Characterization of a class II 5-enopyruvylshikimate-3-phosphate synthase with high tolerance to glyphosate from Sinorhizobium fredii

Novel aroA of Glyphosate-Tolerant Bacterium Pseudomonas sp. Strain HA-09 Isolated from Roundup-Contaminated Garden Soils in Iran

Background

Glyphosate is a non-selective systemic herbicide with a broad spectrum of weed control that inhibits a key enzyme, 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, in the shikimate pathway.

Objectives

Isolation and analysis of the epsps (aroA) gene responsible for glyphosate-tolerance in bacteria from Roundup- contaminated soils was the aim of this study.

Materials and Methods

Sampling was done from the soil of the gardens which were heavily contaminated by Roundup herbicide and then bacterial screening was performed in the presence of high concentrations of glyphosate. The genus of bacterium was identified via molecular methods such as 16S rRNA sequencing. The aroA gene of this bacterium (aroA _HA-09) was isolated using the primers designed-upon specific regions of aroA genes available in NCBI GenBank database. The PCR product was cloned, sequenced and subcloned into pET28a as an expression vector and transferred into E. coli strain BL21(DE3). The cells were inoculated in liquid M9 minimal medium containing IPTG and different concentrations of glyphosate.

Results

The genus of bacterium was identified as Pseudomonas sp. strain HA-09. The isolated aroA _HA-09 gene from this bacterium was approximately 2.2 kb in size. Bioassay of E. coli expressing this gene showed high tolerance to glyphosate (up to 300 mM).

Conclusion

The aroA _HA-09 gene could be considered as a novel and efficient candidate for development of glyphosate-tolerant crop plants.

Impact of a G2-EPSPS & GAT Dual Transgenic Glyphosate-Resistant Soybean Line on the Soil Microbial Community under Field Conditions Affected by Glyphosate Application

In the past thirty years, the biosafety of the aboveground part of crops, including horizontal gene transferal through pollen dispersal and hybridization, has been the focus of research; however, microbial communities in the underground part are attracting increasing attention. In the present study, the soybean root-associated bacterial communities of the G2-EPSPS plus GAT transgenic soybean line Z106, its recipient variety ZH10, and Z106 treated with glyphosate (Z106J) were compared at the seedling, flowering, and seed filling stages by high-throughput sequencing of the V4 hypervariable regions of 16S rRNA gene amplicons using Illumina MiSeq. The results obtained showed no significant differences in the alpha/beta diversities of root-associated bacterial communities at the three stages among ZH10, Z106, and Z106J under field growth conditions; however, the relative abundance of four main nitrogen-fixing bacterial genera significantly differed among ZH10, Z106, and Z106J. Ternary plot results indicated that in the root compartment, the proportional contributions of rhizobial nitrogen-fixing Ensifer fredii and Bradyrhizobium elkanii, which exhibit an extremely broad nodulation host range, markedly differed among the three treatments at the three stages. Thus, the present results indicate that transgenic G2-EPSPS and GAT soybean may induce different changes in functional bacterial species in soil, such as E. fredii and B. elkanii, from ZH10, which were compensated for/enriched at the flowering and seed filling stages, respectively, to some extent through as of yet unknown mechanisms by transgenic soybean treated with glyphosate.

A thermostable 5-enolpyruvylshikimate-3-phosphate synthase from Thermotoga maritima enhances glyphosate tolerance in Escherichia coli and transgenic Arabidopsis

5-Enolpyruvylshikimate-3-phosphate synthase (EPSPS) overexpression, attempting to provide excess EPSPS to combine with glyphosate, is one way to improve glyphosate resistance of plants. The EPSPS in extremophiles which is selected by nature to withstand the evolutionary pressure may possess some potential-specific biological functions. In this study, we reported the cloning, expression and enzymatic characterization of a novel Class II EPSPS AroAT. maritima from Thermotoga maritima MSB8. The enzyme showed low sequence identities with other EPSPSs, and was one of the most thermostable EPSPSs so far, which showed the optimum enzyme activity at 80 °C. The enzyme maintains the activity below 50 °C and in a wide range of pH 4.0-10, which indicated its stability under rough environment, especially in tropical regions and alkaline soil. Excellent Ki/Km value of AroAT. maritima suggested that the enzyme showed powerful competitive binding capacity of PEP over glyphosate and high glyphosate tolerance. Furthermore, aroAT. maritima gene was transformed into Arabidopsis thaliana. The transgenic lines were resistant to 15 mM glyphosate, which proved the application value in the cultivation of glyphosate-tolerant plants.

Effects of herbicide on non-target microorganisms: Towards a new class of biomarkers?

Conventional agriculture still relies on the general use of agrochemicals (herbicides, fungicides and insecticides) to control various pests (weeds, fungal pathogens and insects), to ensure the yield of crop and to feed a constantly growing population. The generalized use of pesticides in agriculture leads to the contamination of soil and other connected environmental resources. The persistence of pesticide residues in soil is identified as a major threat for in-soil living organisms that are supporting an important number of ecosystem services. Although authorities released pesticides on the market only after their careful and thorough evaluation, the risk assessment for in-soil living organisms is unsatisfactory, particularly for microorganisms for which pesticide toxicity is solely considered by one global test measuring N mineralization. Recently, European Food Safety Authority (EFSA) underlined the lack of standardized methods to assess pesticide ecotoxicological effects on soil microorganisms. Within this context, there is an obvious need to develop innovative microbial markers sensitive to pesticide exposure. Biomarkers that reveal direct effects of pesticides on microorganisms are often viewed as the panacea. Such biomarkers can only be developed for pesticides having a mode of action inhibiting a specific enzyme not only found in the targeted organisms but also in microorganisms which are considered as "non-target organisms" by current regulations. This review explores possible ways of innovation to develop such biomarkers for herbicides. We scanned the herbicide classification by considering the mode of action, the targeted enzyme and the ecotoxicological effects of each class of active substance in order to identify those that can be tracked using sensitive microbial markers.

A Novel Naturally Occurring Class I 5-Enolpyruvylshikimate-3-Phosphate Synthase from Janibacter sp. Confers High Glyphosate Tolerance to Rice

As glyphosate is a broad spectrum herbicide extensively used in agriculture worldwide, identification of new aroA genes with high level of glyphosate tolerance is essential for the development and breeding of transgenic glyphosate-tolerant crops. In this study, an aroA gene was cloned from a Janibacter sp. strain isolated from marine sediment (designated as aroAJ. sp). The purified aroAJ. sp enzyme has a Km value of 30 μM for PEP and 83 μM for S3P, and a significantly higher Ki value for glyphosate (373 μM) than aroAE. coli. AroAJ. sp is characterized as a novel and naturally occurring class I aroA enzyme with glyphosate tolerance. Furthermore, we show that aroAJ. sp can be used as an effective selectable marker in both japonica and indica rice cultivar. Transgenic rice lines were tested by herbicide bioassay and it was confirmed that they could tolerate up to 3360 g/ha glyphosate, a dosage four-fold that of the recommended agricultural application level. To our knowledge, it is the first report of a naturally occurring novel class I aroA gene which can be efficiently utilized to study and develop transgenic glyphosate-tolerant crops, and can facilitate a more economical and simplified weed control system.