Endogenous serpin reduces toxicity of Bacillus thuringiensis Cry1Ac against Helicoverpa armigera (Hübner)

Characterization and expression profiling of serine protease inhibitors in the yellow mealworm Tenebrio molitor

Serine protease inhibitors (SPIs) act in diverse biological processes in insects such as immunity, development, and digestion by preventing the unwanted proteolysis. So far, the repertoire of genes encoding SPIs has been identified from few insect species. In this study, 62 SPI genes were identified from the genome of the yellow mealworm, Tenebrio molitor. According to their modes of action, they were classified into three families, serpin (26), canonical SPI (31), and α-macroglobulins (A2M) (5). These SPIs feature eight domains including serpin, Kazal, TIL, Kunitz, WAP, Antistasin, pacifastin, and A2M. In total, 39 SPIs contain a single SPI domain, while the others encode at least two inhibitor units. Based on the amino acids in the cleaved reactive sites, the abilities of these SPIs to inhibit trypsin, chymotrypsin, or elastase-like enzymes are predicted. The expression profiling based on the RNA-seq data showed that these genes displayed stage-specific expression patterns during development, suggesting to us their significance in development. Some of the SPI genes were exclusively expressed in particular tissues such as hemocyte, fat body, gut, ovary, and testis, which may be involved in biological processes specific to the indicated tissues. These findings provide necessary information for further investigation of insect SPIs.

Up-regulated serpin gene involved in Cry1Ac resistance in Helicoverpa armigera

Insect resistance to Bacillus thuringiensis (Bt) is a critical limiting factor for applying the Bt crops. Some studies indicated that decreased protoxin activation because of lower enzymatic activities of trypsin and chymotrypsin and increased expression of serpin might involve in Bt resistance. Our previous study identified an endogenous serpin could inhibit the midgut proteases to activate Cry1Ac and reduce the insecticide activity to Helicoverpa armigera. We hypothesis that up-regulated serpin involve in resistance via inhibiting enzymatic activities of trypsin and chymotrypsin to decrease protoxin activation. Herein, we found the serpin-e gene relative expression in midgut was significantly higher in the LF30 resistant strain than that in the susceptible strain during all developmental stages. Importantly, RNAi-mediated silencing of serpin-e gene expression caused 4.46-fold mortality changes in LF30 strain, but the trypsin and chymotrypsin proteases activities were only changed 0.79-fold and 2.22-fold. In addition, although proteases activities were significantly lower in LF30 strain than that in the susceptible strain, the resistance ratios of LF30 to Cry1Ac protoxin and to activated Cry1Ac toxin were no difference. The results indicated serpins caused insect resistance to Cry1Ac protoxins partly through inhibiting the trypsin and chymotrypsin proteases activities, but it also existed other mechanisms in LF30.

Evaluating the pesticidal impact of plant protease inhibitors: lethal weaponry in the co-evolutionary battle

In the arsenal of plant defense, protease inhibitors (PIs) are well-designed defensive products to counter field pests. PIs are produced in plant tissues by means of 'stable defense metabolite' and triggered on demand as the perception of the signal and well established as a part of plant active defense. PIs have been utilized for approximately four decades, initially as a gene-alone approach that was later replaced by multiple gene pyramiding/gene stacking due to insect adaptability towards the PI alone. By considering the adaptive responses of the pest to the single insecticidal gene, the concept of gene pyramiding gained continuous appreciation for the development of transgenic crops to deal with co-evolving pests. Gene pyramiding approaches are executed to bypass the insect's adaptive responses against PIs. Stacking PIs with additional insecticidal proteins, plastid engineering, recombinant proteinase inhibitors, RNAi-based methods and CRISPR/Cas9-mediated genome editing are the advanced tools and methods for next-generation pest management. Undoubtedly, the domain associated with the mechanism of PIs in the course of plant-pest interactions will occupy a central role for the advancement of more efficient and sustainable pest control strategies. © 2021 Society of Chemical Industry.

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.