Suppression of Calcineurin Enhances the Toxicity of Cry1Ac to Helicoverpa armigera

Reduced expression of the P-glycoprotein gene HaABCB1 is linked to resistance to Bacillus thuringiensis Cry1Ac toxin but not Cry2Ab toxin in Helicoverpa armigera

Rapid evolution of pest resistance to Bt insecticidal proteins presents a serious threat to the sustainable use of Bt crops. The cotton bollworm has been extensively exposed to Bt cotton worldwide and has evolved resistance in laboratory and field. Previous studies have highlighted the significant roles played by the ABC transporter proteins in Bt resistance. In this study, the ORF of HaABCB1 was cloned and analyzed. The expression of HaABCB1 was detected in all developmental stages and tissues, with the highest expression in third instar larvae stage and hindgut tissue. Compared with susceptible strain, a remarkable decrease of HaABCB1 expression in Cry1Ac resistant strain while no significant change in Cry2Ab resistant strain were found. The HaABCB1 expression reduced after susceptible larvae induced by Cry1Ac, but no obvious expression changes after Cry2Ab exposure. RNAi-mediated down-regulation of HaABCB1 could lead to a significant reduction in larval susceptibility to Cry1Ac, but not to Cry2Ab, in susceptible strain. Genetic linkage analysis confirmed that decreased expression of the HaABCB1 mediates resistance to Cry1Ac, but not Cry2Ab resistance. This knowledge contributes to better understanding of the complex molecular mechanisms underlying Bt resistance and provide theoretical foundation for the development of new strategies for pest resistance management.

ATP Synthase Subunit α from Helicoverpa armigera Acts as a Receptor of Bacillus thuringiensis Cry1Ac and Synergizes Cry1Ac Toxicity

Insect resistance to Bacillus thuringiensis (Bt) toxins has led to an urgent need to explore the insecticidal mechanisms of Bt. Previous studies indicated that Helicoverpa armigera ATP synthase subunit α (HaATPs-α) is involved in Cry1Ac resistance. In this study, a real-time quantitative polymerase chain reaction (RT-PCR) confirmed that HaATPs-α expression was significantly reduced in the Cry1Ac-resistant strain (BtR). Cry1Ac feeding induced the downregulated expression of HaATPs-α in the susceptible strain, but not in the BtR strain. Furthermore, the interaction between HaATPs-α and Cry1Ac was verified by ligand blotting and homologous competition experiments. The in vitro gain and loss of function analyses showed HaATPs-α involved in Cry1Ac toxicity by expressing endogenous HaATPs-α and HaATPs-α double-stranded RNAs in Sf9 and midgut cells, respectively. Importantly, purified HaATPs-α synergized Cry1Ac toxicity to H. armigera larvae. These findings provide the first evidence that HaATPs-α is a potential receptor of Cry1Ac, it shows downregulated participation in Cry1Ac resistance, and it exhibits higher enhancement of Cry1Ac toxicity to H. armigera larvae.

Involvement of an Enhanced Immunity Mechanism in the Resistance to Bacillus thuringiensis in Lepidopteran Pests

Bacillus thuringiensis (Bt) is the safest, economically successful entomopathogen to date. It is extensively produced in transgenic crops or used in spray formulations to control Lepidopteran pests. The most serious threat to the sustainable usage of Bt is insect resistance. The resistance mechanisms to Bt toxins depend not only on alterations in insect receptors, but also on the enhancement of insect immune responses. In this work, we review the current knowledge of the immune response and resistance of insects to Bt formulations and Bt proteins, mainly in Lepidopteran pests. We discuss the pattern recognition proteins for recognizing Bt, antimicrobial peptides (AMPs) and their synthetic signaling pathways, the prophenoloxidase system, reactive oxygen species (ROS) generation, nodulation, encapsulation, phagocytosis, and cell-free aggregates, which are involved in immune response reactions or resistance to Bt. This review also analyzes immune priming, which contributes to the evolution of insect resistance to Bt, and puts forward strategies to improve the insecticidal activity of Bt formulations and manage insect resistance, targeting the insect immune responses and resistance.

Cyclosporin A acts as a novel insecticide against Cry1Ac-susceptible and -resistant Helicoverpa armigera

Cotton bollworm (Helicoverpa armigera) is an economically important pest, which is difficult to manage due to its biological and ecological traits, and resistance to most insecticides. Alternative compounds for the sustainable management of H. armigera are needed. As a fungal metabolite, Cyclosporin A (CsA) has not been applied in agriculture pests. Here, CsA was evaluated as a propective insecticide for H. armigera. The results showed that CsA displayed high insecticidal activity against both Cry1Ac-susceptible and -resistant populations of H. armigera. Moreover, lower concentrations of CsA had clear effects, including significantly reduced pupal weight, pupation rate, emergence rate, ovary size, female fecundity and egg hatchability. Further study confirmed that CsA suppressed calcineurin activity and the subsequent expression of endogenous antimicrobial peptide genes (APMs), leading to impaired immunity, ultimately resulting in delayed development and increased mortality. Thus, CsA treatment could control the cotton bollworm population and even showed efficacy against those with Bt resistance. In addition, the morphological changes observed in insects fed CsA with lower concentrations provide insight into insect immunity, regulation of growth and development, regulation of body color, ovary development and sexual selection under external pressure. Overall, our study provides information on biological control potential of Cry1Ac-susceptible and -resistant populations of H. armigera to develop novel bioinsecticides.

Cyclosporin A as a Source for a Novel Insecticidal Product for Controlling Spodoptera frugiperda

The fall armyworm (FAW), Spodoptera frugiperda, causes substantial annual agricultural production losses worldwide due to its resistance to many insecticides. Therefore, new insecticides are urgently needed to more effectively control FAW. Cyclosporin A (CsA) is a secondary metabolite of fungi; little is known about its insecticidal activity, especially for the control of FAW. In this study, we demonstrate that CsA shows excellent insecticidal activity (LC₅₀ = 9.69 μg/g) against FAW through significant suppression of calcineurin (CaN) activity, which is a new target for pest control. Combinations of CsA and indoxacarb, emamectin benzoate, or Vip3Aa showed independent or synergistic toxicity against FAW; however, the combination of CsA and chlorantraniliprole showed no toxicity. Sublethal doses of CsA led to decreases in FAW larval and pupal weight, pupation, emergence, mating rates, adult longevity, extended development of FAW larvae and pupae and the pre-oviposition period of adults, and increases in the proportion of pupal malformation. Importantly, CsA treatment reduced FAW ovarian size and female fecundity, which suggests that it has great potential to suppress FAW colony formation. Taken together, these results indicate that CsA has high potential as an insecticide for controlling FAW.

Cyclosporin A as a Potential Insecticide to Control the Asian Corn Borer Ostrinia furnacalis Guenée (Lepidoptera: Pyralidae)

The long-term use of chemical insecticides has caused serious problems of insect resistance and environmental pollution; new insecticides are needed to solve this problem. Cyclosporin A (CsA) is a polypeptide produced by many fungi, which is used to prevent or treat immune rejection during organ transplantation. However, little is known about the utility of CsA as an insecticide. Therefore, this study evaluated the insecticidal activity of CsA using Ostrinia furnacalis as a model. The results demonstrated that CsA was toxic to O. furnacalis with LC₅₀ values of 113.02 μg/g and 198.70 μg/g for newly hatched neonates and newly molted third-instar larvae, respectively. Furthermore, CsA treatment had sublethal effects on the development of O. furnacalis, and significantly reduced the fecundity of adults; this suggests that CsA has great potential to suppress O. furnacalis populations. Further analysis revealed that CsA suppressed calcineurin activity in larvae. CsA had independent or synergistic toxic effects on O. furnacalis when combined with β-cypermethrin, indoxacarb, emamectin benzoate, azadirachtin, and the Bacillus thuringiensis toxin Cry1Ac, which suggests that CsA can help prevent or manage resistance. Our study provides detailed information on the potential of CsA as an insecticide for controlling lepidopterans.

ABCC2 is a functional receptor of Bacillus thuringiensis Cry1Ca in Spodoptera litura

Spodoptera litura is a serious polyphagous pest in the whole world, which has developed resistance to most conventional insecticides and even some Bacillus thuringiensis (Bt) toxins. Cry1Ca has excellent insecticide activity against S. litura with potential application to control S. litura and delay the development of insect resistance. However, the mode of action of Cry1Ca in S. litura is poorly understood. Here, Cry1Ca-binding proteins were identified from S. litura by using pull down assays and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The results indicated that aminopeptidase-N (APN), ATP binding cassette subfamily C member 2 (ABCC2), polycalin, actin and V-type proton ATPase subunit A may bind with Cry1Ca. Further study confirmed that ABCC2 fragment expressed in vitro can bind to Cry1Ca as demonstrated by Ligand blot and homologous competition experiments. The over-expression of endogenous SlABCC2 in Sf9 cells increased Cry1Ca cytotoxicity. Correspondingly, the vivo loss of function analyses by SlABCC2 small interfering RNAs (siRNAs) in S. litura larvae decreased the toxicity of Cry1Ca to larvae. Altogether, these results show that ABCC2 of S. litura is a functional receptor that is involved in the action mode of Cry1Ca.