Jiahui Xu

Diclofop-methyl affects microbial rhizosphere community and induces systemic acquired resistance in rice

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 100μg/L DM for 2 to 8days 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.

A comparison of the effects of copper nanoparticles and copper sulfate on Phaeodactylum tricornutum physiology and transcription

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 (96h) effects of CuNPs and CuSO4 on Phacodactylum tricornutum growth, photosynthesis, reactive oxygen species production and transcription were assessed. It was found that high concentrations (40μM) 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 96h of treatment with CuNPs and CuSO4. This study delineates the cellular mechanism behind the toxicity of CuNPs and CuSO4 on marine diatoms.

Spatial Variability of Cyanobacteria and Heterotrophic Bacteria in Lake Taihu (China).

Cyanobacterial blooms frequently occur in Lake Taihu (China), but the intertwined relationships between biotic and abiotic factors modulating the frequency and duration of the blooms remain enigmatic. To better understand the relationships between the key abiotic and biotic factors and cyanobacterial blooms, we measured the abundance and diversity of prokaryotic organisms by high-throughput sequencing, the abundance of key genes involved in microcystin production and nitrogen fixation or loss as well as several physicochemical parameters at several stations in Lake Taihu during a cyanobacterial bloom of Microcystis sp.. Measurements of the copy number of denitrification-related genes and 16S rRNA analyses show that denitrification potential and denitrifying bacteria abundance increased in concert with non-diazotrophic cyanobacteria (Microcystis sp.), suggesting limited competition between cyanobacteria and heterotrophic denitrifiers for nutrients, although potential bacteria-mediated N loss may hamper Microcystis growth. The present study provides insight into the importance of different abiotic and biotic factors in controlling cyanobacteria and heterotrophic bacteria spatial variability in Lake Taihu.

Inhibitory effects of tributyl phosphate on algal growth, photosynthesis, and fatty acid synthesis in the marine diatom Phaeodactylum tricornutum

The widely used solvent extractant, tributyl phosphate (TBP), primarily used as a solvent for the conventional processing of nuclear fuel, has come under scrutiny recently due to concerns surrounding potential environmental contamination and toxicity. In this study, we found that, in Phaeodactylum tricornutum, administration of TBP severely inhibited algal cell growth by reducing photosynthetic efficiency and inducing oxidative stress. We further explored the effect of TBP by examining the gene expression of the photosynthetic electron transport chain and its contribution to reactive oxygen species (ROS) burst. Our data revealed that TBP affected both fatty acid content and profile by regulating the transcription of genes related to glycolysis, fatty acid biosynthesis, and β-oxidation. These results demonstrated that TBP did in fact trigger the synthesis of ROS, disrupting the subcellular membrane structure of this aquatic organism. Our study brings new insight into the fundamental mechanism of toxicity exerted by TBP on the marine alga P. tricornutum.

Diuron treatment reveals the different roles of two cyclic electron transfer pathways in photosystem II in Arabidopsis thaliana

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.

Responses of unicellular alga Chlorella pyrenoidosa to allelochemical linoleic acid

Linoleic acid (LA), is the product of secondary metabolism secreted from Microcystis aeruginosa, and it exhibits allelopathic activity against eukaryotic algae. However, information about on the mechanisms associated with the inhibition of algal activity by LA is limited. In this study, Chlorella pyrenoidosa was treated with LA (20-120 μg L-1) for 4 days, and its growth inhibition and physiological responses were examined for potential toxic mechanisms. The photosynthetic efficiency of C. pyrenoidosa was inhibited by LA treatments, and the Fv/Fm parameter decreased significantly compared to that of controls; however, the photosynthetic pigment content did not change significantly. Peroxidase activity was enhanced, relieving oxidative damage in algae after LA treatments. However, superoxide dismutase and catalase were suppressed, ultimately leading to the aggravation of lipid peroxidation. Transcriptome-based gene expression analysis revealed that the 120 μg L-1 LA treatment significantly inhibited the transcription of genes related to photosynthesis, carbon metabolism, and amino acid metabolism in C. pyrenoidosa, suggesting that these genes might be key LA targets in C. pyrenoidosa. Moreover, the expression of genes involved in vitamin, lipid, nitrogen cycling, terpenoid, and ascorbate metabolism was also affected, suggesting that LA inhibits algal cell growth through multiple pathways. The identification of LA-responsive genes in C. pyrenoidosa provides new insight into LA stress responses in eukaryotic algae.

Changes in bacterial community structure and antibiotic resistance genes in soil in the vicinity of a pharmaceutical factory.

China is the largest global producer of antibiotics. With the demand for antibiotics increasing every year, it is necessary to assess potential environmental risks and the spread of antibiotic resistance genes (ARGs) associated with antibiotic production. Here, we investigated the occurrence and distribution of ARGs in soil in the vicinity of a pharmaceutical factory. The results showed that antibiotic concentrations were under the detection limit; however, ARGs were present in soil and tended to be enriched near the factory. A significant correlation between the relative abundance of intI-1 and tetracycline ARGs implied that horizontal gene transfer might play an important role in the spread of ARGs. The occurrence of these ARGs could be the results of previous antibiotic contamination. However, the soil bacterial community structure seemed to be more affected by nutrients or other factors than by antibiotics. Overall, this study supports the viewpoint that long-term pharmaceutical activity might have a negative effect on environmental health, thus, underscoring the need to regulate antibiotic production and management.