Effectiveness of the high dose/refuge strategy for managing pest resistance to Bacillus thuringiensis (Bt) plants expressing one or two toxins

Applying a computer model to evaluate the evolution of resistance by western corn rootworm to multiple Bt traits in transgenic maize

Western corn rootworm, Diabrotica virgifera virgifera (LeConte) (Coleoptera: Chrysomelidae), is a major pest of maize in the United States. Transgenic maize producing insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) have been used to manage this pest since 2003. Refuges of non-Bt maize have been used to delay resistance to Bt maize by western corn rootworm, and are planted in conjunction with maize producing single or multiple (i.e., pyramids) Bt toxins. Two Bt toxins, Cry3Bb1 and Gpp34/Tpp35Ab1, were used individually before being combined as a pyramid, at which point resistance had already evolved to Cry3Bb1. Pyramids targeting western corn rootworm therefore contained at least one toxin to which resistance had evolved. Western corn rootworm has now evolved resistance to all four commercially available Bt toxins used for rootworm management. We used laboratory and field-generated data to parameterize a deterministic model to simulate the effectiveness of refuges and Bt pyramids to delay resistance to Bt maize in western corn rootworm. Resistance to the pyramid of Cry3Bb1 with Gpp34/Tpp35Ab1 evolved more rapidly when resistance to Cry3Bb1 was already present. This effect arose when model conditions affecting initial resistance allele frequency, inheritance of resistance, and fitness costs were varied. Generally, resistance evolved faster when initial resistance allele frequencies were higher, inheritance of resistance was nonrecessive, and fitness costs were absent, which is consistent with previous models that simulated resistance evolution. We conclude that new transgenic pyramids should pair novel, independently acting toxins with abundant refuges to minimize the risk of rapid resistance evolution.

Minimizing IP issues associated with gene constructs encoding the Bt toxin - a case study

BACKGROUND

As part of a publicly funded initiative to develop genetically engineered Brassicas (cabbage, cauliflower, and canola) expressing Bacillus thuringiensis Crystal (Cry)-encoded insecticidal (Bt) toxin for Indian and Australian farmers, we designed several constructs that drive high-level expression of modified Cry1B and Cry1C genes (referred to as Cry1BM and Cry1CM; with M indicating modified). The two main motivations for modifying the DNA sequences of these genes were to minimise any licensing cost associated with the commercial cultivation of transgenic crop plants expressing CryM genes, and to remove or alter sequences that might adversely affect their activity in plants.

RESULTS

To assess the insecticidal efficacy of the Cry1BM/Cry1CM genes, constructs were introduced into the model Brassica Arabidopsis thaliana in which Cry1BM/Cry1CM expression was directed from either single (S4/S7) or double (S4S4/S7S7) subterranean clover stunt virus (SCSV) promoters. The resulting transgenic plants displayed a high-level of Cry1BM/Cry1CM expression. Protein accumulation for Cry1CM ranged from 5.18 to 176.88 µg Cry1CM/g dry weight of leaves. Contrary to previous work on stunt promoters, we found no correlation between the use of either single or double stunt promoters and the expression levels of Cry1BM/Cry1CM genes, with a similar range of Cry1CM transcript abundance and protein content observed from both constructs. First instar Diamondback moth (Plutella xylostella) larvae fed on transgenic Arabidopsis leaves expressing the Cry1BM/Cry1CM genes showed 100% mortality, with a mean leaf damage score on a scale of zero to five of 0.125 for transgenic leaves and 4.2 for wild-type leaves.

CONCLUSIONS

Our work indicates that the modified Cry1 genes are suitable for the development of insect resistant GM crops. Except for the PAT gene in the USA, our assessment of the intellectual property landscape of components presents within the constructs described here suggest that they can be used without the need for further licensing. This has the capacity to significantly reduce the cost of developing and using these Cry1M genes in GM crop plants in the future.

Targeting delta-endotoxin (Cry1Ac) of Bacillus thuringiensis to subcellular compartments increases the protein expression, stability, and biological activity

Evolving insect resistance to delta-endotoxins can be delayed by using a few strategies like high dosage, refugia, and gene stacking which require the expression of delta-endotoxins at sufficiently high levels to kill the resistant insects. In this study, we comparatively analyzed the efficacy of targeting truncated cry1Ac protein to the cytoplasm, endoplasmic reticulum (ER), and chloroplast to obtain high protein expression. mRNA and protein profiling of cry1Ac showed that both ER and chloroplast are efficient targets for expressing high levels of truncated cry1Ac. A maximum of 0.8, 1.6, and 2.0% cry1Ac of total soluble protein were obtained when the truncated cry1Ac was expressed in the cytoplasm, routed through ER, and targeted to the chloroplast. We further showed that not only the protein content but also the biological activity of truncated cry1Ac increases by sub-cellular targeting and the biological activity is slightly greater in the ER routed transgenic lines by conducting different bioassays on Helicoverpa armigera. Using native Western analysis, we demonstrated that the truncated cry1Ac protein could exist as oligomers in plant cells and this oligomerization capability is low in the cytoplasm. In conclusion, routing of delta endotoxins through ER is the first choice to obtain high protein expression and bioactivity.

An Interactive Teaching Tool Describing Resistance Evolution and Basic Economics of Insecticide-Based Pest Management

Simple Summary

To attract bright, creative, and curious students to the academic fields of applied entomology and sustainable food production, instructors of undergraduate and graduate student courses should discuss experiences with lectures and lab sessions and share effective interactive teaching tools. This communication describes how a simple population model in an Excel spreadsheet can be used in teaching both insecticide resistance evolution and basic economics of insecticide-based pest management. A tutorial video and the model as an Excel spreadsheet are freely available. Through hands-on experience with this and similar interactive teaching tools, students will acquire fundamental knowledge about basic structures population models and they will acquire experience with quantitative data interpretation. Teachers can use this tool and accompanying tutorials to demonstrate how models can be used to describe and visualize complex interactions between insect genetics and crop management. Furthermore, data from published studies can be analyzed and discussed using this interactive teaching tool.

Abstract

Effective teaching of complex concepts relies heavily on the ability to establish relevance of topics and to engage students in a constructive dialogue. To connect students with abstract concepts and basic theory, instructors foster and facilitate an engaging teaching environment. Population modeling is a cornerstone in applied entomology. However, it is also a topic and skill set that requires both basic mathematical and biological knowledge, and it may be perceived by students as being abstract and exceedingly theoretical. As a way to introduce entomology students at both that undergraduate and graduate levels to hands-on experience with population modeling, a well-established and widely used deterministic genetic population model is presented as an interactive teaching tool. Moreover, the general model describes three genotypes (SS = homozygous susceptible, SR = heterozygous, and RR = homozygous resistant) during 30 discrete and univoltine generations under a shared population density dependence (carrying capacity). Based on user inputs for each genotype (survival, fitness cost, reproductive rate, emigration, and immigration) and an initial resistance allele frequency, model outputs related to resistance evolution are produced. User inputs related to insecticide-based pest management (pest density action threshold, crop damage rate, insecticide treatment costs, and profit potential) can also be introduced to examine and interpret the basic economic effects of different insect pest management scenarios. The proposed model of resistance evolution and basic economics of pest management relies on a large number of important simplifications, so it may only have limited ability to predict the outcomes of real-world (commercial) scenarios. However, as a teaching tool and to introduce students to a well-known and widely used genetic population model structure, the interactive teaching tool is believed to have considerable utility and relevance.

Convergence of Bar and Cry1Ac Mutant Genes in Soybean Confers Synergistic Resistance to Herbicide and Lepidopteran Insects

Soybean is a globally important crop species, which is subject to pressure by insects and weeds causing severe substantially reduce yield and quality. Despite the success of transgenic soybean in terms of Bacillus thuringiensis (Bt) and herbicide tolerance, unforeseen mitigated performances have still been inspected due to climate changes that favor the emergence of insect resistance. Therefore, there is a need to develop a biotech soybean with elaborated gene stacking to improve insect and herbicide tolerance in the field. In this study, new gene stacking soybean events, such as bialaphos resistance (bar) and pesticidal crystal protein (cry)1Ac mutant 2 (M#2), are being developed in Vietnamese soybean under field condition. Five transgenic plants were extensively studied in the herbicide effects, gene expression patterns, and insect mortality across generations. The increase in the expression of the bar gene by 100% in the leaves of putative transgenic plants was a determinant of herbicide tolerance. In an insect bioassay, the cry1Ac-M#2 protein tested yielded higher than expected larval mortality (86%), reflecting larval weight gain and weight of leaf consumed were less in the T1 generation. Similarly, in the field tests, the expression of cry1Ac-M#2 in the transgenic soybean lines was relatively stable from T0 to T3 generations that corresponded to a large reduction in the rate of leaves and pods damage caused by Lamprosema indicata and Helicoverpa armigera. The transgenic lines converged two genes, producing a soybean phenotype that was resistant to herbicide and lepidopteran insects. Furthermore, the expression of cry1Ac-M#2 was dominant in the T1 generation leading to the exhibit of better phenotypic traits. These results underscored the great potential of combining bar and cry1Ac mutation genes in transgenic soybean as pursuant of ensuring resistance to herbicide and lepidopteran insects.

Does refuge spillover affect arthropod food webs associated with Bt maize?

BACKGROUND

A high dose/refuge combination is the main tactic recommended for mitigating resistance selection of target herbivore species in crops expressing insecticidal proteins of the bacterium Bacillus thuringiensis (i.e. Bt proteins). The tactic consists of the simultaneous use of Bt crops expressing high levels of the Bt protein associated with neighboring areas of refuge of the same non-Bt crop species. Nonetheless, the approach faces controversy regarding its effectiveness and scale of adoption, at least in some regions. One concern focuses on its potential impact on the arthropod community, including its short-term and spatially dependent impact considering the likely biota spillover effect between Bt and non-Bt neighboring areas. Thus, the eventual spillover of Bt maize targeted and non-targeted arthropods was surveyed along transects extending from the refuge border to the center of the Bt maize area.

RESULTS

Arthropods were collected throughout the maize vegetative and reproductive stages. A total of 85 arthropod species were collected, but their richness and abundance did not vary with distance from the refuge. By contrast, cultivation season played a significant role in distinguishing the arthropod communities. Refuge distance from the sampling point within Bt-fields did not significantly affect the food web metrics, unlike season, which affected the number of nodes integrating each food web. Winter maize cultivation exhibited higher arthropod diversity and combined values of species numeric abundance and biomass at each trophic level.

CONCLUSIONS

No arthropod spillover was evident between the refuge edge and Bt maize, adding further controversy to the tactic currently subjected to lower usage in the region with a disputed cost-benefit relationship, because not even the target and its interdependent species were affected. © 2021 Society of Chemical Industry.

Comparison of in Vitro and in Planta Toxicity of Vip3A for Lepidopteran Herbivores

Agricultural pest infestation is as old as domestication of food crops and contributes a major share to the cost of crop production. In a transgenic pest control approach, plant production of Vip3A, an insecticidal protein from Bacillus thuringiensis, is effective against lepidopteran pests. A synthetic Vip3A gene was evaluated for efficacy against Spodoptera litura Fabricius (Lepidoptera: Noctuidae; cotton leafworm), Spodoptera exigua Hübner (Lepidoptera: Noctuidae; beet armyworm), Spodoptera frugiperda Smith (Lepidoptera: Noctuidae; fall armyworm), Helicoverpa armigera Hübner (Lepidoptera: Noctuidae; cotton bollworm), Helicoverpa zea Boddie (Lepidoptera: Noctuidae; corn earworm), Heliothis virescens Fabricius (Lepidoptera: Noctuidae; tobacco budworm), and Manduca sexta L. (Lepidoptera: Sphingidae; tobacco hornworm) in tobacco. In artificial diet assays, the concentration required to achieve 50% mortality was highest for H. zea followed by H. virescens > S. exigua > H. armigera > M. sexta > S. frugiperda > S. litura. By contrast, in bioassays with detached leaves from Vip3A transgenic tobacco, the time until 50% lethality was M. sexta > H. virescens > S. litura > H. zea > H. armigera > S. exigua. There was no significant correlation between the artificial diet and transgenic plant bioassay results. Notably, the two insect species that are best-adapted for growth on tobacco, M. sexta and H. virescens, showed the greatest time to 50% mortality on Vip3A-transgenic tobacco. Together, our results suggest that artificial diet assays may be a poor predictor of Vip3A efficacy in transgenic plants, lepidopteran species vary in their sensitivity to Vip3A in diet-dependent manner, and host plant adaptation of the targeted herbivores should be considered when designing transgenic plants for pest control.

A Framework for Effective Bt Maize IRM Programs: Incorporation of Lessons Learned From Busseola fusca Resistance Development

Bt maize is genetically engineered to express insecticidal proteins from the bacterium Bacillus thuringiensis. Bt maize is used extensively by South African farmers to reduce yield losses caused by lepidopteran larvae. Starting in the 2004/2005 season, severe Busseola fusca-associated damage to Cry1Ab-expressing Bt maize was noted by South African farmers. The unsatisfactory pest control was eventually attributed to the development of insect resistance to the Cry1Ab protein in the Bt maize hybrids. An assessment of the historical events surrounding the development of resistance by B. fusca showed that there was room for improvement both in the insect resistance management (IRM) strategy selected and the implementation of the strategy. With the recent arrival of fall armyworm (Spodoptera frugiperda) in Africa, it is important to have IRM programs that are appropriate for all of the pests that constitute the maize lepidopteran pest complex. After the identification of shortcomings in the IRM programs implemented in South Africa, a framework is proposed for effective Bt maize IRM programs. The IRM framework integrates pre-marketing research, post-marketing monitoring, and two-level remedial action plans (RAPs). The core of the framework is a regulator-approved IRM strategy that is based on comprehensive pre-marketing research and serves to guide stakeholders during the post-marketing phase. The framework will assist technology developers and regulators, especially those with nascent regulatory systems, to select and implement IRM strategies that facilitate sustainable pest management.

The application of self-limiting transgenic insects in managing resistance in experimental metapopulations

The mass release of transgenic insects carrying female lethal self-limiting genes can reduce pest insect populations. Substantial releases are also a novel resistance management tool, since wild type alleles conferring susceptibility to pesticides can dilute resistance alleles in target populations. However, a potential barrier is the need for large-scale area-wide releases. Here, we address whether localized releases of transgenic insects could provide an alternative means of population suppression and resistance management, without serious loss of efficacy.We used experimental mesocosms constituting insect metapopulations to explore the evolution of resistance to the Bacillus thuringiensis toxin Cry1Ac in a high-dose/refugia landscape in the insect Plutella xylostella. We ran two selection experiments, the first compared the efficacy of "everywhere" releases and negative controls to a spatially density-dependent or "whack-a-mole" strategy that concentrated release of transgenic insects in subpopulations with elevated resistance. The second experiment tested the relative efficacy of whack-a-mole and everywhere releases under spatially homogenous and heterogeneous selection pressure.The whack-a-mole releases were less effective than everywhere releases in terms of slowing the evolution of resistance, which, in the first experiment, largely prevented the evolution of resistance. In contrast to predictions, heterogeneous whack-a-mole releases were no more effective under heterogeneous selection pressure. Heterogeneous selection pressure did, however, reduce total insect population sizes.Whack-a-mole releases provided early population suppression, indistinguishable from homogeneous everywhere releases. However, insect population densities tracked the evolution of resistance in this system, as phenotypic resistance provides access to additional diet containing the toxin Cry1Ac. Thus, as resistance levels diverged between treatments, carrying capacities and population sizes increased under the whack-a-mole approach. Synthesis and applications. Spatially density-dependent releases of transgenic insects, particularly those targeting source populations at a landscape level, could suppress pest populations in the absence of blanket area-wide releases. The benefits of self-limiting transgenic insects were reduced in spatially localized releases, suggesting that they are not ideal for "spot" treatment of resistance problems. Nevertheless, spatially homogeneous or heterogeneous releases could be used to support other resistance management interventions.

Stability of expression of Cry1Ac and Cry2Ab2 proteins in Bollgard-II hybrids at different stages of crop growth in different genotypes across cropping seasons and multiple geographies

Bollgard-II cotton expressing Cry1Ac and Cry2Ab2 insecticidal proteins has been commercially cultivated in India since 2006 to control bollworms. These genes were introgressed into parental germplasm of numerous hybrids. Therefore, it is imperative that these insecticidal proteins are expressed in sufficient quantities in different tissues, throughout the season irrespective of genetic background or environmental conditions for effective performance. Here, we document results of a comprehensive study on pattern of expression of Bt proteins across different stages of crop growth in > 2000 cotton hybrids (Gossypium hirsutum), across 12 cropping seasons tested in the Northern, Southern or Central zones in India, in terminal leaf, pre-candle square and boll epicarp tissues. Statistical analysis of variability using Linear mixed effect model was used to estimate factors contributing to variability in expression of Bt proteins. For Cry1Ac, variability was maximally contributed by genotype × season × plant growth stage effect in terminal leaves and boll epicarp, while season effect drove variability in pre-candle square. In Cry2Ab2, season effect drove variability in three tissue types. Pre-candle square tissue had most variability in expression of both proteins followed by terminal leaf and boll epicarp. Further, expression of Bt proteins in 234 G. hirsutum × G. barbadense hybrids showed similar expression patterns as intra specific hybrids though there was a significant difference in expression levels. Cry2Ab2 was expressed in significantly higher amounts when genes were in homozygous state. Bt proteins were also found to be expressed in varied amounts in different tissues and were expressed even when hybrids were grown at sub-optimal temperatures.

Combining the high-dose/refuge strategy and self-limiting transgenic insects in resistance management-A test in experimental mesocosms

The high-dose/refuge strategy has been the primary approach for resistance management in transgenic crops engineered with Bacillus thuringiensis toxins. However, there are continuing pressures from growers to reduce the size of Bt toxin-free refugia, which typically suffer higher damage from pests. One complementary approach is to release male transgenic insects with a female-specific self-limiting gene. This technology can reduce population sizes and slow the evolution of resistance by introgressing susceptible genes through males. Theory predicts that it could be used to facilitate smaller refugia or reverse the evolution of resistance. In this study, we used experimental evolution with caged insect populations to investigate the compatibility of the self-limiting system and the high-dose/refuge strategy in mitigating the evolution of resistance in diamondback moth, Plutella xylostella. The benefits of the self-limiting system were clearer at smaller refuge size, particularly when refugia were inadequate to prevent the evolution of resistance. We found that transgenic males in caged mesocosms could suppress population size and delay resistance development with 10% refugia and 4%-15% initial resistance allele frequency. Fitness costs in hemizygous transgenic insects are particularly important for introgressing susceptible alleles into target populations. Fitness costs of the self-limiting gene in this study (P. xylostella OX4139 line L) were incompletely dominant, and reduced fecundity and male mating competitiveness. The experimental evolution approach used here illustrates some of the benefits and pitfalls of combining mass release of self-limiting insects and the high-dose/refuge strategy, but does indicate that they can be complementary.

Impact of Corn Earworm (Lepidoptera: Noctuidae) on Field Corn (Poales: Poaceae) Yield and Grain Quality

Corn earworm, Helicoverpa zea (Boddie), commonly infests field corn, Zea mays (L.). The combination of corn plant biology, corn earworm behavior in corn ecosystems, and field corn value renders corn earworm management with foliar insecticides noneconomical. Corn technologies containing Bacillus thuringiensis (Bt) Berliner (Bacillales: Bacillaceae) were introduced that exhibit substantial efficacy against corn earworm and may reduce mycotoxin contamination in grain. The first generation Bt traits in field corn demonstrated limited activity on corn earworm feeding on grain. The pyramided corn technologies have greater cumulative protein concentrations and higher expression throughout the plant, so these corn traits should provide effective management of this pest. Additionally, reduced kernel injury may affect physical grain quality. Experiments were conducted during 2011-2012 to investigate corn earworm impact on field corn yield and grain quality. Treatments included field corn hybrids expressing the Herculex, YieldGard, and Genuity VT Triple Pro technologies. Supplemental insecticide treatments were applied every 1-2 d from silk emergence until silk senescence to create a range of injured kernels for each technology. No significant relationship between the number of corn earworm damaged kernels and yield was observed for any technology/hybrid. In these studies, corn earworm larvae did not cause enough damage to impact yield. Additionally, no consistent relationship between corn earworm damage and aflatoxin contamination was observed. Based on these data, the economic value of pyramided Bt corn traits to corn producers, in the southern United States, appears to be from management of other lepidopteran insect pests including European and southwestern corn borer.

Bt Jute Expressing Fused δ-Endotoxin Cry1Ab/Ac for Resistance to Lepidopteran Pests

Jute (Corchorus sp.) is naturally occurring, biodegradable, lignocellulosic-long, silky, golden shiny fiber producing plant that has great demands globally. Paper and textile industries are interested in jute because of the easy availability, non-toxicity and high yield of cellulosic biomass produced per acre in cultivation. Jute is the major and most industrially used bast fiber-producing crop in the world and it needs protection from insect pest infestation that decreases its yield and quality. Single locus integration of the synthetically fused cry1Ab/Ac gene of Bacillus thuringiensis (Bt) in Corchorus capsularis (JRC 321) by Agrobacterium tumefaciens-mediated shoot tip transformation provided 5 potent Bt jute lines BT1, BT2, BT4, BT7 and BT8. These lines consistently expressed the Cry1Ab/Ac endotoxin ranging from 0.16 to 0.35 ng/mg of leaf, in the following generations (analyzed upto T4). The effect of Cry1Ab/Ac endotoxin was studied against 3 major Lepidopteran pests of jute- semilooper (Anomis sabulifera Guenee), hairy caterpillar (Spilarctia obliqua Walker) and indigo caterpillar (Spodoptera exigua Hubner) by detached leaf and whole plant insect bioassay on greenhouse-grown transgenic plants. Results confirm that larvae feeding on transgenic plants had lower food consumption, body size, body weight and dry weight of excreta compared to non-transgenic controls. Insect mortality range among transgenic feeders was 66-100% for semilooper and hairy caterpillar and 87.50% for indigo caterpillar. Apart from insect resistance, the transgenic plants were at par with control plants in terms of agronomic parameters and fiber quality. Hence, these Bt jutes in the field would survive Lepidopteran pest infestation, minimize harmful pesticide usage and yield good quality fiber.

High Expression of Cry1Ac Protein in Cotton (Gossypium hirsutum) by Combining Independent Transgenic Events that Target the Protein to Cytoplasm and Plastids

Transgenic cotton was developed using two constructs containing a truncated and codon-modified cry1Ac gene (1,848 bp), which was originally characterized from Bacillus thuringiensis subspecies kurstaki strain HD73 that encodes a toxin highly effective against many lepidopteran pests. In Construct I, the cry1Ac gene was cloned under FMVde, a strong constitutively expressing promoter, to express the encoded protein in the cytoplasm. In Construct II, the encoded protein was directed to the plastids using a transit peptide taken from the cotton rbcSIb gene. Genetic transformation experiments with Construct I resulted in a single copy insertion event in which the Cry1Ac protein expression level was 2–2.5 times greater than in the Bacillus thuringiensis cotton event Mon 531, which is currently used in varieties and hybrids grown extensively in India and elsewhere. Another high expression event was selected from transgenics developed with Construct II. The Cry protein expression resulting from this event was observed only in the green plant parts. No transgenic protein expression was observed in the non-green parts, including roots, seeds and non-green floral tissues. Thus, leucoplasts may lack the mechanism to allow entry of a protein tagged with the transit peptide from a protein that is only synthesized in tissues containing mature plastids. Combining the two events through sexual crossing led to near additive levels of the toxin at 4–5 times the level currently used in the field. The two high expression events and their combination will allow for effective resistance management against lepidopteran insect pests, particularly Helicoverpa armigera, using a high dosage strategy.

Effect of crop plants on fitness costs associated with resistance to Bacillus thuringiensis toxins Cry1Ac and Cry2Ab in cabbage loopers

Fitness costs associated with resistance to Bacillus thuringiensis (Bt) toxins critically impact the development of resistance in insect populations. In this study, the fitness costs in Trichoplusia ni strains associated with two genetically independent resistance mechanisms to Bt toxins Cry1Ac and Cry2Ab, individually and in combination, on four crop plants (cabbage, cotton, tobacco and tomato) were analyzed, in comparison with their near-isogenic susceptible strain. The net reproductive rate (R₀) and intrinsic rate of increase (r) of the T. ni strains, regardless of their resistance traits, were strongly affected by the host plants. The ABCC2 gene-linked mechanism of Cry1Ac resistance was associated with relatively low fitness costs, while the Cry2Ab resistance mechanism was associated with higher fitness costs. The fitness costs in the presence of both resistance mechanisms in T. ni appeared to be non-additive. The relative fitness of Bt-resistant T. ni depended on the specific resistance mechanisms as well as host plants. In addition to difference in survivorship and fecundity, an asynchrony of adult emergence was observed among T. ni with different resistance mechanisms and on different host plants. Therefore, mechanisms of resistance and host plants available in the field are both important factors affecting development of Bt resistance in insects.

A high-throughput, in-vitro assay for Bacillus thuringiensis insecticidal proteins

A high-throughput, in-vitro assay for Bacillus thuringiensis (Bt) insecticidal proteins designated as Cry was developed and evaluated for screening a large number of Cry protein variants produced by DNA shuffling. This automation-amenable assay exploits an insect cell line expressing a single receptor of Bt Cry proteins. The Cry toxin used to develop this assay is a variant of the Cry1Ab protein called IP1-88, which was produced previously by DNA shuffling. Cell mortality caused by the activated Bt Cry toxin was determined by chemical cell viability assay in 96/384-well microtiter plates utilizing CellTiter 96(®) obtained from Promega. A widely-accepted mode-of-action theory of certain Bt Cry proteins suggests that the activated toxin binds to one or more receptors and forms a pore through the insect gut epithelial cell apical membrane. A number of insect proteins such as cadherin-like protein (Cad), aminopeptidase-N (APN), alkaline phosphatase (ALP) and ABC transporter (ABCC) have been identified as the receptors of Bt Cry toxins. In this study, Bt Cry toxin receptors Ostrinia nubilalis (European corn borer) cadherin-like protein (On-Cad) and aminopeptidase-N 1 and 3 (On-APN1, On-APN3) and Spodoptera frugiperda (fall armyworm) cadherin-like protein (Sf-Cad) were cloned in an insect cell line, Sf21, and a mammalian cell line, Expi293F. It was observed by ligand blotting and immunofluorescence microscopy that trypsin-activated IP1-88 bound to On-Cad and On-APN1, but not Sf-Cad or On-APN3. In contrast, IP1-88 bound only to APN1 in BBMV (Brush Border Membrane Vesicles) prepared from the third and fourth-instar O. nubilalis larval midgut. The sensitivity of the recombinant cells to the toxin was then tested. IP1-88 showed no toxicity to non-recombinant Sf21 and Expi293F. Toxicity was observed only when the On-Cad gene was cloned and expressed. Sf-Cad and On-APN1 were not able to make those cells sensitive to the toxin. Since the expression of On-Cad alone was sufficient to make both insect and mammalian cells sensitive to the IP1-88 toxin, it is not likely that a secondary receptor, which may exist specifically in the Sf21 insect cell but not in the Expi293F cell, is involved in the cytotoxicity of IP1-88.

Transgenic potato plants expressing cry3A gene confer resistance to Colorado potato beetle

The Colorado potato beetle (Leptinotarsa decemlineata Say, CPB) is a fatal pest, which is a quarantine pest in China. The CPB has now invaded the Xinjiang Uygur Autonomous Region and is constantly spreading eastward in China. In this study, we developed transgenic potato plants expressing cry3A gene. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis indicated that the cry3A gene expressed in leaves, stems and roots of the transgenic plants under the control of CaMV 35S promoter, while they expressed only in leaves and stems under the control of potato leaf and stem-specific promoter ST-LS1. The mortality of the larvae was higher (28% and 36%) on the transgenic plant line 35S1 on the 3rd and 4th days, and on ST3 (48%) on the 5th day after inoculation with instar larvae. Insect biomass accumulation on the foliage of the transgenic plant lines 35S1, 35S2 and ST3 was significantly lower (0.42%, 0.43% and 0.42%). Foliage consumption was lowest on transgenic lines 35S8 and ST2 among all plant foliage (7.47 mg/larvae/day and 12.46 mg/larvae/day). The different transgenic plant foliages had varied inhibition to larval growth. The survivors on the transgenic lines obviously were smaller than their original size and extremely weak. The transgenic potato plants with CPB resistance could be used to develop germplasms or varieties for controlling CPB damage and halting its spread in China.

Chloroplast-targeted expression of recombinant crystal-protein gene in cotton: an unconventional combat with resistant pests

Plants transformed with single Bt gene are liable to develop insect resistance and this has already been reported in a number of studies carried out around the world where Bt cotton was cultivated on commercial scale. Later, it was envisaged to transform plants with more than one Bt genes in order to combat with resistant larvae. This approach seems valid as various Bt genes possess different binding domains which could delay the likely hazards of insect resistance against a particular Bt toxin. But it is difficult under field conditions to develop homozygous plants expressing all Bt genes equally after many generations without undergoing recombination effects. A number of researches claiming to transform plants from three to seven transgenes in a single plant were reported during the last decade but none has yet applied for patent of homozygous transgenic lines. A better strategy might be to use hybrid-Bt gene(s) modified for improved lectin-binding domains to boost Bt receptor sites in insect midgut. These recombinant-Bt gene(s) would express different lectin domains in a single polypeptide and it is relatively easy to develop homozygous transgenic lines under field conditions. Enhanced chloroplast-localized expression of hybrid-Bt gene would leave no room for insects to develop resistance. We devised and successfully applied this strategy in cotton (Gossypium hirsutum) and data up to T3 generation showed that our transgenic cotton plants were displaying enhanced chloroplast-targeted Cry1Ac-RB expression. Laboratory and field bioassays gave promising results against American bollworm (Heliothis armigera), pink bollworm (Pictinophora scutigera) and fall armyworm (Spodoptera frugiperda) that otherwise, were reported to have evolved resistance against Cry1Ac toxin. Elevated levels of hybrid-Bt toxin were confirmed by ELISA of chloroplast-enriched protein samples extracted from leaves of transgenic cotton lines. While, localization of recombinant Cry1Ac-RB protein in chloroplast was established through confocal laser scanning microscopy.

Transgenic approaches to western corn rootworm control

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) is a significant corn pest throughout the United States corn belt. Rootworm larvae feed on corn roots causing yield losses and control expenditures that are estimated to exceed US$1 billion annually. Traditional management practices to control rootworms such as chemical insecticides or crop rotation have suffered reduced effectiveness due to the development of physiological and behavioral resistance. Transgenic maize expressing insecticidal proteins are very successful in protecting against rootworm damage and preserving corn yield potential. However, the high rate of grower adoption and early reliance on hybrids expressing a single mode of action and low-dose traits threatens the durability of commercialized transgenic rootworm technology for rootworm control. A summary of current transgenic approaches for rootworm control and the corresponding insect resistance management practices is included. An overview of potential new modes of action based on insecticidal proteins, and especially RNAi targeting mRNA coding for essential insect proteins is provided.

Lengthening of insect development on Bt zone results in adult emergence asynchrony: does it influence the effectiveness of the high dose/refuge zone strategy?

The “High Dose/Refuge” strategy (HD/R) is the currently recommended Insect Resistance Management strategy (IRM) to limit resistance development to Bacillus thuringiensis (Bt) plants. This strategy requires planting a “refuge zone” composed of non-Bt plants suitable for the target insect and in close proximity to a “Bt zone” expressing a high toxin concentration. One of the main assumptions is that enough susceptible adults mate with resistant insects. However, previous studies have suggested that the high toxin concentration produced by Bt plants induces slower insect development, creating an asynchrony in emergence between the refuge and the Bt zone and leading to assortative mating between adults inside each zone. Here, we develop a deterministic model to estimate the impact of toxin concentration, emergence asynchrony and refuge zone size on the effectiveness of the HD/R strategy. We conclude that emergence asynchrony only affects resistance when toxin concentration is high and resistance is recessive. Resistance develops more rapidly and survival of susceptible insects is higher at lower toxin concentration, but in such situations, resistance is insensitive to emergence asynchrony.