Phytoremediation of Metal Contaminated Soil Using Willow: Exploiting Plant-Associated Bacteria to Improve Biomass Production and Metal Uptake

Plant growth-promoting rhizobacterial secondary metabolites in augmenting heavy metal(loid) phytoremediation: An integrated green in situ ecorestorative technology

In recent times, increased geogenic and human-centric activities have caused significant heavy metal(loid) (HM) contamination of soil, adversely impacting environmental, plant, and human health. Phytoremediation is an evolving, cost-effective, environment-friendly, in situ technology that employs indigenous/exotic plant species as natural purifiers to remove toxic HM(s) from deteriorated ambient soil. Interestingly, the plant's rhizomicrobiome is pivotal in promoting overall plant nutrition, health, and phytoremediation. Certain secondary metabolites produced by plant growth-promoting rhizobacteria (PGPR) directly participate in HM bioremediation through chelation/mobilization/sequestration/bioadsorption/bioaccumulation, thus altering metal(loid) bioavailability for their uptake, accumulation, and translocation by plants. Moreover, the metallotolerance of the PGPR and the host plant is another critical factor for the successful phytoremediation of metal(loid)-polluted soil. Among the phytotechniques available for HM remediation, phytoextraction/phytoaccumulation (HM mobilization, uptake, and accumulation within the different plant tissues) and phytosequestration/phytostabilization (HM immobilization within the soil) have gained momentum in recent years. Natural metal(loid)-hyperaccumulating plants have the potential to assimilate increased levels of metal(loid)s, and several such species have already been identified as potential candidates for HM phytoremediation. Furthermore, the development of transgenic rhizobacterial and/or plant strains with enhanced environmental adaptability and metal(loid) uptake ability using genetic engineering might open new avenues in PGPR-assisted phytoremediation technologies. With the use of the Geographic Information System (GIS) for identifying metal(loid)-impacted lands and an appropriate combination of normal/transgenic (hyper)accumulator plant(s) and rhizobacterial inoculant(s), it is possible to develop efficient integrated phytobial remediation strategies in boosting the clean-up process over vast regions of HM-contaminated sites and eventually restore ecosystem health.

Effect of phosphorus fertilizer on phytoextraction using Ricinus communis L. in Cu and Cd co-contaminated soil

Mining activities have led to Cu and Cd contaminated of surrounding agricultural soil. To decrease the Cu and Cd accumulation in crops, the Ricinus communis L. (castor) has been used for phytoremediation. A pot experiment was served to investigate the effect of phosphate fertilizer (Ca(H₂PO₄)₂) on the growth and Cu/Cd uptake of castor in contaminated soil. The results showed that the application of P fertilizer improved the leaf cell morphology, decreased the malonaldehyde (MDA) content of castor leaves, and increased the plant biomass (28.2-34.2%). Besides, phosphate fertilizer still facilitated accumulation Cu and Cd by castor. The addition of phosphate fertilizer increased the contents of Cu in the root of castor, improved the bioconcentration factor (BCF) of Cu, and observably enhanced the accumulation of Cu (up to 201 μg/plant) in castor. Applying phosphorus increased the percentage of residual Cd, diminished the percentage of acid extractable Cd in soil, and the accumulation of Cd in castor was not significantly increased. These results suggest that phosphorus alleviated the stress of heavy metals on castor leaves and enhanced the accumulation of Cu and Cd in castor by promoting the growth of castor.

Isolation and characterization culturable microbes on the surface of ‘Granny Smith’ apples treated with electrolyzed water during cold storage

Response of culturable microbes on the surface of apples treated with slightly alkaline electrolyzed water (SAIEW) is largely unexplored. Thus, the aim of this study was to characterize culturable microbes on the surface of SAIEW treated 'Granny Smith' apples using conventional and molecular approach. Results showed that SAIEW treatments and storage duration influenced culturable microbes isolated from the surface of 'Granny Smith' apples stored at 5 °C for 21 days. Enterobacterial repetitive intergenic consensus (ERIC-PCR) analysis distinctively identified 27 groups of bacteria from 56 plate isolates. Using random amplified polymorphic DNA (RAPD-PCR) typing and RAPD1283 primers, 10 distinct band patterns were identified from 30 fungal isolates. Sequencing of 16S rRNA and intergenic spacer (ITS1 and ITS4) region, identified eight bacteria and four fungi, respectively, to species level. Study showed that SAIEW treatment inhibited growth of Staphylococcus epidermidis, S. capitis, Ochrobactrum soli, and Aspergillus inuii on the surface apples during storage.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-022-01148-2.

Low Indirect Land Use Change (ILUC) Energy Crops to Bioenergy and Biofuels—A Review

Energy crops are dedicated cultures directed for biofuels, electricity, and heat production. Due to their tolerance to contaminated lands, they can alleviate and remediate land pollution by the disposal of toxic elements and polymetallic agents. Moreover, these crops are suitable to be exploited in marginal soils (e.g., saline), and, therefore, the risk of land-use conflicts due to competition for food, feed, and fuel is reduced, contributing positively to economic growth, and bringing additional revenue to landowners. Therefore, further study and investment in R&D is required to link energy crops to the implementation of biorefineries. The main objective of this study is to present a review of the potential of selected energy crops for bioenergy and biofuels production, when cultivated in marginal/degraded/contaminated (MDC) soils (not competing with agriculture), contributing to avoiding Indirect Land Use Change (ILUC) burdens. The selected energy crops are Cynara cardunculus, Arundo donax, Cannabis sativa, Helianthus tuberosus, Linum usitatissimum, Miscanthus × giganteus, Sorghum bicolor, Panicum virgatum, Acacia dealbata, Pinus pinaster, Paulownia tomentosa, Populus alba, Populus nigra, Salix viminalis, and microalgae cultures. This article is useful for researchers or entrepreneurs who want to know what kind of crops can produce which biofuels in MDC soils.

Phytoremediation of Heavy-Metals-Contaminated Soils: A Short-Term Trial Involving Two Willow Species from Gloucester WillowBank in the UK

Phytoremediation, as a bioremediation process in which plants are used to remove contaminants from an environment, has proved to be a practical and low-cost strategy for recovering mining-affected areas. This study aims to assess the potential for use in phytoremediation of two willow species, Salix viminalis and Salix dasyclados, by testing their potential for cleaning-up a range of soils with differing heavy metal concentrations: Pb (111, 141, 192 and 249 mg /kg), Zn (778.6, 1482, 2734 and 4411 mg/kg) and Cd (3.00, 5.03, 9.14 and 16.07 mg/kg). The extracted metals were preferentially translocated to the leaves with considerably higher concentrations and relative BAFs in the case of S. viminalis. The highest recorded Zn concentration of over 0.5% was found in the leaves of S. viminalis growing in soil 4. However, under the conditions of the experiments, S. dasyclados showed greater potential for use in phytoremediation, especially if coupled with use of biomass for energy production. An assessment of the suitability of willow species in this role, with regard to wider aspects involved, such as use of resultant biomass and/or waste management, revealed good potential. Willows are fast growing, grow vigorously from coppiced stumps and have extensive root systems. Therefore, their use in bioenergy production through pyrolysis or combustion, coupled with flue gas screening, is strongly advised.

Water and soil contaminated by arsenic: the use of microorganisms and plants in bioremediation

Owing to their roles in the arsenic (As) biogeochemical cycle, microorganisms and plants offer significant potential for developing innovative biotechnological applications able to remediate As pollutions. This possible use in bioremediation processes and phytomanagement is based on their ability to catalyse various biotransformation reactions leading to, e.g. the precipitation, dissolution, and sequestration of As, stabilisation in the root zone and shoot As removal. On the one hand, genomic studies of microorganisms and their communities are useful in understanding their metabolic activities and their interaction with As. On the other hand, our knowledge of molecular mechanisms and fate of As in plants has been improved by laboratory and field experiments. Such studies pave new avenues for developing environmentally friendly bioprocessing options targeting As, which worldwide represents a major risk to many ecosystems and human health.

Methane production from green and woody biomass using short rotation willow genotypes for bioenergy generation

Short rotation plantations of willow genotypes, harvested in vegetative growth phases, were tested as an alternative biomass for methane production. The substrate characteristics, maximal methane yields (K) and highest methane production rates (µmax) were determined. Leaves and stems from diploid Energo (EN) and tetraploid (PP) plants, harvested in June were superior methane sources to woody tissue. This could be related to the lower lignin contents in green willow. Fermentation of pooled biomasses from tetraploid genotypes harvested in June-August was more efficient than methane production from diploid tissues. Microbial community analyses by 16S rRNA genes showed a dominance of the order Clostridiales. In field study, based on Energo plantation, the maximum in green biomass accumulation was in early month 9 of the vegetation period. A theoretical calculation showed similar or better energy potential per unit area for willow than in the case of maize silage. This study encourages the use of green willow biomass as feedstock in biomethanation processes due to its relatively low production costs and uncomplicated agricultural practice.

Multi-Trait Wheat Rhizobacteria from Calcareous Soil with Biocontrol Activity Promote Plant Growth and Mitigate Salinity Stress

Plant growth promoting rhizobacteria (PGPR) can be functional microbial fertilizers and/or biological control agents, contributing to an eco-spirit and safe solution for chemical replacement. Therefore, we have isolated rhizospheric arylsulfatase (ARS)-producing bacteria, belonging to Pseudomonas and Bacillus genus, from durum wheat crop grown on calcareous soil. These isolates harbouring plant growth promoting (PGP) traits were further evaluated in vitro for additional PGP traits, including indole compounds production and biocontrol activity against phytopathogens, limiting the group of multi-trait strains to eight. The selected bacterial strains were further evaluated for PGP attributes associated with biofilm formation, compatibility, salt tolerance ability and effect on plant growth. In vitro studies demonstrated that the multi-trait isolates, Bacillus (1.SG.7, 5.SG.3) and Pseudomonas (2.SG.20, 2.C.19) strains, enhanced the lateral roots abundance and shoots biomass, mitigated salinity stress, suggesting the utility of beneficial ARS-producing bacteria as potential microbial fertilizers. Furthermore, in vitro studies demonstrated that compatible combinations of multi-trait isolates, Bacillus sp. 1.SG.7 in a mixture coupled with 5.SG.3, and 2.C.19 with 5.SG.3 belonging to Bacillus and Pseudomonas, respectively, may enhance plant growth as compared to single inoculants.

Arsenate-reducing bacteria affect As accumulation and tolerance in Salix atrocinerea

Arsenic (As)-reducing bacteria are able to influence As-speciation and, in this way, change As bio-availability. In consequence, this has an impact on As uptake by plants growing on polluted soil and on the effectiveness of the phytoremediation process. To be able to efficiently utilize these bacteria for As-phytoremediation in the field, a better understanding of the plant-bacterial interactions involved in As-tolerance or toxicity is needed. In this work, seedlings of a clone of Salix atrocinerea derived from a specimen naturally growing on an As-polluted brownfield were grown under gnotobiotic conditions exposed to As, and in the presence or absence of two of its field-associated and in vitro characterized plant growth-promoting (PGP) bacteria. The inoculation with Pantoea sp., induced a moderate reduction of AsV to AsIII in the exposure medium that, together with a coordinated plant response of As uptake, chelation and sequestration, increased As accumulation in roots; which is reflected into a higher phytostabilization. However, inoculation with Rhodococcus erythropolis due to a higher disproportionate reduction of AsV to AsIII in the medium caused less As accumulation in roots that non-bioaugmented plants and despite the lower As content, the concentrations of AsIII present in the medium and the damage suffered in roots and leaves, indicated that As tolerance mechanisms (such as prevention of AsIII uptake and efflux) did not occur in time to avoid physical disturbance and plants growth reduction. Interestingly, by two different metabolic pathways -coordinated by different key transporters mediating As uptake, tolerance, distribution and vacuolar accumulation at the roots- both bacteria limited As accumulation in Salix shoots. Our results provide for the first time a detailed insight in the plant-bacterial responses and physiological changes contributing to As tolerance in S. atrocinerea, that will facilitate the design of effective strategies for exploitation of plant-associated microorganisms for phytoremediation.

Short Rotation Intensive Culture of Willow, Spent Mushroom Substrate and Ramial Chipped Wood for Bioremediation of a Contaminated Site Used for Land Farming Activities of a Former Petrochemical Plant

The aim of this study was to investigate the bioremediation impacts of willows grown in short rotation intensive culture (SRIC) and supplemented or not with spent mushroom substrate (SMS) and ramial chipped wood (RCW). Results did not show that SMS significantly improved either biomass production or phytoremediation efficiency. After the three growing seasons, RCW-amended S. miyabeana accumulated significantly more Zn in the shoots, and greater increases of some PAHs were found in the soil of RCW-amended plots than in the soil of the two other ground cover treatments' plots. Significantly higher Cd concentrations were found in the shoots of cultivar 'SX61'. The results suggest that 'SX61' have reduced the natural attenuation of C10-C50 that occurred in the unvegetated control plots. The presence of willows also tended to increase the total soil concentrations of PCBs. Furthermore, we found that many contaminant concentrations were subject to seasonal oscillations, showing average increases throughout the whole experimental site after a growing period, while showing significantly different variations, such as lesser increases or even decreases, after a dormant period. These observations suggest that contaminants may have leached or degraded faster in untreated conditions, and conversely to have mobilized towards trees through water flow driven by plant transpiration during growing seasons.

Urea-enhanced phytoremediation of cadmium with willow in pyrene and cadmium contaminated soil

The phytoremediation of cadmium (Cd) and pyrene (PYR) in agricultural soil with willow was investigated by carrying out a pot-culture experiment in a greenhouse. The soil was incubated with urea 60 days before it was used for this experiment. The concentrations of Cd and PYR in soil and willow, the bioconcentration and transfer factors, the physiological and biochemical responses, and plant biomass production were determined at the end of the experiment. The phytoremediation with willow based on urea application was effective for enhancing the phytoremediation of Cd and PYR contaminated soil. Urea application did not affect the available Cd but increased the accumulation of soil Cd and the plant biomass of different parts of the willow. The removal rate (77.1-89.5%) of PYR in soil was not significantly affected although urea application decreased the accumulation of PYR in willow root and bark. Urea application significantly promoted the uptake of chlorophyll, carotenoid and malondialdehyde by willow leaves. The results of this study will provide scientific information for the effective phytoremediation of Cd in Cd and PYR contaminated soil.

Core and Differentially Abundant Bacterial Taxa in the Rhizosphere of Field Grown Brassica napus Genotypes: Implications for Canola Breeding

Modifying the rhizosphere microbiome through targeted plant breeding is key to harnessing positive plant-microbial interrelationships in cropping agroecosystems. Here, we examine the composition of rhizosphere bacterial communities of diverse Brassica napus genotypes to identify: (1) taxa that preferentially associate with genotypes, (2) core bacterial microbiota associated with B. napus, (3) heritable alpha diversity measures at flowering and whole growing season, and (4) correlation between microbial and plant genetic distance among canola genotypes at different growth stages. Our aim is to identify and describe signature microbiota with potential positive benefits that could be integrated in B. napus breeding and management strategies. Rhizosphere soils of 16 diverse genotypes sampled weekly over a 10-week period at single location as well as at three time points at two additional locations were analyzed using 16S rRNA gene amplicon sequencing. The B. napus rhizosphere microbiome was characterized by diverse bacterial communities with 32 named bacterial phyla. The most abundant phyla were Proteobacteria, Actinobacteria, and Acidobacteria. Overall microbial and plant genetic distances were highly correlated (R = 0.65). Alpha diversity heritability estimates were between 0.16 and 0.41 when evaluated across growth stage and between 0.24 and 0.59 at flowering. Compared with a reference B. napus genotype, a total of 81 genera were significantly more abundant and 71 were significantly less abundant in at least one B. napus genotype out of the total 558 bacterial genera. Most differentially abundant genera were Proteobacteria and Actinobacteria followed by Bacteroidetes and Firmicutes. Here, we also show that B. napus genotypes select an overall core bacterial microbiome with growth-stage-related patterns as to how taxa joined the core membership. In addition, we report that sets of B. napus core taxa were consistent across our three sites and 2 years. Both differential abundance and core analysis implicate numerous bacteria that have been reported to have beneficial effects on plant growth including disease suppression, antifungal properties, and plant growth promotion. Using a multi-site year, temporally intensive field sampling approach, we showed that small plant genetic differences cause predictable changes in canola microbiome and are potential target for direct and indirect selection within breeding programs.

Integrative response of arsenic uptake, speciation and detoxification by Salix atrocinerea

Despite arsenic (As) being very toxic with deleterious effects on metabolism, it can be tolerated and accumulated by some plants. General genetic mechanisms responsible for As tolerance in plants, including Salix species, have been described in transcriptomic analysis, but further experimental verification of the significance of particular transcripts is needed. In this study, a Salix atrocinerea clone, able to thrive in an As-contaminated brownfield, was grown hydroponically in controlled conditions under an As concentration similar to the bioavailable fraction of the contaminated area (18 mg kg^-1) for 30 days. At different time points, i.e. short-term and long-term exposure, biometric data, As accumulation, phytochelatin synthesis, non-protein thiol production and expression of target genes related to these processes were studied. Results showed that S. atrocinerea presents a great tolerance to As and accumulates up to 2400 mg As kg^-1 dry weight in roots and 25 mg As kg^-1 dry weight in leaves. Roots reduce As V to As III rapidly, with As III being the predominant form of As accumulated in root tissues, whereas in the leaves it is As V. After 1 d of As exposure, roots and leaves show de novo synthesis and an increase in non-protein thiols as compared to the control. Integrating these data on As accumulation in the plant and its speciation, non-protein thiol production and the kinetic gene expression of related target genes, a fundamental role is highlighted for these processes in As accumulation and tolerance in S. atrocinerea. As such, this study offers new insights in the plant tolerance mechanisms to As, which provides important knowledge for future application of high-biomass willow plants in phytoremediation of As-polluted soils.

New Caledonian ultramafic conditions structure the features of Curtobacterium citreum strains that play a role in plant adaptation

The present study focused on the characterization of 10 Curtobacterium citreum strains isolated from the rhizosphere of pioneer plants growing on ultramafic soils from New Caledonia. Taxonomic status was investigated using a polyphasic approach. Three strains (BE, BB, and AM) were selected in terms of multiple-metal resistance and plant-growth-promoting traits. They were tested on sorghum growing on ultramafic soil and compared with the reference strain C. citreum DSM20528^T. To better understand the bacterial mechanisms involved, biosorption, bioaccumulation, and biofilm formation were investigated for the representative strain of the ultramafic cluster (strain BE) versus C. citreum DSM20528^T. The polyphasic approach confirmed that all native isolates belong to the same cluster and are C. citreum. The inoculation of sorghum with strains BE and BB significantly reduced Ni content in shoots compared with inoculation with C. citreum DSM20528^T and control values. This result was related to the higher Ni tolerance of the ultramafic strains compared with C. citreum DSM20528^T. Ni biosorption and bioaccumulation showed that BE exhibited a lower Ni content, which is explained by the ability of this strain to produce exopolysaccharides involved in Ni chelation. We suggested that ultramafic C. citreum strains are more adapted to this substrate than is C. citreum DSM20528^T, and their features allow them to enhance plant metal tolerance.

Biomass growth variation and phytoextraction potential of four Salix varieties grown in contaminated soil amended with lime and wood ash

In a greenhouse experiment, plant growth and copper (Cu) and zinc (Zn) uptake by four Salix cultivars grown in Cu and Zn contaminated soils collected from a mining area in Finland were tested to assess their suitability for phytoextraction. The cultivars displayed tolerance to heavily contaminated soils throughout the experiment. After uptake, total mean Cu concentrations in the leaves, shoots and roots in all cultivars and treatments ranged from 163 to 474 mg kg^-1 and mean Zn concentrations ranged from 776 to 1823 mg kg^-1. Lime and wood ash addition increased dry biomass growth (25-43%), chlorophyll fluorescence (Fv/Fm) values (3-6%), the translocation factor (TF) (15-60% for Cu; 10-25% for Zn), the bio-concentration factor (BCF) (40-85% for Cu; 70-120% for Zn), and metal uptake (55-70% for Cu; 50-65% for Zn) compared to unamended treatment across all cultivars. The results revealed that Salix cultivars have the potential to take up and accumulate significant amounts of Cu and Zn. Cultivar Klara (Salix viminalis × S. schwerinii × S. dasyclados) was found to be the most effective cultivar for phytoextraction since it displayed greater dry biomass production, Fv/Fm, TF, BCF values and uptake percentage rates of Cu and Zn compared to the other three cultivars. This study indicates that further research is needed to clarify the wider phytoextraction capabilities of different Salix cultivars.

Capability of amendments (biochar, compost and garden soil) added to a mining technosol contaminated by Pb and As to allow poplar seed (Populus nigra L.) germination

The germination capacity of poplar seeds has never been studied in the context of metal(loid)-contaminated soils, even though poplars are present over a vast geographical area. In this study, black poplar seeds from the Loire Valley (France) were grown for 28 days in mesocosm on a heavily polluted soil that was subjected to different amendments. This phytomanagement process aimed to allow the revegetation of an As and Pb-contaminated mining soil by adding appropriate amendments, resulting in metal(loid) soil stabilisation and efficient plant growth. The objectives were to evaluate the effect of three amendments (garden soil, compost and biochar) when added alone or combined to a technosol on (i) the soil physicochemical properties, (ii) the mobility of As and Pb in the soil pore water (SPW), (iii) the capacity of poplar seeds to germinate and to grow and (iv) the metal(loid) distribution within the plant organs. The addition of amendments alone or combined allowed a 90% decrease in SPW Pb concentrations, while the arsenic concentrations were between 18 and 416 times higher. However, we were only able to obtain seed germination and plant growth on amended soils. These promising results will allow us to explore the use of such amendments in rehabilitating areas that are sources of significant metal(loid) dissemination, as well as allowing a natural plant recolonisation of these sites by seeds from the surrounding environment.

Opportunities and challenges in the remediation of metal-contaminated soils by using tobacco (Nicotiana tabacum L.): a critical review

The successful phytoextraction of potentially toxic elements (PTEs) from polluted soils can be achieved by growing non-food and industrial crops. Tobacco (Nicotiana tabacum L.) is one of the main industrial crops and is widely grown in many countries. Tobacco can uptake high concentrations of PTEs especially in aboveground biomass without suffering from toxicity. This review highlighted the potential of tobacco for the phytoextraction of heavy metals and tolerance mechanisms under metal stress. Different management practices have been discussed which can enhance the potential of this plant for metal extraction. Finally, suitable options for the management/disposal of biomass enriched in excess metal have been elaborated to prevent secondary pollution.

Effects of soil amendments on the growth response and phytoextraction capability of a willow variety (S. viminalis × S. schwerinii × S. dasyclados) grown in contaminated soils

This study was conducted to evaluate the effects of lime and bisphosphonates (BPs) such as N10O chelate amendment on the growth, physiological and biochemical parameters, and phytoextraction potential of the willow variety Klara (Salix viminalis × S. schwerinii × S. dasyclados) grown in soils heavily contaminated with copper (Cu), nickel (Ni) and zinc (Zn). The plants were irrigated with tap or processed water (mine wastewater). The results suggest that the combined effects of the contaminated soil and processed water inhibited growth parameters, gas exchange parameters and chlorophyll fluorescence (Fv/Fm) values. In contrast, malondialdehyde (MDA) content, organic acids, total phenolic and total flavonoid contents, and the accumulation of metals/metalloids in the plant tissues were increased compared to the control. When the soil was supplemented with lime and N10O; growth, physiological, biochemical parameters, and resistance capacity were significantly higher compared to unamended soil treatments, especially in the contaminated soil treatments. The combined lime‒ and N10O‒amended soil treatment produced higher growth rates, resistance capacity, photosynthesis rates and phytoextraction efficiency levels relative to either the lime‒amended or the N10O‒amended soil treatments. This study provides practical evidence of the efficient chelate‒assisted phytoextraction capability of Klara and highlights its potential as a viable and inexpensive novel approach for in situ remediation of Cu‒, Ni‒ and Zn‒contaminated soils and mine wastewaters.

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

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

Intensify production, transform biomass to energy and novel goods and protect soils in Europe-A vision how to mobilize marginal lands

The rapid increase of the world population constantly demands more food production from agricultural soils. This causes conflicts, since at the same time strong interest arises on novel bio-based products from agriculture, and new perspectives for rural landscapes with their valuable ecosystem services. Agriculture is in transition to fulfill these demands. In many countries, conventional farming, influenced by post-war food requirements, has largely been transformed into integrated and sustainable farming. However, since it is estimated that agricultural production systems will have to produce food for a global population that might amount to 9.1 billion by 2050 and over 10 billion by the end of the century, we will require an even smarter use of the available land, including fallow and derelict sites. One of the biggest challenges is to reverse non-sustainable management and land degradation. Innovative technologies and principles have to be applied to characterize marginal lands, explore options for remediation and re-establish productivity. With view to the heterogeneity of agricultural lands, it is more than logical to apply specific crop management and production practices according to soil conditions. Cross-fertilizing with conservation agriculture, such a novel approach will provide (1) increased resource use efficiency by producing more with less (ensuring food security), (2) improved product quality, (3) ameliorated nutritional status in food and feed products, (4) increased sustainability, (5) product traceability and (6) minimized negative environmental impacts notably on biodiversity and ecological functions. A sustainable strategy for future agriculture should concentrate on production of food and fodder, before utilizing bulk fractions for emerging bio-based products and convert residual stage products to compost, biochar and bioenergy. The present position paper discusses recent developments to indicate how to unlock the potentials of marginal land.

Phytoextraction with Salix viminalis in a moderately to strongly contaminated area

We tested the suitability of Salix viminalis for phytoextraction with the analysis of selected elements in soil, root, and leaf, and by visual tree condition assessment in an area with varying levels of contamination. Bioconcentration factor (BCF) and translocation factor (TF) were used to assess the phytoextraction potential of willows. The middle part of the study area was strongly contaminated, while the northern and southern parts were moderately contaminated. We found increasing element concentrations toward deeper layers. Mean concentrations of elements in roots were similar among the three parts, while in leaves the highest concentrations were found in the strongly contaminated part of the study area. Tree condition scores were the lowest in the strongly contaminated part of the study area, which was caused by Al, Ca, K, Mg, Ni, Sr, and Zn concentration. These elements induced leaf disease and leaf feeders. The highest BCF values were found for Cu, Fe, Mn, and Zn in root, and for Cd and Zn in leaves, indicating that S. viminalis had high accumulation potential of these elements. Furthermore, TF values were high for Cd, Mn, Sr, and Zn. Our results also demonstrated that soil element composition has major influence on the condition of S. viminalis individuals. Furthermore, visual condition assessment was found to be a useful tool to assess the phytoextraction potential of trees.

Plant Growth Promoting Rhizobacteria in Amelioration of Salinity Stress: A Systems Biology Perspective

Salinity affects plant growth and is a major abiotic stress that limits crop productivity. It is well-understood that environmental adaptations and genetic traits regulate salinity tolerance in plants, but imparting the knowledge gained towards crop improvement remain arduous. Harnessing the potential of beneficial microorganisms present in the rhizosphere is an alternative strategy for improving plant stress tolerance. This review intends to elucidate the understanding of salinity tolerance mechanisms attributed by plant growth promoting rhizobacteria (PGPR). Recent advances in molecular studies have yielded insights into the signaling networks of plant-microbe interactions that contribute to salt tolerance. The beneficial effects of PGPR involve boosting key physiological processes, including water and nutrient uptake, photosynthesis, and source-sink relationships that promote growth and development. The regulation of osmotic balance and ion homeostasis by PGPR are conducted through modulation of phytohormone status, gene expression, protein function, and metabolite synthesis in plants. As a result, improved antioxidant activity, osmolyte accumulation, proton transport machinery, salt compartmentalization, and nutrient status reduce osmotic stress and ion toxicity. Furthermore, in addition to indole-3-acetic acid and 1-aminocyclopropane-1-carboxylic acid deaminase biosynthesis, other extracellular secretions of the rhizobacteria function as signaling molecules and elicit stress responsive pathways. Application of PGPR inoculants is a promising measure to combat salinity in agricultural fields, thereby increasing global food production.

Effect of Pseudomonas fluorescens RB4 and Bacillus subtilis 189 on the phytoremediation potential of Catharanthus roseus (L.) in Cu and Pb-contaminated soils

The remediation of heavy metal-contaminated soils has become a critical issue due to toxic effects of these metals on living organisms. The current research was conducted to study the effect of Pseudomonas fluorescens RB4 and Bacillus subtilis 189 on the growth and phytoremediation potential of Catharanthus roseus in Cu- and Pb-contaminated soils. The bacterial strains exhibited significantly higher level of water-extractable Pb and Cu in Pb, Cu, and Cu+Pb-contaminated. The P. fluorescens RB4 inoculated plants, produced 102%, 48%, and 45% higher fresh weight (FW) in soils contaminated with Cu, Pb, and both elements, respectively, as compared to un-inoculated control plants. Similarly, B. subtilis 189 inoculated plants produced 108%, 43%, and 114% more FW in the presence of Cu, Pb, and both elements. The plants co-cultivated with both bacteria exhibited 121%, 102%, and 177% higher FW, in Cu, Pb, and both elements contaminated soils, as compared to respective un-inoculated control. Co-cultivation of P. fluorescens RB4, B. subtilis 189, and P. fluorescens RB4 + B. subtilis 189 resulted in higher accumulation of Cu and Pb in shoots of the C. roseus grown in contaminated soils as compared to un-inoculated control. Bacterial treatments also improved the translocation and metal bioconcentration factors. The growth and phytoextraction capability of C. roseus was improved by inoculation of P. fluorescens RB4 and B. subtilis 189.

Evaluating phytoextraction efficiency of two high-biomass crops after soil amendment and inoculation with rhizobacterial strains

We evaluated the effect of compost amendment and/or bacterial inoculants on the growth and metal accumulation of Salix caprea (clone BOKU 01 AT-004) and Nicotiana tabacum (in vitro-bred clone NBCu10-8). Soil was collected from an abandoned Pb/Zn mine and rhizobacterial inoculants were previously isolated from plants growing at the same site. Plants were grown in untreated or compost-amended (5% w/w) soil and were inoculated with five rhizobacterial strains. Non-inoculated plants were also established as a control. Compost addition increased the shoot DW yield of N. tabacum but not S. caprea, while it decreased soil metal availability and lowered shoot Cd/Zn concentrations in tobacco plants. Compost amendment enhanced the shoot Cd/Zn removal due to the growth promotion of N. tabacum or to the increase in metal concentration in S. caprea leaves. Bacterial inoculants increased photosynthetic efficiency (particularly in N. tabacum) and sometimes modified soil metal availability, but this did not lead to a significant increase in Cd/Zn removal. Compost amendment was more effective in improving the Cd and Zn phytoextraction efficiency than bioaugmentation.

Phytoremediation: State-of-the-art and a key role for the plant microbiome in future trends and research prospects

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 land in 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.

Effects of Cd- and Pb-resistant endophytic fungi on growth and phytoextraction of Brassica napus in metal-contaminated soils

Metal-resistant endophytic fungi from roots improved phytoremediation efficacy of host plants; however, the effects of endophytic fungi from plant aerial parts on host plants are unknown. The aim of this study was to develop a feasible method to screen fungal endophytes from stems and roots of Brassica napus and to investigate effects of the endophytic fungi on growth and phytoremediation efficiency of the plant. Endophytic Fusarium sp. CBRF44, Penicillium sp. CBRF65, and Alternaria sp. CBSF68 with different traits were isolated from roots and stems of rapes grown in a metal-contaminated soil. Fusarium sp. CBRF44 (resistant to 5 mM Cd and 15 mM Pb, isolated from roots) and Alternaria sp. CBSF68 (resistant to 1 mM Cd and 10 mM Pb, isolated from stems) could produce indole-3-acetic acid (IAA) and siderophore; Penicillium sp. CBRF65 (tolerate 2 mM Cd and 20 mM Pb, isolated from roots) could not produce IAA and siderophore but showed the highest phosphate-solubilizing activities. Fusarium sp. CBRF44 and Penicillium sp. CBRF65 significantly increased the rape biomass and promoted the extraction efficacy of Pb and Cd, while Alternaria sp. CBSF68 did not show similar results. Penicillium sp. CBRF65 and Fusarium sp. CBRF44 could be frequently recovered from inoculated rape roots, while Alternaria sp. CBSF68 was scarcely recovered. The results indicate that the colonizing capacity of endophytic fungi in roots is important to improve phytoremediation efficacy of host plants.

Endophytic Phytoaugmentation: Treating Wastewater and Runoff Through Augmented Phytoremediation

Limited options exist for efficiently and effectively treating water runoff from agricultural fields and landfills. Traditional treatments include excavation, transport to landfills, incineration, stabilization, and vitrification. In general, treatment options relying on biological methods such as bioremediation have the ability to be applied in situ and offer a sustainable remedial option with a lower environmental impact and reduced long-term operating expenses. These methods are generally considered ecologically friendly, particularly when compared to traditional physicochemical cleanup options. Phytoremediation, which relies on plants to take up and/or transform the contaminant of interest, is another alternative treatment method which has been developed. However, phytoremediation is not widely used, largely due to its low treatment efficiency. Endophytic phytoaugmentation is a variation on phytoremediation that relies on augmenting the phytoremediating plants with exogenous strains to stimulate associated plant-microbe interactions to facilitate and improve remediation efficiency. In this review, we offer a summary of the current knowledge as well as developments in endophytic phytoaugmentation and present some potential future applications for this technology. There has been a limited number of published endophytic phytoaugmentation case studies and much remains to be done to transition lab-scale results to field applications. Future research needs include large-scale endophytic phytoaugmentation experiments as well as the development of more exhaustive tools for monitoring plant-microbe-pollutant interactions.

Engineering the Rhizosphere

All components of the rhizosphere can be engineered to promote plant health and growth, two features that strongly depend upon the interactions of living organisms with their environment. This review describes the progress in plant and microbial molecular genetics and ecology that has led to a wealth of potential applications. Recent efforts especially deal with the plant defense machinery that is instrumental in engineering plant resistance to biotic stresses. Another approach involves microbial population engineering rather than single strain engineering. More generally, the plants (and the associated microbes) are no longer seen as 'individual' but rather as a holobiont, in other words a unit of selection in evolution, a concept that holds great promise for future plant breeding programs.

Inoculation methods using Rhodococcus erythropolis strain P30 affects bacterial assisted phytoextraction capacity of Nicotiana tabacum

In this study different bacterial inoculation methods were tested for tobacco plants growing in a mine-soil contaminated with Pb, Zn, and Cd. The inoculation methods evaluated were: seed inoculation, soil inoculation, dual soil inoculation event, and seed+soil inoculation. Each inoculum was added at two bacterial densities (10(6) CFUs mL(-1) and 10(8) CFUs mL(-1)). The objectives were to evaluate whether or not the mode of inoculation or the number of applied microorganisms influences plant response. The most pronounced bacterial-induced effect was found for biomass production, and the soil inoculation treatment (using 10(6) CFUs mL(-1)) led to the highest increase in shoot dry weight yield (up to 45%). Bacterial-induced effects on shoot metal concentrations were less pronounced; although a positive effect was found on shoot Pb concentration when using 10(8) CFUs mL(-1) in the soil inoculation (29% increase) and in the seed+soil inoculation (34% increase). Also shoot Zn concentration increased by 24% after seed inoculation with 10(6) CFUs mL(-1). The best effects on the total metal yield were not correlated with an increasing number of inoculated bacteria. In fact the best results were found after a single soil inoculation using the lower cellular density of 10(6) CFUs mL(-1).