Transcriptomic Responses to Different Cry1Ac Selection Stresses in Helicoverpa armigera

Abstract

Helicoverpa armigera can develop resistance to Bacillus thuringiensis (Bt), which threaten the long-term success of Bt crops. In the present study, RNAseq was employed to investigate the midgut genes response to strains with different levels of resistance (LF5, LF10, LF20, LF30, LF60, and LF120) in H. armigera. Results revealed that a series of differentially expressed unigenes (DEGs) were expressed significantly in resistant strains compared with the LF-susceptible strain. Nine trypsin genes, ALP2, were downregulated significantly in all the six resistant strains and further verified by qRT-PCR, indicating that these genes may be used as markers to monitor and manage pest resistance in transgenic crops. Most importantly, the differences in DEG functions in the different resistant strains revealed that different resistance mechanisms may develop during the evolution of resistance. The immune and detoxification processes appear to be associated with the low-level resistance (LF5 strain). Metabolic process-related macromolecules possibly lead to resistance to Cry1Ac in the LF10 and LF20 strains. The DEGs involved in the “proton-transporting V-type ATPase complex” and the “proton-transporting two-sector ATPase complex” were significantly expressed in the LF30 strain, probably causing resistance to Cry1Ac in the LF30 strain. The DEGs involved in binding and iron ion homeostasis appear to lead to high-level resistance in the LF60 and LF120 strains, respectively. The multiple genes and different pathways seem to be involved in Cry1Ac resistance depending on the levels of resistance. Although the mechanisms of resistance are very complex in H. armigera, a main pathway seemingly exists, which contributes to resistance in each level of resistant strain. Altogether, the findings in the current study provide a transcriptome-based foundation for identifying the functional genes involved in Cry1Ac resistance in H. armigera.

Polycalin is involved in the toxicity and resistance to Cry1Ac toxin in Helicoverpa armigera (Hübner)

“…The significantly downregulated of APN1 in the Cry1Ac‐resistant Trichoplusia ni lead to the resistance to Cry1Ac (Tiewsiri & Wang, 2011). And we found that the downregulated of ALP and APN generally existed in the Cry1Ac‐resistant strain (Wei et al, 2018b). Here we found the relative expression level of polycalin significantly reduced both at the protein level and the transcript level in the Cry1Ac‐resistant H. armigera strain.…”

“…Cadherin fragment from Anopheles gambiae and Manduca sexta could enhance the insecticidal activity of Cry4Ba and Cry1Ab, respectively (Peng, Xu, Ye, Yu, & Sun, 2010). Overexpression ALP and APN in the insect cells could significantly increase the toxicity of Cry1Ac (Wei, Liang et al, 2016; Wei, Yang et al 2018). Stevens, Song, Bruning, Choo, and Baxter (2017) reported they expressed P. xylostella ABCC2 gene in transgenic Drosophila caused this transgenic Drosophila susceptible to Cry1Ac.…”

“…Meanwhile, RNA interference (RNAi) and CRISPR/Cas9 technology recently were used to reduce the expression of target genes or proteins to confirm the function of the Cry toxins candidate receptor proteins. Reduce the expression of cadherin, ALP, APN, and ABCC2 could significantly reduce insecticidal activity of Cry1Ac by RNAi or CRISPR/Cas9 technology (Wang, Wang, Liu, Liu, & Wu, 2017; Wei, Liang et al, 2016, Wei, Zhang et al 2016; Wei, Yang et al, 2018; Xiao et al, 2016,). Equally important, antisera blocking technology was also employed to study the function of the Cry toxins candidate receptors.…”

“…Cadherin fragment from Anopheles gambiae and Manduca sexta could enhance the insecticidal activity of Cry4Ba and Cry1Ab, respectively (Peng, Xu, Ye, Yu, & Sun, 2010). Overexpression ALP and APN in the insect cells could significantly increase the toxicity of Cry1Ac Wei, Yang et al 2018 (Wang, Wang, Liu, Liu, & Wu, 2017;, Wei, Zhang et al 2016Wei, Yang et al, 2018;Xiao et al, 2016,). Equally important, antisera blocking technology was also employed to study the function of the Cry toxins candidate receptors.…”

Alkaline phosphatase 2 is a functional receptor of Cry1Ac but not Cry2Ab in Helicoverpa zea

“…Likewise, Cry1Ac resistance was found to be tightly linked to downregulation of ALP in Plutella xylostella . The fact that HaALP1 and HaALP2 were upregulated and downregulated (Wei et al, 2018), respectively, in several Cry1Ac-resistant strains of this pest, suggests that HaALP1 is probably a soluble ALP and functions as a Cry1Ac sequesterer, whereas HaALP2 is an m-ALP and acts as a Cry1Ac receptor. In two Cry1Ac-resistant strains of H. zea, resistance probably resulted from a 10-fold increase of a soluble ALP (~68 kDa) secreted into the midgut lumen, which sequestered many of the ingested Cry1Ac toxins in the midgut lumen, thus not only preventing them from binding to the midgut receptors, but also exposing them for additional proteolysis degradation to occur (Caccia et al, 2012).…”