Comprehensive Analysis of Gene Expression Profiles of the Beet Armyworm Spodoptera exigua Larvae Challenged with Bacillus thuringiensis Vip3Aa Toxin

Abstract

Host-pathogen interactions result in complex relationship, many aspects of which are not completely understood. Vip proteins, which are Bacillus thuringensis (Bt) insecticidal toxins produced during the vegetative stage, are selectively effective against specific insect pests. This new group of Bt proteins represents an interesting alternative to the classical Bt Cry toxins because current data suggests that they do not share the same mode of action. We have designed and developed a genome-wide microarray for the beet armyworm Spodoptera exigua, a serious lepidopteran pest of many agricultural crops, and used it to better understand how lepidopteran larvae respond to the treatment with the insecticidal protein Vip3Aa. With this approach, the goal of our study was to evaluate the changes in gene expression levels caused by treatment with sublethal doses of Vip3Aa (causing 99% growth inhibition) at 8 and 24 h after feeding. Results indicated that the toxin provoked a wide transcriptional response, with 19% of the microarray unigenes responding significantly to treatment. The number of up- and down-regulated unigenes was very similar. The number of genes whose expression was regulated at 8 h was similar to the number of genes whose expression was regulated after 24 h of treatment. The up-regulated sequences were enriched for genes involved in innate immune response and in pathogen response such as antimicrobial peptides (AMPs) and repat genes. The down-regulated sequences were mainly unigenes with homology to genes involved in metabolism. Genes related to the mode of action of Bt Cry proteins were found, in general, to be slightly overexpressed. The present study is the first genome-wide analysis of the response of lepidopteran insects to Vip3Aa intoxication. An insight into the molecular mechanisms and components related to Vip intoxication will allow designing of more effective management strategies for pest control.

Bacterial Vegetative Insecticidal Proteins (Vip) from Entomopathogenic Bacteria

“…In an attempt to understand how midgut cells respond to intoxication by Vip3 proteins, gene expression profiles of S. exigua larvae treated with a sublethal dose of Vip3Aa were obtained by using a genome-wide microarray that included Ͼ29,000 unigenes (unique assembled sequences obtained from a transcriptome) (99). No alteration in the expression levels of the two Vip3A-binding proteins described above (S2 and the tenascin of the X-tox type) was found, nor were there alterations in the transcription levels of genes related to the mode of action of the Cry proteins.…”

Intestinal regeneration as an insect resistance mechanism to entomopathogenic bacteria

“…For instance, the response to pathogens (REPAT) genes identified in Spodoptera exigua have been commonly reported to display differential expression in response to Cry [51 • ] or vegetative insecticidal proteins (Vip) [52 • ] from Bt. While the specific role for REPAT genes in gut healing is unknown, their activity as transcription factors, high sequence diversity, and specific expression profiles in response to selected pathogens, suggest that they are involved in multiple defensive processes [53].…”

“…Furthermore, the role of arylphorin in midgut healing is supported by levels increasing after ingestion of bacteria [56], although this increase may be related to a role as inducible effector protein in insect immunity [57]. However, arylphorin transcripts were highly downregulated in larval midguts of L. dispar after ingestion of a Bt pesticide [58], after intoxication of S. exigua larvae with Vip toxin [52 • ] or in Ostrinia nubilalis larvae exposed to Cry1Fa protoxin [59]. While these reports may contradict a role for arylphorin in midgut healing, it is important to consider that the mitogenic effect of arylphorin is highly dependent on concentration.…”

“…While not experimentally tested to date, the observation that selected REPAT and arylphorin genes appear to display similar expression profiles during exposure to Bt toxins [52 • ,67], and that REPAT genes have been proposed as transcriptional regulators [71] may be suggestive of REPAT genes participating in arylphorin expression. Interestingly, silencing expression of the ATP binding cassette (ABCC) transporter gene expression in S. exigua larvae resulted in up-regulation of the same REPAT and arylphorin genes that respond to Bt intoxication and were found constitutively up-regulated in the Xentari-resistant strain [72].…”

Iflavirus increases its infectivity and physical stability in association with baculovirus

“…A 44K Agilent oligonucleotide microarray was designed to study different aspects of the interaction of S. exigua larvae with viral (iflavirus and baculovirus) and bacterial pathogens (Jakubowska, Vogel & Herrero, 2013;Bel et al, 2013). In that sense, the array mainly comprised probes representing unigenes from the S. exigua transcriptome (Pascual et al, 2012;Jakubowska, Vogel & Herrero, 2013), but also included 60-mer non-overlapping tiling probes covering both strands of the SeIV1 and SeMNPV genomes.…”