Selenium in soil-plant system: Transport, detoxification and bioremediation.

Selenium (Se) is an essential micronutrient for humans and a beneficial element for plants. However, high Se doses always exhibit hazardous effects. Recently, Se toxicity in plant-soil system has received increasing attention. This review will summarize (1) Se concentration in soils and its sources, (2) Se bioavailability in soils and influencing factors, (3) mechanisms on Se uptake and translocation in plants, (4) toxicity and detoxification of Se in plants and (5) strategies to remediate Se pollution. High Se concentration mainly results from wastewater discharge and industrial waste dumping. Selenate (Se [VI]) and selenite (Se [IV]) are the two primary forms absorbed by plants. Soil conditions such as pH, redox potential, organic matter and microorganisms will influence Se bioavailability. In plants, excessive Se will interfere with element uptake, depress photosynthetic pigment biosynthesis, generate oxidative damages and cause genotoxicity. Plants employ a series of strategies to detoxify Se, such as activating antioxidant defense systems and sequestrating excessive Se in the vacuole. In order to alleviate Se toxicity to plants, some strategies can be applied, including phytoremediation, OM remediation, microbial remediation, adsorption technique, chemical reduction technology and exogenous substances (such as Methyl jasmonate, Nitric oxide and Melatonin). This review is expected to expand the knowledge of Se toxicity/detoxicity in soil-plant system and offer valuable insights into soils Se pollution remediation strategies.

Mangroves leaves phyllosphere bacteria community and its ability to survive under pyrene stress during the acclimation process.

Plants in general and mangroves in particular can harbor hyper-diverse microorganisms in their different compartments including the phyllosphere area. This study used the leaves of three mangrove species; black mangrove (Avicenia germinans), red mangrove (Rhizophora mangle) and mangrove apple (Sonneratia alba) in order to evaluate the phyllosphere epiphytic bacterial community on their leaves surface and assess the ability of some epiphytic bacteria to tolerate and survive under pyrene stress. Through the 16S rRNA genes sequencing, 380203, 405203 and 344863 OTUs were identified respectively in the leaves of mangroves apple, black and red mangroves. The identified OTUs was positively correlated with leaves-wax (p < 0.05, r2 = 0.904), nitrogen (r2 = 0.72), phosphorus content (r2 = 0.62) and the water factor (r2 = 0.93). It was however highly and negatively correlated with the canopy cover (r2 = 0.93). The pyrene degradation rate in the mineral salt medium (MSM) containing pyrene as external stress was different in each mangrove species and varied depending on various factors. Therefore, through the succession culture in MSM, several bacteria strain belonging to Rhizobiales and Enterobacteres were found to be abundant in red mangroves. Bacteria belonging to Bacilliales and Sphingobacteriales were more abundant in mangroves apples and bacteria from Xanthomonadales and Sphingomonadales were more presents in back mangroves. The important finding was to reveal that the black mangrove at the non-submerged substrate, recorded the highest number of OTU, coinciding with its highest leaf's nitrogen and phosphorus content and most importantly, its highest rate of pyrene degradation. The general result of this study join previous research results and get place in the mangrove agenda, as part of a better understanding insight into the role of plant identity in driving the phyllosphere epiphytic microbial community structures in mangrove ecosystems.

Aerobic and Anaerobic Bacterial and Fungal Degradation of Pyrene: Mechanism Pathway Including Biochemical Reaction and Catabolic Genes.

Microbial biodegradation is one of the acceptable technologies to remediate and control the pollution by polycyclic aromatic hydrocarbon (PAH). Several bacteria, fungi, and cyanobacteria strains have been isolated and used for bioremediation purpose. This review paper is intended to provide key information on the various steps and actors involved in the bacterial and fungal aerobic and anaerobic degradation of pyrene, a high molecular weight PAH, including catabolic genes and enzymes, in order to expand our understanding on pyrene degradation. The aerobic degradation pathway by Mycobacterium vanbaalenii PRY-1 and Mycobactetrium sp. KMS and the anaerobic one, by the facultative bacteria anaerobe Pseudomonas sp. JP1 and Klebsiella sp. LZ6 are reviewed and presented, to describe the complete and integrated degradation mechanism pathway of pyrene. The different microbial strains with the ability to degrade pyrene are listed, and the degradation of pyrene by consortium is also discussed. The future studies on the anaerobic degradation of pyrene would be a great initiative to understand and address the degradation mechanism pathway, since, although some strains are identified to degrade pyrene in reduced or total absence of oxygen, the degradation pathway of more than 90% remains unclear and incomplete. Additionally, the present review recommends the use of the combination of various strains of anaerobic fungi and a fungi consortium and anaerobic bacteria to achieve maximum efficiency of the pyrene biodegradation mechanism.

Hyunsoonleella sp. HU1-3 Increased the Biomass of Ulva fasciata.

Green algae are photosynthetic organisms and play an important role in coastal environment. The microbial community on the surface of green algae has an effect on the health and nutrition of the host. However, few species of epiphytic microbiota have been reported to play a role in promoting the growth of algae. In this study, 16S rDNA sequencing was used to study the changes of microbial composition on the surface of Ulva fasciata at different growth stages. Some growth promoting bacteria were identified. The possible growth-promoting behavior of the strains were verified by co-culture of pure bacteria obtained from the surface of U. fasciata with its sterile host. Among the identified species, a new bacterial species, Hyunsoonleella sp. HU1-3 (belonging to the family Flavobacteriaceae) significantly promoted the growth of U. fasciata. The results also showed that there were many genes involved in the synthesis of growth hormone and cytokinin in the genome of Hyunsoonleella sp. HU1-3. This study identified the bacterium Hyunsoonleella sp. HU1-3 for the first time, in which this bacterium has strong growth-promoting effects on U. fasciata. Our findings not only provide insights on the establishment of the surface microbiota of U. fasciata, but also indicate that Hyunsoonleella sp. HU1-3 is one of the important species to promote the growth of U. fasciata.

Role of Ferrous Sulfate (FeSO4) in Resistance to Cadmium Stress in Two Rice (Oryza sativa L.) Genotypes.

The impact of heavy metal, i.e., cadmium (Cd), on the growth, photosynthetic pigments, gas exchange characteristics, oxidative stress biomarkers, and antioxidants machinery (enzymatic and non-enzymatic antioxidants), ions uptake, organic acids exudation, and ultra-structure of membranous bounded organelles of two rice (Oryza sativa L.) genotypes (Shan 63 and Lu 9803) were investigated with and without the exogenous application of ferrous sulfate (FeSO4). Two O. sativa genotypes were grown under different levels of CdCl2 [0 (no Cd), 50 and 100 µM] and then treated with exogenously supplemented ferrous sulfate (FeSO4) [0 (no Fe), 50 and 100 µM] for 21 days. The results revealed that Cd stress significantly (p < 0.05) affected plant growth and biomass, photosynthetic pigments, gas exchange characteristics, affected antioxidant machinery, sugar contents, and ions uptake/accumulation, and destroy the ultra-structure of many membranous bounded organelles. The findings also showed that Cd toxicity induces oxidative stress biomarkers, i.e., malondialdehyde (MDA) contents, hydrogen peroxide (H2O2) initiation, and electrolyte leakage (%), which was also manifested by increasing the enzymatic antioxidants, i.e., superoxidase dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) and non-enzymatic antioxidant compounds (phenolics, flavonoids, ascorbic acid, and anthocyanin) and organic acids exudation pattern in both O. sativa genotypes. At the same time, the results also elucidated that the O. sativa genotypes Lu 9803 are more tolerant to Cd stress than Shan 63. Although, results also illustrated that the exogenous application of ferrous sulfate (FeSO4) also decreased Cd toxicity in both O. sativa genotypes by increasing antioxidant capacity and thus improved the plant growth and biomass, photosynthetic pigments, gas exchange characteristics, and decrease oxidative stress in the roots and shoots of O. sativa genotypes. Here, we conclude that the exogenous supplementation of FeSO4 under short-term exposure of Cd stress significantly improved plant growth and biomass, photosynthetic pigments, gas exchange characteristics, regulate antioxidant defense system, and essential nutrients uptake and maintained the ultra-structure of membranous bounded organelles in O. sativa genotypes.

Soil phosphorus transformation characteristics in response to molybdenum supply in leguminous crops.

Phosphorus (P) is one of the most restrictive essential elements to crop growth and development due to less availability in the soil system. Previous studies have reported the synergistic effects between molybdenum (Mo) and P fertilizer on P uptake in various crops. However, an induced long term effect of Mo on soil P dynamics in the rhizosphere and non-rhizosphere has not been reported yet in leguminous crops. In this study, a long term field experiment was conducted to explore the P transformation characteristics and bioavailability in Mo-deficient (-Mo) and Mo-enriched (+Mo) soil under leguminous (broad bean-soybean) cropping system. The results indicated that long-term Mo application increased the plant dry matter accumulation (14.23%-35.27%, for broad bean; 24.40%-37.46%, for soybean) from March-September. In rhizosphere soil, the percent decrease in pH (8.10%) under +Mo treatment of the soybean crop was recorded more during September as compared to broad bean crop. Under Mo supply, H2O-Pi fraction increased up to 28.53% and 43.67% while for NaHCO3-Pi this increase was up to 5.61% and 11.98%, respectively in the rhizosphere soil of broad bean and soybean, whereas, residual-P exhibited the highest proportion of P fractions. Moreover, compared with -Mo, +Mo treatments significantly increased the soil acid phosphatase (broad bean = 17.43 µmol/d/g; soybean = 28.60 µmol/d/g), alkaline phosphatase (broad bean = 3.34 µmol/d/g; soybean 6.35 µmol/d/g) and phytase enzymes activities (broad bean = 2.45 µmol/min/g; soybean = 5.91 µmol/min/g), transcript abundance of phoN/phoC genes and microbial biomass P (MBP) in rhizosphere soil. In crux, the findings of this study suggest that long term Mo application enhanced P bioavailability through increased available P, MBP, P related enzymes activities and their genes expressions which may represent a strategy of Mo to encounter P deficiencies in the soil system.

Molybdenum-induced effects on photosynthetic efficacy of winter wheat (Triticum aestivum L.) under different nitrogen sources are associated with nitrogen assimilation.

Different nitrogen (N) sources have been reported to significantly affect the photosynthesis (Pn) and its attributes. However, molybdenum (Mo) induced effects on photosynthetic efficacy of winter wheat under different N sources have not been investigated. A hydroponic study was carried out comprising of two winter wheat cultivars '97003' and '97014' as Mo-efficient and Mo-inefficient, respectively to underpin the effects of Mo supply (0 and 1 µM) on photosynthetic efficacy of winter wheat under different N sources (NO3̶, NH4NO3 or NH4+). The results revealed that Mo-induced increases in dry weight, gas exchange parameters, chlorophyll contents, NR activities, NO3̶ assimilation, total N contents and transcripts of TaNR and TaNRT1.1 genes under different N sources followed the trend of NH4NO3 > NO3̶ > NH4+, suggesting that Mo has more complementary effects to nitrate nutrition than sole ammonium. Interestingly, under Mo-deprivation environments, cultivar '97003' recorded more pronounced alterations in Mo-dependent parameters than '97014' cultivar. Moreover, Mo application significantly improved the chlorophyll contents and chloroplast configuration in all N sources showing that Mo has a key role in chlorophyll biosynthesis and chloroplast integrity. The results also highlighted that Mo-induced enhancements in total N contents and photosynthetic characteristics followed the same order as NH4NO3 > NO3- > NH4+, suggesting that Mo might affect Pn through N metabolism. In crux, our study findings imply that Mo supply increased Pn not only through chlorophyll synthesis and chloroplast configuration but also by N uptake and assimilation which may represent a strategy of Mo fertilizer to strengthen the photosynthetic machinery.

Molybdenum-Induced Effects on Nitrogen Metabolism Enzymes and Elemental Profile of Winter Wheat (Triticum aestivum L.) Under Different Nitrogen Sources.

Different nitrogen (N) sources have been reported to significantly affect the activities and expressions of N metabolism enzymes and mineral elements concentrations in crop plants. However, molybdenum-induced effects in winter wheat cultivars have still not been investigated under different N sources. Here, a hydroponic study was carried out to investigate these effects on two winter wheat cultivars ('97003' and '97014') as Mo-efficient and Mo-inefficient, respectively, under different N sources (NO3-, NH4NO3, and NH4+). The results revealed that the activities of nitrate reductase (NR) and nitrite reductase (NiR) followed the order of NH4NO3 > NO3- > NH4+ sources, while glutamine synthetase (GS) and glutamate synthase (GOGAT) followed the order of NH4+ > NH4NO3 > NO3- in both the wheat cultivars. However, Mo-induced effects in the activities and expressions of N metabolism enzymes under different N sources followed the order of NH4NO3 > NO3- > NH4+ sources, indicating that Mo has more complementary effects towards nitrate nutrition than the sole ammonium source in winter wheat. Interestingly, under -Mo-deprived conditions, cultivar '97003' recorded more pronounced alterations in Mo-dependent parameters than '97014' cultivar. Moreover, Mo application increased the proteins, amino acids, ammonium, and nitrite contents while concomitantly decreasing the nitrate contents in the same order of NH4NO3 > NO3- > NH4+ sources that coincides with the Mo-induced N enzymes activities and expressions. The findings of the present study indicated that Mo plays a key role in regulating the N metabolism enzymes and assimilatory products under all the three N sources; however, the extent of complementation exists in the order of NH4NO3 > NO3- > NH4+ sources in winter wheat. In addition, it was revealed that mineral elements profiles were mainly affected by different N sources, while Mo application generally had no significant effects on the mineral elements contents in the winter wheat leaves under different N sources.

Modified Rice Straw Enhanced Cadmium (II) Immobilization in Soil and Promoted the Degradation of Phenanthrene in Co-Contaminated Soil.

Very limited information is available about heavy metal-polycyclic aromatic hydrocarbons (PAHs) depollution involving the modified natural material in soil. Using phenanthrene and cadmium (Cd) as model, this study investigated the effect(s) of modified rice straw by a NaOH solution and on PAHs, heavy metal availability, and their interactions. Treatment included chemical contaminant with/without modified/unmodified rice straw. Fourier Transform Infrared (FTIR) analysis revealed that certain functional groups including anionic matters groups, which can a complex with Cd2+, were exposed on the modified rice straw surfaces. Therefore, Cd concentration was significantly reduced by about 60%, 57%, 62.5 %, and, 64% in the root, shoot, CaCl2, diethylenetriaminepentaacetic acid (DTPA), and extractable Cd, respectively. Subsequently, the prediction of the functional profile of the soil metagenome using Clusters Orthologous Groups (COGs) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) database revealed that the significantly changed individual COGs belonged to the carbohydrate metabolism, ion transports, and signaling (including cytochrome P450s) categories. This indicated that ion transports might be involved in Cd management, while carbohydrate metabolism, including bisphenol, benzoate, ethylbenzene degradation, and cytochrome P450s, were rather involved in phenanthrene metabolism. The exposed functional group might serve as an external substrate, and P450s might serve as a catalyst to activate and initiate phenanthrene metabolism process. These finding offer confirmation that modified straw could promote the reduction of heavy metal and the degradation of PAHs in soil.

Selenium induces changes of rhizosphere bacterial characteristics and enzyme activities affecting chromium/selenium uptake by pak choi (Brassica campestris L. ssp. Chinensis Makino) in chromium contaminated soil.

Understanding the chemical response and characteristics of bacterial communities in soil is critical to evaluate the effects of selenium (Se) supplement on plant growth and chromium (Cr)/Se uptake in Cr contaminated soil. The rhizosphere soil characteristics of pak choi (Brassica campestris L. ssp. Chinensis Makino) were investigated in soil contaminated with different levels and forms of Cr when supplemented with Se. Although inhibition of plant growth caused by Cr stress was not completely alleviated by Se, Cr content in plant tissues decreased in Cr(VI)120Se5 treatment (Cr(VI): 120 mg kg-1 soil; Se: 5 mg kg-1 soil) and its bioavailability in soil decreased in Cr(III)200Se5 (Cr(III): 200 mg kg-1 soil; Se: 5 mg kg-1 soil) treatment. Moreover, antagonism of Cr and Se on soil enzyme activities and bacterial communities were revealed. Notably, results of Cr(VI) reduction and Se metabolism functional profiles confirmed that bacterial communities play a critical role in regulating Cr/Se bioavailability. Additionally, the increases of Se bioavailability in Cr contaminated soil were ascribed to oxidation of Cr(VI) and reduction of Se reductases proportions, as well as the enhancing of pH in soil. These findings reveal that Se has the potential capacity to sustain the stability of microdomain in Cr contaminated soil.

Cadmium in plants: uptake, toxicity, and its interactions with selenium fertilizers.

Cd is the third major contaminant of greatest hazard to the environment after mercury and lead and is considered as the only metal that poses health risks to both humans and animals at plant tissue concentrations that are generally not phytotoxic. Cd accumulation in plant shoots depends on Cd entry through the roots, sequestration within root vacuoles, translocation in the xylem and phloem, and Cd dilution within the plant shoot throughout its growth. Several metal transporters, processes, and channels are involved from the first step of Cd reaching the root cells and until its final accumulation in the edible parts of the plant. It is hard to demonstrate one step as the pivotal factor to decide the Cd tolerance or accumulation ability of plants since the role of a specific transporter/process varies among plant species and even cultivars. In this review, we discuss the sources of Cd pollutants, Cd toxicity to plants, and mechanisms of Cd uptake and redistribution in plant tissues. The metal transporters involved in Cd transport within plant tissues are also discussed and how their manipulation can control Cd uptake and/or translocation. Finally, we discuss the beneficial effects of Se on plants under Cd stress, and how it can minimize or mitigate Cd toxicity in plants.

Earthworms, Rice Straw, and Plant Interactions Change the Organic Connections in Soil and Promote the Decontamination of Cadmium in Soil.

The joint effects of earthworms and crop straw on toxic metal speciation are not clear, and very limited information is available regarding the effects of their interaction on Cd mobility in Cd contaminated soil or in remediation processes involving plants. This study evaluated their impacts on Cd mobile form changes in soil and their effects on Cd uptake by plants. Treatments included both planted and unplanted-Cd-contaminated soil with or without rice straw and/or earthworms. The results revealed that earthworms, rice straw, and plant interactions change the Cd mobile forms in soil. The order of Cd concentration of different chemical forms was as follows: exchangeable > residual > bound to Fe-Mn oxide > bound to organic matter for earthworms, and exchangeable > bound to organic matter > residual > bound to Fe-Mn oxide for rice straw treatment, with a recovery rate of 96 ± 3%. The accumulation of Cd in plants increased in the presence of earthworms and decreased in the presence of rice straw. FT-IR spectra indicated that the degradation of rice straw increases C⁻O, C⁻O⁻H, C⁻H, and O⁻H functional groups which could complex with Cd ions. These findings highlighted that earthworms' activities and crop straw can modify soil properties and structure and promote the remediation of heavy metal. This study suggests that the ecological context of remediation instead of being limiting on soil-earthworms-plant interaction, should integrate the natural resources forsaken which can provide a positive influence on both plant health and the remediation of heavy metal in contaminated soil.

Phenanthrene Mitigates Cadmium Toxicity in Earthworms Eisenia fetida (Epigeic Specie) and Aporrectodea caliginosa (Endogeic Specie) in Soil.

In classical toxicology studies, the interaction of combined doses of chemicals with dissimilar modes of toxic action in soil is complex and depending on the end point investigated and the experimental protocol employed. This study was used to examine the interactive effect of phenanthrene and Cadmium on two ecologically different species of earthworms; Eisenia. fetida and Aporrectodea. caliginosa. This interactive effect was scrutinized by using the acute toxicity test with the concentrations of 2.51 mg kg-1 and 3.74 mg kg-1, respectively, being lethal for 50% of E. fetida and A. caliginosa. The results showed that in the mixture treatment, phenanthrene at 5, 10, 15 and 20 mg kg-1 significantly mitigated both earthworms species mortality and body-mass loss. Moreover, the factor of Cd accumulated in E. fetida and A. caliginosa tissues was significantly decreased by about 12% and 16%, respectively. Linear regression correlation coefficient revealed that the reduction of both earthworm species mortality was negatively and significantly correlated (r² = 0.98 ± 0.40 and 1 ± 3.9 p < 0.001) with phenanthrene concentration in soil. However, over 20 mg kg-1 of phenanthrene, both organisms mortality rate increased again, as was the Bioaccumulation factor of phenanthrene. Thus, this study proposes that the antagonistical effect of phenanthrene on Cd at a degree of concentration can be used to mitigate Cd effect on soil living organisms. However, as an implication of these results, the interpretation of standardized toxicity bioassays, including whole effluent toxicity tests and single-compound toxicity tests, should be performed with caution. In addition, risk assessment protocols for environment pollution by a mixture of metals and polycyclic aromatic hydrocarbons should include robust methods that can detect possible interactive effects between contaminants to optimize environmental protection.

Can Selenium and Molybdenum Restrain Cadmium Toxicity to Pollen Grains in Brassica napus?

Cadmium (Cd) is highly toxic, even at very low concentrations, to both animals and plants. Pollen is extremely sensitive to heavy metal pollutants; however, less attention has been paid to the protection of this vital part under heavy metal stress. A pot experiment was designed to investigate the effect of foliar application of Se (1 mg/L) and Mo (0.3 mg/L) either alone or in combination on their absorption, translocation, and their impact on Cd uptake and its further distribution in Brassica napus, as well as the impact of these fertilizers on the pollen grains morphology, viability, and germination rate in B. napus under Cd stress. Foliar application of either Se or Mo could counteract Cd toxicity and increase the plant biomass, while combined application of Se and Mo solutions on B. napus has no significant promotional effect on plant root and stem, but reduces the seeds' weight by 10⁻11%. Se and Mo have decreased the accumulated Cd in seeds by 6.8% and 9.7%, respectively. Microscopic studies, SEM, and pollen viability tests demonstrated that pollen grains could be negatively affected by Cd, thus disturbing the plant fertility. Se and Mo foliar application could reduce the toxic symptoms in pollen grains when the one or the other was sprayed alone on plants. In an in vitro pollen germination test, 500 µM Cd stress could strongly inhibit the pollen germination rate to less than 2.5%, however, when Se (10 µM) or Mo (1.0 µM) was added to the germination medium, the rate increased, reaching 66.2% and 39.4%, respectively. At the molecular level, Se and Mo could greatly affect the expression levels of some genes related to Cd uptake by roots (IRT1), Cd transport (HMA2 and HMA4), Cd sequestration in plant vacuoles (HMA3), and the final Cd distribution in plant tissue at the physiological level (PCS1).