Chitosan Oligosaccharides Mitigate Flooding Stress Damage in Rice by Affecting Antioxidants, Osmoregulation, and Hormones.

Rice (Oryza sativa L.) is one of the most important food crops worldwide. However, during direct seeding, rice is extremely vulnerable to flooding stress, which impairs rice's emergence and seedling growth and results in a significant yield loss. According to our research, chitosan oligosaccharides have the potential to be a chemical seed-soaking agent that greatly increases rice's resistance to flooding. Chitosan oligosaccharides were able to enhance seed energy supply, osmoregulation, and antioxidant capacity, according to physiological index assessments. Using transcriptome and metabolomic analysis, we discovered that important differential metabolites and genes were involved in the signaling pathway for hormone synthesis and antioxidant capacity. Exogenous chitosan oligosaccharides specifically and significantly inhibit genes linked to auxin, jasmonic acid, and abscisic acid. This suggested that applying chitosan oligosaccharides could stabilize seedling growth and development by controlling associated hormones and reducing flooding stress by enhancing membrane stability and antioxidant capacity. Finally, we verified the effectiveness of exogenous chitosan oligosaccharides imbibed in seeds by field validation, demonstrating that they can enhance rice seedling emergence and growth under flooding stress.

Combined pesticides in field doses weaken honey bee (Apis cerana F.) flight ability and analyses of transcriptomics and metabolomics.

Imidacloprid, chlorpyrifos, and glyphosate rank among the most extensively employed pesticides worldwide. The effects of these pesticides and their combined on the flight capability of Apis cerana, and the potential underlying mechanisms remain uncertain. To investigate these effects, we carried out flight mill, transcriptome, and metabolome experiments. Our findings reveal that individual acute oral treatments with pesticides, specifically 20 µL of 10 ng/g imidacloprid (0.2 ng per bee), 30 ng/g chlorpyrifos (0.6 ng per bee), and 60 ng/g glyphosate (1.2 ng per bee), did not impact the flight capability of the bees. However, when bees were exposed to a combination of two or three pesticides, a notable reduction in flight duration and distance was observed. In the transcriptomic and metabolomic analyses, we identified 307 transcripts and 17 metabolites that exhibited differential expression following exposure to combined pesticides, primarily associated with metabolic pathways involved in energy regulation. Our results illuminate the intricate effects and potential hazards posed by combined pesticide exposures on bee behavior. These findings offer valuable insights into the synergistic potential of pesticide combinations and their capacity to impair bee behavior. Understanding these complex interactions is essential for comprehending the broader consequences of pesticide formulations on honey bee populations.

A reference genome of Commelinales provides insights into the commelinids evolution and global spread of water hyacinth (Pontederia crassipes).

Commelinales belongs to the commelinids clade, which also comprises Poales that includes the most important monocot species, such as rice, wheat, and maize. No reference genome of Commelinales is currently available. Water hyacinth (Pontederia crassipes or Eichhornia crassipes), a member of Commelinales, is one of the devastating aquatic weeds, although it is also grown as an ornamental and medical plant. Here, we present a chromosome-scale reference genome of the tetraploid water hyacinth with a total length of 1.22 Gb (over 95% of the estimated size) across 8 pseudochromosome pairs. With the representative genomes, we reconstructed a phylogeny of the commelinids, which supported Zingiberales and Commelinales being sister lineages of Arecales and shed lights on the controversial relationship of the orders. We also reconstructed ancestral karyotypes of the commelinids clade and confirmed the ancient commelinids genome having 8 chromosomes but not 5 as previously reported. Gene family analysis revealed contraction of disease-resistance genes during polyploidization of water hyacinth, likely a result of fitness requirement for its role as a weed. Genetic diversity analysis using 9 water hyacinth lines from 3 continents (South America, Asia, and Europe) revealed very closely related nuclear genomes and almost identical chloroplast genomes of the materials, as well as provided clues about the global dispersal of water hyacinth. The genomic resources of P. crassipes reported here contribute a crucial missing link of the commelinids species and offer novel insights into their phylogeny.

Bioremediation of paddy soil with amphitropic mixture markedly attenuates rice cadmium: Effect of soil cadmium removal and Fe/S-cycling bacteria in rhizosphere.

Cadmium (Cd) pollution in paddy soil can easily lead to excessive Cd in rice, thereby considerably threatening human health. Microbial leaching is an effective pathway for the mobilization and removal of Cd from soil. In this study, an amphitropic mixture (AM) composed of autotrophic and heterotrophic microbial strains was used to leach Cd-contaminated paddy fields. Chemical analysis showed that the AM effectively removed 52 % of the total Cd, 39 % of the available Cd, and 60 % of the exchangeable and carbonate-bound Cd from the paddy soil. After bioleaching, the Cd in the discarded AM solution was adsorbed using a metal adsorbent. Effects of remediation on the soil nutrients or secondary pollution were not significant. Microbial analysis showed that >96 % and 67 % of the indigenous bacteria and fungi, respectively, remained in the AM-remediated soil. Double-cropped rice was cultivated to evaluate the Cd removal efficiency of grains using AM remediation. The Cd in early and late brown rice decreased by 86 % and 56 %, respectively, which was higher than that found for a series of biochemical remediation materials reported in other studies. Furthermore, the AM remediation promoted the growth of iron (Fe)- and sulfur (S)-cycling bacteria in the rice rhizosphere, such as Sulfuricurvum, Desulfurivibrio and Geobacter etc., which reduced the Cd availability in the soil and rice uptake. This study shows that AM has potential applications in the remediation of Cd-contaminated paddy fields and provides a new pathway for safe rice production.

Detoxification mechanism of herbicide in Polypogon fugax and its influence on rhizosphere enzyme activities.

The excessive use of chemical herbicides has resulted in evolution of herbicide-resistant weeds. Cytochrome P450 monooxygenases (P450s) are vital detoxification enzymes for herbicide-resistant weeds. Herein, we confirmed a resistant (R) Polypogon fugax population showing resistance to quizalofop-p-ethyl, acetolactate synthase (ALS)-inhibiting herbicide pyroxsulam, and several other ACCase (acetyl-CoA carboxylase)-inhibiting herbicides. Molecular analysis revealed no target-site gene mutations in the R population. Foliar spraying with malathion clearly reversed the quizalofop-p-ethyl phytotoxicity. Higher level of quizalofop-p-ethyl degradation was confirmed in the R population using HPLC analysis. Subsequently, RNA-Seq transcriptome analysis indicated that the overexpression of CYP89A2 gene appeared to be responsible for reducing quizalofop-p-ethyl phytotoxicity. The molecular docking results supported a metabolic effect of CYP89A2 protein on most herbicides tested. Furthermore, we found that low doses of herbicides stimulated the rhizosphere enzyme activities in P. fugax and the increase of rhizosphere dehydrogenase of R population may be related to its resistance mechanism. In summary, our research has shown that metabolic herbicide resistance mediated by CYP89A2, contributes to quizalofop-p-ethyl resistance in P. fugax.

Genomic analyses provide insights into the polyploidization-driven herbicide adaptation in Leptochloa weeds.

Polyploidy confers a selective advantage under stress conditions; however, whether polyploidization mediates enhanced herbicide adaptation remains largely unknown. Tetraploid Leptochloa chinensis is a notorious weed in the rice ecosystem, causing severe yield loss in rice. In China, L. chinensis has only one sister species, the diploid L. panicea, whose damage is rarely reported. To gain insights into the effects of polyploidization on herbicide adaptation, we first assembled a high-quality genome of L. panicea and identified genome structure variations with L. chinensis. Moreover, we identified herbicide-resistance genes specifically expanded in L. chinensis, which may confer a greater herbicide adaptability in L. chinensis. Analysis of gene retention and loss showed that five herbicide target-site genes and several herbicide nontarget-site resistance gene families were retained during polyploidization. Notably, we identified three pairs of polyploidization-retained genes including LcABCC8, LcCYP76C1 and LcCYP76C4 that may enhance herbicide resistance. More importantly, we found that both copies of LcCYP76C4 were under herbicide selection during the spread of L. chinensis in China. Furthermore, we identified another gene potentially involved in herbicide resistance, LcCYP709B2, which is also retained during polyploidization and under selection. This study provides insights into the genomic basis of the enhanced herbicide adaptability of Leptochloa weeds during polyploidization and provides guidance for the precise and efficient control of polyploidy weeds.

A standalone incompatible insect technique enables mosquito suppression in the urban subtropics.

The strong suppression of Aedes albopictus on two Guangzhou islands in China has been successfully achieved by releasing males with an artificial triple-Wolbachia infection. However, it requires the use of radiation to sterilize residual females to prevent population replacement. To develop a highly effective tool for dengue control, we tested a standalone incompatible insect technique (IIT) to control A. albopictus in the urban area of Changsha, an inland city where dengue recently emerged. Male mosquitoes were produced in a mass rearing facility in Guangzhou and transported over 670 km under low temperature to the release site. After a once-per-week release with high numbers of males (phase I) and a subsequent twice-per-week release with low numbers of males (phase II), the average numbers of hatched eggs and female adults collected weekly per trap were reduced by 97% and 85%, respectively. The population suppression caused a 94% decrease in mosquito biting at the release site compared to the control site. Remarkably, this strong suppression was achieved using only 28% of the number of males released in a previous trial. Despite the lack of irradiation to sterilize residual females, no triple-infected mosquitoes were detected in the field post release based on the monitoring of adult and larval A. albopictus populations for two years, indicating that population replacement was prevented. Our results support the feasibility of implementing a standalone IIT for dengue control in urban areas.

Germination Characteristics Associated With Glutathione S-Transferases Endowed Quizalofop-p-Ethyl Resistance in Polypogon fugax.

Quantification of germination characteristics between herbicide-resistant and -susceptible weeds might provide methods to control resistant weeds and permit better prediction of evolution and persistence of herbicide resistance. This study aimed to compare the germination characteristics of Asian minor bluegrass (Polypogon fugax) populations that are resistant or susceptible to quizalofop-p-ethyl under controlled conditions, which the resistance mechanism is involved in glutathione S-transferases (GSTs) metabolism-based resistance. No major differences in seed germination were found at diverse temperatures, pH ranges, and light conditions. However, a significant difference that seed response to a gradient of osmotic and salt stress between the resistant and susceptible P. fugax populations were found. Two stress response genes (P5CS-1 and CDPK-2) in P. fugax were likely involved in germination rate as well as germination speed in response to these stresses. Subsequently, population verification demonstrated that P5CS-1 and CDPK-2 genes may be linked to the resistance mechanism. Additionally, the two genes play an important role in response to salt stress and osmotic stress as shown by transcript abundance after stress treatments. Our findings suggest that the variation of the germination characteristics in P. fugax associates with the presence of GST-endowed resistance mechanism.

Genomic insights into the origin, adaptive evolution, and herbicide resistance of Leptochloa chinensis, a devastating tetraploid weedy grass in rice fields.

Chinese sprangletop (Leptochloa chinensis), belonging to the grass subfamily Chloridoideae, is one of the most notorious weeds in rice ecosystems. Here, we report a chromosome-scale reference genome assembly and a genomic variation map of the tetraploid L. chinensis. The L. chinensis genome is derived from two diploid progenitors that diverged ∼10.9 million years ago, and its two subgenomes display neither fractionation bias nor overall gene expression dominance. Comparative genomic analyses reveal substantial genome rearrangements in L. chinensis after its divergence from the common ancestor of Chloridoideae and, together with transcriptome profiling, demonstrate the important contribution of tetraploidization to the gene sources for the herbicide resistance of L. chinensis. Population genomic analyses of 89 accessions from China reveal that L. chinensis accessions collected from southern/southwestern provinces have substantially higher nucleotide diversity than those from the middle and lower reaches of the Yangtze River, suggesting that L. chinensis spread in China from the southern/southwestern provinces to the middle and lower reaches of the Yangtze River. During this spread, L. chinensis developed significantly increased herbicide resistance, accompanied by the selection of numerous genes involved in herbicide resistance. Taken together, our study generated valuable genomic resources for future fundamental research and agricultural management of L. chinensis, and provides significant new insights into the herbicide resistance as well as the origin and adaptive evolution of L. chinensis.

Application of mixotrophic acidophiles for the bioremediation of cadmium-contaminated soils elevates cadmium removal, soil nutrient availability, and rice growth.

A major challenge in radically alleviating the threats posed by Cd-contaminated paddy fields to human health is to reduce the Cd levels in both soils and rice grains. In this study, the microbial extraction (ME) treatment using a mixotrophic acidophilic consortium was used for the bioremediation of Cd-contaminated soils. The results showed that the ME treatment enhanced the total Cd (40%) and diethylenetriamine pentaacetic acid-soluble Cd (DTPA-Cd, 64%) removal efficiencies in contaminated soils. In addition, ME treatment decreased the levels of Cd acid-soluble and reducible fractions and thereby reduced Cd uptake in rice tissues. Microbial community analysis indicated that the indigenous soil microbial diversity and composition were not changed after the ME treatment, but the relative abundance of functional microbes associated with Cd removal was improved. Notably, soil available nutrient levels were elevated upon inoculation with mixotrophic acidophiles, resulting in an increase in rice growth and grain weight. This study provides a scientific basis for the potential application and evaluation of ME treatment in the field for remediating Cd-contaminated paddy soils.

Cultivar-dependent rhizobacteria community and cadmium accumulation in rice: Effects on cadmium availability in soils and iron-plaque formation.

The association between the rhizospheric microbial community and Cd accumulation in rice is poorly understood. A field trial was conducted to investigate the different rhizobacterial communities of two rice cultivars with high Cd accumulation (HA) and low Cd accumulation (LA) at four growth stages. Results showed that the Cd content in the roots of the HA cultivar was 1.23 - 27.53 higher than that of the LA cultivar (0.08 - 10.5 µg/plant) at four stages. The LA cultivar had a significantly lower Cd availability in rhizosphere and a higher quantity of iron plaque (IP) on the root surface than the HA cultivar at four stages. This resulted in the reduction of Cd concentration in IPs and Cd translocation from IP-to-root. Microbial analysis indicated that the LA cultivar formed a distinct rhizobacterial community from the HA cultivar and had less α-diversity. The rhizosphere of the LA cultivar was enriched in specific bacterial taxa (e.g., Massilia and Bacillus) involved in Cd immobilization by phosphate precipitation and IP formation by iron oxidization. However, the rhizosphere in the HA cultivar assembled abundant sulfur-oxidizing bacteria (e.g., Sulfuricurvum) and iron reduction bacteria (Geobacter). They promoted Cd mobilization and reduced IP formation via the metal redox process. This study reveals a potential approach in which specific rhizobacteria decrease or increase Cd accumulation in rice on contaminated soil and provides a new perspective for secure rice production.

First Asp-2078-Gly Mutation Conferring Resistance to Different ACCase Inhibitors in a Polypogon fugax Population from China.

Asia minor bluegrass (Polypogon fugax) is a common and problematic weed throughout China. P. fugax that is often controlled by acetyl-CoA carboxylase (ACCase) inhibitors in canola fields. Herein, we confirmed a P. fugax population (R) showing resistance to all ACCase inhibitors tested with resistance indexes ranging from 5.4-18.4. We further investigated the resistance mechanisms of this R population. Molecular analyses revealed that an amino acid mutation (Asp-2078-Gly) was present in the R population by comparing ACCase gene sequences of the sensitive population (S). In addition, differences in susceptibility between the R and S population were unlikely to be related to herbicide metabolism. Furthermore, a new derived cleaved amplified polymorphic sequence (dCAPS) method was developed for detecting the Asp-2078-Gly mutation in P. fugax efficiently. We found that 93.75% of plants in the R population carried the Asp-2078-Gly mutation, and all the herbicide-resistant phenotype of this R population is inseparable from this mutation. This is the first report of cross resistance to ACCase inhibitors conferred by the Asp-2078-Gly target-site mutation in P. fugax. The research suggested the urgent need to improve the diversity of weed management practices to prevent the widespread evolution of herbicide resistance in P. fugax in China.

Cadmium Speciation Distribution Responses to Soil Properties and Soil Microbes of Plow Layer and Plow Pan Soils in Cadmium-Contaminated Paddy Fields.

Cadmium (Cd) speciation ratio in arable land determines the Cd exposure risk and Cd uptake in crops. However, the driving mechanisms of Cd speciation change on the vertical scale of paddy fields remain poorly understood. In this study, the effects of plow layer and plow pan on Cd speciation distribution were investigated in a long-term Cd-contaminated rice ecosystem. The Cd accumulative effect within rice grain was enhanced with high levels of activated Cd speciation ratios in soils. Activated Cd speciation ratios were higher in plow layer soils, while stabilized Cd speciation ratios were elevated in plow pan soils. Soil physicochemical properties and soil microbes synergistically affected the Cd speciation changes in different ways between the two soil layers. Soil pH and organic elements in plow layer environment directly hindered the transformation of stabilized Cd speciation, while in plow pan environment, soil pH and organic elements indirectly decreased activated Cd speciation ratios and resulted in the accumulation of stabilized Cd speciation via regulating the predominant bacterial taxa. This study will improve our understanding of how soil environments regulate Cd speciation distributions in rice ecosystems and help to seek effective remediation methods of Cd-contaminated paddy fields to reduce the Cd accumulation in rice.

Can the multi-walled carbon nanotubes be used to alleviate the phytotoxicity of herbicides in soils?

Herbicides are commonly used globally. However, residual herbicides in soils for ages often result in phytotoxicity and serious yield loss to subsequent crops. In this paper, the multi-walled carbon nanotubes (MWCNTs) were utilized to amend the herbicide polluted soil, and the adsorption performance of herbicides to MWCNTs amended soil was studied. Results indicate efficient alleviation of herbicide-induced phytotoxicity to rice and tobacco due to MWCNTs amendment. When 0.4% MWCNTs were applied, the concentration of sulfentrazone that inhibited the same rice height by 50% (IC50) increased to more than 3 times that of pure soil. When the MWCNTs were used to alleviate the phytotoxicity of quinclorac to tobacco, the MWCNTs not only alleviated the phytotoxicity of quinclorac but also promoted the growth of tobacco. The MWCNTs amended soil significantly increased the adsorption of herbicide to soil than biochar. The soil microbial analysis shows that MWCNTs had no significant effect on soil microbial community diversity, but the long-term exposure to MWCNTs could change the structure of the soil microbial community. Above all, our results highlighted the potential implication of the MWCNTs to ensure crop production by promoting crop growth and reducing the residual bioavailability of herbicides.

Whole transcriptome analysis resulted in the identification of Chinese sprangletop (Leptochloa chinensis) genes involved in cyhalofop-butyl tolerance.

BACKGROUND: Chinese sprangletop [Leptochloa chinensis (L.) Nees] is an annual malignant weed, which can often be found in paddy fields. Cyhalofop-butyl is a specialized herbicide which is utilized to control L. chinensis. However, in many areas, L. chinensis has become tolerant to this key herbicide due to its continuous long-term use. RESULTS: In this study, we utilized a tolerant (LC18002) and a sensitive (LC17041) L. chinensis populations previously identified in our laboratory, which were divided into four different groups. We then employed whole transcriptome analysis to identify candidate genes which may be involved in cyhalofop-butyl tolerance. This analysis resulted in the identification of six possible candidate genes, including three cytochrome P450 genes and three ATP-binding cassette transporter genes. We then carried out a phylogenetic analysis to identify homologs of the differentially expressed cytochrome P450 genes. This phylogenetic analysis indicated that all genes have close homologs in other species, some of which have been implicated in non-target site resistance (NTSR). CONCLUSIONS: This study is the first to use whole transcriptome analysis to identify herbicide non-target resistance genes in L. chinensis. The differentially expressed genes represent promising targets for better understanding herbicide tolerance in L. chinensis. The six genes belonging to classes already associated in herbicide tolerance may play important roles in the metabolic resistance of L. chinensis to cyhalofop-butyl, although the exact mechanisms require further study.

New Lead Discovery of Herbicide Safener for Metolachlor Based on a Scaffold-Hopping Strategy.

The use of herbicide safeners can significantly alleviate herbicide injury to protect crop plants and expand the application scope of the existing herbicides in the field. Sanshools, which are well known as spices, are N-alkyl substituted compounds extracted from the Zanthoxylum species and have several essential physiological and pharmacological functions. Sanshools display excellent safener activity for the herbicide metolachlor in rice seedlings. However, the high cost of sanshools extraction and difficulties in the synthesis of their complicated chemical structures limit their utilization in agricultural fields. Thus, the present study designed and synthesized various N-alkyl amide derivatives via the scaffold-hopping strategy to solve the challenge of complicated structures and find novel potential safeners for the herbicide metolachlor. In total, 33 N-alkyl amide derivatives (2a-k, 3a-k, and 4a-k) were synthesized using amines and saturated and unsaturated fatty acids as starting materials through acylation and condensation. The identity of all the target compounds was well confirmed by 1H-NMR, 13C-NMR, and high-resolution mass spectrometry (HRMS). The primary evaluation of safener activities for the compounds by the agar method indicated that most of the target compounds could protect rice seedlings from injury caused by metolachlor. Notably, compounds 2k and 4k displayed excellent herbicide safener activities on plant height and demonstrated relatively similar activities to the commercialized compound dichlormid. Moreover, we showed that compounds 2k and 4k had higher glutathione S-transferase (GST), superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and polyphenol oxidase (PPO) activities in rice seedlings, compared to the metolachlor treatment. In particular, 2k and 4k are safer for aquatic organisms than dichlormid. Results from the current work exhibit that compounds 2k and 4k have excellent crop safener activities toward rice and can, thus, be promising candidates for further structural optimization in rice protection.

The Genomes of the Allohexaploid Echinochloa crus-galli and Its Progenitors Provide Insights into Polyploidization-Driven Adaptation.

The hexaploid species Echinochloa crus-galli is one of the most detrimental weeds in crop fields, especially in rice paddies. Its evolutionary history is similar to that of bread wheat, arising through polyploidization after hybridization between a tetraploid and a diploid species. In this study, we generated and analyzed high-quality genome sequences of diploid (E. haploclada), tetraploid (E. oryzicola), and hexaploid (E. crus-galli) Echinochloa species. Gene family analysis showed a significant loss of disease-resistance genes such as those encoding NB-ARC domain-containing proteins during Echinochloa polyploidization, contrary to their significant expansionduring wheat polyploidization, suggesting that natural selection might favor reduced investment in resistance in this weed to maximize its growth and reproduction. In contrast to the asymmetric patterns of genome evolution observed in wheat and other crops, no significant differences in selection pressure were detected between the subgenomes in E. oryzicola and E. crus-galli. In addition, distinctive differences in subgenome transcriptome dynamics during hexaploidization were observed between E. crus-galli and bread wheat. Collectively, our study documents genomic mechanisms underlying the adaptation of a major agricultural weed during polyploidization. The genomic and transcriptomic resources of three Echinochloa species and new insights into the polyploidization-driven adaptive evolution would be useful for future breeding cereal crops.

Quizalofop-p-ethyl resistance in Polypogon fugax involves glutathione S-transferases.

BACKGROUND: Asia minor bluegrass (Polypogon fugax) is one of the main weeds invading Chinese canola fields. The P. fugax resistant population SC-R, which survived quizalofop-p-ethyl at the field-recommended rate (67.5 g ha-1 ), was collected from a canola field in Qingsheng County in China. The present study aimed to (1) characterize the SC-R resistance pattern to acetyl-CoA carboxylase (ACCase)-inhibiting herbicides, and (2) investigate the mechanism of quizalofop-p-ethyl resistance in this population. RESULTS: Dose-response studies showed that resistance to quizalofop-p-ethyl and haloxyfop occurred in the SC-R population. Four transcripts/genes encoding the plastidic ACCase carboxyl-transferase domain were isolated from the P. fugax plants. No mutations in the four ACCase genes were detected in the SC-R population compared to the SC-S population. Pre-treatment with the known glutathione S-transferase (GST) inhibitor 4-chloro-7-nitrobenzoxadiazole (NBDCl), reversed resistance to quizalofop-p-ethyl and partially reversed resistance to haloxyfop-R-methyl in the resistant population (SC-R). However, the cytochrome P450 inhibitor malathion did not reverse the resistance. There was no difference in basal GST activity (using CDNB as a substrate), but there was higher inducible GST activity in SC-R relative to SC-S. Two GST genes, GST2c and GSTL3, were constitutively overexpressed in the resistant SC-R population. CONCLUSION: This study confirmed that resistance to quizalofop-p-ethyl in the resistant P. fugax population is likely nontarget-site based involving GST, and this resistance mechanism also partially confers haloxyfop-R-methyl resistance. © 2020 Society of Chemical Industry.

Bioremediation of cadmium-contaminated paddy soil using an autotrophic and heterotrophic mixture.

Cadmium (Cd) pollution poses a serious risk to human health and ecological security. Bioremediation can be a promising and effective remediation technology for treating Cd contaminated soils. In this study, seven heterotrophic strains were isolated from Cd contaminated soil and 7 autotrophic strains were isolated from acid mine drainage. Cd removal efficiencies were compared after leaching with autotrophic bacteria (Att-sys), heterotrophic isolates (Htt-sys) and cooperative leaching systems (Co-sys) in laboratory agitating reactors. The results indicated that Cd removal efficiency of Co-sys (32.09%) was significantly higher than that of Att-sys (23.24%) and Htt-sys (0.74%). By analyzing the soil microbial community in different bioleaching systems, we found that the addition of heterotrophic isolates significantly promoted the growth of some heavy metal resistant inhabitants (Massilia, Alicyclobacillus, Micromonospora, etc.), and Co-sys had a minor effect on the growth of soil indigenous microbes. In Co-sys, the content of the four Cd fractions all decreased compared with other leaching systems. The analysis of soil physicochemical parameters during the leaching process showed that pH and ORP (oxidation reduction potential) were not the only determinants for Cd removal efficiency in Co-sys, synergistic metabolic activities of autotrophic and heterotrophic strains may be other determinants. This study demonstrated that cooperative bioremediation may prove to be a safe and efficient technique for field application in heavy metal soil pollution.

Spatial distribution and risk assessment of heavy metals in contaminated paddy fields - A case study in Xiangtan City, southern China.

An extensive investigation on spatial distribution and environmental risk assessment based on total content and fractions of heavy metals, as well as the cancer risk of Cd from seven adjacent contaminated paddy fields at Xiangtan City, southern China, was conducted in this study. A total of 63 soil samples were analyzed for soil physical properties and concentrations of eight heavy metals (Cd, Cr, Co, Cu, Mn, Ni, Pb, Zn). The results showed that concentrations of metals except for Cr, Mn and Ni exceeded the background values to varying degrees, and particularly, content of Cd was as 57.4-612 times higher than background values. Principal components analysis and correlation analysis revealed three groups: industry activities for Cd and Zn; natural sources mainly for Cu, Pb, Ni and Cr, with some slight anthropogenic activities for Cu and Pb accumulation; and manganese ore associated with cobalt for Co and Mn. Combined with different indices, Cd and Zn were the major contributors to the ecological risk, and cancer risk of Cd indicated an unacceptable degree in this area. Altogether, results from this study will facilitate a better understanding of metals distribution characteristics and provide a scientific basis for further comprehensive management for these paddy fields. Combination of functional microbial agent and plants promises to be a feasible and effective remediation method for cadmium pollution in the study area.