Use of a dietary exposure system for screening of insecticidal compounds for their toxicity to the planthopper Laodelphax striatellus

Impacts of Bt rice on non-target organisms assessed by the hazard quotient (HQ)

The potential risk of Bt (Bacillus thuringiensis) crops on non-target organisms (NTOs) has drawn a lot of public concerns. Despite a series of risk assessments of Bt crops on NTOs has been conducted, a quantitative approach which could support a precise judgment of their safety is required. In the present work, hazard quotient (HQ) was applied in the safety evaluation of three Bt rice events (Cry1Ab, Cry1C and Cry2Aa rice) on NTOs. Eight NTOs in different functional guilds associated with Bt rice were selected to conduct the tests. The results showed that the HQs of three Bt rice events for eight NTOs were all below the trigger value 1, while the HQ of Cry1Ab rice for one target pest Chilo suppressalis was three times higher than 1. Our results assured the reliability of the HQ and indicated that the three Bt rice events would pose no risks to the eight NTOs. Further testing of three Bt proteins on biological parameters of one NTO Nasonia virtipennis under no observed adverse effect concentration (NOAEC) confirmed the robustness of HQ assessment. We recommend that the HQ could be applied in tier-1 risk assessments of Bt crops on NTOs as a reference data standard, which would provide more clear and credible safety information of transgenic crops for the public and policy makers.

Cry78Ba1, One Novel Crystal Protein from Bacillus thuringiensis with High Insecticidal Activity against Rice Planthopper

The rice planthopper is a very important hemipteran pest that preys on rice and substantially affects the safety of rice production. Moreover, the long-term prevention and control of these pests with chemical pesticides has led to an increase in the resistance of the rice planthopper as well as serious environmental pollution and food safety problems. Bacillus thuringiensis (Bt) has been used for the efficient and green control of a variety of rice pests. Therefore, based on the high-throughput screening of Bt strains that are active against the rice planthopper, we found that Bt strain B4F11 showed certain insecticidal activity against Laodelphax striatellus Fallén, and we have identified a novel insecticidal protein Cry78Ba1 from the Bt strain B4F11, which is expected to provide the specific and safe control of the rice planthopper. The Cry78Ba1 protein is composed of 380 amino acid residues with a molecular weight of 42.55 kDa and contains conserved Ricin_B_Lectin and Toxin_10 superfamily domains. It displays high insecticidal activity against L. striatellus with a lethal concentration (LC₅₀) of 9.723 μg/mL. More importantly, this Toxin_10-like protein does not display sequence homology to any known allergen and can be degraded and inactivated rapidly when heated at 90 °C and in simulated gastrointestinal fluid. In summary, Cry78Ba1 has great potential for applications in the efficient and safe prevention and control of the rice planthopper.

Cry78Aa, a novel Bacillus thuringiensis insecticidal protein with activity against Laodelphax striatellus and Nilaparvata lugens

Transgenic plants expressing insecticidal proteins originating from Bacillus thuringiensis (Bt) have successfully been used to control lepidopteran and coleopteran pests with chewing mouthparts. However, only a handful of Bt proteins have been identified that have bioactivity against sap sucking pests (Hemiptera), including aphids, whiteflies, plant bugs and planthoppers. A novel Bt insecticidal protein with significant toxicity against a hemipteran insect pest is described here. The gene encoding the 359 amino acid, 40.7 kDa protein was cloned from strain C9F1. After expression and purification of the toxin, its median lethal concentration (LC₅₀) values against Laodelphax striatellus and Nilaparvata lugens were determined as 6.89 μg/mL and 15.78 μg/mL respectively. Analysis of the toxin sequence revealed the presence of both Toxin_10 and Ricin_B_Lectin domains.

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.

Cry64Ba and Cry64Ca, Two ETX/MTX2-Type Bacillus thuringiensis Insecticidal Proteins Active against Hemipteran Pests

Genetically modified crops that express insecticidal Bacillus thuringiensis (Bt) proteins have become a primary approach for control of lepidopteran (moth) and coleopteran (beetle) pests that feed by chewing the plants. However, the sap-sucking insects (Hemiptera) are not particularly susceptible to Bt toxins. In this study, we describe two Cry toxins (Cry64Ba and Cry64Ca) from Bt strain 1012 that showed toxicity against two important hemipteran rice pests, Laodelphax striatellus and Sogatella furcifera Both of these proteins contain an ETX/MTX2 domain and share common sequence features with the β-pore-forming toxins. Coexpression of cry64Ba and cry64Ca genes in the acrystalliferous Bt strain HD73^- resulted in high insecticidal activity against both hemipteran pests. No toxicity was observed on other pests such as Ostrinia furnacalis, Plutella xylostella, or Colaphellus bowringi Also, no hemolytic activity or toxicity against cancer cells was detected. Binding assays showed specific binding of the Cry64Ba/Cry64Ca toxin complex to brush border membrane vesicles isolated from L. striatellus Cry64Ba and Cry64Ca are Bt Cry toxins highly effective against hemipteran pests and could provide a novel strategy for the environmentally friendly biological control of rice planthoppers in transgenic plants.IMPORTANCE In Asia, rice is an important staple food, whose production is threatened by rice planthoppers. To date, no effective Bacillus thuringiensis (Bt) protein has been shown to have activity against rice planthoppers. We cloned two Bt toxin genes from Bt strain 1012 that showed toxicity against small brown planthoppers (Laodelphax striatellus) and white-backed planthoppers (Sogatella furcifera). To our knowledge, the proteins encoded by the cry64Ba and cry64Ca genes are the most efficient insecticidal Bt Cry proteins with activity against hemipteran insects reported so far. Cry64Ba and Cry64Ca showed no toxicity against some lepidopteran or coleopteran pests. These two proteins should be able to be used for integrated hemipteran pest management.

Cloning, Expression, Purification, and Insecticidal Activity of a Novel Cry1Na3 Toxin From Bacillus thuringiensis BRC-ZYR2

Bacillus thuringiensis produces a variety of insecticidal crystal proteins (ICPs). Genome sequencing is a promising strategy for detecting and identifying B. thuringiensis ICPs, which are of great interest to the biocontrol field. In this study, a novel ICP gene was cloned from B. thuringiensis BRC-ZYR2 based on genomic data from 454 GS-FLX Titanium sequencing and an analysis of the results using the B. thuringiensis Toxin_Scanner ( http://bcam.hzaubmb.net/BtToxin_scanner/index.php ). cry1Na3 designated by the B. thuringiensis Toxin Nomenclature Committee, encoded a 601-amino acid, 68.0-kDa protein that exhibited 95% identity with Cry1Na1 and 99% identity with Cry1Na2. Cry1Na3 contained three conserved domains commonly found in three-domain ICPs. Cry1Na3 was toxic to Plutella xylostella (L.) and Ostrinia furnacalis (Guenée), with LC 50 values of 3.69 μg/ml and 31.30 μg/ml, respectively. However, Laodelphax striatellus (Fallén) nymphs were unaffected when fed purified Cry1Na3 (250 μg/ml) in their diet. Spodoptera exigua (Hübner) and Colaphellus bowringi (Baly) larvae survived even when the concentration of Cry1Na3 protein reached 500 μg/ml. Cry1Na3 is a promising agent for the control of lepidopteran insect pests.

Transgenic cry1C or cry2A rice has no adverse impacts on the life-table parameters and population dynamics of the brown planthopper, Nilaparvata lugens (Hemiptera: Delphacidae)

BACKGROUND

Transgenic rice producing the insecticidal protein from Bacillus thuringiensis Berliner (Bt) is protected from damage by lepidopteran insect pests. However, one of the main concerns about Bt rice is the potential impact on non-target herbivores. In the present study, the ecological impacts of two Bt rice lines, T1C-19 expressing Cry1C protein and T2A-1 expressing Cry2A protein, on the non-target herbivore brown planthopper (BPH), Nilaparvata lugens (Stål), were evaluated under laboratory and field conditions. The purpose was to verify whether these Bt rice lines could affect the performance of BPH at individual and population scales.

RESULTS

Laboratory results showed that most of the fitness parameters (development duration, survival rate, fecundity, fertility, amount of honeydew excreted) of BPH were not significantly affected by the two tested Bt rice lines, although the development duration of fourth-instar nymphs fed on T1C-19 was distinctly longer compared with that on T2A-1 and non-Bt rice plants. Five life-table parameters did not significantly differ among rice types. Two-year field trials also revealed no significant difference in population dynamics of BPH among rice types.

CONCLUSION

It is inferred that the tested Bt rice lines are unlikely to affect the population growth of BPH.

Consumption of Bt rice pollen containing Cry1C or Cry2A does not pose a risk to Propylea japonica (Thunberg) (Coleoptera: Coccinellidae)

As a pollen feeder, Propylea japonica would be directly exposed to Cry proteins in Bacillus thuringiensis (Bt)-transgenic rice fields. The effect of Cry1C- or Cry2A-containing transgenic rice pollen on the fitness of P. japonica was assessed using two dietary-exposure experiments in the laboratory. In the first experiment, larval developmental time of P. japonica was significantly longer when fed pollen from Bt rice lines rather than control pollen but other life table parameters were not significantly affected. In the second experiment, P. japonica was not affected when fed a rapeseed pollen-based diet containing purified Cry1C or Cry2A at concentrations that were >10-times higher than in pollen, but P. japonica was affected when the diet contained E-64 as a positive control. In both experiments, the stability and bioactivity of the Cry proteins in the food sources and the uptake of the proteins by P. japonica were confirmed. The results show that P. japonica is not sensitive to Cry1C or Cry2A proteins; the effect observed in the first experiment was likely attributable to unknown differences in the nutritional composition of Bt rice pollen. Overall, the data indicate that the growing of Cry1C- or Cry2A-transgenic rice should pose a negligible risk to P. japonica.