Physico-chemical parameters, hydrological conditions, and microbial interactions can affect the growth and persistence of cyanobacteria, but the interacting effects among these bloom-forming factors are still poorly known. This hampers our capacity to predict the occurrence of cyanobacterial bloom accurately. Here, we studied the relationship between temperature, N and P cycles, and the microbial community abundance and diversity at 0.5 m under the surface of West Lake (China) from January 21 to November 20, 2015, in order to better understand the key factors regulating temporal changes in the cyanobacterial community. Using high throughput sequencing of the 16S rRNA gene V3-V4 region, we studied the diversity and abundance of bacteria. In parallel, we measured physico-chemical parameters and followed the abundance of key genes involved in N fixation, denitrification, and nutrient uptake. Multivariate analyses suggest that P concentration and water temperature are the key factors controlling the outbreak of summer cyanobacterial bloom. RT-qPCR analyses of the bacterial community and measurements of the copy number of denitrification-related gene (nirK, nosZ, nirS) show that denitrification potential and denitrifying bacteria relative abundance (Pseudomonas and Bacillus) increased in concert with diazotrophic cyanobacterial genera (Anabaena, Nostoc, Aphanizomenon flos-aquae) and the common bloom-forming non-diazotrophic cyanobacterium genus Microcystis. The present study brings new insights on the complex interplay between physico-chemical parameters, heterotrophic bacterial community composition, nitrogen cycle, and cyanobacteria dominance in a eutrophic lake.

•Herbicide enantiomers behave differently toxic effects in vascular plants and algae.•We review the current state of knowledge of herbicide enantiomers biodegradation and toxicity mechanism.•Gathering knowledge on the enantioselective effects of herbicides will be of interest for pesticide risk assessment.•Future work needs to develop “omics” techniques to clarify enantioselective mechanisms in chiral herbicide.Chiral herbicides are often used in agriculture as racemic mixtures, although studies have shown that the fate and toxicity of herbicide enantiomers to target and non-target plants can be enantioselective and that herbicide toxicity can be mediated by only one enantiomer. If one enantiomer is active against the target plant, the use of enantiomer-rich herbicide mixtures instead of racemic herbicides could decrease the amount of herbicide applied to a crop and the cost of herbicide application, as well as unintended toxic herbicide effects in the environment. Such a change in the management of herbicide applications requires in-depth knowledge and a critical analysis of the fate and effects of herbicide enantiomers in the environment. This review article first synthesizes the current state of knowledge on soil and plant biodegradation of herbicide enantiomers. Second, we discuss our understanding of the biochemical toxicity mechanisms associated with both enantiomers in target and non-target plants gained from state-of-the-art genomic, proteomic and metabolomic tools. Third, we present the emerging view on the “side effects” of herbicides in the root microbiome and their repercussions on target or non-target plant metabolism. Although our review of the literature indicates that the toxicity of herbicide enantiomers is highly variable depending on plant species and herbicides, we found general trends in the enantioselective toxic effects of different herbicides in vascular plants and algae. The present study will be helpful for pesticide risk assessments as well as for the management of applying enriched-enantiomer herbicides.Download high-res image (238KB)Download full-size image

•The effect of CuNPs on P. tricornutum was studied at the level of gene transcription.•The toxicity between CuNPs and CuSO4 under same concentrations was compared.•An experiment of the effect of salinity on DLS and zeta potential of CuNPs in culture medium was conducted.•The high dissolved Cu explained the similar toxic responses between CuNPs and CuSO4 treatments.Copper nanoparticles (CuNPs) have been used in a broad range of applications. However, they are inevitably released into the marine environment, making it necessary to evaluate their potential effects on marine phytoplankton. In this study, the short-term (96 h) effects of CuNPs and CuSO4 on Phacodactylum tricornutum growth, photosynthesis, reactive oxygen species production and transcription were assessed. It was found that high concentrations (40 μM) of CuNPs and CuSO4 significantly inhibited the growth, photosynthesis and induced oxidative stress of P. tricornutum, while lower concentrations caused a hormetic response as indicated by a slight stimulation in algal growth. The high percentage of dissolved Cu (78–100%) in culture medium suggested that the dissolved Cu was the main driver of toxicity during CuNPs treatment. The algal cells upregulated electron transport chain-related genes to produce more energy and restore photosynthesis after 96 h of treatment with CuNPs and CuSO4. This study delineates the cellular mechanism behind the toxicity of CuNPs and CuSO4 on marine diatoms.Download high-res image (149KB)Download full-size image

The growing demand for and production of commercial silver nanoparticles (AgNPs) inevitably increases the risk for their environmental release and soil accumulation, which could have deleterious effects on plant growth and soil microorganism communities. However, to date, little is known about how AgNPs impact plant growth, seed quality, and soil microbial communities. We therefore evaluated wheat growth and seed quality after exposure to low concentration of AgNPs while characterizing the composition of the associated soil microbial community by high-throughput sequencing of 16S rRNA genes. Our results showed that low concentration of AgNPs (1 mg/kg in fresh soil) neither inhibited wheat seedling growth nor changed the amino acid content in wheat seeds. Interestingly, the soil microorganisms in the wheat-planted group had more diversity and richness than those in the bulk-soil group. The structure of the bacterial community was affected by AgNP exposure, most significantly during the transition from the seedling to the vegetative stage of the wheat, but recovered to normal level after 49 days of treatment. In conclusion, the results from this study highlight that the environmental risks associated with low concentration of AgNPs, which have clear bioeffects on soil microorganisms, warrant further investigation.

•The concentration of 98 μM Al nanoparticles stimulated root growth of Arabidopsis.•Al nanoparticles increase transcription of genes involved in root growth.•At the same total initial concentration, ionic Al was toxic while Al nanoparticle was beneficial on growth.•Ionic Al induces the transcription of genes related to the SA signaling pathway.Nano-aluminium oxide (nAl2O3) is one of the most widely used nanomaterials. However, nAl2O3 toxicity mechanisms and potential beneficial effects on terrestrial plant physiology remain poorly understood. Such knowledge is essential for the development of robust nAl2O3 risk assessment. In this study, we studied the influence of a 10-d exposure to a total selected concentration of 98 μM nAl2O3 or to the equivalent molar concentration of ionic Al (AlCl3) (196 μM) on the model plant Arabidopsis thaliana on the physiology (e.g., growth and photosynthesis, membrane damage) and the transcriptome using a high throughput state-of-the-art technology, RNA-seq. We found no evidence of nAl2O3 toxicity on photosynthesis, growth and lipid peroxidation. Rather the nAl2O3 treatment stimulated root weight and length by 48% and 39%, respectively as well as photosynthesis opening up the door to the use of nAl2O3 in biotechnology and nano agriculture. Transcriptomic analyses indicate that the beneficial effect of nAl2O3 was related to an increase in the transcription of several genes involved in root growth as well as in root nutrient uptake (e.g., up-regulation of the root hair-specific gene family and root development genes, POLARIS protein). By contrast, the ionic Al treatment decreased shoot and root weight of Arabidopsis thaliana by 57.01% and 45.15%, respectively. This toxic effect was coupled to a range of response at the gene transcription level including increase transcription of antioxidant-related genes and transcription of genes involved in plant defense response to pathogens. This work provides an integrated understanding at the molecular and physiological level of the effects of nAl2O3 and ionic Al in Arabidopsis.Download high-res image (211KB)Download full-size image

The widespread application of copper oxide nanoparticles (nCuO) results in ecological risk when nanoparticles enter the environment. This study clarifies the mechanism of nCuO toxicity in Arabidopsis thaliana seedlings via comparison with copper (Cu) ion bioeffects. Under the same culture conditions, Cu2+ ion exposure exerted a stronger inhibitory effect on plant fresh weight and growth and caused stronger oxidative disruption (measured by malondialdehyde, MDA) than nCuO exposure. The Cu2+ ions also showed a stronger induction effect than did nCuO on the activity of antioxidant enzymes and the transcription of antioxidant-related genes. Dissolved Cu2+ ions contributed a minority of the toxicity of nCuO, implying that nCuO itself showed relative strong phytotoxicity. The work presented here will help increase our understanding of the toxicity of metal nanoparticles in plants.

Photosynthesis is a very important metabolic pathway for plant growth and crop yield. This report investigated the effect of the herbicide imazethapyr on photosynthesis in the Arabidopsis thaliana pnsB3 mutant (a defect in the NDH pathway) and pgr5 mutant (a defect in the PGR5 pathway) to determine which cyclic electron transport chain (CET) of the NDH and PGR5 pathways is more important for protecting the photosynthetic system under herbicide stress. The results showed that 20 μg/L imazethapyr markedly inhibited the growth of the three ecotypes of A. thaliana and produced more anthocyanins and reactive oxygen species (ROS), particularly in the pgr5 mutant. The chlorophyll fluorescence results showed that PSII was severely damaged in the pgr5 mutant. Additionally, the CET was significantly stimulated to protect the photosynthetic system from light damage in Wt and the pnsB3 mutant but not the pgr5 mutant. The real-time PCR analysis indicated that imazethapyr treatment considerably decreased the transcript levels of most photosynthesis-related genes in the three treated groups. Several genes in the PGR5 pathway were significantly induced in the pnsB3 mutant, but no genes in the NDH pathway were induced in the pgr5 mutant. The gene transcription analysis showed that the pgr5 mutant cannot compensate for the deficit in the PGR5 pathway by stimulating the NDH pathway, whereas the pnsB3 mutant can compensate for the deficit in the CET cycle by regulating the PGR5 pathway. The iTRAQ analyses also showed that the photosynthesis system, glycolysis, and TCA cycle suffered the most severe damage in the pgr5 mutant. All of these results showed that the PGR5 pathway is more critical for electron transfer around PSI than the NDH pathway to resist herbicide stress.

Trace aluminum (Al) concentrations can be toxic to marine phytoplankton, the basis of the marine food web, but the fundamental Al toxicity and detoxification mechanisms at the molecular levels are poorly understood. Using an array of proteomic, transcriptomic, and biochemical techniques, we describe in detail the cellular response of the model marine diatom Phaeodactylum tricornutum to a short-term sublethal Al stress (4 h of exposure to 200 μM total initial Al). A total of 2204 proteins were identified and quantified by isobaric tags for relative and absolute quantification (iTRAQ) in response to the Al stress. Among them, 87 and 78 proteins performing various cell functions were up- and down-regulated after Al treatment, respectively. We found that photosynthesis was a key Al toxicity target. The Al-induced decrease in electron transport rates in thylakoid membranes lead to an increase in reactive oxygen species (ROS) production, which cause increased lipid peroxidation. Several ROS-detoxifying proteins were induced to help decrease Al-induced oxidative stress. In parallel, glycolysis and pentose phosphate pathway were up-regulated in order to produce cell energy (NADPH, ATP) and carbon skeleton for cell growth, partially circumventing the Al-induced toxicity effects on photosynthesis. These cellular responses to Al stress were coordinated by the activation of various signal transduction pathways. The identification of Al-responsive proteins in the model marine phytoplankton P. tricornutum provides new insights on Al stress responses as well as a good start for further exploring Al detoxification mechanisms.

Effects of sublethal levels of the photosystem-interfering herbicides atrazine (Atr) and methyl viologen (MV) on photosynthetic electron transport were investigated in Arabidopsis thaliana mutants with defects in cyclic electron transfer (CET) activity. Analysis based on chlorophyll fluorescence parameters showed that pgr5 mutant (a defect in the PGR5 pathway) was more sensitive to both Atr and MV than wild type (Wt) and pnsB3 mutant (a defect in the NDH pathway). Real-time PCR (polymerase chain reaction) analysis of transcripts indicated that Wt plants showed marked increases in transcripts in the PRG5 and NDH pathways under treatment with either Atr or MV. In contrast, Atr increased the gene transcripts in CET, but MV decreased them in pnsB3 mutant plants. Atr did not increase the transcripts, while MV down-regulated them in pgr5 mutant. Immunoblot analysis partially supported the changes in the transcripts; that is, the protein levels of PGRL1 and PGR5 were increased in pnsB3 mutant, while no protein level was increased in pgr5 mutant after the herbicide treatment. The present results suggest that cyclic electron transport is very sensitive to photosystem-interference induced by chemicals and that the PGR5 pathway is very critical for regulation. Thus, pgr5 mutants may be useful plants for monitoring photosystem-interfering herbicides.

Residual soil concentrations of the herbicide diclofop-methyl (DM) can be toxic to other nontarget plant species, but the toxicity mechanisms at play are not fully understood. In the present study, we analyzed the toxic effect of DM on root growth and metabolism in the rice species Oryza sativa. The results show that a 48-h exposure to a trace level (5 μg/L) of DM inhibits rice root growth by almost 70%. A 48-h exposure to 5 μg/L DM also leads to an ≈2.5-fold increase in citrate synthase (CS) activity (and CS gene transcription) and an ≈2-fold decrease in the citrate lyase gene transcripts, which lead to an increase in the intracellular concentration of citrate and in citrate exudation rate. Addition of a specific inhibitor of cell membrane anion channel, anthracene-9-carboxylic acid, decreased citrate release in the culture, suggesting that DM-induced citrate loss from the cells is mediated by a specific membrane-bound channel protein. This study brings new insights into the key biochemical mechanisms leading to DM toxicity in rice.

Copper (Cu) is one of the most toxic metals in phytoplankton but the toxicity mechanisms of this metal are still not fully understood. This study examines the toxicity targets of Cu in the modeled marine diatom, Phaeodactylum tricornutum, at the physiological (cell division, DNA cell cycle), biochemical [pigments synthesis, reactive oxygen species (ROS) and malondialdehyde (MDA)], structural (subcellular observation by flow cytometry) and molecular (transcription of several metalloprotein genes) level. Cu toxicity was detectable at all these levels after 48 h of exposure to ≥20 μM total initial added Cu. The order of sensitivity of all the studied Cu toxicity endpoints was: G2/M phase > MDA > metalloproteins RNA of the photosynthetic electron transport chain (ETC) > metalloproteins RNA of the respiratory chains > G0/G1 phase > pigments ≈ S phase > propidium iodide > estimated cell yield > ROS. The relatively sensitive decrease of the transcription of metalloproteins RNA of the ETC in response to Cu exposure, if associated to an effective decrease in the expression of the proteins composing the ETC, may help to initially mitigate the ROS-mediated toxic effects of Cu in P. tricornutum. However, this cellular response to Cu was only transitory and the transcription of virtually all genes involved in redox electron transfer chains was up-regulated within an interval of 2 days. This study brings new insights into the cellular mechanisms of Cu toxicity by documenting the sensitivity and kinetics of multiple Cu-cellular targets in one marine alga.

Ascorbic acid (AsA) is one of the most important soluble antioxidant molecules in plants, but excess AsA is a type of stress factor that inhibits plant growth. The exposure of Arabidopsis thaliana seedlings to 2 and 8 mM AsA decreased fresh weight to 78.6 and 64.3% of the control, respectively, after 5 days of treatment. A more than fivefold increase in the MDA content following the exposure to AsA suggests that the plant cellular structure is severely damaged by an increase in the ROS content. We also found that the transcripts of several antioxidant genes were down-regulated, which resulted in decreased activities of several antioxidant enzymes. These events caused an imbalance between oxidants and the antioxidant system. Real-time PCR demonstrated that the exogenous AsA reduced the transcript abundance of several aquaporins, whereas 2 mM exogenous AsA increased the transcripts of four aquaporins after 5 days of exposure. A high concentration of AsA significantly down-regulated aquaporins compared to a low concentration of AsA, especially PIP 2;1 and PIP 2;2, which were only 6 and 10% of the control, respectively. These results demonstrated that exogenous AsA was a stress factor that caused ROS overproduction, inhibited antioxidant ability, regulated aquaporin gene expression, and inhibited plant growth.

This study investigated the effects of glufosinate, a widely used herbicide, on the marine diatom Phaeodactylum tricornutum through short-term toxicity tests at the physiological and gene transcriptional levels. Glufosinate (4 mg L−1) decreased the amount of pigments but increased reactive oxygen species (ROS) and malondialdehyde levels. As a glutamine synthetase (GS) inhibitor, glufosinate affected the transcripts and activities of key enzymes related to nitrogen assimilation. Transcript levels of GS and nitrate reductase (NR) in P. tricornutum decreased to only 57 and 26 % of the control. However, transcript levels of nitrate transporter (NRT) and the small subunit of glutamate synthase (GltD) were 1.79 and 1.76 times higher than that of the control. The activities of NRT, GS and GOGAT were consistent with gene expression except for NR, which was regulated mainly by post-translational modification. Furthermore, the results of electron microscopy showed that chloroplast structure was disrupted in response to glufosinate exposure. These results demonstrated that glufosinate first disturbed nitrogen metabolism and caused a ROS burst, which disrupted chloroplast ultrastructure. Ultimately, the growth of P. tricornutum was greatly inhibited by glufosinate.

•Two herbicide candidates (IIa and IIr) showed strong herbicidal activity.•IIa and IIr altered its root structure.•IIa and IIr affected the absorption of nitrogen and phosphorus.•IIa and IIr down-regulated the transcripts of N and P metabolism related genes.Both 2-[(2,4-dichlorophenoxy)acetoxy](methy)lmethyl-5,5-dimethyl-1,3,2-dioxaphosphinan-2-one (termed as IIa) and 2-[(4-chloro-2-methyl-phenoxy)-acetoxy](methyl)methyl-5,5-dimethyl-1,3,2-dioxaphosphinan-2-one (termed as IIr) are novel herbicide candidates that positively affect herbicidal activity via the introduction of a phosphorus-containing heterocyclic ring. This report investigated the mechanism of IIa and IIr on weed control in the model plant Arabidopsis thaliana at physiological, ultrastructural and molecular levels. IIa and IIr significantly inhibited the growth of A. thaliana and altered its root structure by inhibiting energy metabolism and lipid or protein biosynthesis. These compounds also significantly affected the absorption of nitrogen and phosphorus by down-regulating the transcripts of nitrate transporter-related genes, ammonium transporter-related genes and phosphorus transporter-related genes.Download full-size image

•Rice is sensitive to bentazon during the initial exposure period.•2D-DIGE reveals that bentazon primarily suppressed photosynthesis processes.•2D-DIGE reveals that several stress response proteins are induced in response to bentazon.••A 519-kD transcription factor is identified to drive expression of detoxification genes.Bentazon is a widely used herbicide that selectively removes broad-leaf weeds by competing with plastoquinone for the binding site in the D1 protein and interrupting the PET (photosynthetic electron transfer) chain. However, monocotyledonous plants, such as rice, show strong resistance to bentazon due to CYP81A6 induction, which results in herbicide detoxification. Here, we confirmed that rice was sensitive to bentazon treatment during the initial exposure period, in which bentazon rapidly inhibited photosynthesis efficiency and electron transfer, based on results of chlorophyll fluorescence analysis. In order to gain a comprehensive, pathway-oriented, mechanistic understanding of the effects directly induced by bentazon, we employed 2D-DIGE (two-dimensional difference gel electrophoresis) to analyze the leaf proteome after 8 h of bentazon treatment coupled with individual protein identification by MALDI-TOF (Matrix assisted laser desorption/ionization-time of flight) MS/MS. Proteomic analyses revealed that bentazon induced the relative upregulation or downregulation of 30 and 71 proteins (by 1.5-fold or more, p < 0.05), respectively. The pathways involved include photosynthesis processes, carbohydrate metabolism, antioxidant systems, and DNA stabilization and protein folding. Protein analysis data revealed that bentazon primarily suppressed photosynthesis processes, and showed inhibitory effects on carbohydrate metabolism and ATP synthesis, whereas several stress response proteins were induced in response to bentazon. Importantly, we identified a 519 kD protein containing two histidine kinase-like ATPase domains and a C3HC4 RING type zinc finger domain which may function as a transcript factor to drive expression of detoxification genes such as CYP81A6, leading to bentazon tolerance. This study identifies, for the first time, a candidate transcription factor that could up-regulate CYP81A6 expression, and provides a foundation for further research to advance our knowledge of mechanisms of bentazon resistance in rice.Download full-size image

Diclofop-methyl (DM), a widely used herbicide in food crops, may partly contaminate the soil surface of natural ecosystems in agricultural area and exert toxic effects at low dose to nontarget plants. Even though rhizosphere microorganisms strongly interact with root cells, little is known regarding their potential modulating effect on herbicide toxicity in plants. Here we exposed rice seedlings (Xiushui 63) to 100 μg/L DM for 2 to 8 days and studied the effects of DM on rice rhizosphere microorganisms, rice systemic acquired resistance (SAR) and rice-microorganisms interactions. The results of metagenomic 16S rDNA Illumina tags show that DM increases bacterial biomass and affects their community structure in the rice rhizosphere. After DM treatment, the relative abundance of the bacterium genera Massilia and Anderseniella increased the most relative to the control. In parallel, malate and oxalate exudation by rice roots increased, potentially acting as a carbon source for several rhizosphere bacteria. Transcriptomic analyses suggest that DM induced SAR in rice seedlings through the salicylic acid (but not the jasmonic acid) signal pathway. This response to DM stress conferred resistance to infection by a pathogenic bacterium, but was not influenced by the presence of bacteria in the rhizosphere since SAR transcripts did not change significantly in xenic and axenic plant roots exposed to DM. The present study provides new insights on the response of rice and its associated microorganisms to DM stress.Download high-res image (387KB)Download full-size image

•Diuron adversely affected the fresh weight and chlorophyll content of the plants.•pgr5 mutant was more sensitive to diuron than Wt and the ndf4 mutant.•Deficiencies in the PGR5 pathway could not be counteracted by the NDH pathway.•Deficiencies in the NDH pathway could be overcome by stimulating PGR5.Three ecotypes of Arabidopsis thaliana, ecotype Columbia (Wild type, Wt) and two mutants (pgr5 and ndf4), were used to evaluate the effects of diuron on photosynthetic activity of A. thaliana. It was found that diuron adversely affected the fresh weight and chlorophyll content of the plants. Chlorophyll fluorescence studies determined that the pgr5 mutant was more sensitive to diuron than Wt and the ndf4 mutant. Gene expression analysis revealed different roles for the two cyclic electron transfer (CET) pathways, NAD(P)H dehydrogenase (NDH) and proton gradient regulation (PGR5) pathways, in the plant after diuron treatment. For example, a gene in the NDH pathway, lhca5, was activated in the low dose (LD) group in the pgr5 mutant, but was down-regulated in the moderate dose (MD) group, along with two other NDH-related genes (ppl2 and ndhH). In the PGR5 pathway, the pgr5 gene was functional under conditions of increased stress (MD group), and was up-regulated to a greater extent in the ndf4 mutant than that in the Wt and pgr5 mutant. Our results suggest that the PGR5 pathway in plants is more important than the NDH pathway during resistance to environmental stress. Deficiencies in the PGR5 pathway could not be counteracted by the NDH pathway, but deficiencies in the NDH pathway could be overcome by stimulating PGR5.Download high-res image (103KB)Download full-size image

Plant growth and development are strongly affected by environmental pollutants, such as herbicides. Widely used herbicides can remain in soil or aquatic systems for long periods of time. Herbicide pollutants have been reported to heavily affect global plant growth and pose a significant challenge to agriculture. However, it is unclear whether herbicides affect plant flowering. Here, we demonstrated that imazethapyr (IM), a chiral herbicide, can enantioselectively promote flowering in Arabidopsis thaliana. We clarified the possible mechanism by which IM promotes flowering and found that the photoperiod pathway may play an important role in propagating the IM stress signal. IM enantiomers decreased the amplitude of core oscillators (CIRCADIAN CLOCK ASSOCIATED 1 and LATE ELONGATED HYPOCOTYL) and utilized the up-regulation of the GIGANTEA-(CONSTANS)-FLOWERING LOCUS T pathway to induce floral gene, APETALA1 over-expression enantioselectively; this treatment ultimately caused early flowering. Our findings provide new insight into the method by which plants control reproductive timing in response to herbicide stress. Flowering time is an important trait in crops and affects the life cycles of pollinator species. The persistence of herbicides in the biosphere will alter plant life cycles and diversity.