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Salt marshes are highly dynamic, biologically diverse ecosystems with a broad range of ecological functions. We investigated the endophytic bacterial community of surface sterilized seeds of the holoparasitic Cistanche phelypaea growing in coastal salt marshes of the Iberian Peninsula in Portugal. C. phelypaea is the only representative of the genus Cistanche that was reported in such habitat. Using high-throughput sequencing methods, 23 bacterial phyla and 263 different OTUs on genus level were found. Bacterial strains belonging to phyla Proteobacteria and Actinobacteriota were dominating. Also some newly classified or undiscovered bacterial phyla, unclassified and unexplored taxonomic groups, symbiotic Archaea groups inhabited the C. phelypaea seeds. γ-Proteobacteria was the most diverse phylogenetic group. Sixty-three bacterial strains belonging to Bacilli, Actinomycetes, α-, γ- and β-Proteobacteria and unclassified bacteria were isolated. We also investigated the in vitro PGP traits and salt tolerance of the isolates. Among the Actinobacteria, Micromonospora spp. showed the most promising endophytes in the seeds. Taken together, the results indicated that the seeds were inhabited by halotolerant bacterial strains that may play a role in mitigating the adverse effects of salt stress on the host plant. In future research, these bacteria should be assessed as potential sources of novel and unique bioactive compounds or as novel bacterial species. [ABSTRACT FROM AUTHOR]

Endophytes within plants are known to be crucial for plant fitness, and while their presence and functions in many compartments have been studied in depth, the research on seed endophytes is still limited. This work aimed to characterize the seed endophytic and rhizospheric bacterial community of two Noccaea caerulescens Pb-Zn hyperaccumulator populations, growing on two heavy-metal-polluted sites in Belgium. Cultured representatives were evaluated for their potential to enhance seed germination and root length of the model species Arabidopsis thaliana. The results indicated that the community structure within the seed is conserved between the two locations, comprising mainly of Proteobacteria (seeds), and Actinobacteria in the bulk soil. Root length of A. thaliana was significantly increased when inoculated with Sphingomonas vulcanisoli. The results of this paper offer insights into the importance of the selection of the core seed endophytic microbiome and highlight the precarious symbiotic relationship they have with the plant and seed. [ABSTRACT FROM AUTHOR]

Background: A diverse community of microbes naturally exists on the phylloplane, the surface of leaves. It is one of the most prevalent microbial habitats on earth and bacteria are the most abundant members, living in communities that are highly dynamic. Today, one of the key challenges for microbiologists is to develop strategies to culture the vast diversity of microorganisms that have been detected in metagenomic surveys. Results: We isolated bacteria from the phylloplane of Hedera helix (common ivy), a widespread evergreen, using five growth media: Luria–Bertani (LB), LB01, yeast extract–mannitol (YMA), yeast extract–flour (YFlour), and YEx. We also included a comparison with the uncultured phylloplane, which we showed to be dominated by Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes. Inter-sample (beta) diversity shifted from LB and LB01 containing the highest amount of resources to YEx, YMA, and YFlour which are more selective. All growth media equally favoured Actinobacteria and Gammaproteobacteria, whereas Bacteroidetes could only be found on LB01, YEx, and YMA. LB and LB01 favoured Firmicutes and YFlour was most selective for Betaproteobacteria. At the genus level, LB favoured the growth of Bacillus and Stenotrophomonas, while YFlour was most selective for Burkholderia and Curtobacterium. The in vitro plant growth promotion (PGP) profile of 200 isolates obtained in this study indicates that previously uncultured bacteria from the phylloplane may have potential applications in phytoremediation and other plant-based biotechnologies. Conclusions: This study gives first insights into the total bacterial community of the H. helix phylloplane, including an evaluation of its culturability using five different growth media. We further provide a collection of 200 bacterial isolates underrepresented in current databases, including the characterization of PGP profiles. Here we highlight the potential of simple strategies to obtain higher microbial diversity from environmental samples and the use of high-throughput sequencing to guide isolate selection from a variety of growth media. [ABSTRACT FROM AUTHOR]

Exploiting the potential of bacteria in phytoremediation for the removal of organic and inorganic pollutants from soils and (ground)water holds great promise. Besides bacteria, mycorrhizal fungi and free-living saprotrophs are well known for their strong degradative capacities and plant growth promotion effects, which makes them of high interest for use in different bioremediation strategies. To further increase the efficiency and successes of phytoremediation, interactions between plants and their associated microorganisms, both bacteria and fungi, should be further investigated, in addition to the close interactions between bacteria and fungi. Benefitting from an increased understanding of microbial community structure and assembly allows us to better understand how the holobiont can be modified to improve pollutant degradation and plant growth. In this review, we present an overview of insights in plant-bacteria-fungi interactions and the opportunities of exploiting these tripartite interactions to enhance the effectiveness of phytoremediation of organic pollutants. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Efficient N2-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 Ca3(PO4)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. [ABSTRACT FROM AUTHOR]

Many endophytic bacteria exert beneficial effects on their host, but still little is known about the bacteria associated with plants growing in areas heavily polluted by hydrocarbons. The aim of the study was characterization of culturable hydrocarbon-degrading endophytic bacteria associated with Lotus corniculatus L. and Oenothera biennis L. collected in long-term petroleum hydrocarbon-polluted site using culture-dependent and molecular approaches. A total of 26 hydrocarbon-degrading endophytes from these plants were isolated. Phylogenetic analyses classified the isolates into the phyla Proteobacteria and Actinobacteria. The majority of strains belonged to the genera Rhizobium, Pseudomonas, Stenotrophomonas, and Rhodococcus. More than 90% of the isolates could grow on medium with diesel oil, approximately 20% could use n-hexadecane as a sole carbon and energy source. PCR analysis revealed that 40% of the isolates possessed the P450 gene encoding for cytochrome P450-type alkane hydroxylase (CYP153). In in vitro tests, all endophytic strains demonstrated a wide range of plant growth-promoting traits such as production of indole-3-acetic acid, hydrogen cyanide, siderophores, and phosphate solubilization. More than 40% of the bacteria carried the gene encoding for the 1-aminocyclopropane-1-carboxylic acid deaminase ( acdS). Our study shows that the diversity of endophytic bacterial communities in tested plants was different. The results revealed also that the investigated plants were colonized by endophytic bacteria possessing plant growth-promoting features and a clear potential to degrade hydrocarbons. The properties of isolated endophytes indicate that they have the high potential to improve phytoremediation of petroleum hydrocarbon-polluted soils. [ABSTRACT FROM AUTHOR]

Phytoremediation is increasingly adopted as a more sustainable approach for soil remediation. However, significant advances in efficiency are still necessary to attain higher levels of environmental and economic sustainability. Current interventions do not always give the expected outcomes in field settings due to an incomplete understanding of the multicomponent biological interactions. New advances in -omics are gradually implemented for studying microbial communities of polluted landin situ. This opens new perspectives for the discovery of biodegradative strains and provides us new ways of interfering with microbial communities to enhance bioremediation rates. This review presents retrospectives and future perspectives for plant microbiome studies relevant to phytoremediation, as well as some knowledge gaps in this promising research field. The implementation of phytoremediation in soil clean-up management systems is discussed, and an overview of the promoting factors that determine the growth of the phytoremediation market is given. Continuous growth is expected since elimination of contaminants from the environment is demanded. The evolution of scientific thought from a reductionist view to a more holistic approach will boost phytoremediation as an efficient and reliable phytotechnology. It is anticipated that phytoremediation will prove the most promising for organic contaminant degradation and bioenergy crop production on marginal land. [ABSTRACT FROM PUBLISHER]

The association of plants and rhizospheric bacteria provides a successful strategy to clean up contaminated soils. The purpose of this work was to enhance diesel degradation in Lupinus luteus rhizosphere by inoculation with selected bacterial strains: a diesel degrader (D), plant growth-promoting (PGP) strains, or a combination (D+PGP). Plants were set up in pots with the A or B horizon of an umbric Cambisol (ACamb and BCamb) spiked with diesel (1.25%, w/w). After 1 mo, the dissipation of diesel range organics (DRO) with respect to t = 0 (i.e., 1 wk after preparing the pots with the seedlings) concentration was significantly higher in inoculated than in noninoculated (NI) pots: The highest DRO losses were found in ACamb D+PGP pots (close to 15-20% higher than NI) and in BCamb D pots (close to 10% higher). The water-extractable DRO fraction was significantly higher at t = 30 d (15-25%) compared with t = 0 (<5%), probably due to the effects of plant root exudates and biosurfactants produced by the degrader strain. The results of this experiment reflect the importance of the partnerships between plants and bacterial inoculants and demonstrate the relevance of the effect of bacterial biosurfactants and plant root exudates on contaminant bioavailability, a key factor for enhancing diesel rhizodegradation. The association of lupine with D and PGP strains resulted in a promising combination for application in the rhizoremediation of soils with moderate diesel contamination. [ABSTRACT FROM AUTHOR]

Phytoremediation is a promising technology to clean-up contaminated soils based on the synergistic actions of plants and microorganisms. However, to become a widely accepted, and predictable remediation alternative, a deeper understanding of the plant-microbe interactions is needed. A number of studies link the success of phytoremediation to the plant-associated microbiome functioning, though whether the microbiome can exist in alternative, functional states for soil remediation, is incompletely understood. Moreover, current approaches that target the plant host, and environment separately to improve phytoremediation, potentially overlook microbial functions and properties that are part of the multiscale complexity of the plant-environment wherein biodegradation takes place. In contrast, in situ studies of phytoremediation research at the metaorganism level (host and microbiome together) are lacking. Here, we discuss a competition-driven model, based on recent evidence from the metagenomics level, and hypotheses generated by microbial community ecology, to explain the establishment of a catabolic rhizosphere microbiome in a contaminated soil. There is evidence to ground that if the host provides the right level and mix of resources (exudates) over which the microbes can compete, then a competitive catabolic and plant-growth promoting (PGP) microbiome can be selected for as long as it provides a competitive superiority in the niche. The competition-driven model indicates four strategies to interfere with the microbiome. Specifically, the rhizosphere microbiome community can be shifted using treatments that alter the host, resources, environment, and that take advantage of prioritization in inoculation. Our model and suggestions, considering the metaorganism in its natural context, would allow to gain further knowledge on the plant-microbial functions, and facilitate translation to more effective, and predictable phytotechnologies. [ABSTRACT FROM AUTHOR]

Although the importance of plant-associated microorganisms for plant growth and health was getting more recognition recently, the role of seed-associated microorganisms, and especially seed endophytic bacteria, still is underestimated. Nevertheless, these associations could be beneficial for germination and seedling establishment as seed endophytic bacteria are already present in these very early plant growth stages. Moreover, bacteria with beneficial characteristics can be selected by the plants and could be transferred via the seed to benefit the next generation. In this paper, the current literature concerning bacterial endophytes that have been isolated from seeds of different plant species is reviewed. Their colonization routes, localization inside seeds and mode of transmission as well as their role and fate during germination and seedling development are discussed. At the end, some examples of bacterial seed endophytes with applications as a plant growth-promoting or biocontrol agent are given. [ABSTRACT FROM AUTHOR]

Background and aims: Numerous microorganisms have been isolated from trinitrotoluene (TNT)-contaminated soils, however TNT tends to persist, indicating that the microbial biomass or activity is insufficient for degradation. Deep-rooting trees at military sites have been found to take-up contaminants from groundwater, and the extensive root and endosphere provide ideal niches for microbial TNT-transformations. Methods: We characterised the rhizosphere, root endosphere and endo-phyllosphere bacteria of Acer pseudoplatanus growing at a historically TNT-contaminated location, using 16S rRNA gene fingerprinting, bacteria isolation, oxidoreductase gene-cloning, in planta growth-promotion (PGP) tests, inoculation, plant physiology measurements and microscopy. Results: Based on terminal-restriction-fragment-length-polymorphism analysis, bulk soil and rhizosphere samples were highly clustered. Proteo- and Actinobacteria dominated the rhizosphere and root endosphere, whereas Alphaproteobacteria were more abundant in shoots and Actinobacteria in leaves. We isolated multiple PGP-bacteria and cloned 5 flavin-oxidoreductases belonging to the Old Yellow Enzyme family involved in TNT-reduction from 3 Pseudomonas spp., the leaf symbiont Stenotrophomonas chelatiphaga and the root endophyte Variovorax ginsengisola. Conclusions: The inoculation with a selection of these strains, consortium CAP9, which combines efficient TNT-transformation capabilities with beneficial PGP-properties, has the ability to detoxify TNT in the bent grass ( Agrostis capillaris) rhizosphere, stimulate plant growth and improve plant health under TNT stress. [ABSTRACT FROM AUTHOR]

The presence of explosives in soils and the interaction with drought stress and nutrient limitation are among the environmental factors that severely affect plant growth on military soils. In this study, we seek to isolate and identify the cultivable bacteria of a 2,4-dinitrotoluene ( DNT) contaminated soil ( DS) and an adjacent grassland soil ( GS) of a military training area aiming to isolate new plant growth-promoting ( PGP) and 2,4- DNT-degrading strains. Metabolic profiling revealed disturbances in Ecocarbon use in the bare DS; isolation of cultivable strains revealed a lower colony-forming-unit count and a less diverse community associated with DS in comparison with GS. New 2,4- DNT-tolerant strains were identified by selective enrichments, which were further characterized by auxanography for 2,4- DNT use, resistance to drought stress, cold, nutrient starvation and PGP features. By selecting multiple beneficial PGP and abiotic stress-resistant strains, efficient 2,4- DNT-degrading consortia were composed. After inoculation, consortium UHasselt Sofie 3 with seven members belonging to B urkholderia, V ariovorax, B acillus, P seudomonas and R alstonia species was capable to successfully enhance root length of A rabidopsis under 2,4- DNT stress. After 9 days, doubling of main root length was observed . Our results indicate that beneficial bacteria inhabiting a disturbed environment have the potential to improve plant growth and alleviate 2,4- DNT stress. [ABSTRACT FROM AUTHOR]

Phytoextraction has been reported as an economically and ecologically sound alternative for the remediation of metal-contaminated soils. Willow is a metal phytoextractor of interest because it allows to combine a gradual contaminant removal with production of biomass that can be valorized in different ways. In this work two willow clones growing on a metal-contaminated site were selected: ' Belgisch Rood' ( BR) with a moderate metal extraction capacity and ' Tora' ( TO) with a twice as high metal accumulation. All cultivable bacteria associated with both willow clones were isolated and identified using 16SrDNA ARDRA analysis followed by 16SrDNA sequencing. Further all isolated bacteria were investigated for characteristics that might promote plant growth (production of siderophores, organic acids and indol acetic acid) and for their metal resistance. The genotypic and phenotypic characterization of the isolated bacteria showed that the TO endophytic bacterial population is more diverse and contains a higher percentage of metal-resistant plant growth promoting bacteria than the endophytic population associated with BR. We hypothesize that the difference in the metal accumulation capacity between BR and TO clones might be at least partly related to differences in characteristics of their associated bacterial population. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study was to investigate if bacteria with beneficial properties that were isolated from willow growing on a metal-contaminated site can be further equipped with genes coding for a specific degradation pathway to finally obtain transconjugants that can be inoculated in willow to improve phytoremediation efficiency of mixed contaminations. Materials and methods: Cultivable rhizosphere bacteria and root endophytes were isolated from willow (cv. Tora) growing on a metal-contaminated soil. All isolated strains were tested for their metal resistance and potential to promote plant growth. The two most promising strains were selected and were equipped with the pTOM plasmid coding for toluene degradation. Both transconjugants were inoculated separately and combined in willow cuttings exposed to mixed Cd-toluene contamination, and their effect on phytotoxicity, Cd uptake, and toluene evapotranspiration was evaluated. Results and discussion: Many of the isolated strains tested positive for the production of siderophores, organic acids, and indole acetic acid (IAA) and showed increased Cd resistance. The Cd-resistant, siderophore-producing rhizosphere strain Burkholderia sp. HU001 and the Cd-resistant root endophyte Pseudomonas sp. HU002, able to produce siderophores, organic acids, and IAA, were selected as receptors for conjugation with the toluene-degrading Burkholderia vietnamiensis BU61 as a donor of the pTOM-TCE plasmid. Although inoculation with the individual transconjugant strains had no effect on plant growth and negatively affected Cd uptake, their combined inoculation resulted in an increased shoot biomass upon Cd-toluene exposure did not affect Cd uptake and strongly reduced evapotranspiration of toluene to the atmosphere. Conclusions: In this study, inoculation of willow with a consortium of plant-associated bacteria equipped with the appropriate characteristics resulted in an improved phytoremediation of a mixed Cd-toluene contamination: the degradation of toluene was improved leading to a decreased toxicity and evapotranspiration, while Cd uptake and translocation were not affected. [ABSTRACT FROM AUTHOR]

This study aimed to investigate effects on growth and development and alterations in the nutrient profiles for Arabidopsis thaliana seedlings following uranium exposure. Seventeen-day-old Arabidopsis thaliana seedlings, grown in hydroponics, were exposed to 0, 0.1, 1, 10 and 100 μM uranium for one, three and seven days. Fresh weight of leaves and roots decreased after exposure to 100 μM uranium for one and three days and after exposure to 1, 10 and 100 μM uranium for seven days. Anthocyanous-colored leaves and stunted yellow roots were observed after exposure to 100 μM uranium. The uranium content of the roots highly increased but the root-to-shoot transfer of uranium was limited. Uranium exposure resulted in disturbed macro- and micronutrient profiles, especially following exposure to 100 μM. This study indicates that elevated uranium concentrations can cause important morphological and physiological effects and disturb the nutrient profiles in Arabidopsis thaliana seedlings. [ABSTRACT FROM PUBLISHER]

Inoculation of plants with their associated microorganisms is a promising strategy for improving phytoremediation of organic contaminants. Isolation and characterisation of these organisms from plants growing in contaminated sites will permit the identification of candidate strains for re-inoculation studies. The diversity of culturable endophytic and rhizoplane bacteria found in association with Cytisus striatus plants growing at a hexachlorocyclohexane (HCH)-contaminated site was studied. A total of 97 strains of endophytic bacteria were isolated from the root, stem and leaf tissues, and 49 from the rhizoplane. They were further characterised genotypically (BOX-PCR, 16S rDNA sequencing, presence of linA and linB genes) and phenotypically (trace metal tolerance, capacity to produce biosurfactants and plant growth promoting (PGP) traits). Proteobacteria and Actinobacteria dominated the isolate collection, and taxonomic diversity was strongly tissue-specific. The linA and linB genes were not detected in the isolate collection. The majority of isolates had at least one of the PGP traits tested, whereas biosurfactant-producing strains were less frequent. Resistance to more than one trace metal was generally restricted to endophytes isolated from shoot tissues. The PGP characteristics found in an important number of the bacterial isolates obtained in this study could be particularly useful for exploiting the phytoremediation potential of C. striatus. [ABSTRACT FROM AUTHOR]

Enterobacter sp. 638 is an endophytic plant growth promoting gamma-proteobacterium that was isolated from the stem of poplar (Populus trichocarpa6deltoides cv. H11-11), a potentially important biofuel feed stock plant. The Enterobacter sp. 638 genome sequence reveals the presence of a 4,518,712 bp chromosome and a 157,749 bp plasmid (pENT638-1). Genome annotation and comparative genomics allowed the identification of an extended set of genes specific to the plant niche adaptation of this bacterium. This includes genes that code for putative proteins involved in survival in the rhizosphere (to cope with oxidative stress or uptake of nutrients released by plant roots), root adhesion (pili, adhesion, hemagglutinin, cellulose biosynthesis), colonization/establishment inside the plant (chemiotaxis, flagella, cellobiose phosphorylase), plant protection against fungal and bacterial infections (siderophore production and synthesis of the antimicrobial compounds 4- hydroxybenzoate and 2-phenylethanol), and improved poplar growth and development through the production of the phytohormones indole acetic acid, acetoin, and 2,3-butanediol. Metabolite analysis confirmed by quantitative RT-PCR showed that, the production of acetoin and 2,3-butanediol is induced by the presence of sucrose in the growth medium. Interestingly, both the genetic determinants required for sucrose metabolism and the synthesis of acetoin and 2,3- butanediol are clustered on a genomic island. These findings point to a close interaction between Enterobacter sp. 638 and its poplar host, where the availability of sucrose, a major plant sugar, affects the synthesis of plant growth promoting phytohormones by the endophytic bacterium. The availability of the genome sequence, combined with metabolome and transcriptome analysis, will provide a better understanding of the synergistic interactions between poplar and its growth promoting endophyte Enterobacter sp. 638. This information can be further exploited to improve establishment and sustainable production of poplar as an energy feedstock on marginal, non-agricultural soils using endophytic bacteria as growth promoting agents. [ABSTRACT FROM AUTHOR]

Seed endophyte inoculation can enhance the plant tolerance to pollutants, which allows plant cultivation on trace element (TE) polluted soils. Methylobacterium sp. CP3 and Kineococcus endophyticus CP19 were tested in vitro for their tolerance to Zn and Cd and their plant growth promotion traits. The in vivo effects of bioaugmentation with individual strains or both strains were tested using two poplar cultivars, Populus deltoides x (P. trichocarpa x P. maximowiczii) 'Dender' and 'Marke', grown in TE polluted soil for six weeks. Methylobacterium sp. was found to grow on media enriched with 0.4 and 0.8 mM Cd, and both endophytes tolerated 0.6 and 1 mM Zn, due to the presence of genes involved in Zn and Cd tolerance and transport. Methylobacterium sp. showed an extracellular ion sequestration mechanism. Production of indole-3-acetic acid by Methylobacterium sp. and K. endophyticus, as well as phosphorus solubilization by Methylobacterium sp. were observed. Bioaugmentation with both endophytes increased the shoot length of Populus 'Marke' and enhanced the Mg uptake in both cultivars. Inoculation with Methylobacterium sp. reduced the bioaccumulation of Zn in 'Marke', conferring it an excluder strategy. Methylobacterium sp. and K. endophyticus seemed to improve the plant nutritional status, which can alleviate abiotic stress. [ABSTRACT FROM AUTHOR]

Environmental pollutants can generate stress in plants causing increased ethylene production that leads to the inhibition of plant growth. Ethylene production by the stressed plant may be lowered by Plant Growth-Promoting Bacteria (PGPB) that metabolizes the immediate precursor of ethylene 1-aminocyclopropane-1-carboxylate (ACC). Thus, engineering PGPB with ACC deaminase activity can be a promising alternative to mitigate the harmful effects of pollutants and thus enhance plant production. Here we show that the aromatics-degrading and metal-resistant Azoarcus sp. CIB behaves as a PGP-bacterium when colonizing rice as an endophyte, showing a 30% increment in plant weight compared to non-inoculated plants. The cloning and expression of an acdS gene led to a recombinant strain, Azoarcus sp. CIB (pSEVA237acdS), possessing significant ACC deaminase activity (6716 nmol mg−1 h−1), constituting the first PGPB of the Rhodocyclaceae family equipped with this PGP trait. The recombinant CIB strain acquired the ability to protect inoculated rice plants from the stress induced by cadmium (Cd) exposure and to increase the Cd concentration in rice seedlings. The observed decrease of the levels of reactive oxygen species levels in rice roots confirms such a protective effect. The broad-host-range pSEVA237acdS plasmid paves the way to engineer PGPB with ACC deaminase activity to improve the growth of plants that might face stress conditions. [ABSTRACT FROM AUTHOR]

Lead (Pb) and cadmium (Cd) are major environmental pollutants, and the accumulation of these elements in soils and plants is of great concern in agricultural production due to their toxic effects on crop growth. Also, these elements can enter into the food chain and severely affect human and animal health. Bioaugmentation with plant growth-promoting bacteria (PGPB) can contribute to an environmentally friendly and effective remediation approach by improving plant survival and promoting element phytostabilization or extraction under such harsh conditions. We isolated and characterised Pb and Cd-tolerant root-associated bacteria from Helianthus petiolaris growing on a Pb/Cd polluted soil in order to compose inoculants that can promote plant growth and also ameliorate the phytostabilization or phytoextraction efficiency. One hundred and five trace element-tolerant rhizospheric and endophytic bacterial strains belonging to eight different genera were isolated from the aromatic plant species Helianthus petiolaris. Most of the strains showed multiple PGP-capabilities, ability to immobilise trace elements on their cell wall, and promotion of seed germination. Bacillus paramycoides ST9, Bacillus wiedmannii ST29, Bacillus proteolyticus ST89, Brevibacterium frigoritolerans ST30, Cellulosimicrobium cellulans ST54 and Methylobacterium sp. ST85 were selected to perform bioaugmentation assays in greenhouse microcosms. After 2 months, seedlings of sunflower (H. annuus) grown on polluted soil and inoculated with B. proteolyticus ST89 produced 40% more biomass compared to the non-inoculated control plants and accumulated 20 % less Pb and 40% less Cd in the aboveground plant parts. In contrast, B. paramycoides ST9 increased the bioaccumulation factor (BAF) of Pb three times and of Cd six times without inhibiting plant growth. Our results indicate that, depending on the strain, bioaugmentation with specific beneficial bacteria can improve plant growth and either reduce trace element mobility or enhance plant trace element uptake. [ABSTRACT FROM AUTHOR]

Abstract: To improve the knowledge about the use of plants for the removal of toxic metals from contaminated soils, metabolic and transcriptional responses of Brassica carinata to different forms of copper (Cu) were studied. Two-week-old hydroponically grown seedlings were exposed for 24h to 30μM CuSO4 or CuEDDS. CuSO4 appeared to be more toxic than CuEDDS as roots showed higher levels of thiobarbituric acid reactive substances (TBARS) and increased relative leakage ratios (RLR), although the superoxide dismutase (SOD, EC 1.15.1.1) activity increased following both exposures. In CuSO4-exposed seedlings the higher toxicity was underlined by increased transcription of lipoxygenases (EC 1.13.11.12) and NADPH oxidases (EC 1.6.99.6) and by the higher Cu accumulation in both tissues compared to CuEDDS exposure. The presence of EDDS increased Cu translocation, which resulted 5-times higher than when exposed to CuSO4. Decreases in catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11) and glutathione reductase (GR, EC 1.6.4.2) activities together with increases of reduced glutathione (GSH) and tocopherols and a reduction of lipoic acid (LA) were observed in roots of CuSO4-exposed seedlings. On the contrary, CuEDDS exposure induced a general increase in enzyme activities and decreases in ascorbate (AsA) and tocopherol levels. In the primary leaves, in both exposures Cu differently affected the oxidative stress responses indicating that the cellular redox balance was anyway maintained. EDDS plays a crucial role in B. carinata tolerance to oxidative stress induced by Cu and might be proposed to improve the efficiency of Cu phytoextraction. [Copyright &y& Elsevier]

Plant growth promoting (PGP) bacterial strains possess different mechanisms to improve plant development under common environmental stresses, and are therefore often used as inoculants in soil phytoremediation processes. The aims of the present work were to study the effects of a collection of plant growth promoting bacterial strains on plant development, antioxidant enzyme activities and nutritional status of Cytisus striatus and/or Lupinus luteus plants a) growing in perlite under non-stress conditions and b) growing in diesel-contaminated soil. For this, two greenhouse experiments were designed. Firstly, C. striatus and L. luteus plants were grown from seeds in perlite, and periodically inoculated with 6 PGP strains, either individually or in pairs. Secondly, L. luteus seedlings were grown in soil samples of the A and B horizons of a Cambisol contaminated with 1.25% (w/w) of diesel and inoculated with best PGP inoculant selected from the first experiment. The results indicated that the PGP strains tested in perlite significantly improved plant growth. Combination treatments provoked better growth of L. luteus than the respective individual strains, while individual inoculation treatments were more effective for C. striatus . L. luteus growth in diesel-contaminated soil was significantly improved in the presence of PGP strains, presenting a 2-fold or higher increase in plant biomass. Inoculants did not provoke significant changes in plant nutritional status, with the exception of a subset of siderophore-producing and P-solubilising bacterial strains that resulted in significantly modification of Fe or P concentrations in leaf tissues. Inoculants did not cause significant changes in enzyme activities in perlite experiments, however they significantly reduced oxidative stress in contaminated soils suggesting an improvement in plant tolerance to diesel. Some strains were applied to non-host plants, indicating a non-specific performance of their plant growth promotion. The use of PGP strains in phytoremediation may help plants to overcome contaminant and other soil stresses, increasing phytoremediation efficiency. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) induces the generation of reactive oxygen species (ROS) and stimulates ethylene biosynthesis. The phytohormone ethylene is a regulator of many developmental and physiological plant processes as well as stress responses. Previous research indicated various links between ethylene signalling and oxidative stress. Our results support a correlation between the Cd-induced oxidative challenge and ethylene signalling in Arabidopsis thaliana leaves. The effects of 24 or 72 h exposure to 5 μM Cd on plant growth and several oxidative stress-related parameters were compared between wild-type (WT) and ethylene insensitive mutants ( etr1-1 , ein2-1 , ein3-1 ). Cadmium-induced responses observed in WT plants were mainly affected in etr1-1 and ein2-1 mutants, of which the growth was less inhibited by Cd exposure as compared to WT and ein3-1 mutants. Both etr1-1 and ein2-1 showed a delayed response in the glutathione (GSH) metabolism, including GSH levels and transcript levels of GSH synthesising and recycling enzymes. Furthermore, the expression of different oxidative stress marker genes was significantly lower in Cd-exposed ein2-1 mutants, evidencing that ethylene signalling is involved in early responses to Cd stress. A model for the cross-talk between ethylene signalling and oxidative stress is proposed. [ABSTRACT FROM AUTHOR]

The impact of soil metal pollution on plant communities has been studied extensively in the past. However, very little is known about the fungal species that co-occur with these plant communities on metal polluted soils. We characterized the belowground fungal community in a heavy metal polluted and a non-polluted soil using 454 pyrosequencing. The fungal communities at both study sites were shown to consist mainly of the same ectomycorrhizal species, but a consistent shift in the relative abundances of these species was observed, whereas no differences in fungal diversity were found. In metal polluted soil, root tips of young pines were initially largely colonized by stress-tolerant dark Ascomycota that were mostly replaced by metal-tolerant Basidiomycota within 2 years. Compared to older forests, a low belowground fungal diversity was observed in the two pioneer stands. [ABSTRACT FROM AUTHOR]

The biosphere is constantly exposed to ionising radiation, due to cosmic radiation and the presence of natural and anthropogenic radionuclides in the environment. The biological effects of the resulting radiation exposure are currently poorly understood in plants, due to a large influence of parameters such as species, cultivar, experimental setup and plant age on the response. In this study, 7-, 10- and 14-day - old Arabidopsis thaliana seedlings were exposed to 96 h and 168 h of 100 mGy h −1 chronic γ-radiation in a hydroponic setup. The response of growth, photosynthesis and the transcription of genes involved in DNA repair, cell cycle and signalling were measured. The results show that there is a difference in growth response between plants of different ages and decreasing radiosensitivity with increasing seedling age. This is linked to differences in regulation of DNA repair and cell cycle control at the transcriptional level. [ABSTRACT FROM AUTHOR]

To study the impact of environmental uranium (U) contamination, effects should be analysed at different environmentally relevant pH levels as the speciation of U, and hence its toxicity, is strongly dependent on the pH. As photosynthesis is a major energy producing process in plants intimately connected to plant growth and known to be susceptible to metal stress, the effects of different U concentrations on photosynthesis in 18-day-old Arabidopsis thaliana (Columbia ecotype) are investigated at two contrasting pH levels, pH 4.5 and pH 7.5. At pH 4.5, U is highly taken up by the roots but is poorly translocated to the shoots, while at pH 7.5, less U is taken up but the translocation is higher. The lower U concentrations in the shoots at pH 4.5 are accompanied by a more reduced leaf growth as compared to pH 7.5. In addition, U does not influence the photosynthetic machinery at pH 7.5, while an optimization of the photosynthesis takes place after U exposure at pH 4.5. As such, more of the absorbed quanta are effectively used for photosynthesis accompanied by a decreased non-photochemical quenching and an increased electron transport rate. Since the enhanced photosynthesis at pH 4.5 is accompanied by a decreased growth, we suggest that the energy produced during photosynthesis is used for defence reactions against U-induced oxidative stress rather than for growth. As such, a high discrepancy was observed between the two pH levels, with an optimized photosynthetic apparatus at pH 4.5 and almost no effects at pH 7.5. [ABSTRACT FROM AUTHOR]

Abstract: Plant growth and development can be highly restricted by environmental stressors such as cadmium (Cd) pollution. The mitochondrial non-phosphorylating alternative respiratory pathway, mediated by alternative oxidase (AOX), alternative NAD(P)H dehydrogenases (NDs) and uncoupling protein (UCP), was suggested to be crucial in the acclimation of plants to fluctuating environmental conditions. Therefore, we examined the effects of environmentally realistic Cd exposure (5 and 10μM) on the alternative respiratory chain in Arabidopsis thaliana using a kinetic exposure setup. We demonstrated that during exposure to Cd, Arabidopsis seedlings show a mitochondrial oxidative challenge to which they acutely respond by increasing the transcript level of several AOX, ND and UCP isoforms in both roots and leaves. In addition, AOX protein levels increased during acute Cd exposure (2 and 24h). Based on our data, we suggest the formation of a condensed non-phosphorylating electron transport chain (ETC) functioning through cytosolic NDs and AOX, with co-regulation of ND and AOX expression during Cd stress. Therefore, both enzymes might cooperate in the potential acclimation of Arabidopsis seedlings to environmentally realistic Cd exposure by modulating the extent of mitochondrial ROS production. [Copyright &y& Elsevier]

Abstract: Lipoxygenases (LOXes, EC 1.13.11.12) are involved in growth, development and responses to stress. Earlier results suggested a role in stress generation, signalling and/or responses when Arabidopsis thaliana is exposed to cadmium (Cd), and expression of the cytosolic LOX1 was highly upregulated in the roots after Cd exposure. To investigate the involvement of LOX1 in early metal stress responses, three-week-old wild-type and lox1-1 mutant A. thaliana plants were acutely (24 h) exposed to realistic Cd concentrations (5 and 10 μM) and several oxidative stress and signalling related parameters were studied at transcriptional and biochemical levels. Transcription of several genes encoding ROS producing and scavenging enzymes failed to be induced up to wild-type levels after Cd exposure. Expression of 9-LOX enzymes was inhibited in lox1-1 mutant roots due to lack of functional LOX1 and downregulated LOX5 expression, and the lox1-1 mutation also interfered with the expression of genes involved in jasmonate biosynthesis. LOX1 and RBOHD may be involved in stress signalling from roots to shoots, as the induction of APX2 expression, which is dependent on RBOHD activity, was disrupted in lox1-1 while RBOHD failed to be upregulated. A different pattern of H2O2 production and ascorbate and glutathione levels in lox1-1 mutants after Cd exposure may have indirectly influenced gene expression patterns. Although indirect effects of the lox1-1 mutation on gene expression complicate the determination of exact sensing – signalling – response pathways, the results presented here outline a more refined LOX1 functioning in Cd-induced stress responses that could be used in studies determining the exact involvement of LOX1 in these pathways. [Copyright &y& Elsevier]

Abstract: Plants exposed to cadmium (Cd) show morphological and physiological disorders. To increase our knowledge regarding Cd-induced signalling, most often the effects of acute exposure are investigated. However, this does not allow in-depth analysis of morphological effects. Therefore, we chronically exposed Arabidopsis thaliana plants to environmentally realistic Cd concentrations (5 or 10 μM) and, using a described phenotypic framework methodology, we determined the impact of Cd on the plant’s ability to complete its life cycle and produce germinative seeds. Visible Cd-induced morphological changes were observed within a short exposure period, with chlorotic and anthocyanous leaf colouring occurring dose-dependently. Although rosette growth was severely reduced in Cd-exposed plants, all plants were able to emerge inflorescences and produce siliques containing germinative seeds, thus confirming the non-lethality of the used Cd concentrations. Although the growth inhibition of Cd-exposed plants was dependent on the dose, both concentrations had similar effects on inflorescence height and silique counts. In conclusion, vegetative growth of plants chronically exposed to Cd is inhibited in a concentration-dependent manner. However, the effect on plant regeneration is clearly stress-determined but independent on the Cd concentration applied. In Arabidopsis thaliana, vegetative and reproductive growth are differentially influenced by Cd. [Copyright &y& Elsevier]

A promising field to exploit plant–endophyte partnerships is the remediation of contaminated soils and (ground) water. Many plant growth promoting endophytes can assist their host plant to overcome contaminant-induced stress responses, thus providing improved plant growth. During phytoremediation of organic contaminants, plants can further benefit from endophytes possessing appropriate degradation pathways and metabolic capabilities, leading to more efficient contaminant degradation and reduction of both phytotoxicity and evapotranspiration of volatile contaminants. For phytoremediation of toxic metals, endophytes possessing a metal-resistance/sequestration system can lower metal phytotoxicity and affect metal translocation to the above-ground plant parts. Furthermore, endophytes that can degrade organic contaminants and deal with or, even better, improve extraction of the metals offer promising ways to improve phytoremediation of mixed pollution. [Copyright &y& Elsevier]

Pollution of surface waters is a worldwide problem for people and wildlife. Remediation and phytoremediation approaches can offer a solution to deal with specific scenarios. Lemna minor, commonly known as duckweed, can absorb and accumulate pollutants in its biomass. To evaluate if L. minor could be applied for phytoremediation purposes, it is necessary to further investigate its remediation capability and to identify which parameters affect the remediation process. Such a model must include both plant growth and pollutant exchange. A remediation model based on a robust experimental study can help to evaluate L. minor as a proper remediation strategy and to predict the outcome of a L. minor based remediation system. To set up this model, this paper focusses on a detailed experimental study and a comprehensive mathematical modelling approach to represent L. minor growth as a function of biomass, temperature, light irradiation and variable nutrient concentrations. The influence of environmental conditions on L. minor growth was studied, by composing 7 days growth curves. Plants were grown under predefined environmental conditions (25°C, 14h photoperiod, 220 μmol m−2 s−1 light intensity and a modified Hoagland solution with 23.94 mg N L−1 and 3.10 mg P L−1 (N:P ratio of 7.73)) as standard for all experiments. The influence of different temperatures (6, 10, 15, 20, 25, 30 and 35°C), light intensities (63, 118, 170, 220 and 262 μmol m−2 s−1), photoperiods (12h and 14h) and N:P ratios (1.18, 3.36, 7.73 and 29.57) were tested in the model. As a result, a growth model was optimised using separate datasets for temperature, light intensity, photoperiod and nutrients and validated by further integrated testing. The growth model is a stable platform for application in phytoremediation of radionuclides in contaminated water, to be extended in future studies with information of pollutant uptake, pollutant-nutrient interactions and transfer to the biomass. • Lemna minor growth model established and validated experimentally. • Studied effects of temperature, light and nutrients on logistic growth. • Impact of temperature on growth modelled with improved power function. • Nitrogen and phosphorus nutrient impact on growth modelled. • Approach forms basis for future remediation impact studies. [ABSTRACT FROM AUTHOR]

Poplar, a plant species frequently used for phytoremediation of groundwater contaminated with organic solvents, was inoculated with the endophyte Burkholderia cepacia VM1468. This strain, whose natural host is yellow lupine, contains the pTOM-Bu61 plasmid coding for constitutively expressed toluene degradation. Noninoculated plants or plants inoculated with the soil bacterium B. cepacia Bu61(pTOM-Bu61) were used as controls. Inoculation of poplar had a positive effect on plant growth in the presence of toluene and reduced the amount of toluene released via evapotranspiration. These effects were more dramatic for VM1468, the endophytic strain, than for Bu61. Remarkably, none of the strains became established at detectable levels in the endophytic community, but there was horizontal gene transfer of pTOM-Bu61 to different members of the endogenous endophytic community, both in the presence and in the absence of toluene. This work is the first report of in planta horizontal gene transfer among plant-associated endophytic bacteria and demonstrates that such transfer could be used to change natural endophytic microbial communities in order to improve the remediation of environmental insults. [ABSTRACT FROM AUTHOR]