Effect and localization of phenanthrene in maize roots

Phenanthrene metabolism in Panicum miliaceum: anatomical adaptations, degradation pathway, and computational analysis of a dioxygenase enzyme

Polycyclic aromatic compounds (PAHs) are persistent organic pollutants of environmental concern due to their potential impacts on food chain, with plants being particularly vulnerable. While plants can uptake, transport, and transform PAHs, the precise mechanisms underlying their localization and degradation are not fully understood. Here, a cultivation experiment conducted with Panicum miliaceum exposed different concentrations of phenanthrene (PHE). Intermediate PHE degradation compounds were identified via GC-MS analysis, leading to the proposal of a phytodegradation pathway featuring three significant benzene ring cleavage steps. Our results showed that P. miliaceum exhibited the ability to effectively degrade high levels of PHE, resulting in the production of various intermediate products through several chemical changes. Examination of the localization and anatomical characteristics revealed structural alterations linked to PHE stress, with an observed enhancement in PHE accumulation density in both roots and shoots as treatment levels increased. Following a 2-week aging period, a decrease in the amount of PHE accumulation was observed, along with a change in its localization. Bioinformatics analysis of the P. miliaceum 2-oxoglutarate-dependent dioxygenase (2-ODD) DAO-like protein revealed a 299 amino acid structure with two highly conserved domains, namely 2OG-FeII_Oxy and DIOX_N. Molecular docking analysis aligned with experimental results, strongly affirming the potential link and direct action of 2-ODD DAO-like protein with PHE. Our study highlights P. miliaceum capacity for PAHs degradation and elucidates the mechanisms behind enhanced degradation efficiency. By integrating experimental evidence with bioinformatics analysis, we offer valuable insights into the potential applications of plant-based remediation strategies for PAHs-contaminated environments.

Laccase as a useful assistant for maize to accelerate the phenanthrene degradation in soil

Polycyclic aromatic hydrocarbon (PAH) pollution has attracted much attention due to their wide distribution in soil environment and serious harm to human health. In order to establish an efficient and eco-friendly technology for remediation of PAH-contaminated soil, phytoremediation utilizing maize assisted with enzyme remediation was explored in this study. The results showed that the participation of laccase could promote the degradation of phenanthrene (PHE) from soil and significantly reduce the accumulation of PHE in maize. The degradation efficiency of PHE in soil could reach 77.19% under laccase-assisted maize remediation treatment, while the accumulation of PHE in maize roots and leaves decreased by 41.23% and 74.63%, respectively, compared to that without laccase treatment, after 24 days of maize cultivation. Moreover, it was found that laccase addition shifted the soil microbial community structure and promoted the relative abundance of some PAH degrading bacteria, such as Pseudomonas and Sphingomonas. In addition, the activities of some enzymes that were involved in PAH degradation process and soil nutrient cycle increased with the treatment of laccase enzyme. Above all, the addition of laccase could not only improve the removal efficiency of PHE in soil, but also alter the soil environment and reduce the accumulation of PHE in maize. This study provided new perspective for exploring the efficiency of the laccase-assisted maize in the remediation of contaminated soil, evaluating the way for reducing the risk of secondary pollution of plants in the phytoremediation process.

The role of microplastics in the process of laccase-assisted phytoremediation of phenanthrene-contaminated soil

Polycyclic aromatic hydrocarbons (PAHs) are highly toxic organic pollutants widely distributed in terrestrial environments and laccase was considered as an effective enzyme in PAHs bioremediation. However, laccase-assisted phytoremediation of PAHs-contaminated soil has not been reported. Moreover, the overuse of plastic films in agriculture greatly increased the risk of co-existence of PAHs and microplastics in soil. Microplastics can adsorb hydrophobic organics, thus altering the bioavailability of PAHs and ultimately affecting the removal of PAHs from soil. Therefore, this study aimed to evaluate the efficiency of laccase-assisted maize (Zea mays L.) in the remediation of phenanthrene (PHE)-contaminated soil and investigate the effect of microplastics on this remediation process. The results showed that the combined application of laccase and maize achieved a removal efficiency of 83.47 % for soil PHE, and laccase significantly reduced the accumulation of PHE in maize. However, microplastics significantly inhibited the removal of soil PHE (10.88 %) and reduced the translocation factor of PHE in maize (87.72 %), in comparison with PHE + L treatment. Moreover, microplastics reduced the laccase activity and the relative abundance of some PAHs-degrading bacteria in soil. This study provided an idea for evaluating the feasibility of the laccase-assisted plants in the remediation of PAHs-contaminated soil, paving the way for reducing the risk of secondary pollution in the process of phytoremediation.

Microplastics reduced bioavailability and altered toxicity of phenanthrene to maize (Zea mays L.) through modulating rhizosphere microbial community and maize growth

Due to its large specific surface area and great hydrophobicity, microplastics can adsorb polycyclic aromatic hydrocarbons (PAHs), affecting the bioavailability and the toxicity of PAHs to plants. This study aimed to evaluate the effects of D550 and D250 (with diameters of 550 μm and 250 μm) microplastics on phenanthrene (PHE) removal from soil and PHE accumulation in maize (Zea mays L.). Moreover, the effects of microplastics on rhizosphere microbial community of maize grown in PHE-contaminated soil would also be determined. The results showed that D550 and D250 microplastics decreased the removal of PHE from soil by 6.5% and 2.7% and significantly reduced the accumulation of PHE in maize leaves by 64.9% and 88.5%. Interestingly, D550 microplastics promoted the growth of maize and enhanced the activities of soil protease and alkaline phosphatase, while D250 microplastics significantly inhibited the growth of maize and decreased the activities of soil invertase, alkaline phosphatase and catalase, in comparison with PHE treatment. In addition, microplastics changed the rhizosphere soil microbial community and reduced the relative abundance of PAHs degrading bacteria (Pseudomonas, Massilia, Proteobacteria), which might further inhibit the removal of PHE from soil. This study provided a new perspective for evaluating the role of microplastics on the bioavailability of PHE to plants and revealing the combined toxicity of microplastics and PHE to soil microcosm and plant growth.

Response of Poplar and Associated Fungal Endophytic Communities to a PAH Contamination Gradient

Microbial populations associated to poplar are well described in non-contaminated and metal-contaminated environments but more poorly in the context of polycyclic aromatic hydrocarbon (PAH) contamination. This study aimed to understand how a gradient of phenanthrene (PHE) contamination affects poplar growth and the fungal microbiome in both soil and plant endosphere (roots, stems and leaves). Plant growth and fitness parameters indicated that the growth of Populus canadensis was impaired when PHE concentration increased above 400 mg kg^-1. Values of alpha-diversity indicators of fungal diversity and richness were not affected by the PHE gradient. The PHE contamination had a stronger impact on the fungal community composition in the soil and root compartments compared to that of the aboveground organs. Most of the indicator species whose relative abundance was correlated with PHE contamination decreased along the gradient indicating a toxic effect of PHE on these fungal OTUs (Operational Taxonomic Units). However, the relative abundance of some OTUs such as Cadophora, Alternaria and Aspergillus, potentially linked to PHE degradation or being plant-beneficial taxa, increased along the gradient. Finally, this study allowed a deeper understanding of the dual response of plant and fungal communities in the case of a soil PAH contamination gradient leading to new perspectives on fungal assisted phytoremediation.

Soil PAHs contamination effect on the cellular and subcellular organelle changes of Phragmites australis Cav

The concentrations of ∑₁₆ priority polycyclic aromatic hydrocarbons (PAHs) for soils, roots, and above-ground parts of reed (Phragmites australis Cav.) were determined on different monitoring plots located near the city of Kamensk-Shakhtinsky, southern Russia, where historically received industrial sewage and sludge. The total PAHs concentration in monitoring soil plots was significantly higher than those in the background site which situated at the distance of 2 km from the contamination source. Accordingly, the maximum accumulation was found for phenanthrene and chrysene among the 16 priority PAHs in most of the plant samples collected in the impact zone. The effects of PAHs' pollution on changes of Phragmites australis Cav. cellular and subcellular organelles in the studied monitoring sites were also determined using optical and electron microscopy, respectively. The obtained data showed that increasing of PAHs contamination negatively affected the ultrastructural changes of the studied plants. Phragmites australis Cav. showed a high level of adaptation to the effect of stressors by using tissue and cell levels. In general, the detected alterations under the PAHs effect were possibly connected to changes in biochemical and histochemical parameters as a response for reactive oxygen species and as a protective response against oxidative stress. The obtained results introduce innovative findings of cellular and subcellular changes in plants exposed to ∑₁₆ priority PAHs as very persistent and toxic contaminants.

Uptake mechanism, subcellular distribution, and uptake process of perfluorooctanoic acid and perfluorooctane sulfonic acid by wetland plant Alisma orientale

Perfluoroalkyl substances (PFASs) are of particular environmental concern due to their environmental persistence and potential toxicity. Phytoremediation may be used to remove PFASs from wastewater. Here we investigated the uptake mechanism, subcellular distribution, and uptake process of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate acid (PFOS) in the wetland plant Alisma orientale by using a series of hydroponic experiments. Active uptake facilitated by water transporters and anion channels was involved in the uptake of PFASs by plant roots. PFOA and PFOS were mainly distributed in the water-soluble fraction (46.2-70.8%) and in cell walls (45.6-58.4%), respectively. The uptake process was proposed as follows: PFOS and PFOA were first distributed in the soluble fraction; a proportion of PFOS and PFOA were adsorbed gradually by the cell wall, and a proportion of PFOS and PFOA in the cell wall passed through the cell wall and plasmalemma and bind with organelles. PFOS and PFOA were transported from the external solution to the vascular bundle of the plant root through both symplastic and apoplastic routes.

Impact of phenanthrene on primary metabolite profiling in root exudates and maize mucilage

This study was conducted to assess the impact of polycyclic aromatic hydrocarbon on the composition of rhizodeposits. Maize was submitted to increasing phenanthrene (PHE) concentrations in the substrate (0, 25, 50, and 100 mg PHE.kg^-1 of dry sand). After 6 weeks of cultivation, two types of rhizodeposit solution were collected. The first one, called rhizospheric sand extract, resulted from the extraction of root adhering sand in order to collect mucilage and associated compounds. The second one, the diffusate solution, was collected by the diffusion of exudates from roots soaked in water. The impact of phenanthrene on maize morphology and functioning was measured prior to the analysis of the main components of the rhizodeposit solutions, by measuring total carbon, protein, amino acid, and sugars as well as by determining about 40 compounds using GC-MS and LC-MS. As maize exposure to PHE increased, different trends were observed in the two rhizodeposit solutions. In the diffusate solution, we measured a global increase of metabolites exudation like carbohydrates, amino acids, and proteins except for some monoglycerides and organic acids which exudation decreased in the presence of PHE. In the rhizospheric sand extract, we witnessed a decrease in carbohydrates and amino acids secretion as well as in fatty and organic acids when plants were exposed to PHE. Many of the compounds measured, like organic acids, carbohydrates, amino acids, or fatty acids, could directly or indirectly drive PAHs availability in soils with particular consequences for their degradation.

Profiling of main metabolites in root exudates and mucilage collected from maize submitted to cadmium stress

The aim of this study was to characterize qualitatively and quantitatively the composition of the main rhizodeposits emitted from maize (Zea mays) under Cd stress, in order to discuss their role in Cd availability and tolerance. Maize was grown for 6 weeks in sand at four Cd exposure levels (0, 10, 20, and 40 μM Cd in nutrient solution) and two types of rhizodeposits were collected at the end of cultivation period. Mucilage and other molecules adhering to rhizospheric sand were extracted with a buffer before root exudates were collected by diffusion into water. Total carbon, proteins, amino acids, and sugars were analyzed for both rhizodeposit types and about 40 molecules were identified using GC-MS and LC-MS. Cadmium effect on plant morphology and functioning was slight, but consistent with previous works on Cd toxicity. However, rhizodeposition did tend to be impacted, with a decrease in total carbon, sugars, and amino acids correlating with an increasing Cd content. Such a decrease was not noticeable for proteins in root exudates. These observations were confirmed by the same trends in individual compound contents, although the results were generally not statistically significant. Many of the molecules determined are well-known to modify, whether directly or indirectly, Cd speciation and dynamics in the soil and could play a role in Cd tolerance.

Apoplastic and symplastic uptake of phenanthrene in wheat roots

The contamination of agricultural crops by polycyclic aromatic hydrocarbons (PAHs) has drawn considerable attention due to their carcinogenicity, mutagenicity, and toxicity. However, the uptake process of PAHs in plant roots has not been clearly understood. In this work, we first study the radial uptake of phenanthrene in hydroculture wheat roots by vacuum-infiltration-centrifugation method. The concentration-dependent kinetics of apoplastic and symplastic uptake at phenanthrene concentrations of 0-6.72 μM for 4 h can be described with the Langmuir and Michaelis-Menten equations, respectively; whereas, their time-dependent kinetics at 5.60 μM phenanthrene for 36 h follow the Elovich equation. The apoplastic and symplastic uptake increases with temperature of 15-35 °C. The apparent Arrhenius activation energies for apoplastic and symplastic uptake are 77.5 and 9.39 KJ mol^-1, respectively. The symplastic uptake accounts for over 55% of total phenanthrene uptake, suggesting that symplast is the dominant pathway for wheat root phenanthrene uptake. Larger volume of symplast in roots and lower activation energy lead to the greater contribution of symplast to total uptake of phenanthrene. Our results provide not only novel insights into the mechanisms on the uptake of PAHs by plant roots, but also the help to optimize strategies for crop safety and phytoremediation of PAH-contaminated soil/water.

Retention of CdS/ZnS Quantum Dots (QDs) on the Root Epidermis of Woody Plant and Its Implications by Benzo[a]pyrene: Evidence from the in Situ Synchronous Nanosecond Time-Resolved Fluorescence Spectra Method

The retention of CdS/ZnS QDs on the epidermis has been confirmed to be one of the core procedures during the root uptake process. However, the retention mechanisms of QDs on the epidermis of woody plant were poorly understood for lacking of an appropriate QD quantitative method. In this study, a novel method for in situ determination of CdS/ZnS QDs retained on the root epidermis was established using synchronous nanosecond time-resolved fluorescence spectroscopy. No correlations between K_f values of oleylamine-CdS/ZnS QDs retained on the epidermal tissues and the surface/bulk composition of mangrove root were observed (p > 0.05) due to the existence of endocytosis mechanisms during the QD uptake processes. Moreover, the difference of the CdS/ZnS QDs in water and further translocated to xylem/phloem of root rather than the combination with cell wall/membranes was the predominant reason that caused the K_f values to follow the sequence of PEG-COOH-CdS/ZnS QDs < PEG-NH₂-CdS/ZnS QDs ≪ oleylamine-CdS/ZnS QDs.

Roots alterations in presence of phenanthrene may limit co-remediation implementation with Noccaea caerulescens

Co-phytoremediation of both trace elements and polycyclic aromatic hydrocarbons (PAH) is an emerging technique to treat multi-contaminated soils. In this study, root morphological and structural features of the heavy metal hyperaccumulator Noccaea caerulescens, exposed to a model PAH phenanthrene (PHE) in combination with cadmium (Cd), were observed. In vitro cultivated seedlings were exposed to 2 mM of PHE and/or 5 μM of Cd for 1 week. Co-phytoremediation effectiveness appeared restricted because of a serious inhibition (about 40%) of root and shoot biomass production in presence of PHE, while Cd had no significant adverse effect on these parameters. The most striking effects of PHE on roots were a decreased average root diameter, the inhibition of cell and root hair elongation and the promotion of lateral root formation. Moreover, endodermal cells with suberin lamellae appeared closer to the root apex when exposed to PHE compared to control and Cd treatments, possibly due to modified lateral root formation. The stage with well-developed suberin lamellae was not influenced by PHE whereas peri-endodermal layer development was impaired in PHE-treated plants. Many of these symptoms were similar to a water-deficit response. These morphological and structural root modifications in response to PHE exposition might in turn limit Cd phytoextraction by N. caerulescens in co-contaminated soils.

The retention and distribution of parent, alkylated, and N/O/S-containing polycyclic aromatic hydrocarbons on the epidermal tissue of mangrove seedlings

The polycyclic aromatic hydrocarbons (PAHs) located on the epidermal tissues showed distinctive toxic effects to root, while the retention and distribution of PAHs on mangrove seedlings poorly understood. Our results confirmed that the partition coefficients (K_f) of the PAHs retained on the epidermal tissue of mangrove roots, such as Kandelia obovata, Avicennia marina and Aegiceras corniculatum, were much higher than the Poaceae plants roots, for example wheat and maize (Wild et al., 2005). Moreover, to the parent and alkyl PAHs, a well negative correlation was observed between the surface polarity of these three species of mangrove root and the K_f values (p < 0.05). To the N/O/S containing PAHs, these relationships were not obviously due to existing of the π-π, n-π interactions and hydrogen bonding between the N/O/S-containing PAHs and epidermal tissues. The PAHs retained on these three species of mangrove root epidermal tissues formed larger clusters than that of on Poaceae plants, such as wheat and maize (Wild et al., 2005) due to the limitation of the suberization of the root exodermis and endodermis. After exposure of 30 d, rhizo- and endophytic bacteria degraded parts of the N/O/S-containing PAHs to medium-lifetime fluorescence substances. To our knowledge, this is the first time to assess the retention of PAHs on the epidermal tissue of mangrove root, which will improve our understanding of the root uptake PAHs process.

Phytoavailability and mechanism of bound PAH residues in filed contaminated soils

Understanding the phytoavailability of bound residues of polycyclic aromatic hydrocarbons (PAHs) in soils is essential to assessing their environmental fate and risks. This study investigated the release and plant uptake of bound PAH residues (reference to parent compounds) in field contaminated soils after the removal of extractable PAH fractions. Plant pot experiments were performed in a greenhouse using ryegrass (Lolium multiflorum Lam.) to examine the phytoavailablility of bound PAH residues, and microcosm incubation experiments with and without the addition of artificial root exudates (AREs) or oxalic acid were conducted to examine the effect of root exudates on the release of bound PAH residues. PAH accumulation in the ryegrass after a 50-day growth period indicated that bound PAH residues were significantly phytoavailable. The extractable fractions, including the desorbing and non-desorbing fractions, dominated the total PAH concentrations in vegetated soils after 50 days, indicating the transfer of bound PAH residues to the extractable fractions. This transfer was facilitated by root exudates. The addition of AREs and oxalic acid to test soils enhanced the release of bound PAH residues into their extractable fractions, resulting in enhanced phytoavailability of bound PAH residues in soils. This study provided important information regarding environmental fate and risks of bound PAH residues in soils.

Dynamics of PAHs and derived organic compounds in a soil-plant mesocosm spiked with 13C-phenanthrene

Polycyclic Aromatic Hydrocarbons (PAHs) are ubiquitous and persistent soil pollutants. Their fate and the influence of the plant rhizosphere on their dynamics has been extensively studied, but studies mainly focused on their dissipation rate. We conducted a plant-soil mesocosm experiment to study the fate and distribution of PAHs or derived compounds in the extractable fraction, the residual soil, the shoot biomass and the root biomass. The experiment was conducted for 21 days using ryegrass and a forest soil spiked with ¹³C-labeled phenanthrene (PHE), using combined IRMS and NanoSIMS for analyses. Almost 90% of the initial extractable PHE content was dissipated within 3 weeks, but no rhizospheric effect was highlighted on PHE dissipation. More than 40% of ¹³C-PHE was still in the soil at the end of the experiment, but not as PHE or PAH-derived compounds. Therefore it was under the form of new compounds (metabolites) and/or had been incorporated into the microbial biomass. About 0.36% of the initial ¹³C-PHE was recovered in the root and shoot tissues, representing similar ¹³C enrichment (E¹³C) as in the soil (E¹³C ≈ 0.04 at.%). Using NanoSIMS, ¹³C was also localized at the microscale in the roots and their close environment. Global ¹³C enrichment confirmed the results obtained by IRMS. Some hotspots of ¹³C enrichment were found, with a high ³²S/¹²C¹⁴N ratio. Comparing the ratios, sizes and shapes of these hotspots suggested that they could be bacteria.

Degradation of some representative polycyclic aromatic hydrocarbons by the water-soluble protein extracts from Zea mays L. cv PR32-B10

The ability of the water-soluble protein extracts from Zea mais L. cv. PR32-B10 to degrade some representative polycyclic aromatic hydrocarbons (PAHs), has been evaluated. Surface sterilized seeds of corn (Zea mais L. Pioneer cv. PR32-B10) were hydroponically cultivated in a growth chamber under no-stressful conditions. The water-soluble protein extracts isolated from maize tissues showed peroxidase, polyphenol oxidase and catalase activities. Incubation of the extracts with naphthalene, fluorene, phenanthrene and pyrene, led to formation of oxidized and/or degradation products. GC-MS and TLC monitoring of the processes showed that naphthalene, phenanthrene, fluorene and pyrene underwent 100%, 78%, 92% and 65% oxidative degradation, respectively, after 120 min. The chemical structure of the degradation products were determined by (1)H NMR and ESI-MS spectrometry.