Differential impact of Bt-transgenic rice plantings on bacterial community in three niches over consecutive years

Impacts of AlaAT3 transgenic poplar on rhizosphere soil chemical properties, enzyme activity, bacterial community, and metabolites under two nitrogen conditions

Poplar stands as one of the primary afforestation trees globally. We successfully generated transgenic poplar trees characterized by enhanced biomass under identical nutrient conditions, through the overexpression of the pivotal nitrogen assimilation gene, pxAlaAT3. An environmental risk assessment was conducted for investigate the potential changes in rhizosphere soil associated with these overexpressing lines (OL). The results show that acid phosphatase activity was significantly altered under ammonium in OL compared to the wild-type control (WT), and a similar difference was observed for protease under nitrate. 16SrDNA sequencing indicated no significant divergence in rhizosphere soil microbial community diversity between WT and OL. Metabolomics analysis revealed that the OL caused minimal alterations in the metabolites of the rhizosphere soil, posing no potential harm to the environment. With these findings in mind, we anticipate that overexpressed plants will not adversely impact the surrounding soil environment.

Metagenomic Analysis of Rhizospheric Bacterial Community of Citrus Trees Expressing Phloem-Directed Antimicrobials

Huanglongbing, also known as citrus greening, is currently the most devastating citrus disease with limited success in prevention and mitigation. A promising strategy for Huanglongbing control is the use of antimicrobials fused to a carrier protein (phloem protein of 16 kDa or PP16) that targets vascular tissues. This study investigated the effects of genetically modified citrus trees expressing Citrus sinensis PP16 (CsPP16) fused to human lysozyme and β-defensin-2 on the soil microbiome diversity using 16S amplicon analysis. The results indicated that there were no significant alterations in alpha diversity, beta diversity, phylogenetic diversity, differential abundance, or functional prediction between the antimicrobial phloem-overexpressing plants and the control group, suggesting minimal impact on microbial community structure. However, microbiota diversity analysis revealed distinct bacterial assemblages between the rhizosphere soil and root environments. This study helps to understand the ecological implications of crops expressing phloem-targeted antimicrobials for vascular disease management, with minimal impact on soil microbiota.

New roles for Bacillus thuringiensis in the removal of environmental pollutants

For a long time, the well-known Gram-positive bacterium Bacillus thuringiensis (Bt) has been extensively studied and developed as a biological insecticide for Lepidoptera and Coleoptera pests due to its ability to secrete a large number of specific insecticidal proteins. In recent years, studies have found that Bt strains can also potentially biodegrade residual pollutants in the environment. Many researchers have isolated Bt strains from multiple sites polluted by exogenous compounds and characterized and identified their xenobiotic-degrading potential. Furthermore, its pathway for degradation was also investigated at molecular level, and a number of major genes/enzymes responsible for degradation have been explored. At present, a variety of xenobiotics involved in degradation in Bt have been reported, including inorganic pollutants (used in the field of heavy metal biosorption and recovery and precious metal recovery and regeneration), pesticides (chlorpyrifos, cypermethrin, 2,2-dichloropropionic acid, etc.), organic tin, petroleum and polycyclic aromatic hydrocarbons, reactive dyes (congo red, methyl orange, methyl blue, etc.), and ibuprofen, among others. In this paper, the biodegrading ability of Bt is reviewed according to the categories of related pollutants, so as to emphasize that Bt is a powerful agent for removing environmental pollutants.

Impact of Insect-Resistant Transgenic Maize 2A-7 on Diversity and Dynamics of Bacterial Communities in Rhizosphere Soil

Artificial modification of Bacillus thuringiensis (Bt) proteins can effectively improve their resistance to target pests, but the effect of such modification on the diversity of rhizosphere microorganisms remains unclear. Transgenic maize 2A-7 contains two artificially modified Bt proteins, mCry1Ab and mCry2Ab. These proteins can enter soil and pose a potential threat to soil microbial diversity. To assess their impacts on rhizosphere bacteria communities, the contents of the two Bt proteins and changes in bacterial community diversity in the rhizosphere soils of transgenic maize 2A-7 and its control variety were analyzed at different growth stages in 2020. The results showed that the two Bt proteins were detected at low levels in the rhizosphere soils of 2A-7 plants. No significant differences in soil bacterial diversity were detected between 2A-7 and its control variety at any of the growth stages. Bioinformatics analysis indicated that the growth stage, rather than the cultivar, was the main factor causing changes in bacterial communities. This research provides valuable data for understanding the impact of Bt crops on the soil microbiome, and establishes a theoretical basis for evaluation of their safety.

Evaluation of the Ecological Environment Affected by Cry1Ah1 in Poplar

Populus is a genus of globally significant plantation trees used widely in industrial and agricultural production. Poplars are easily damaged by Micromelalopha troglodyta and Hyphantria cunea, resulting in decreasing quality. Bt toxin-encoded by the Cry gene has been widely adopted in poplar breeding because of its strong insect resistance. There is still no comprehensive and sufficient information about the effects of Cry1Ah1-modified (CM) poplars on the ecological environment. Here, we sampled the rhizosphere soils of field-grown CM and non-transgenic (NT) poplars and applied 16S rRNA and internal transcribed spacer amplicon Illumina MiSeq sequencing to determine the bacterial community associated with the CM and NT poplars. Based on the high-throughput sequencing of samples, we found that the predominant taxa included Proteobacteria (about 40% of the total bacteria), Acidobacteria (about 20% of the total bacteria), and Actinobacteria (about 20% of the total bacteria) collected from the natural rhizosphere of NT and CM poplars. In addition, studies on the microbial diversity of poplar showed that Cry1Ah1 expression has no significant influence on rhizosphere soil alkaline nitrogen, but significantly affects soil phosphorus, soil microbial biomass nitrogen, and carbon. The results exhibited a similar bacterial community structure between CM varieties affected by the expression of Cry1Ah1 and non-transgenic poplars. In addition, Cry1Ah1 expression revealed no significant influence on the composition of rhizosphere microbiomes. These results broadly reflect the effect of the Bt toxin-encoded by Cry1Ah1 on the ecology and environment and provide a clear path for researchers to continue research in this field in the future.

Effects of Insect-Resistant Maize 2A-7 Expressing mCry1Ab and mCry2Ab on the Soil Ecosystem

Transgenic maize 2A-7 expressing mCry1Ab and mCry2Ab has excellent resistance to lepidopteran pests. Previous studies have investigated the effects of several Bacillus thuringiensis (Bt) proteins on the soil. However, the effects of artificially modified Bt proteins on soil ecosystems are still unclear. To evaluate the effects of transgenic maize 2A-7 on soil, the physicochemical properties, enzyme activities and functional diversities of the microbial communities in rhizosphere soils from 2A-7 and its near-isogenic non-transgenic control Dongdan 6531 were analyzed at different developmental stages under field conditions. The alteration of six physicochemical properties (pH, total nitrogen, total phosphorus, organic matter, available phosphorus and alkali-hydrolyzed nitrogen) and six functional enzymes (catalase, alkaline phosphatase, sucrase, acid phosphatase, urease and alkaline protease) activities in the rhizosphere soils between the two maize cultivars were drastically correlated with plant growth stage, but not affected by the artificially modified Bt transgenes. An analysis of time-course Biolog data revealed that the functional diversity of microbial communities in the rhizosphere soil of 2A-7 and its control were similar at each developmental stage. The results suggest that transgenic maize 2A-7 has no significant impact on the soil ecosystem and provide valuable information on scientific safety assessments of 2A-7 and its commercial applications.

Differential Microbial Communities in Paddy Soils Between Guiyang Plateaus and Chengdu Basins Drive the Incidence of Rice Bacterial Diseases

Southwest China has the most complex rice-growing regions in China. With great differences in topography, consisting mainly of basins and plateaus, ecological factors differ greatly between regions. In this study, bulk paddy soils collected from long-term rice fields in Chengdu (basins) and Guiyang (plateaus) were used to study the correlation between microbial diversity and the incidence of rice bacterial diseases. Results showed that the microbial community composition in paddy soils and the microbial functional categories differed significantly between basins and plateaus. They shared >70% of the dominant genera (abundance >1%), but the abundance of the dominant genera differed significantly. Functional analysis found that bulk paddy soils from Chengdu were significantly enriched in virulence factor-related genes; soils from Guiyang were enriched in biosynthesis of secondary metabolites, especially antibiotics. Correspondingly, Chengdu was significantly enriched in leaf bacterial pathogens Acidovorax, Xanthomonas, and Pseudomonas. Greenhouse experiments and correlation analysis showed that soil chemical properties had a greater effect on microbial community composition and positively correlated with the higher incidence of rice bacterial foot rot in Guiyang, whereas temperature had a greater effect on soil microbial functions and positively correlated with the higher severity index of leaf bacterial diseases in Chengdu. Our results provide a new perspective on how differences in microbial communities in paddy soils can influence the incidence of rice bacterial diseases in areas with different topographies.

Environmental Behaviors of Bacillus thuringiensis (Bt) Insecticidal Proteins and Their Effects on Microbial Ecology

Bt proteins are crystal proteins produced by Bacillus thuringiensis (Bt) in the early stage of spore formation that exhibit highly specific insecticidal activities. The application of Bt proteins primarily includes Bt transgenic plants and Bt biopesticides. Transgenic crops with insect resistance (via Bt)/herbicide tolerance comprise the largest global area of agricultural planting. After artificial modification, Bt insecticidal proteins expressed from Bt can be released into soils through root exudates, pollen, and plant residues. In addition, the construction of Bt recombinant engineered strains through genetic engineering has become a major focus of Bt biopesticides, and the expressed Bt proteins will also remain in soil environments. Bt proteins expressed and released by Bt transgenic plants and Bt recombinant strains are structurally and functionally quite different from Bt prototoxins naturally expressed by B. thuringiensis in soils. The former can thus be regarded as an environmentally exogenous substance with insecticidal toxicity that may have potential ecological risks. Consequently, biosafety evaluations must be conducted before field tests and production of Bt plants or recombinant strains. This review summarizes the adsorption, retention, and degradation behavior of Bt insecticidal proteins in soils, in addition to their impacts on soil physical and chemical properties along with soil microbial diversity. The review provides a scientific framework for evaluating the environmental biosafety of Bt transgenic plants, Bt transgenic microorganisms, and their expression products. In addition, prospective research targets, research methods, and evaluation methods are highlighted based on current research of Bt proteins.

Heterogeneity of soil bacterial and bacteriophage communities in three rice agroecosystems and potential impacts of bacteriophage on nutrient cycling

BACKGROUND

As genetic entities infecting and replicating only in bacteria, bacteriophages can regulate the community structure and functions of their host bacteria. The ecological roles of bacteriophages in aquatic and forest environments have been widely explored, but those in agroecosystems remains limited. Here, we used metagenomic sequencing to analyze the diversity and interactions of bacteriophages and their host bacteria in soils from three typical rice agroecosystems in China: double cropping in Guangzhou, southern China, rice-wheat rotation cropping in Nanjing, eastern China and early maturing single cropping in Jiamusi, northeastern China. Enterobacter phage-NJ was isolated and its functions on soil nitrogen cycling and effect on soil bacterial community structure were verified in pot inoculation experiments and 16S rRNA gene sequencing.

RESULTS

Soil bacterial and viral diversity and predicted functions varied among the three agroecosystems. Genes detected in communities from the three agroecosystems were associated with typical functions: soil bacteria in Jiamusi were significantly enriched in genes related to carbohydrate metabolism, in Nanjing with xenobiotics biodegradation and metabolism, and in Guangzhou with virulence factors and scarce in secondary metabolite biosynthesis, which might lead to a significant occurrence of rice bacterial diseases. The virus community structure varies significantly among the three ecosystems, only 13.39% of the total viral species were shared by the three rice agroecosystems, 59.56% of the viral species were specific to one agroecosystem. Notably, over-represented auxiliary carbohydrate-active enzyme (CAZyme) genes were identified in the viruses, which might assist host bacteria in metabolizing carbon, and 67.43% of these genes were present in Jiamusi. In bacteriophage isolation and inoculation experiments, Enterobacter bacteriophage-NJ reduced the nitrogen fixation capacity of soil by lysing N-fixing host bacteria and changed the soil bacterial diversity and community structure.

CONCLUSION

Our results showed that diversity and function predicted of paddy soil bacteria and viruses varied in the three agroecosystems. Soil bacteriophages can affect nutrient cycling by boosting host metabolism through the carried auxiliary metabolic genes (AMGs) and lysing the host bacteria that are involved in biogeochemical cycles. These findings form a basis for better understanding bacterial and bacteriophage diversity in different rice agroecosystems, laying a solid foundation for further studies of soil microbial communities that support ecofriendly production of healthy rice.

A Three-Year Plant Study of Salt-Tolerant Transgenic Maize Showed No Effects on Soil Enzyme Activity and Nematode Community

The environmental effects of genetically modified crops are now a global concern. It is important to monitor the potential environmental impact of transgenic corn after commercial release. In rhizosphere soil, plant roots interact with soil enzymes and microfauna, which can be affected by the transgenes of genetically modified crops. To determine the long-term impact of transgenic plant cultivation, we conducted a field study for 3 consecutive years (2018–2020) and observed the enzyme activities and nematode populations in plots planted with transgenic maize BQ-2, non-transgenic wild-type maize (Qi319), and inbred line B73. We took soil samples from three cornfields at four different growth stages (V3, V9, R1, and R6 stages); determined soil dehydrogenase, urease, and sucrase activities; and collected and identified soil nematodes to the genus level. The results demonstrated seasonal variations in dehydrogenase, urease, and sucrase activities. However, there was a consistent trend of change. The generic composition and diversity indices of the soil nematodes did not significantly differ, although significant seasonal variation was found in the individual densities of the principal trophic groups and the diversity indices of the nematodes in all three cornfields. The results of the study suggest that a 3-year cultivation of transgenic corn had no significant effects on soil enzyme activity and the soil nematode community. This study provides a theoretical basis for the environmental impact monitoring of transgenic corn.