against mercury

Microbial mediated remediation of heavy metals toxicity: mechanisms and future prospects

Heavy metal pollution has become a serious concern across the globe due to their persistent nature, higher toxicity, and recalcitrance. These toxic metals threaten the stability of the environment and the health of all living beings. Heavy metals also enter the human food chain by eating contaminated foods and cause toxic effects on human health. Thus, remediation of HMs polluted soils is mandatory and it needs to be addressed at higher priority. The use of microbes is considered as a promising approach to combat the adverse impacts of HMs. Microbes aided in the restoration of deteriorated environments to their natural condition, with long-term environmental effects. Microbial remediation prevents the leaching and mobilization of HMs and they also make the extraction of HMs simple. Therefore, in this context recent technological advancement allowed to use of bioremediation as an imperative approach to remediate polluted soils. Microbes use different mechanisms including bio-sorption, bioaccumulation, bioleaching, bio-transformation, bio-volatilization and bio-mineralization to mitigate toxic the effects of HMs. Thus, keeping in the view toxic HMs here in this review explores the role of bacteria, fungi and algae in bioremediation of polluted soils. This review also discusses the various approaches that can be used to improve the efficiency of microbes to remediate HMs polluted soils. It also highlights different research gaps that must be solved in future study programs to improve bioremediation efficency.

Critical review on biogeochemical dynamics of mercury (Hg) and its abatement strategies

Mercury (Hg) is among the naturally occurring heavy metal with elemental, organic, and inorganic distributions in the environment. Being considered a global pollutant, high pools of Hg-emissions ranging from >6000 to 8000 Mg Hg/year get accumulated by the natural and anthropogenic activities in the atmosphere. These toxicants have high persistence, toxicity, and widespread contamination in the soil, water, and air resources. Hg accumulation inside the plant parts amplifies the traces of toxic elements in the linking food chains, leads to Hg exposure to humans, and acts as a potential genotoxic, neurotoxic and carcinogenic entity. However, excessive Hg levels are equally toxic to the plant system and severely disrupt the physiological and metabolic processes in plants. Thus, a plausible link between Hg-concentration and its biogeochemical behavior is highly imperative to analyze the plant-soil interactions. Therefore, it is requisite to bring these toxic contaminants in between the acceptable limits to safeguard the environment. Plants efficiently incorporate or absorb the bioavailable Hg from the soil thus a constructive understanding of Hg uptake, translocation/sequestration involving specific heavy metal transporters, and detoxification mechanisms are drawn. Whereas recent investigations in biological remediation of Hg provide insights into the potential associations between the plants and microbes. Furthermore, intense research on Hg-induced antioxidants, protein networks, metabolic mechanisms, and signaling pathways is required to understand these bioremediations techniques. This review sheds light on the mercury (Hg) sources, pollution, biogeochemical cycles, its uptake, translocation, and detoxification methods with respect to its molecular approaches in plants.

Molecular and Physiological Mechanisms to Mitigate Abiotic Stress Conditions in Plants

Agriculture production faces many abiotic stresses, mainly drought, salinity, low and high temperature. These abiotic stresses inhibit plants' genetic potential, which is the cause of huge reduction in crop productivity, decrease potent yields for important crop plants by more than 50% and imbalance agriculture's sustainability. They lead to changes in the physio-morphological, molecular, and biochemical nature of the plants and change plants' regular metabolism, which makes them a leading cause of losses in crop productivity. These changes in plant systems also help to mitigate abiotic stress conditions. To initiate the signal during stress conditions, sensor molecules of the plant perceive the stress signal from the outside and commence a signaling cascade to send a message and stimulate nuclear transcription factors to provoke specific gene expression. To mitigate the abiotic stress, plants contain several methods of avoidance, adaption, and acclimation. In addition to these, to manage stress conditions, plants possess several tolerance mechanisms which involve ion transporters, osmoprotectants, proteins, and other factors associated with transcriptional control, and signaling cascades are stimulated to offset abiotic stress-associated biochemical and molecular changes. Plant growth and survival depends on the ability to respond to the stress stimulus, produce the signal, and start suitable biochemical and physiological changes. Various important factors, such as the biochemical, physiological, and molecular mechanisms of plants, including the use of microbiomes and nanotechnology to combat abiotic stresses, are highlighted in this article.

Comparative Genomic Analyses of the Genus Photobacterium Illuminate Biosynthetic Gene Clusters Associated with Antagonism

The genus Photobacterium is known for its ecophysiological versatility encompassing free-living, symbiotic, and pathogenic lifestyles. Photobacterium sp. CCB-ST2H9 was isolated from estuarine sediment collected at Matang Mangrove, Malaysia. In this study, the genome of CCB-ST2H9 was sequenced, and the pan-genome of 37 Photobacterium strains was analysed. Phylogeny based on core genes showed that CCB-ST2H9 clustered with P. galatheae, forming a distinct clade with P. halotolerans, P. salinisoli, and P. arenosum. The core genome of Photobacterium was conserved in housekeeping functions, while the flexible genome was well represented by environmental genes related to energy production and carbohydrate metabolism. Genomic metrics including 16S rRNA sequence similarity, average nucleotide identity, and digital DNA–DNA hybridization values were below the cut-off for species delineation, implying that CCB-ST2H9 potentially represents a new species. Genome mining revealed that biosynthetic gene clusters (BGCs) involved in producing antimicrobial compounds such as holomycin in CCB-ST2H9 could contribute to the antagonistic potential. Furthermore, the EtOAc extract from the culture broth of CCB-ST2H9 exhibited antagonistic activity against Vibrio spp. Intriguingly, clustering based on BGCs profiles grouped P. galatheae, P. halotolerans, P. salinisoli, P. arenosum, and CCB-ST2H9 together in the heatmap by the presence of a large number of BGCs. These BGCs-rich Photobacterium strains represent great potential for bioactive secondary metabolites production and sources for novel compounds.

Conjoint application of novel bacterial isolates on dynamic changes in oxidative stress responses of axenic Brassica juncea L. in Hg-stress soils

This experimental study explores the possible role of three Hg-resistant bacterial strains in the enhanced growth of the mustard plant (Brassica juncea) under Hg-stress conditions. Under different concentrations of Hg, a pot scale experiment with Brassica juncea L. was performed to investigate the potential of bacterial strains for phytoremediation under Hg stress conditions. The results showed that all three strains, as well as their consortium, were capable of stimulating plant growth, biomass, and anti-oxidative enzyme activities. In comparison to the individual strains, the consortiums of all three strains were more prominent in the intensification of Brassica juncea L. physiological activity. Under Hg-stress conditions, all three strains increased the level of antioxidative content in Brassica juncea, indicating an increase in enzyme activity to cope with oxidative stress. Among all the three strains, Citrobacter Freundii (IITISM25) showed the highest accumulation potential in B. juncea followed by Morganella morganii (IITISM23) and Brevundimonas Dimunta (IITISM22). Hence, the results suggest that the IITISM22, IITISM23, IITISM25 strains and their consortium are very effective in phytoremediation and promote Brassica juncea growth under Hg-stress conditions.

Heavy Metal Stress Alleviation Through Omics Analysis of Soil and Plant Microbiome

Heavy metal (HM) contamination of soil and water resources is a global concern, which not only limits crop yield and quality, but also has serious environmental effects. Due to the non-biodegradable nature and toxicity, high concentration of HMs in food and environment is a serious threat to the entire ecosystem. Moreover, the target of supplying safe and quality food to the rising human population (expected to reach ~9–10 bn by the year 2050), necessitates effective treatment of the HM-contaminated soil. Various microbe-mediated bioremediation strategies such as biosorption, bioprecipiation, biostimulation, etc., have been found to be effective in uptake and conversion of HMs to less toxic forms. Further, in the past few years, the use of soil and plant-associated microbiome for HM stress alleviation is gaining attention among the scientific community. In general, microbes are spectacular in being dynamic and more responsive to environmental conditions in comparison to their host plants. Moreover, with the advancements in high throughput sequencing technologies, the focus is eventually shifting from just structural characterization to functional insights into the microbiome. The microbes inhabiting the HM-contaminated environments or associated with HM-tolerant plants are a source for exploring HM-tolerant microbial communities, which could be used for enhancing bioremediation efficiency and conferring HM tolerance in plants. This review discusses the application of omics techniques including metagenomics, metatranscriptomics, metaproteomics, and metabolomics, for rapid and robust identification of HM-tolerant microbial communities, mining novel HM resistance genes, and fabricating the HM resistome.

Plant Growth Promoting and Stress Mitigating Abilities of Soil Born Microorganisms

Abiotic stresses affect the plant growth in different ways and at different developmental stages that reduce the crop yields. The increasing world population continually demands more crop yields; therefore it is important to use low-cost technologies against abiotic stresses to increase crop productivity. Soil microorganisms survive in the soil associated with plants in extreme condition. It was demonstrated that these beneficial microorganisms promote plant growth and development under various stresses. The soil microbes interact with the plant through rhizospheric or endophytic association and promote the plant growth through different processes such as nutrients mobilization, disease suppression, and hormone secretions. The microorganisms colonized in the rhizospheric region and imparted the abiotic stress tolerance by producing 1-aminocyclopropane-1- carboxylate (ACC) deaminase, antioxidant, and volatile compounds, inducing the accumulation of osmolytes, production of exopolysaccharide, upregulation or downregulation of stress genes, phytohormones and change the root morphology. A large number of these rhizosphere microorganisms are now patented. In the present review, an attempt was made to throw light on the mechanism of micro-organism that operates during abiotic stresses and promotes plant survival and productivity.

Role of Two Plant Growth-Promoting Bacteria in Remediating Cadmium-Contaminated Soil Combined with Miscanthus floridulus (Lab.)

Miscanthus spp. are energy plants and excellent candidates for phytoremediation approaches of metal(loid)s-contaminated soils, especially when combined with plant growth-promoting bacteria. Forty-one bacterial strains were isolated from the rhizosphere soils and roots tissue of five dominant plants (Artemisia argyi Levl., Gladiolus gandavensis Vaniot Houtt, Boehmeria nivea L., Veronica didyma Tenore, and Miscanthus floridulus Lab.) colonizing a cadmium (Cd)-contaminated mining area (Huayuan, Hunan, China). We subsequently tested their plant growth-promoting (PGP) traits (e.g., production of indole-3-acetic acid, siderophore, and 1-aminocyclopropane-1-carboxylate deaminase) and Cd tolerance. Among bacteria, two strains, Klebsiella michiganensis TS8 and Lelliottia jeotgali MR2, presented higher Cd tolerance and showed the best results regarding in vitro growth-promoting traits. In the subsequent pot experiments using soil spiked with 10 mg Cd·kg⁻¹, we investigated the effects of TS8 and MR2 strains on soil Cd phytoremediation when combined with M. floridulus (Lab.). After sixty days of planting M. floridulus (Lab.), we found that TS8 increased plant height by 39.9%, dry weight of leaves by 99.1%, and the total Cd in the rhizosphere soil was reduced by 49.2%. Although MR2 had no significant effects on the efficiency of phytoremediation, it significantly enhanced the Cd translocation from the root to the aboveground tissues (translocation factor > 1). The combination of K. michiganensis TS8 and M. floridulus (Lab.) may be an effective method to remediate Cd-contaminated soils, while the inoculation of L. jeotgali MR2 may be used to enhance the phytoextraction potential of M. floridulus.

Comparative Analysis of Bacterial Diversity and Community Structure in the Rhizosphere and Root Endosphere of Two Halophytes, Salicornia europaea and Glaux maritima, Collected from Two Brackish Lakes in Japan

Microbial community structures associated with halophytes and their compositions among different habitats, particularly natural saline sites, have not yet been investigated in detail. In the present study, we examined the diversity and composition of the rhizosphere and root endosphere bacteria of two halophytes, Salicornia europaea L. and Glaux maritima L., collected from two adjacent brackish lakes, Lake Notoro and Lake Tofutsu, in Japan. The bacterial species richness and diversity indices of the two halophytes collected from both lakes showed no significant differences in the rhizosphere or root endosphere. In contrast, beta diversity and taxonomic analyses revealed significant differences in the bacterial communities from each halophyte between the two lakes even though the two locations were natural saline sites, indicating that the bacterial communities for S. europaea and G. maritima both fluctuated in a manner that depended on the geographical location. Common and abundant genera associated with each halophyte across the two lakes were then identified to verify the bacterial genera specifically inhabiting each plant species. The results obtained showed that the composition of abundant genera inhabiting each halophyte across two lakes was distinct from that reported previously in other saline soil areas. These results suggest that each halophyte in different geographical sites had an individual complex bacterial community.

Phytoremediation and Microorganisms-Assisted Phytoremediation of Mercury-Contaminated Soils: Challenges and Perspectives

Mercury (Hg) pollution is a global threat to human and environmental health because of its toxicity, mobility and long-term persistence. Although costly engineering-based technologies can be used to treat heavily Hg-contaminated areas, they are not suitable for decontaminating agricultural or extensively-polluted soils. Emerging phyto- and bioremediation strategies for decontaminating Hg-polluted soils generally involve low investment, simple operation, and in situ application, and they are less destructive for the ecosystem. Current understanding of the uptake, translocation and sequestration of Hg in plants is reviewed to highlight new avenues for exploration in phytoremediation research, and different phytoremediation strategies (phytostabilization, phytoextraction and phytovolatilization) are discussed. Research aimed at identifying suitable plant species and associated-microorganisms for use in phytoremediation of Hg-contaminated soils is also surveyed. Investigation into the potential use of transgenic plants in Hg-phytoremediation is described. Recent research on exploiting the beneficial interactions between plants and microorganisms (bacteria and fungi) that are Hg-resistant and secrete plant growth promoting compounds is reviewed. We highlight areas where more research is required into the effective use of phytoremediation on Hg-contaminated sites, and conclude that the approaches it offers provide considerable potential for the future.

Unlocking the potential of plant growth-promoting rhizobacteria on soil health and the sustainability of agricultural systems

The concept of soil health refers to specific soil properties and the ability to support and sustain crop growth and productivity, while maintaining long-term environmental quality. The key components of healthy soil are high populations of organisms that promote plant growth, such as the plant growth promoting rhizobacteria (PGPR). PGPR plays multiple beneficial and ecological roles in the rhizosphere soil. Among the roles of PGPR in agroecosystems are the nutrient cycling and uptake, inhibition of potential phytopathogens growth, stimulation of plant innate immunity, and direct enhancement of plant growth by producing phytohormones or other metabolites. Other important roles of PGPR are their environmental cleanup capacities (soil bioremediation). In this work, we review recent literature concerning the diverse mechanisms of PGPR in maintaining healthy conditions of agricultural soils, thus reducing (or eliminating) the toxic agrochemicals dependence. In conclusion, this review provides comprehensive knowledge on the current PGPR basic mechanisms and applications as biocontrol agents, plant growth stimulators and soil rhizoremediators, with the final goal of having more agroecological practices for sustainable agriculture.

Endophytic bacteria stimulate mercury phytoremediation by modulating its bioaccumulation and volatilization

The quantification, efficiency, and possible mechanisms of mercury phytoremediation by endophytic bacteria are poorly understood. Here we selected 8 out of 34 previously isolated endophytic bacterial strains with a broad resistance profile to metals and 11 antibiotics: Acinetobacter baumannii BacI43, Bacillus sp. BacI34, Enterobacter sp. BacI14, Klebsiella pneumoniae BacI20, Pantoea sp. BacI23, Pseudomonas sp. BacI7, Pseudomonas sp. BacI38, and Serratia marcescens BacI56. Except for Klebsiella pneumoniae BacI20, the other seven bacterial strains promoted maize growth on a mercury-contaminated substrate. Acinetobacter baumannii BacI43 and Bacillus sp. BacI34 increased total dry biomass by approximately 47%. The bacteria assisted mercury remediation by decreasing the metal amount in the substrate, possibly by promoting its volatilization. The plants inoculated with Serratia marcescens BacI56 and Pseudomonas sp. BacI38 increased mercury volatilization to 47.16% and 62.42%, respectively. Except for Bacillus sp. BacI34 and Pantoea sp. BacI23, the other six bacterial strains favored mercury bioaccumulation in plant tissues. Endophytic bacteria-assisted phytoremediation contributed to reduce the substrate toxicity assessed in different model organisms. The endophytic bacterial strains selected herein are potential candidates for assisted phytoremediation that shall help reduce environmental toxicity of mercury-contaminated soils.

Variations of soil bacterial diversity and metabolic function with tidal flat elevation gradient in an artificial mangrove wetland

Understanding the sensitivity of soil bacteria to environmental fluctuations can enhance the management of microbial ecosystem services in artificial mangrove wetlands. In this study, the variation in bacterial diversity and metabolic functions in different compartments (bulk soil, rhizosphere soil, and rhizoplane) of the soil and mangrove plant along the tidal elevation gradient was studied in Xiatanwei (Xiamen China) mangrove wetland park, a Kandelia obovata-dominated artificial mangrove stand. With the increase of the tidal flat elevation, the soil pH, total organic matter, and soil moisture contents decreased significantly, while the soil electric conductivity and redox potential increased significantly. The bacterial diversity in the bulk soil and the rhizosphere soil both decreased with the elevation of tidal levels. The relative abundance of the dominant phyla in the bulk and rhizosphere soils decreased with the rise of the tidal flat level. A significant rhizosphere effect was observed in the roots of K. obovata that the rhizosphere soil had higher bacterial diversity and richness than that in the bulk soil nearby. The rhizosphere soil of K. obovata at the low-tidal flat was enriched with the genera Nitrospira and Planctomycetes, which are valuable for the mangrove ecosystem. The Chao1 estimator and Shannon index of the bacterial community in the rhizoplane of K. obovata were much lower than that in the rhizosphere and bulk soils. Results of Biolog-Eco assay show that the bacterial groups in low tidal flat bulk soil had the highest ability in utilizing the carbon sources, which was indicated by the high values of average well color development and the high McIntosh index, and the utilization ability of carbon source decreased with the increase of tidal flat levels. The variation of the soil humidity and Eh jointly shaped the diversity and metabolic function of soil bacterial communities along the tidal flat elevation gradient.

Characterization of cadmium-resistant rhizobacteria and their promotion effects on Brassica napus growth and cadmium uptake

Excessive cadmium (Cd) accumulation in soil can adversely affect plants, animals, microbes, and humans; therefore, novel and uncharacterized Cd-resistant plant-growth-promoting rhizobacteria (PGPR) are required to address this issue. In the paper, 13 bacteria were screened, their partial 16S rRNA sequences determined, and the isolates, respectively, clustered into Curtobacterium (7), Chryseobacterium (4), Cupriavidus (1), and Sphingomonas (1). Evaluation of PGP traits, including indole-3-acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, siderophore secretion, and cyanhydric acid production, identified Cupriavidus necator GX_5, Sphingomonas sp. GX_15, and Curtobacterium sp. GX_31 as promising candidates for PGPR based on high IAA or ACC deaminase production. Additionally, root-elongation assays indicated that inoculating GX_5, _15, or _31 increased Brassica napus root length both in the presence and absence of Cd by 19.75-29.96% and 19.15-31.69%, respectively. Pot experiments indicated that inoculating B. napus with GX_5, _15, and _31 significantly increased the dry weight of above-ground tissues and root biomass by 40.97-85.55% and 18.99-103.13%, respectively. Moreover, these isolates significantly increased Cd uptake in the aerial parts and root tissue of B. napus by 7.38-11.98% and 48.09-79.73%, respectively. These results identified GX_5, _15, or _31 as excellent promoters of metal remediation by using microorganism-associated phytoremediation.

Microbial Community Succession and Nutrient Cycling Responses following Perturbations of Experimental Saltwater Aquaria

Saltwater aquaria are living systems that support a complex biological community of fish, invertebrates, and microbes. The health and maintenance of saltwater tanks are pressing concerns for home hobbyists, zoos, and professionals in the aquarium trade; however, we do not yet understand the underlying microbial species interactions and community dynamics which contribute to tank setup and conditioning. This report provides a detailed view of ecological succession and changes in microbial community assemblages in two saltwater aquaria which were sampled over a 3-month period, from initial tank setup and conditioning with “live rocks” through subsequent tank cleanings and water replacement. Our results showed that microbial succession appeared to be consistent and replicable across both aquaria. However, changes in microbial communities did not always correlate with water chemistry measurements, and aquarium microbial communities appear to have shifted among multiple stable states without any obvious buildup of undesirable nitrogen compounds in the tank environment.

Although aquaria are common features of homes and other buildings, little is known about how environmental perturbations (i.e., tank cleaning, water changes, addition of habitat features) impact the diversity and succession of aquarium microbial communities. In this study, we sought to evaluate the hypotheses that newly established aquaria show clear microbial successional patterns over time and that common marine aquarium-conditioning practices, such as the addition of ocean-derived “live rocks” (defined as any “dead coral skeleton covered with crustose coralline algae” transferred into an aquarium from open ocean habitats) impact the diversity of microbial populations as well as nitrogen cycling in aquaria. We collected water chemistry data alongside water and sediment samples from two independent and newly established saltwater aquaria over a 3-month period. Microbial communities in samples were assessed by DNA extraction, amplification of the 16S rRNA gene, and Illumina MiSeq sequencing. Our results showed clear and replicable patterns of community succession in both aquaria, with the existence of multiple stable states for aquarium microbial assemblages. Notably, our results show that changes in aquarium microbial communities do not always correlate with water chemistry measurements and that operational taxonomic unit (OTU)-level patterns relevant to nitrogen cycling were not reported as statistically significant. Overall, our results demonstrate that aquarium perturbations have a substantial impact on microbial community profiles of aquarium water and sediment and that the addition of live rocks improves nutrient cycling by shifting aquarium communities toward a more typical saltwater assemblage of microbial taxa.

IMPORTANCE Saltwater aquaria are living systems that support a complex biological community of fish, invertebrates, and microbes. The health and maintenance of saltwater tanks are pressing concerns for home hobbyists, zoos, and professionals in the aquarium trade; however, we do not yet understand the underlying microbial species interactions and community dynamics which contribute to tank setup and conditioning. This report provides a detailed view of ecological succession and changes in microbial community assemblages in two saltwater aquaria which were sampled over a 3-month period, from initial tank setup and conditioning with “live rocks” through subsequent tank cleanings and water replacement. Our results showed that microbial succession appeared to be consistent and replicable across both aquaria. However, changes in microbial communities did not always correlate with water chemistry measurements, and aquarium microbial communities appear to have shifted among multiple stable states without any obvious buildup of undesirable nitrogen compounds in the tank environment.

Comparative Genomics Reveals High Genomic Diversity in the Genus Photobacterium

Vibrionaceae is a large marine bacterial family, which can constitute up to 50% of the prokaryotic population in marine waters. Photobacterium is the second largest genus in the family and we used comparative genomics on 35 strains representing 16 of the 28 species described so far, to understand the genomic diversity present in the Photobacterium genus. Such understanding is important for ecophysiology studies of the genus. We used whole genome sequences to evaluate phylogenetic relationships using several analyses (16S rRNA, MLSA, fur, amino-acid usage, ANI), which allowed us to identify two misidentified strains. Genome analyses also revealed occurrence of higher and lower GC content clades, correlating with phylogenetic clusters. Pan- and core-genome analysis revealed the conservation of 25% of the genome throughout the genus, with a large and open pan-genome. The major source of genomic diversity could be traced to the smaller chromosome and plasmids. Several of the physiological traits studied in the genus did not correlate with phylogenetic data. Since horizontal gene transfer (HGT) is often suggested as a source of genetic diversity and a potential driver of genomic evolution in bacterial species, we looked into evidence of such in Photobacterium genomes. Genomic islands were the source of genomic differences between strains of the same species. Also, we found transposase genes and CRISPR arrays that suggest multiple encounters with foreign DNA. Presence of genomic exchange traits was widespread and abundant in the genus, suggesting a role in genomic evolution. The high genetic variability and indications of genetic exchange make it difficult to elucidate genome evolutionary paths and raise the awareness of the roles of foreign DNA in the genomic evolution of environmental organisms.

Abiotic Stress Responses and Microbe-Mediated Mitigation in Plants: The Omics Strategies

Abiotic stresses are the foremost limiting factors for agricultural productivity. Crop plants need to cope up adverse external pressure created by environmental and edaphic conditions with their intrinsic biological mechanisms, failing which their growth, development, and productivity suffer. Microorganisms, the most natural inhabitants of diverse environments exhibit enormous metabolic capabilities to mitigate abiotic stresses. Since microbial interactions with plants are an integral part of the living ecosystem, they are believed to be the natural partners that modulate local and systemic mechanisms in plants to offer defense under adverse external conditions. Plant-microbe interactions comprise complex mechanisms within the plant cellular system. Biochemical, molecular and physiological studies are paving the way in understanding the complex but integrated cellular processes. Under the continuous pressure of increasing climatic alterations, it now becomes more imperative to define and interpret plant-microbe relationships in terms of protection against abiotic stresses. At the same time, it also becomes essential to generate deeper insights into the stress-mitigating mechanisms in crop plants for their translation in higher productivity. Multi-omics approaches comprising genomics, transcriptomics, proteomics, metabolomics and phenomics integrate studies on the interaction of plants with microbes and their external environment and generate multi-layered information that can answer what is happening in real-time within the cells. Integration, analysis and decipherization of the big-data can lead to a massive outcome that has significant chance for implementation in the fields. This review summarizes abiotic stresses responses in plants in-terms of biochemical and molecular mechanisms followed by the microbe-mediated stress mitigation phenomenon. We describe the role of multi-omics approaches in generating multi-pronged information to provide a better understanding of plant-microbe interactions that modulate cellular mechanisms in plants under extreme external conditions and help to optimize abiotic stresses. Vigilant amalgamation of these high-throughput approaches supports a higher level of knowledge generation about root-level mechanisms involved in the alleviation of abiotic stresses in organisms.

Phytolacca acinosa Roxb. with Arthrobacter echigonensis MN1405 enhances heavy metal phytoremediation

The growth and metal-extraction efficiency of plants when exposed to toxic metals can be enhanced by inoculating with certain bacteria, but the mechanisms of this process remain unclear. We report results from glasshouse experiments on the effect of Arthrobacter echigonensis MN1405 in promoting Phytolacca acinosa Roxb. growth when exposed to 100 mg/L Mn solution. Mn removal efficiency in solution was significantly enhanced by bacterial inoculation; Mn was accumulated in the root of P. acinosa Roxb. plant. The bacteria oxidized the Mn on root surface, which formed a Mn plaque to serve as a barrier or a containment to prevent metal toxicity. In this process, pH condition was an important factor on the effects of microbial-assisted heavy metal phytoremediation. Our finding suggests that A. echigonensis MN1405 assisted P. acinosa to achieve high remediation efficiency of Mn removal and accumulation in Mn contamination area.

Genomic sequence analysis of a plant-associated Photobacterium halotolerans MELD1: from marine to terrestrial environment?

Mercury impacts the function and development of the central nervous system in both humans and wildlife by being a potent neurotoxin. Microbial bioremediation is an important means of remediation of mercury-contaminated soil. The rhizospheric Photobacterium halotolerans strain MELD1 was isolated from mercury and dioxin contaminated site from Tainan, Taiwan. It has been shown to reduce Hg²⁺ to Hg⁰. The 4,758,027 bp genome of P. halotolerans MELD1 has a G + C content of 50.88 % and contains 4198 protein-coding and 106 RNA genes. Genomic analysis revealed the presence of a number of interesting gene cluster that maybe involved in heavy metal resistance, rhizosphere competence and colonization of the host plant.

Genome Sequence of Photobacterium halotolerans MELD1, with Mercury Reductase (merA), Isolated from Phragmites australis

Here, we present the whole-genome sequence of Photobacterium halotolerans strain, MELD1, isolated from the roots of a terrestrial plant Phragmites australis grown in soil heavily contaminated with mercury and dioxin. The genome provides further insight into the adaptation of bacteria to the toxic environment from where it was isolated.