Interaction of plant defense compounds with the insect gut: new insights from genomic and molecular analyses

Resistance of Bt and Non-Bt Soybean Cultivars Adapted to Novel Growing Regions of Brazil to Chrysodeixis includens and Spodoptera frugiperda

Soybean is a highly valuable commodity crop for Brazil's economy. However, it faces significant threats from the attack of a complex of lepidopteran pests, particularly Chrysodeixis includens (Walker) and Spodoptera frugiperda (J. E. Smith). These pests have been managed primarily using transgenic Bt soybeans, but limited knowledge exists about the resistance levels of Bt and non-Bt cultivars adapted to novel soybean-growing areas in Brazil, such as the Minas Gerais state. This study evaluated the resistance levels of Bt and non-Bt soybean cultivars to C. includens and S. frugiperda, and whether the Bt cultivars can differentially affect these pests across larval stages. No-choice bioassays were conducted using Bt (NS6010 IPRO and P97R50 IPRO) and non-Bt soybeans (UFLA 6301 RR, P96R90 RR, and ANsc 80111 RR) at V4-stage in the laboratory with neonate (24 h) and third-instar larvae. Larvae were fed leaf discs in Petri dishes, recording the mortality, leaf consumption, and weight gain after 7 days. There was high mortality of C. includens neonates on the Bt cultivars, but this trend was not observed for older larvae. For S. frugiperda neonates, there was high mortality on the Bt cultivar NS 6010 IPRO and non-Bt cultivar UFLA 6301 RR, but only the former was effective for older larvae. Although the Bt cultivars did not kill the third instars, antinutritional effects were found, such that leaf tissue consumed was not converted to larval weight gain. These findings are important for defining regional strategies of integrated and resistance management of C. includens and S. frugiperda in expanding regions of soybean cultivation in Brazil.

Comparative proteome analysis of Spodoptera litura-infested Zea mays reveals a robust defense strategy targeting insect peritrophic membrane

Herbivorous insects such as Spodoptera litura, pose a constant threat to agricultural crops. The incompetence of contemporary pest management tools and techniques stipulates unravelling of molecular dogma, that drives pest-plant interaction. From our previous observations, we inferred that despite being a voracious polyphagous herbivore, S. litura growth and adaptability is severely hampered on maize foliage diet. In this investigation we explored further and demonstrated the impact of maize diet on the insect gut peritrophic membrane (PM, a crucial membrane involved in compartmentalizing digestive events and absorption of nutrients), its structural analysis using scanning electron microscopy (SEM) revealed damaged and perforated PM. Further, this study delves into the intricate resistance mechanism adapted by Z. mays against S. litura by conducting a comparative proteome analysis. We have detected 345 differentially abundant proteins (DAPs) at p < 0.05 and fold change ≥1. The DAPs were categorized as plant defense, secondary metabolite synthesis, redox homeostasis, cytoskeleton/cell wall biosynthesis, primary metabolism, transport and molecular processes. We remarkably report differential expression of proteolysis- and defense-related proteins that have potential to target insect gut, digestion and absorption of nutrients. Our findings contribute to a deeper understanding of the molecular dynamics governing maize resistance against S. litura. Understanding of such intricate molecular dialogues at these interfaces could provide valuable information on the arms race between plants and herbivores, it may pave the way for innovative pest management strategies.

The comparison of gut gene expression and bacterial community in Diaphorina citri (Hemiptera: Liviidae) adults fed on Murraya exotica and 'Shatangju' mandarin (Citrus reticulate cv. Shatangju)

BACKGROUND

Diaphorina citri Kuwayama is an important citrus pest. It serves as the vector for the transmission of Candidatus Liberibacter asiaticus (CLas), which induced a destructive disease, Huanglongbing, and caused huge economic losses. During the interaction between insects and plants, insects have evolved a series of mechanisms to adapt to various host plants. Murraya exotica and 'Shatangju' mandarin (Citrus reticulate cv. Shatangju) are the Rutaceae species from different genera that have been discovered as suitable hosts for D. citri adults. While the adaptation mechanism of this pest to these two host plants is unclear.

RESULTS

In this study, RNA-seq and 16 S rDNA amplification sequencing were performed on the gut of D. citri adults reared on M. exotica and 'Shatangju' mandarin. RNA-seq results showed that a total of 964 differentially expressed genes were found in different gut groups with two host plant treatments. The impacted genes include those that encode ribosomal proteins, cathepsins, and mitochondrial respiratory chain complexes. According to 16 S rDNA sequencing, the compositions of the gut bacterial communities were altered by different treatments. The α and β diversity analyses confirmed that the host plant changes influenced the gut microbial diversity. The functional classification analysis by Tax4Fun revealed that 27 KEGG pathways, mostly those related to metabolism, including those for nucleotide metabolism, energy metabolism, metabolism of cofactors and vitamins, amino acid metabolism, carbohydrate metabolism, xenbiotics biodegradation and metabolism, lipid metabolism, and biosynthesis of other secondary metabolites, were significantly altered.

CONCLUSION

Our preliminary findings shed light on the connection between D. citri and host plants by showing that host plants alter the gene expression profiles and bacterial community composition of D. citri adults.

Tomato Chemical Defenses Intensify Corn Earworm (Helicoverpa zea) Mortality from Opportunistic Bacterial Pathogens

Insect herbivores face multiple challenges to their ability to grow and reproduce. Plants can produce a series of defenses that disrupt and damage the herbivore digestive system, which are heightened upon injury by insect feeding. Additionally, insects face threats from virulent microorganisms that can incur their own set of potential costs to hosts. Microorganisms that invade through the digestive system may function in concert with defenses generated by plants, creating combined assailments on host insects. In our study, we evaluated how tomato defenses interact with an enteric bacterial isolate, Serratia marcescens, in the corn earworm (Helicoverpa zea). We performed bioassays using different tomato cultivars that were induced by methyl jasmonate and larvae orally inoculated with a S. marcescens isolate. Untreated corn earworm larval mortality was low on constitutive tomato, while larvae inoculated with S. marcescens exhibited > 50% mortality within 5 days. Induction treatments elevated both control mortality (~ 45%) and in combination with S. marcescens (> 95%). Larvae also died faster when encountering induced defenses and Serratia. Using a tomato mutant, foliar polyphenol oxidase activity likely had stronger impacts on S. marcescens-mediated larval mortality. Induction treatments also elevated the number of bacterial colony-forming units in the hemolymph of larvae inoculated with Serratia. Larval mortality by S. marcescens was low (< 10%) on artificial diets. Our results demonstrate that plant chemical defenses enhance larval mortality from an opportunistic gut microbe. We propose that the combined damage from both the plant and microbial agent overwhelm the herbivore to increase mortality rates and expedite host death.

Evaluating the Toxic Effects of Tannic Acid Treatment on Hyphantria cunea Larvae

To increase the development potential of botanical pesticides, it is necessary to expand the toxicology research on plant secondary metabolites. Herein, the Hyphantria cunea larvae were exposed to tannic acid concentrations consistent with those found in larch needles, and, subsequently, the growth and nutrient utilization, oxidative damage, and detoxification abilities in the larval midgut, as well as the changes in the gut microbiome, were analyzed. Our results revealed that tannic acid treatment significantly increased the mortality of H. cunea larvae and inhibited larval growth and food utilization. The contents of malondialdehyde and hydrogen peroxide in the larval midgut were significantly elevated in the treatment group, along with a significant decrease in the activities of antioxidant enzymes and detoxifying enzymes. However, the non-enzymatic antioxidants showed a significant increase in the tannic acid-treated larvae. From gut microbiome analysis in the treatment group, the abundance of gut microbiota related to toxin degradation and nutrient metabolism was significantly reduced, and the enrichment analysis also suggested that all pathways related to nutritional and detoxification metabolism were substantially inhibited. Taken together, tannic acid exerts toxic effects on H. cunea larvae at multiple levels and is a potential botanical pesticide for the control of H. cunea larvae.

Functional identification and characterization of GST genes in the Asian gypsy moth in response to poplar secondary metabolites

The Asian gypsy moth, Lymantria dispar, as one of the most important forest pests in the world, can feed on more than 500 species of host plants, causing serious damage to the forests. Poplar is one of the favorite host plants of L. dispar. The present study aimed to explore the effects of poplar secondary metabolites on the growth and detoxification function of L. dispar larvae. We also aimed to study the expression of glutathione S-transferase (GST) genes in different developmental stages and in response to treatment with secondary metabolites. Six kinds of main secondary metabolites and three groups of characteristic mixed secondary metabolites were selected as follows: Caffeic acid, salicin, rutin, quercetin, catechol, flavone, mixture 1 (salicin and flavone), mixture 2 (salicin, caffeic acid and catechol), and mixture 3 (flavone, caffeic acid and catechol) according to the content changes of secondary metabolites in poplar. The thirteen GST genes were selected as candidate genes to study the expression of GST genes in different developmental stages and after treatment with secondary metabolites using quantitative real-time reverse transcription PCR. The LdGSTe4 and LdGSTo1 genes could be induced by secondary metabolites and were screened to explore their detoxification function against secondary metabolites using RNA interference technology. The results showed that salicin and rutin significantly induced the expression of LdGSTe4 and LdGSTo1. Under the stress of secondary metabolites, LdGSTe4 silencing affected the adaptability of L. dispar larvae to salicin and rutin. LdGSTe4 silencing resulted in a significant decrease in the body weight of L. dispar, but had little effect on the relative growth rate, relative consumption rate, efficiency of conversion of ingested food, efficiency of conversion of digested food, and approximate digestibility, as well as the survival rate and development time. These results provide a deeper understanding of the adaptive mechanism of L. dispar to host plants, form the foundation for the further research into the host resistance mechanism, and identify target genes for breeding resistant transgenic poplar.

Down-regulation of lysosomal protein ABCB6 increases gossypol susceptibility in Helicoverpa armigera

Cotton bollworm (Helicoverpa armigera) is the major insect herbivore of cotton plants. As its larvae feed and grow on cotton, H. armigera can likely tolerate gossypol, the main defense metabolite produced by cotton plants, through detoxification and sequestration mechanisms. Recent reports have shown that various P450 monooxygenases and UDP-glycosyltransferases in H. armigera are involved in gossypol detoxification, while the roles of ABC transporters, another gene family widely associated with metabolite detoxification, remain to be elucidated. Here, we show that ingestion of gossypol-infused artificial diet and cotton leaves significantly induced the expression of HaABCB6 in H. armigera larvae. Knockdown and knockout of HaABCB6 increased sensitivity of H. armigera larvae to gossypol. Moreover, HaABCB6-GFP fusion protein was localized on lysosomes in Hi5 cells and its overexpression significantly enhanced gossypol tolerance in vitro. These experimental results strongly support that HaABCB6 plays an important role in gossypol detoxification by H. armigera.

Effects of a Reserve Protein on Spodoptera frugiperda Development: A Biochemical and Molecular Approach to the Entomotoxic Mechanism

Talisin is a storage protein from Talisia esculenta seeds that presents lectin-like and peptidase inhibitor properties. These characteristics suggest that talisin plays a role in the plant defense process, making it a multifunctional protein. This work aimed to investigate the effects of chronic intake of talisin on fifth instar larvae of Spodoptera frugiperda, considered the main insect pest of maize and the cause of substantial economic losses in several other crops. The chronic intake of talisin presented antinutritional effects on the larvae, reducing their weight and prolonging the total development time of the insects. In addition, talisin-fed larvae also showed a significant reduction in the activity of trypsin-like enzymes. Midgut histology analysis of talisin-fed larvae showed alterations in the intestinal epithelium and rupture of the peritrophic membrane, possibly causing an increase of aminopeptidase activity in the midgut lumen. Talisin also proved to be resistant to degradation by the digestive enzymes of S. frugiperda. The transcription profile of trypsin, chymotrypsin and aminopeptidase genes was also analyzed through qPCR technique. Talisin intake resulted in differential expression of at least two genes from each of these classes of enzymes. Molecular docking studies indicated a higher affinity of talisin for the less expressed enzymes.

Detoxification of Plant Aromatic Abietanoids via Cleavage of the Benzene Ring into 11,12-Seco-diterpene Polyenes by a Specialist Insect of Leucosceptrum canum

Leaves of Leucosceptrum canum harbor abundant toxic aromatic abietanoids, and they are rarely attacked by insect herbivores, except for the larvae of Nacna malachitis. The excrements of the insect that fed on L. canum leaves were investigated, leading to the isolation and identification of two unprecedented 11,12-seco-abietane diterpene polyenes: nacnabietanins A (1) and B (2). This discovery heralds a unique detoxification mechanism of plant aromatic abietanoids by insects through enzymatic cleavage of stable benzene rings into more easily degraded polyenes.

Adaptive regulation of detoxification enzymes in Helicoverpa armigera to different host plants

Cotton plants produce gossypol as a major secondary metabolite to resist insect herbivores and pathogens. Helicoverpa armigera may employ multigene families of detoxification enzymes to deal with this metabolite. So far, the strength of the transcriptional response to gossypol detoxification in the cotton bollworms remains poorly understood. Here, we investigated the genomewide transcriptional changes that occur in cotton bollworm larvae after one generation feeding on various host plants (cotton, corn, soybean and chili) or an artificial diet. Six genes potentially involved in detoxification of xenobiotics were highly upregulated in bollworms fed on cotton, and the expression of five of these differed significantly in insects that fed on gossypol diet compared with the artificial diet. When these six genes were downregulated using RNA interference, downregulation only of CYP4L11, CYP6AB9 and CCE001b led to reduced growth of bollworm larvae feeding on gossypol diets. These data suggest that the three genes are involved in response of H. armigera to gossypol of cotton. Our results proved that H. armigera may have a broad mechanism for gossypol detoxification.

Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites

Tomato plants are attacked by diverse herbivorous arthropods, including by cell-content-feeding mites, such as the extreme generalist Tetranychus urticae and specialists like Tetranychus evansi and Aculops lycopersici. Mite feeding induces plant defense responses that reduce mite performance. However, T. evansi and A. lycopersici suppress plant defenses via poorly understood mechanisms and, consequently, maintain a high performance on tomato. On a shared host, T. urticae can be facilitated by either of the specialist mites, likely due to the suppression of plant defenses. To better understand defense suppression and indirect plant-mediated interactions between herbivorous mites, we used gene-expression microarrays to analyze the transcriptomic changes in tomato after attack by either a single mite species (T. urticae, T. evansi, A. lycopersici) or two species simultaneously (T. urticae plus T. evansi or T. urticae plus A. lycopersici). Additionally, we assessed mite-induced changes in defense-associated phytohormones using LC-MS/MS. Compared to non-infested controls, jasmonates (JAs) and salicylate (SA) accumulated to higher amounts upon all mite-infestation treatments, but the response was attenuated after single infestations with defense-suppressors. Strikingly, whereas 8 to 10% of tomato genes were differentially expressed upon single infestations with T. urticae or A. lycopersici, respectively, only 0.1% was altered in T. evansi-infested plants. Transcriptome analysis of dual-infested leaves revealed that A. lycopersici primarily suppressed T. urticae-induced JA defenses, while T. evansi dampened T. urticae-triggered host responses on a transcriptome-wide scale. The latter suggests that T. evansi not solely down-regulates plant gene expression, but rather directs it back towards housekeeping levels. Our results provide valuable new insights into the mechanisms underlying host defense suppression and the plant-mediated facilitation of competing herbivores.

Mechanisms of aphid adaptation to host plant resistance

Host-plant resistant (HPR) crops can play a major role in preventing insect damage, but their durability is limited due to insect adaptation. Research in basal plant resistance provides a framework to investigate adaptation against HPR. Resistance and adaptation are predicted to follow the gene-for-gene and zigzag models of plant defense. These models also highlight the importance of insect effectors, which are small molecules that modulate host plant defense signaling. We highlight research in insect adaptation to plant resistance, and then draw parallels to virulence adaptation. We focus on virulent biotype evolution within the Aphididae, since this group has the highest number of described virulent biotypes. Understanding how virulence occurs will lead to more durable insect management strategies and enhance food production and security.

Comparative transcriptome analysis reveals host-associated differentiation in Chilo suppressalis (Lepidoptera: Crambidae)

The striped stem borer, Chilo suppressalis Walker (Lepidoptera: Crambidae), is one of the most serious rice pests. Besides attacking rice, it also feeds on an economically important vegetable crop, water-oat Zizania latifolia. The species feeding on water-oat has higher growth and survival rate than those on rice, suggesting their success in adaptation to the new host plant. However, little is known about the molecular mechanisms of host plant adaptation. Here we investigated the midgut transcriptome responses of C. suppressalis larvae reared on rice and water-oat. A total of 1,633 differentially expressed genes were identified, with a greater number up-regulated on the more delicious new host. The up-regulation of most digestive and detoxification-related genes may be the result of adaptation to the changes in nutritional requirements and toxic chemicals during host shift. In contrast, down-regulation of ribosomal genes may be related to their better development performance when feeding on the new host. In conclusion, our results suggest that transcriptional regulation of genes related to digestion, detoxification and ribosome may play an important role in adaptation of C. suppressalis to a new host plant.