Toxicological and biochemical analyses demonstrate no toxic effect of Cry1C and Cry2A to Folsomia candida

Neutral Dietary Effects of Two MicroRNAs, Csu-Novel-260 and Csu-Mir-14, on the Non-Target Arthropod Folsomia candida

RNA interference (RNAi) that is triggered by small or short RNAs has shown enormous potential in the development of pest control strategies. Two microRNAs (miRNAs), Csu-novel-260 and Csu-miR-14, were used in insect-resistant genetically engineered (IRGE) rice lines to confer resistance to Chilo suppressalis. However, a risk assessment of RNAi-based products is essential to determine the safety of a biopesticide or IRGE crop for commercialization. The non-target organism Folsomia candida, which plays an important ecological role as a soil decomposer in agricultural ecosystems, was used to assess the risk of miRNAs Csu-novel-260 and Csu-miR-14. In this study, a dietary miRNA toxicity assay system was established in F. candida. The expression levels of target genes, survival rate, fecundity and body size were investigated to evaluate the effects of the miRNAs on F. candida under the worst-case scenario. The results showed that the dietary miRNA toxicity assay system could be used for risk assessment of miRNA in F. candida. The target genes of miRNAs were influenced by miRNA at some time points. However, no significant differences were observed in the life-table parameters in F. candida fed with a diet containing miRNAs. The dietary effects of two miRNAs on F. candida are neutral.

Impact of Transgenic Cry1Ab/2Aj Maize on Abundance of Non-Target Arthropods in the Field

Transgenic Bacillus thuringiensis (Bt) maize has broad prospects for application in China. Before commercialization, it is necessary to assess possible ecological impacts, including impacts on non-target arthropods (NTAs) in the field. In the present study, transgenic Bt maize expressing cry1Ab/2Aj and its corresponding non-transformed near isoline were planted under the same environmental and agricultural conditions, and arthropods in the field were collected during the three main growth stages of maize. In a one year trial, the results showed the composition of NTA communities in the transgenic and control maize fields were similar. There were no significant differences for community-level parameters of species richness (S), Shannon-Wiener diversity index (H'), evenness index (J) and Simpson's dominant concentration (C) between the two types of maize fields. Likewise, a Bray-Curtis dissimilarity and distance analysis showed that Cry1Ab/2Aj toxin exposure did not increase community dissimilarities between Bt and non-Bt maize plots and that the structure of the NTAs community was similar on the two maize varieties. Furthermore, planting of the transgenic cry1Ab/2Aj maize did not affect the density or composition of non-target decomposers, herbivores, predators, parasitoids and pollinator guilds. In summary, our results showed that planting of Bt maize producing Cry1Ab/Cry2Aj proteins do not adversely affect population dynamics and diversity of NTAs.

Climate change did not alter the effects of Bt maize on soil Collembola in northeast China

Bt maize is being increasingly cultivated worldwide as the effects of climate change are increasing globally. Bt maize IE09S034 and its near-isogenic non-Bt maize Zong 31 were used to investigate whether climate change alters the effects of Bt maize on soil Collembola. Warming and drought conditions were simulated using open-top chambers (OTC), and their effects on soil Collembola were evaluated. We found that the maize type had no significant effect on Collembola; however, the abundance and diversity of Collembola were significantly higher in the OTC than outside at the seedling stage; they were significantly lower in the OTC at the heading and mature stages. The interactions of the maize type with the OTC had no effect on these parameters. Therefore, Bt maize had no significant effect on soil Collembola, and the effects of climate warming and drought on soil Collembola depended on the ambient climatic conditions. When the temperature was low, collembolan abundance and diversity were promoted by warming; however, when the temperature was high and the humidity was low, collembolan abundance and diversity were inhibited by warming and drought. The climate changes simulated by the OTC did not alter the effects of Bt maize on soil Collembola.

Biochemical analyses demonstrate that Bt maize has no adverse effects on Eisenia fetida

The potential effects of Bt (Bacillus thuringiensis) maize on non-target organisms should be evaluated before such maize is commercially planted. Earthworms play an indispensable role in the soil ecosystem; act as important bio-indicators of soil quality and environmental pollution. Therefore, earthworms are often used as the object to evaluate the non-target effect of Bt maize. To accelerate the commercialization of transgenic maize in China, a 90-day Eisenia fetida feeding experiment was conducted to evaluate the potential effects of Bt maize line, BT799—which was developed by China Agricultural University and contains the Cry1Ac gene—and its non-Bt conventional isoline—Zheng 58—on E. fetida. Our results showed that the Bt maize line had no significant effects on the growth, reproduction, or enzymatic activities of these earthworms. In summary, Bt maize had no toxic effects on E. fetida.

Efficient Transformation of Catalpa bungei Shows Crystal Genes Conferring Resistance to the Shoot Borer Omphisa plagialis

Although Catalpa bungei is a forest plant with considerable economic and ornamental value in China, its wood and decorative qualities are constrained by insect pests such as the shoot borer Omphisa plagialis (Lepidoptera). Overexpressing insect resistance genes such as crystal genes to develop an insect-resistant variety of C. bungei is an environmental and ecological approach. However, genotype limitations and low regeneration rates of embryogenic calli (EC) inhibit the development of transformation and the insect-resistant gene expression system in C. bungei. Here, we first established embryogenic callus induction and regeneration systems of five genotypes using mature seed and stem segment explants; the highest induction and regeneration rates of EC were 39.89 and 100%, respectively. Next, an efficient and stable Agrobacterium-mediated genetic transformation system was developed from EC and its positive frequency was up to 92.31%. Finally, using the transformation system, 15 and 22 transgenic C. bungei lines that expressed Cry2A and Cry9Aa-like were generated, respectively. These transgenic lines that exhibited significantly higher resistance to O. plagialis in the laboratory and field have great promise for meeting the challenge of future pest management under changing climatic conditions. Additionally, this efficient, fast, and stable transformation system could be a potential tool for gene function analysis and forest tree genetic improvement.

Toxicological and biochemical analyses demonstrate no toxic effect of Bt maize on the Folsomia candida

The potential effects of Bt (Bacillus thuringiensis) maize on non-target organisms must be conducted before the Bt maize is commercially planted. Folsomia candida is one of the non-target organisms of Bt maize, also as an important indicator of soil quality and environmental pollution. In this study, a 90-day F. candida feeding test were conducted to evaluate the potential effects of two Bt maize lines IE09S034 and BT799 and their non-Bt conventional isolines Zong 31 and Zheng 58. The results show that Bt maize lines had no significant effects on the survival rate, reproduction, adult body length, larval body length, and the activities of acetyl cholinesterase, catalase and superoxide dismutase on the F. candida. Namely, Bt maize had no toxic effects on the F. candida.

A multi-generational risk assessment of Cry1F on the non-target soil organism Folsomia candida (Collembola) based on whole transcriptome profiling

The Bacillus thuringiensis toxin Cry1F has been used to develop insect-resistant genetically engineered crops. There has been great interest in evaluating its potential risk to non-target organisms (NTOs). However, the majority of previous risk assessments only examined one generation of NTOs using several physiological indicators, which cannot comprehensively detect some potential sub-lethal effects at the molecular level. In this study, we conducted a laboratory-based, multi-generational risk assessment of Cry1F for the collembolan Folsomia candida, an important representative of soil arthropods in terms of survival, reproduction, and differentially expressed genes (DEGs) identified from whole transcriptome profiles. Our results demonstrated that Cry1F was continuously ingested by collembolans over three consecutive generations, but it did not affect the survival or reproduction of F. candida. There were no significant differences in the global gene expression between F. candida—fed diets with and without Cry1F, and no consistent co-expressed DEGs over three generations. In addition, Cry1F did not obviously alter the expression profiles of seven sensitive biological markers. Our composite data indicates that Cry1F had no long-term harmful effects on collembolan F. candida.

Effects of straw leachates from Cry1C-expressing transgenic rice on the development and reproduction of Daphnia magna

The transgenic rice line T1C-19 provides high resistance to lepidopteran pests because of the synthesis of the Bacillus thuringiensis (Bt) insecticidal protein Cry1C. It thus shows good prospect for commercial planting in China. Species of Cladocera, an order of aquatic arthropods commonly found in aquatic ecosystems such as rice paddies, might be exposed to the insecticidal protein released from Bt-transgenic rice-straw residues. For the study reported herein, we used Daphnia magna (water flea) as a representative of Cladocera to evaluate whether aquatic arthropods are adversely affected when exposed to Bt rice-straw leachates. We exposed D. magna to M4 medium containing various volume percentages of medium that had been incubated with T1C-19 rice straw or rice straw from its non-transformed near-isoline Minghui 63 (MH63) for 21 days. Compared with pure M4 medium (control), the fitness and developmental and reproduction parameters of D. magna decreased significantly when exposed to rice-straw leachates; conversely, no significant differences between the T1C-19 and MH63 rice-straw leachate treatments were observed, indicating that the Bt rice straw leachate did not adversely affect this non-target species.

Establishment of a dietary exposure assay for evaluating the toxicity of insecticidal compounds to Apolygus lucorum (Hemiptera: Miridae)

With the commercialization of transgenic cotton that expresses Bt (Bacillus thuringiensis) insecticidal proteins, mirid bugs have become key pests in cotton and maize fields in China. Genetically engineered (GE) crops for controlling mirids are unavailable owing to a lack of suitable insecticidal genes. In this study, we developed and validated a dietary exposure assay for screening insecticidal compounds and for assessing the potential effects of insecticidal proteins produced by GE plants on Apolygus lucorum, one of the main mirid pests of Bt cotton and Bt maize. Diets containing potassium arsenate (PA) or the cysteine protease inhibitor E-64 were used as positive controls for validating the efficacy of the dietary exposure assay. The results showed that with increasing concentrations of PA or E-64, A. lucorum larval development time was prolonged and adult weight and fecundity were decreased, suggesting that the dietary exposure assay was useful for detecting the toxicity of insecticidal compounds to A. lucorum. This assay was then used to assess the toxicity of Cry1Ab, Cry1Ac, Cry1F, Cry2Aa, and Cry2Ab proteins, which have been transformed into several crops, against A. lucorum. The results showed that A. lucorum did not show a negative effect by feeding on an artificial diet containing any of the purified Cry proteins. No significant changes in the activities of digestive, detoxifying, or antioxidant enzymes were detected in A. lucorum that fed on a diet containing Cry proteins, but A. lucorum fitness was reduced when the insect fed on a diet containing E-64 or PA. These results demonstrate that A. lucorum is not sensitive to the tested Cry proteins and that the dietary exposure assay is useful for evaluating the toxicity of insecticidal compounds to this species.

Safety Assessment of Bacillus thuringiensis Insecticidal Proteins Cry1C and Cry2A with a Zebrafish Embryotoxicity Test

As a result of the large-scale planting of transgenic Bacillus thuringiensis (Bt) crops, fish would be exposed to freely soluble Bt insecticidal protein(s) that are released from Bt crop tissues into adjacent bodies of water or by way of direct feeding on deposited plant material. To assess the safety of two Bt proteins Cry1C and Cry2A to fish, we used zebrafish as a representative species and exposed their embryos to 0.1, 1, and 10 mg/L of the two Cry proteins until 132 h post-fertilization and then several developmental, biochemical, and molecular parameters were evaluated. Chlorpyrifos (CPF), a known toxicant to aquatic organisms, was used as a positive control. Although CPF exposure resulted in significant developmental, biochemical, and molecular changes in the zebrafish embryos, there were almost no significant differences after Cry1C or Cry2A exposure. Thus, we conclude that zebrafish embryos are not sensitive to Cry1C and Cry2A insecticidal proteins at test concentrations.

Toxicological and Biochemical Analyses Demonstrate the Absence of Lethal or Sublethal Effects of cry1C- or cry2A-Expressing Bt Rice on the Collembolan Folsomia candida

Assessing the potential effects of insect-resistant genetically engineered (GE) plants on collembolans is important because these common soil arthropods may be exposed to insecticidal proteins produced in GE plants by ingestion of plant residues, crop pollen, or root exudates. Laboratory studies were conducted to evaluate the potential effects of two Bacillus thuringiensis (Bt)-rice lines expressing Cry1C and Cry2A in pollen and leaves and of their non-Bt conventional isolines on the fitness of the collembolan Folsomia candida and on the activities of its antioxidant-related enzymes, superoxide dismutase and peroxidase, and of its detoxification-related enzymes, glutathione reductase and glutathione S-transferase. Survival, development, reproduction, and the intrinsic rate of increase (r_m) were not significantly reduced when F. candida fed on the Bt rice pollen or leaf powder than on the non-Bt rice materials; these parameters, however, were significantly reduced when F. candida fed on non-Bt rice pollen or non-Bt leaf-based diets containing the protease inhibitor E-64 at 75 μg/g. The activities of the antioxidant-related and detoxification-related enzymes in F. candida were not significantly affected when F. candida fed on the Bt rice materials, but were significantly increased when F. candida fed on the non-Bt rice materials containing E-64. The results demonstrate that Cry1C and Cry2A are not toxic to F. candida, and also indicate the absence of unintended effects on the collembolan caused by any change in plant tissue nutritional composition due to foreign gene transformation.

Safety of Bacillus thuringiensis Cry1C protein for Daphnia magna based on different functional traits

Cry1C is a Bacillus thuringiensis (Bt) insecticidal protein and it can be produced by transgenic rice lines developed in China. Cladocera species are common aquatic arthropods that may be exposed to insecticidal proteins produced in Bt-transgenic plants through ingestion of pollen or crop residues in water. As the cladoceran Daphnia magna plays an important role in the aquatic food chain, it is important to assess the possible effects of Bt crops to this species. To evaluate the safety of the Cry1C protein for D. magna, individuals were exposed to different concentrations of purified Cry1C protein in M4 medium for 21 days. Potassium dichromate (K₂Cr₂O₇), a known toxicant to D. magna, was added to M4 medium as a positive control treatment, and pure M4 medium was used as a negative control. Our results show that developmental, reproductive, and biochemical parameters of D. magna were not significantly different between Cry1C and negative control treatments but were significantly inhibited by the positive control. We thus conclude that D. magna is insensitive to Cry1C.

Ingestion of Bt rice pollen does not reduce the survival or hypopharyngeal gland development of Apis mellifera adults

Because of its ecological and economic importance, the honey bee Apis mellifera is commonly used to assess the environmental risk of insect-resistant, genetically modified plants. In the present study, feeding-exposure experiments were used to determine whether pollen from transgenic rice harms A. mellifera worker bees. In 1 experiment, the survival and mean acinus diameter of hypopharyngeal glands of adult bees were similar when bees were fed on pollen from Bt rice lines or from a non-Bt rice line, but bee survival was significantly reduced when they received pollen that was mixed with potassium arsenate as a positive control. In a second experiment, bee survival and hypopharyngeal gland development were not reduced when adult bees were fed on non-Bt pollen and a sucrose solution supplemented with Cry2A at 400 µg/g, Cry1C at 50 µg/g, or bovine serum albumin (BSA) at 400 µg/g, but bee survival and hypopharyngeal gland development were reduced when the diet was supplemented with soybean trypsin inhibitor as a positive control. In both experiments, the uptake of Cry proteins by adult bees was confirmed. Overall, the results indicate that the planting of Bt rice lines expressing Cry2A or Cry1C protein poses a negligible risk to A. mellifera worker bees. Environ Toxicol Chem 2017;36:1243-1248. © 2016 SETAC.

Consumption of Bt Maize Pollen Containing Cry1Ie Does Not Negatively Affect Propylea japonica (Thunberg) (Coleoptera: Coccinellidae)

Propylea japonica (Thunberg) (Coleoptera: Coccinellidae) are prevalent predators and pollen feeders in East Asian maize fields. They are therefore indirectly (via prey) and directly (via pollen) exposed to Cry proteins within Bt-transgenic maize fields. The effects of Cry1Ie-producing transgenic maize pollen on the fitness of P. japonica was assessed using two dietary-exposure experiments in the laboratory. In the first experiment, survival, larval developmental time, adult fresh weight, and fecundity did not differ between ladybirds consuming Bt or non-Bt maize pollen. In the second experiment, none of the tested lethal and sublethal parameters of P. japonica were negatively affected when fed a rapeseed pollen-based diet containing Cry1Ie protein at 200 μg/g dry weight of diet. In contrast, the larval developmental time, adult fresh weight, and fecundity of P. japonica were significantly adversely affected when fed diet containing the positive control compound E-64. In both experiments, the bioactivity of the Cry1Ie protein in the food sources was confirmed by bioassays with a Cry1Ie-sensitive lepidopteran species. These results indicated that P. japonica are not affected by the consumption of Cry1Ie-expressing maize pollen and are not sensitive to the Cry1Ie protein, suggesting that the growing of Bt maize expressing Cry1Ie protein will pose a negligible risk to P. japonica.

A laboratory assessment of the potential effect of Cry1Ab/Cry2Aj-containing Bt maize pollen on Folsomia candida by toxicological and biochemical analyses

The common soil arthropod Folsomia candida can survive well when fed only maize pollen and thus may be exposed to insecticidal proteins by ingesting insect-resistant genetically engineered maize pollen containing Bacillus thuringiensis (Bt) proteins when being released into the soil. Laboratory experiments were conducted to assess the potential effects of Cry1Ab/Cry2Aj-producing transgenic Bt maize (Shuangkang 12-5) pollen on F. candida fitness. Survival, development, and the reproduction were not significantly reduced when F. candida fed on Bt maize pollen rather than on non-Bt maize pollen, but these parameters were significantly reduced when F. candida fed on non-Bt maize pollen containing the protease inhibitor E-64 at 75 μg/g pollen. The intrinsic rate of increase (r_m) was not significantly reduced when F. candida fed on Bt maize pollen but was significantly reduced when F. candida fed on non-Bt maize pollen containing E-64. The activities of antioxidant-related enzymes in F. candida were not significantly affected when F. candida fed on Bt maize pollen but were significantly increased when F. candida fed on non-Bt pollen containing E-64. The results demonstrate that consumption of Bt maize pollen containing Cry1Ab/Cry2Aj has no lethal or sublethal effects on F. candida.

The tiered-evaluation of the effects of transgenic cry1c rice on Cyrtorhinus lividipennis, a main predator of Nilaparvata lugens

T1C-19, a newly developed transgenic cry1C rice line, expresses cry1C under the control of the maize ubiquitin promoter, and is highly resistant to lepidopteran pests of rice. Cyrtorhinus lividipennis is the major predator of the eggs and young nymphs of Nilaparvata lugens, which is the main non-target sap-sucking insect pest of Bt rice. C. lividipennis may be exposed to Cry1C protein, thus biosafety evaluations of transgenic cry1C rice on C. lividipennis should be conducted before the commercialization of T1C-19. In the current study, we tested the direct toxicity of elevated doses of Cry1C to C. lividipennis, effects of T1C-19 on the life-table parameters of C. lividipennis via preying planthoppers, and effects of T1C-19 on the population density and dynamics in rice fields. No detrimental effects on development, survival, female ratio and body weight of C. lividipennis were caused by direct exposure to elevated doses of the Cry1C protein or prey-mediated exposure to realistic doses of the protein. The population density and dynamics did not significantly differ between C. lividipennis in T1C-19 and non-transgenic rice fields. Thus, transgenic cry1C rice had no negative effects on C. lividipennis. This is the first report of the effects of transgenic cry1C rice on C. lividipennis.

Proteolytic Activation of Bacillus thuringiensis Cry2Ab through a Belt-and-Braces Approach

Proteolytic processing of Bacillus thuringiensis (Bt) crystal toxins by insect midgut proteases plays an essential role in their insecticidal toxicities against target insects. In the present study, proteolysis of Bt crystal toxin Cry2Ab by Plutella xylostella L. midgut proteases (PxMJ) was evaluated. Both trypsin and chymotrypsin were identified involving the proteolytic activation of Cry2Ab and cleaving Cry2Ab at Arg(139) and Leu(144), respectively. Three Cry2Ab mutants (R139A, L144A, and R139A-L144A) were constructed by replacing residues Arg(139), Leu(144), and Arg(139)-Leu(144) with alanine. Proteolysis assays revealed that mutants R139A and L144A but not R139A-L144A could be cleaved into 50 kDa activated toxins by PxMJ. Bioassays showed that mutants R139A and L144A were highly toxic against P. xylostella larvae, while mutant R139A-L144A was almost non-insecticidal. Those results demonstrated that proteolysis by PxMJ was associated with the toxicity of Cry2Ab against P. xylostella. It also revealed that either trypsin or chymotrypsin was enough to activate Cry2Ab protoxin. This characteristic was regarded as a belt-and-braces approach and might contribute to the control of resistance development in target insects. Our studies characterized the proteolytic processing of Cry2Ab and provided new insight into the activation of this Bt toxin.

Assessment of Potential Risks of Dietary RNAi to a Soil Micro-arthropod, Sinella curviseta Brook (Collembola: Entomobryidae)

RNAi-based genetically engineered (GE) crops for the management of insect pests are likely to be commercialized by the end of this decade. Without a workable framework for conducting the ecological risk assessment (ERA) and a standardized ERA protocol, however, the utility of RNAi transgenic crops in pest management remains uncertain. The overall goal of this study is to assess the risks of RNAi-based GE crops on a non-target soil micro-arthropod, Sinella curviseta, which could be exposed to plant-protected dsRNAs deposited in crop residues. Based on the preliminary research, we hypothesized that insecticidal dsRNAs targeting at the western corn rootworm, Diabrotica virgifera virgifera, a billion-dollar insect pest, has no adverse impacts on S. curviseta, a soil decomposer. Following a tiered approach, we tested this risk hypothesis using a well-designed dietary RNAi toxicity assay. To create the worst-case scenario, the full-length cDNA of v-ATPase subunit A from S. curviseta were cloned and a 400 bp fragment representing the highest sequence similarity between target pest and non-target arthropods was selected as the template to synthesize insecticidal dsRNAs. Specifically, 10-days-old S. curviseta larvae were subjected to artificial diets containing v-ATPase A dsRNAs from both D. v. virgifera (dsDVV) and S. curviseta (dsSC), respectively, a dsRNA control, β-glucuronidase, from plant (dsGUS), and a vehicle control, H2O. The endpoint measurements included gene expression profiles, survival, and life history traits, such as developmental time, fecundity, hatching rate, and body length. Although, S. curviseta larvae developed significantly faster under the treatments of dsDVV and dsSC than the vehicle control, the combined results from both temporal RNAi effect study and dietary RNAi toxicity assay support the risk hypothesis, suggesting that the impacts of ingested arthropod-active dsRNAs on this representative soil decomposer are negligible.

Toxicological, Biochemical, and Histopathological Analyses Demonstrating That Cry1C and Cry2A Are Not Toxic to Larvae of the Honeybee, Apis mellifera

The honey bee, Apis mellifera, is commonly used as a test species for the regulatory risk assessment of insect-resistant genetically engineered (IRGE) plants. In the current study, a dietary exposure assay was developed, validated, and used to assess the potential toxicity of Cry1C and Cry2A proteins from Bacillus thuringiensis (Bt) to A. mellifera larvae; Cry1C and Cry2A are produced by different IRGE crops. The assay, which uses the soybean trypsin inhibitor (SBTI) as a positive control and bovine serum albumin (BSA) as a negative control, was used to measure the responses of A. mellifera larvae to high concentrations of Cry1C and Cry2A. Survival was reduced and development was delayed when larvae were fed SBTI (1 mg/g diet) but were unaffected when larvae were fed BSA (400 μg/g), Cry1C (50 μg/g), or Cry2A (400 μg/g). The enzymatic activities of A. mellifera larvae were not altered and their midgut brush border membranes (BBMs) were not damaged after being fed with diets containing BSA, Cry1C, or Cry2A; however, enzymatic activities were increased and BBMs were damaged when diets contained SBTI. The study confirms that Cry1C and Cry2A have no acute toxicity to A. mellifera larvae at concentrations >10 times higher than those detected in pollen from Bt plants.