Search

Trans-generational adaptation is important to respond rapidly to environmental challenges and increase overall plant fitness. Besides well-known mechanisms such as epigenetic modifications, vertically transmitted endophytic bacteria might contribute to this process. The cultivable and total endophytic communities of several generations of Arabidopsis thaliana seeds harvested from plants exposed to cadmium (Cd) or not exposed were investigated. The diversity and richness of the seed endophytic community decreased with an increasing number of generations. Aeromicrobium and Pseudonocardia were identified as indicator species in seeds from Cd-exposed plants, while Rhizobium was abundantly present in both seed types. Remarkably, Rhizobium was the only genus that was consistently detected in seeds of all generations, which suggests that the phenotypic characteristics were more important as selection criteria for which bacteria are transferred to the next plant generation than the actual genera. Production of IAA was an important trait for endophytes from both seed types, while ACC deaminase activity and Cd tolerance were mainly associated with seed endophytes from Cd-exposed plants. Understanding how different factors influence the seed endophytic community can help us to improve seed quality and plant growth through different biotechnological applications. [ABSTRACT FROM AUTHOR]

In order to stimulate selection for plant-associated bacteria with the potential to improve Cd phytoextraction, yellow lupine plants were grown on a metal-contaminated field soil. It was hypothesised that growing these plants on this contaminated soil, which is a source of bacteria possessing different traits to cope with Cd, could enhance colonisation of lupine with potential plant-associated bacteria that could then be inoculated in Cd-exposed plants to reduce Cd phytotoxicity and enhance Cd uptake. All cultivable bacteria from rhizosphere, root and stem were isolated and genotypically and phenotypically characterised. Many of the rhizobacteria and root endophytes produce siderophores, organic acids, indole-3-acetic acid ( IAA) and aminocyclopropane-1-carboxylate ( ACC) deaminase, as well as being resistant to Cd and Zn. Most of the stem endophytes could produce organic acids (73.8%) and IAA (74.3%), however, only a minor fraction (up to 0.7%) were Cd or Zn resistant or could produce siderophores or ACC deaminase. A siderophore- and ACC deaminase-producing, highly Cd-resistant Rhizobium sp. from the rhizosphere, a siderophore-, organic acid-, IAA- and ACC deaminase-producing highly Cd-resistant Pseudomonas sp. colonising the roots, a highly Cd- and Zn-resistant organic acid and IAA-producing Clavibacter sp. present in the stem, and a consortium composed of these three strains were inoculated into non-exposed and Cd-exposed yellow lupine plants. Although all selected strains possessed promising in vitro characteristics to improve Cd phytoextraction, inoculation of none of the strains (i) reduced Cd phytotoxicity nor (ii) strongly affected plant Cd uptake. This work highlights that in vitro characterisation of bacteria is not sufficient to predict the in vivo behaviour of bacteria in interaction with their host plants. [ABSTRACT FROM AUTHOR]

We examined whether long-term Cd exposure leads to beneficial changes in the cultivable endophytic bacteria present in the seeds ofAgrostis capillaris. Therefore the cultivable seed endophytes ofAgrostis capillarisgrowing on a long-term Cd/Ni-contaminated plot (Cd/Ni seeds) were compared with those originating from a non-contaminated plot (control seeds). We observed plant- and contaminant-dependent effects on the population composition between control and Cd/Ni seeds. Also differences in phenotypic characteristics were found: endophytes from Cd/Ni seeds exhibited more ACC deaminase activity and production of siderophores and IAA, while endophytes from control seeds, very surprisingly, showed more metal tolerance. Finally, the 3 most promising seed endophytes were selected based on their metal tolerance and plant growth promoting potential, and inoculated inAgrostis capillarisseedlings. In case of non-exposed plants, inoculation resulted in a significantly improved plant growth; after inoculation of Cd-exposed plants an increased Cd uptake was achieved without affecting plant growth. This indicates that inoculation ofAgrostiswith its seed endophytes might be beneficial for its establishment during phytoextraction and phytostabilisation of Cd-contaminated soils. [ABSTRACT FROM AUTHOR]

Plant-associated bacteria can have beneficial effects on the growth and health of their host. Nevertheless, the role of endophytic bacteria present in seeds has not been investigated in depth. In this study, the cultivable endophytic population of seeds from Arabidopsis thaliana exposed to 2 μ m cadmium for several generations ( Cd seeds) was compared with a population isolated from seeds of plants that were never exposed to Cd (control seeds). We observed obvious differences between the two types of seed concerning genera present and phenotypic characteristics of the different isolates. Sinorhizobium sp. and Micrococcus sp. were only found in control seeds, while Pseudomonas sp., Bosea sp. and Paenibacillus sp. were only found in Cd seeds. Sphingomonas sp., Rhizobium sp., Acidovorax sp., Variovorax sp., Methylobacterium sp., Bacillus sp. and Staphylococcus sp. occurred in varying numbers in both types of seed. Metal tolerance and 1-aminocyclopropane-1-carboxylate deaminase activity were predominantly found in strains isolated from Cd seeds, while the production of siderophores, indole-3-acetic acid and organic acids was more prevalent in endophytes isolated from control seeds. These data support the hypothesis that certain endophytes are selected for transfer to the next generation and that their presence might be important for subsequent germination and early seedling development. [ABSTRACT FROM AUTHOR]

Cultivable bacterial strains associated with field-grown Brassica napus L. (soil, rhizosphere and roots) from a trace elements ( Cd, Zn and Pb) contaminated field and a non-contaminated control field were characterized genotypically and phenotypically. Correspondence analysis of the genotypic data revealed a correlation between soil and rhizosphere communities isolated from the same field, indicating that local conditions play a more important role in influencing the composition of (rhizosphere) soil bacterial communities than root exudates. In contrast, endophytic communities of roots showed a correlation between fields, suggesting that plants on the two fields contain similar obligate endophytes derived from a common seed endophytic community and/or can select bacteria from the rhizosphere. The latter seemed not very likely since, despite the presence of several potential endophytic taxa in the rhizosphere, no significant correlation was found between root and rhizosphere communities. The majority of Cd/ Zn tolerant strains capable of phosphorus solubilization, nitrogen fixation, indole-3-acetic acid production and showing 1-aminocyclopropane-1-carboxylate deaminase capacity were found in the rhizosphere and roots of plants growing on the contaminated field. [ABSTRACT FROM AUTHOR]

ABSTRACT Phytoextraction has a promising potential as an environmentally friendly clean-up method for soils contaminated with toxic metals. To improve the development of efficient phytoextraction strategies, better knowledge regarding metal uptake, translocation and detoxification in planta is a prerequisite. This review highlights our current understanding on these mechanisms, and their impact on plant growth and health. Special attention is paid to the central role of glutathione (GSH) in this process. Because of the high affinity of metals to thiols and as a precursor for phytochelatins (PCs), GSH is an essential metal chelator. Being an important antioxidant, a direct link between metal detoxification and the oxidative challenge in plants growing on contaminated soils is observed, where GSH could be a key player. In addition, as redox couple, oxidized and reduced GSH transmits specific information, in this way tuning cellular signalling pathways under environmental stress conditions. Possible improvements of phytoextraction could be achieved by using transgenic plants or plant-associated microorganisms. Joined efforts should be made to cope with the challenges faced with phytoextraction in order to successfully implement this technique in the field. [ABSTRACT FROM AUTHOR]

Background: The beneficial use of nanoparticle silver or nanosilver may be confounded when its potent antimicrobial properties impact non-target members of natural microbiomes such as those present in soil or the plant rhizosphere. Agricultural soils are a likely sink for nanosilver due to its presence in agrochemicals and land-applied biosolids, but a complete assessment of nanosilver's effects on this environment is lacking because the impact on the natural soil microbiome is not known. In a study assessing the use of nanosilver for phytopathogen control with maize, we analyzed the metatranscriptome of the maize rhizosphere and observed multiple unintended effects of exposure to 100 mg kg -1 nanosilver in soil during a growth period of 117 days., Results: We found several unintended effects of nanosilver which could interfere with agricultural systems in the long term. Firstly, the archaea community was negatively impacted with a more than 30% decrease in relative abundance, and as such, their involvement in nitrogen cycling and specifically, nitrification, was compromised. Secondly, certain potentially phytopathogenic fungal groups showed significantly increased abundances, possibly due to the negative effects of nanosilver on bacteria exerting natural biocontrol against these fungi as indicated by negative interactions in a network analysis. Up to 5-fold increases in relative abundance have been observed for certain possibly phytopathogenic fungal genera. Lastly, nanosilver exposure also caused a direct physiological impact on maize as illustrated by increased transcript abundance of aquaporin and phytohormone genes, overall resulting in a stress level with the potential to yield hormetically stimulated plant root growth., Conclusions: This study indicates the occurrence of significant unintended effects of nanosilver use on corn, which could turn out to be negative to crop productivity and ecosystem health in the long term. We therefore highlight the need to include the microbiome when assessing the risk associated with nano-enabled agriculture. Video Abstract.

Chlorendic acid is a recalcitrant, highly chlorinated organic pollutant for which no microbial degrader has yet been identified. To address this knowledge gap, fungi were isolated from bulk soil, rhizosphere, and roots of the common bent ( Agrostis capillaris ) and the hybrid poplar [ Populus deltoides × ( Populus trichocarpa × P. deltoides ) cv. Grimminge], both of which grow on a chlorendic acid polluted site in Belgium. Isolates were taxonomically identified and phenotypically screened for chlorendic acid degradation. Several fungal isolates could degrade chlorendic acid in liquid media up to 45%. The chlorendic acid degrading fungal isolates produced higher levels of hydroxyl radicals when exposed to the pollutant when compared to non-exposed controls, suggesting that the oxidative degradation of chlorendic acid occurs through production of Fenton-mediated hydroxyl radicals. In addition, the isolated Ascomycete Penicillium sp. 1D-2a degraded 58% of the original chlorendic acid concentration in the soil after 28 days. This study demonstrates that the presence of fungi in a chlorendic acid polluted soil can degrade this highly chlorinated organic pollutant. These results indicate that recalcitrant, seemingly non-biologically degradable organic pollutants, such as chlorendic acid, can be remediated by using bioremediation, which opens new perspectives for in situ bioremediation.

The aerial surfaces of plants harbor diverse communities of microorganisms. The rising awareness concerning the potential roles of these phyllosphere microbiota for airborne pollutant remediation and plant growth promotion, advocates for a better understanding of their community structure and dynamics in urban ecosystems. Here, we characterized the epiphytic microbial communities on leaves of Platanus × hispanica trees in the city centre of Hasselt (Belgium), and the nearby forest area of Bokrijk, Genk (Belgium). We compared the influences of season, site, and air pollutants concentration variations on the tree's phyllosphere microbiome by determining the intra- and inter-individual variation in leaf bacterial communities. High-throughput amplicon sequencing of the 16S rRNA gene revealed large variation in the bacterial community structure and diversity throughout the years but also allowed to discriminate an environment effect on community assembly. Partial drivers for this environment effect on composition can be correlated with the huge differences in ultrafine particulate matter (UFP) and black carbon on the leaves. A change in bacterial community composition was noted for trees growing in the city center compared to the natural site, and also more human-associated genera were found colonizing the leaves from the city center. These integrated results offer an original and first insight in the Platanus phyllomicrobiota, which can offer new opportunities to use phyllosphere microorganisms to enhance air pollution degradation.

Phytoextraction could be a potential management option for diffusely Cd-Zn-Pb-polluted agricultural land in Northeast Belgium. The use of high yielding crops with a sufficiently high metal accumulation is preferred as these are expected to both gradually decontaminate the soil while generating an income through biomass valorization. To find out which high biomass crop possessed the highest and most constant (in time) phytoextraction potential on these soils, different plant species and different mutants or clones of each species, were evaluated during consecutive years. Biomass production and metal accumulation of pre-selected tobacco somaclonal variants ( Nicotiana tabacum L.) and pre-selected sunflower mutants ( Helianthus annuus L.) were investigated for two productivity years, while the phytoextraction potential of experimental poplar ( Populus ) and willow ( Salix ) in short rotation coppice (SRC) was assessed at the end of the second cutting cycle (after two times four growing seasons). The tobacco clones and the sunflower mutants showed efficient extraction of, respectively, Cd and Zn, while the highest simultaneous extractions of Cd and Zn were gained with some SRC clones. Variation in biomass production and metal accumulation were high for all crops over the years. The highest biomass production was observed for the experimental poplar clone of the crossing type Populus deltoides ( P. maximowiczii x P. trichocarpa ) with 9.9 ton DW per ha per year. The remediation period to reach legal threshold values for the pseudo-total content of Cd in this specific soil was estimated to be at least 60 years. Combining estimated phytoextraction potential and economic and environmental aspects, the SRC option is proposed as the most suitable crop for implementing metal phytoextraction in the investigated area.

Plants and their associated bacteria play a crucial role in constructed wetlands. In this study, the impact of different levels of pollution and bioaugmentation with indigenous strains individually or in consortia was investigated on the composition of the endophytic microbial communities of Juncus acutus . Five treatments were examined and compared in where the wetland plant was exposed to increasing levels of metal pollution (Zn, Ni, Cd) and emerging pollutants (BPA, SMX, CIP), enriched with different combinations of single or mixed endophytic strains. High levels of mixed pollution had a negative effect on alpha diversity indices of the root communities; moreover, the diversity indices were negatively correlated with the increasing metal concentrations. It was demonstrated that the root communities were separated depending on the level of mixed pollution, while the family Sphingomonadaceae exhibited the higher relative abundance within the root endophytic communities from high and low polluted treatments. This study highlights the effects of pollution and inoculation on phytoremediation efficiency based on a better understanding of the plant microbiome community composition.

Military activities have worldwide introduced toxic explosives into the environment with considerable effects on soil and plant-associated microbiota. Fortunately, these microorganisms, and their collective metabolic activities, can be harnessed for site restoration via in situ phytoremediation. We characterized the bacterial communities inhabiting the bulk soil and rhizosphere of sycamore maple ( Acer pseudoplatanus ) in two chronically 2,4,6-trinitrotoluene (TNT) polluted soils. Three hundred strains were isolated, purified and characterized, a majority of which showed multiple plant growth promoting (PGP) traits. Several isolates showed high nitroreductase enzyme activity and concurrent TNT-transformation. A 12-member bacterial consortium, comprising selected TNT-detoxifying and rhizobacterial strains, significantly enhanced TNT removal from soil compared to non-inoculated plants, increased root and shoot weight, and the plants were less stressed than the un-inoculated plants as estimated by the responses of antioxidative enzymes. The sycamore maple tree (SYCAM) culture collection is a significant resource of plant-associated strains with multiple PGP and catalytic properties, available for further genetic and phenotypic discovery and use in field applications.

Efficient N 2 -fixing Leguminosae nodulating bacteria resistant to As may facilitate plant growth on As-contaminated sites. In order to identify bacteria holding these features, 24 strains were isolated from nodules of the trap species Crotalaria spectabilis (12) and Stizolobium aterrimum (12) growing on an As-contaminated gold mine site. 16S rRNA gene sequencing revealed that most of the strains belonged to the group of α-Proteobacteria, being representatives of the genera Bradyrhizobium, Rhizobium, Inquilinus, Labrys, Bosea, Starkeya, and Methylobacterium. Strains of the first four genera showed symbiotic efficiency with their original host, and demonstrated in vitro specific plant-growth-promoting (PGP) traits (production of organic acids, indole-3-acetic-acid and siderophores, 1-aminocyclopropane-1-carboxylate deaminase activity, and Ca 3 (PO 4 ) 2 solubilization), and increased resistance to As, Zn, and Cd. In addition, these strains and some type and reference rhizobia strains exhibited a wide resistance spectrum to β-lactam antibiotics. Both intrinsic PGP abilities and multi-element resistance of rhizobia are promising for exploiting the symbiosis with different legume plants on trace-element-contaminated soils.

Bioremediation of polluted soils is a promising technique with low environmental impact, which uses soil organisms to degrade soil contaminants. In this study, 19 bacterial strains isolated from a diesel-contaminated soil were screened for their diesel-degrading potential, biosurfactant (BS) production, and biofilm formation abilities, all desirable characteristics when selecting strains for re-inoculation into hydrocarbon-contaminated soils. Diesel-degradation rates were determined in vitro in minimal medium with diesel as the sole carbon source. The capacity to degrade diesel range organics (DROs) of strains SPG23 (Arthobacter sp.) and PF1 (Acinetobacter oleivorans) reached 17-26% of total DROs after 10 days, and 90% for strain GK2 (Acinetobacter calcoaceticus). The amount and rate of alkane degradation decreased significantly with increasing carbon number for strains SPG23 and PF1. Strain GK2, which produced BSs and biofilms, exhibited a greater extent, and faster rate of alkane degradation compared to SPG23 and PF1. Based on the outcomes of degradation experiments, in addition to BS production, biofilm formation capacities, and previous genome characterizations, strain GK2 is a promising candidate for microbial-assisted phytoremediation of diesel-contaminated soils. These results are of particular interest to select suitable strains for bioremediation, not only presenting high diesel-degradation rates, but also other characteristics which could improve rhizosphere colonization.

Plant growth promoting endophytic bacteria (PGPB) isolated from Brassica napus were inoculated in two cultivars of Helianthus tuberosus (VR and D19) growing on sand supplemented with 0.1 mM Cd or 1 mM Zn. Plant growth, concentrations of metals and thiobarbituric acid (TBA) reactive compounds were determined. Colonization of roots of H. tuberosus D19 by Pseudomonas sp. 262 was evaluated using confocal laser scanning microscopy. Pseudomonas sp. 228, Serratia sp. 246 and Pseudomonas sp. 262 significantly enhanced growth of H. tuberosus D19 exposed to Cd or Zn. Pseudomonas sp. 228 significantly increased Cd concentrations in roots. Serratia sp. 246, and Pseudomonas sp. 256 and 228 resulted in significantly decreased contents of TBA reactive compounds in roots of Zn exposed D19 plants. Growth improvement and decrease of metal-induced stress were more pronounced in D19 than in VR. Pseudomonas sp. 262 - green fluorescent protein (GFP) colonized the root epidermis/exodermis and also inside root hairs, indicating that an endophytic interaction was established. H. tuberosus D19 inoculated with Pseudomonas sp. 228, Serratia sp. 246 and Pseudomonas sp. 262 holds promise for sustainable biomass production in combination with phytoremediation on Cd and Zn contaminated soils., Competing Interests: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

The aim of this study was to investigate the potential of indigenous arsenic-tolerant bacteria to enhance arsenic phytoremediation by the autochthonous pseudometallophyte Betula celtiberica The first goal was to perform an initial analysis of the entire rhizosphere and endophytic bacterial communities of the above-named accumulator plant, including the cultivable bacterial species. B. celtiberica 's microbiome was dominated by taxa related to Flavobacteriales , Burkholderiales , and Pseudomonadales , especially the Pseudomonas and Flavobacterium genera. A total of 54 cultivable rhizobacteria and 41 root endophytes, mainly affiliated with the phyla Proteobacteria , Bacteroidetes , Firmicutes , and Actinobacteria , were isolated and characterized with respect to several potentially useful features for metal plant accumulation, such as the ability to promote plant growth, metal chelation, and/or mitigation of heavy-metal stress. Seven bacterial isolates were further selected and tested for in vitro accumulation of arsenic in plants; four of them were finally assayed in field-scale bioaugmentation experiments. The exposure to arsenic in vitro caused an increase in the total nonprotein thiol compound content in roots, suggesting a detoxification mechanism through phytochelatin complexation. In the contaminated field, the siderophore and indole-3-acetic acid producers of the endophytic bacterial consortium enhanced arsenic accumulation in the leaves and roots of Betula celtiberica , whereas the rhizosphere isolate Ensifer adhaerens strain 91R mainly promoted plant growth. Field experimentation showed that additional factors, such as soil arsenic content and pH, influenced arsenic uptake in the plant, attesting to the relevance of field conditions in the success of phytoextraction strategies. IMPORTANCE Microorganisms and plants have developed several ways of dealing with arsenic, allowing them to resist and metabolize this metalloid. These properties form the basis of phytoremediation treatments and the understanding that the interactions of plants with soil bacteria are crucial for the optimization of arsenic uptake. To address this in our work, we initially performed a microbiome analysis of the autochthonous Betula celtiberica plants growing in arsenic-contaminated soils, including endosphere and rhizosphere bacterial communities. We then proceeded to isolate and characterize the cultivable bacteria that were potentially better suited to enhance phytoextraction efficiency. Eventually, we went to the field application stage. Our results corroborated the idea that recovery of pseudometallophyte-associated bacteria adapted to a large historically contaminated site and their use in bioaugmentation technologies are affordable experimental approaches and potentially very useful for implementing effective phytoremediation strategies with plants and their indigenous bacteria., (Copyright © 2017 American Society for Microbiology.)