The mutation R423S in the Bacillus thuringiensis hybrid toxin CryAAC slightly increases toxicity for Mamestra brassicae L

Multiple dsRNases Involved in Exogenous dsRNA Degradation of Fall Armyworm Spodoptera frugiperda

RNAi is regarded as a promising technology for pest control. However, not all insects are sensitive to RNAi. Studies have confirmed that insect dsRNases are one of key factors affecting RNAi efficiency. In the current study, we identified four genes coding for dsRNases from the Spodoptera frugiperda genome. Spatial and temporal expression analysis showed that those dsRNases were highly expressed in the midgut and old larvae. Then a delivery method was applied for inducing efficient RNAi based on dsRNA encapsulated by liposome. Furthermore, we assessed degradation efficiency by incubation with dsRNA with gut juice or hemocoel to characterize potential roles of different SfdsRNases after suppression of SfdsRNase. The result showed that interferenced with any sfdsRNase reduced the degradation of exogenous dsRNA in midgut, interfered with sfdsRNase1 and sfdsRNase3 slowed down the degradation of exogenous dsRNA in hemolymph. Our data suggest the evolutionary expansion and multiple high activity dsRNase genes would take part in the RNAi obstinate in S. frugiperda, besides we also provide an efficient RNAi method for better use of RNAi in S. frugiperda.

Proteolytic Activation of Bacillus thuringiensis Cry3Aa Toxin in the Red Palm Weevil (Coleoptera: Curculionidae)

The red palm weevil (RPW), Rhynchophorus ferrugineus (Oliver) is an important pest of palms that causes significant damage by boring into and feeding within palm stem tissues. Here, we studied the proteolytic process of Cry3Aa in the RPW to understand the mechanism of Cry toxicity. The bioassays showed that Cry3Aa toxin is weakly toxic to the RPW. Proteolytic activation assays indicated the Cry3Aa protein is digested into smaller fragments than the 55-kDa activated fragments under different conditions. In particular, at higher mass ratios of gut protease and Cry3Aa protein (5:1, 2:1, and 1:1, respectively), and at 36.9°C for 16 h in a solution of pH 8.6, the Cry3Aa protoxin is over-digested by the gut proteases of weevil larvae. Moreover, the zymogram analysis of the gut proteases revealed the RPW larvae harbors intestinal digestive enzymes mainly composed of serine proteases. This study describes the proteolytic activation process of Cry3Aa in the midgut of RPW larvae.

Functional expression of a peritrophin A-like SfPER protein is required for larval development in Spodoptera frugiperda (Lepidoptera: Noctuidae)

Peritrophins are associated with structural and functional integrity of peritrophic membranes (PM), structures composed of chitin and proteins. PM lines the insect midgut and has roles in digestion and protection from toxins. We report the full-length cDNA cloning, molecular characterization and functional analysis of SfPER, a novel PM peritrophin A protein, in Spodoptera frugiperda. The predicted amino acid sequence indicated SfPER's domain structure as a CMCMC-type, consisting of a signal peptide and three chitin-binding (C) domains with two intervening mucin-like (M) domains. Phylogenetic analysis determined a close relationship between SfPER and another S. frugiperda PM peritrophin partial sequence. SfPER transcripts were found in larvae and adults but were absent from eggs and pupae. Chitin affinity studies with a recombinant SfPER-C1 peritrophin A-type domain fused to SUMO/His-tag confirmed that SfPER binds to chitin. Western blots of S. frugiperda larval proteins detected different sized variants of SfPER along the PM, with larger variants found towards the posterior PM. In vivo suppression of SfPER expression did not affect susceptibility of larvae to Bacillus thuringiensis toxin, but significantly decreased pupal weight and adult emergence, possibly due to PM structural alterations impairing digestion. Our results suggest SfPER could be a novel target for insect control.

Transcriptional response of BmToll9-1 and RNAi machinery genes to exogenous dsRNA in the midgut of Bombyx mori

Injection of dsRNA is widely applied to silence endogenous genes and study gene function in insects. However, it is not yet clear to what extent it can also exert non-specific effects, for instance by interference with the innate immune response. In this study, we report on the transcriptional response of BmToll9-1 to lipopolysaccharide (LPS) and dsRNA in the silkmoth, Bombyx mori. BmToll9-1 encodes a Toll receptor highly expressed in midgut tissue and that shows limited similarity to the mammalian TLR3 endolysosome receptor for dsRNA; while Dcr2 and Ago2 encode two key components of the RNAi machinery. An expression pattern study of all 14 Toll receptors in B. mori showed that BmToll9-1 was expressed in different larval and pupal tissues with the highest expression level detected in the midgut, indicating a possible function in immunity against pathogens taken up by the food. In order to investigate the response of BmToll9-1, different ways to deliver dsRNA, specific for GFP (dsGFP), and LPS were applied in Bombyx 5th instar larvae. The feeding experiments suggested that dsGFP did not suppress the expression of BmToll9-1 significantly, while LPS could suppress the expression of BmToll9-1 after 3h of feeding. On the other hand, the injection experiments showed that dsGFP, as well as LPS, could significantly inhibit the expression of BmToll9-1 in 3h. Bacteria that constantly expressed dsGFP could also down-regulate the expression of BmToll9-1 to a greater extent than bacteria that do not express dsGFP. The failure of dsGFP by feeding to affect the expression of BmToll9-1 was correlated with the rapid degradation of dsGFP by dsRNase in the midgut juice. Expression of the RNAi machinery genes Dcr2 and Ago2, as well as dsRNase, was also affected by injection of dsRNA and not by feeding, but in these cases an induction was observed instead of a down-regulation. Because LPS is a well-known pathogen-associated molecular pattern (PAMP), it suggested that the decrease in BmToll9-1 expression is a consequence of the activation of the innate immune response by LPS. The similar response of BmToll9-1 between the two triggers, LPS and dsRNA, suggests that dsRNA can also act as a PAMP in the midgut of Bombyx. Furthermore, induction of the genes Dcr2, Ago2 and dsRNase may also constitute a defense mechanism against invading dsRNA.

A recombinant immunosuppressive protein from Pimpla hypochondriaca (rVPr1) increases the susceptibility of Lacanobia oleracea and Mamestra brassicae larvae to Bacillus thuringiensis

The precise mechanisms underlying Bacillus thuringiensis-mediated killing of pest insects are not clear. In some cases, death may be due to septicaemia caused by Bt and/or gut bacteria gaining access to the insect haemocoel. Since insects protect themselves from microbes using an array of cellular and humoral immune defences, we aimed to determine if a recombinant immunosuppressive wasp venom protein (rVPr1) could increase the susceptibility of two pest Lepidoptera (Lacanobia oleracea and Mamestra brassicae) to Bt. Bio-assays indicated that injection of 6 microl of rVPr1 into the haemocoel of both larvae caused similar levels of mortality (less than 38%). On the other hand, the LD(30-40) of Bt for M. brassicae larvae was approximately 20 times higher than that for L. oleracea larvae. Furthermore, in bio-assays where larvae were injected with rVPr1, then fed Bt, a significant reduction in survival of larvae for both species occurred compared to each treatment on its own (P<0.001); and for L. oleracea larvae, this effect was more than additive. The results are discussed within the context of insect immunity and protection against Bt.

Molecular characterization of Spodoptera frugiperda-Bacillus thuringiensis Cry1Ca toxin interaction

The use of Bacillus thuringiensis Cry delta-endotoxins as bioinsecticides is threatened by the possibility of pest resistance. Determining transcriptional profiles of midgut cells early in Cry toxin poisoning is crucial for understanding the biochemical and molecular aspects of insect detoxification and for sustained use of such toxins. In this study, transcriptional responses of midgut cells from Spodoptera frugiperda third-instar larvae following treatment with Cry1Ca were investigated. Suppression subtractive hybridization (SSH) on insect midguts dissected at different time intervals during the first 24h of exposure to a sublethal concentration of Cry1Ca was used to isolate and identify S. frugiperda gut genes that change in expression on intoxication. After differential screening by membrane-based hybridization, 86 cDNA fragments were selected, sequenced, and analyzed in databases using BLASTN/BLASTX. The cDNA collection comprised a repertoire of genes mainly associated with metabolism, defence and oxidative stress. The expression of a subset of these genes was further investigated. Northern blot analysis confirmed the differential expression patterns between intoxicated and control larvae. The transcript accumulation rate at six different times taken after the initiation of the intoxication point was also examined. Differential expression of most genes examined was detected within 15 min after toxin challenge, where defence and oxidative stress-related genes were transcriptionally enhanced and metabolic-related genes were repressed.