The improved resistance against gypsy moth in Larix olgensis seedlings exposed to Cd stress association with elemental and chemical defenses

Changes in the Biology and Susceptibility of Weevil (Cylas formicarius) to the Insecticide Spinetoram as a Response to Cadmium Contamination

The sweet potato weevil Cylas formicarius is a notorious underground pest in sweet potato (Ipomoea batatas L.). However, little is known about the effects of cadmium (Cd) stress on weevil biology and resistance to pesticides and biotic agents. Therefore, we fed sweet potato weevils with Cd-contaminated sweet potato and assessed adult food intake and survival and larval developmental duration and mortality rates, as well as resistance to the insecticide spinetoram and susceptibility to the entomopathogenic fungus Beauveria bassiana. With increasing Cd concentration, the number of adult weevil feeding holes, adult survival and life span, and larval developmental duration decreased significantly, whereas larval mortality rates increased significantly. However, at the lowest Cd concentration (30 mg/L), adult feeding was stimulated. Resistance of adult sweet potato weevils to spinetoram increased at low Cd concentration, whereas Cd contamination did not affect sensitivity to B. bassiana. Thus, Cd contamination affected sweet potato weevil biology and resistance, and further studies will investigate weevil Cd accumulation and detoxification mechanisms.

Advances in the Involvement of Metals and Metalloids in Plant Defense Response to External Stress

Plants, as sessile organisms, uptake nutrients from the soil. Throughout their whole life cycle, they confront various external biotic and abiotic threats, encompassing harmful element toxicity, pathogen infection, and herbivore attack, posing risks to plant growth and production. Plants have evolved multifaceted mechanisms to cope with exogenous stress. The element defense hypothesis (EDH) theory elucidates that plants employ elements within their tissues to withstand various natural enemies. Notably, essential and non-essential trace metals and metalloids have been identified as active participants in plant defense mechanisms, especially in nanoparticle form. In this review, we compiled and synthetized recent advancements and robust evidence regarding the involvement of trace metals and metalloids in plant element defense against external stresses that include biotic stressors (such as drought, salinity, and heavy metal toxicity) and abiotic environmental stressors (such as pathogen invasion and herbivore attack). We discuss the mechanisms underlying the metals and metalloids involved in plant defense enhancement from physiological, biochemical, and molecular perspectives. By consolidating this information, this review enhances our understanding of how metals and metalloids contribute to plant element defense. Drawing on the current advances in plant elemental defense, we propose an application prospect of metals and metalloids in agricultural products to solve current issues, including soil pollution and production, for the sustainable development of agriculture. Although the studies focused on plant elemental defense have advanced, the precise mechanism under the plant defense response still needs further investigation.

Defense response of Fraxinus mandshurica seedlings to Hyphantria cunea larvae under Cd stress: A contradiction between attraction and resistance

Heavy metal pollution, as a common and serious environmental problem worldwide, has been regarded as an abiotic stimulus that can affect plant insect resistance and pest occurrence. This study evaluated the defense response of Fraxinus mandshurica seedlings to Hyphantria cunea larvae under Cd stress, with consideration given to chemical defense, physical defense, and elemental defense. Our results showed that the H. cunea larvae had a strong preference for Cd-treated F. mandshurica seedlings, but there was a significant reduction in body weight and survival rate in larvae that fed on leaves of Cd-treated seedlings. Under Cd treatment, the increase in attractant metabolites (e.g., styrene, dibutyl phthalate, and d-limonene) and the decrease in repellent metabolites (e.g., aromadendrene, heptadecane, and camphene) in leaf volatiles were responsible for the high attractant activity to H. cunea larvae. Based on leaf physicochemical properties, tissue structure, and phenolic acid content, an overall reduction in physical defense, chemical defense and their combination in F. mandshurica seedlings exposed to Cd stress was identified by Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) analysis. Elemental defense occurred in Cd-treated F. mandshurica seedlings, as evidenced by the high concentration of Cd in leaves and H. cunea larvae under Cd treatment. Taken together, these findings demonstrate that under Cd stress, elemental defense replaces the dominant role of basic defense in F. mandshurica seedlings and accounts for the enhanced ability to defend against H. cunea larvae.

Cadmium contributes to heat tolerance of a hyperaccumulator plant species Sedum alfredii

Hyperaccumulators are plant species that tolerate and accumulate very high concentrations of toxic metals, including Cd. Hyperaccumulation of heavy metals is reported to benefit plant biotic resistance; however, no prior study has examined the possible role of toxic metals on abiotic stress resistance in hyperaccumulators. A preliminary experiment found that Cd significantly improved plant growth of a hyperaccumulator, Sedum alfredii Hance, under heat stress. This study investigated the possible role of Cd in S. alfredii's heat resistance, using infrared thermography, transmission electron microscopy (TEM), real-time quantitative polymerase chain reaction (RTqPCR), and high-throughput sequencing. The results showed that high temperatures irreversibly damaged stomatal function, chloroplast structure, photosynthesis in S. alfredii, and lowered survival rates to 25%. However, Cd application significantly decreased the leaf temperature of S. alfredii and increased the survival rate to 75%. Cd penetrated the guard cells, restored stomatal function, and mitigated excessive water loss from S. alfredii under heat stress. Moreover, it activated antioxidant enzymes, promoted phytohormone biosynthesis, and upregulated a series of unigenes, thereby augmenting heat resistance in S. alfredii. These results indicate that Cd effectively improved thermotolerance in S. alfredii by regulating stomatal movement and antioxidant systems via upregulation of phytohormones and heat shock proteins.

Cd exposure-triggered susceptibility to Bacillus thuringiensis in Lymantria dispar involves in gut microbiota dysbiosis and hemolymph metabolic disorder

The immunotoxicity induced by heavy metals on herbivorous insects reflect the alterations of the susceptibility to entomopathogenic agents in herbivorous insects exposed to heavy metal. In the present study, the susceptibility of gypsy moth larvae to Bacillus thuringiensis under Cd treatment at low and high dosages was investigated, and the gut microbiome-hemolymph metabolome responses that affected larval disease susceptibility caused by Cd exposure were examined. Our results showed that mortality of gypsy moth larvae caused by B. thuringiensis was significantly higher in larvae pre-exposed to Cd stress, and there was a synergistic effect between Cd pre-exposure and bacterial infection. Exposure to Cd significantly decreased the abundance of several probiotics (e.g., Serratia for the low Cd dosage and Weissella, Aeroonas, and Serratia for the high Cd dosage) and increased the abundances of several pathogenic bacteria (e.g., Stenotrophomonas, Gardnerella, and Cutibacterium for the low Cd dosage and Pluralibacter and Tsukamurella for the high Cd dosage) compared to the controls. Moreover, metabolomics analysis indicated that amino acid biosynthesis and metabolism were significantly perturbed in larval hemolymph under Cd exposure at both the low and high dosages. Correlation analysis demonstrated that several altered metabolites in larval hemolymph were significantly correlated with changes in the gut microbial community. The results demonstrate that prior exposure to Cd increases the susceptibility of gypsy moth larvae to B. thuringiensis in a synergistic fashion due to gut microbiota dysbiosis and hemolymph metabolic disorder, and thus microbial-based biological control may be the best pest control strategy in heavy metal-polluted areas.

The immunotoxicity of Cd exposure to gypsy moth larvae: An integrated analysis of cellular immunity and humoral immunity

Heavy metal exposure-triggered growth retardation and physiology disorder in phytophagous insects have been widely understood, but only a few studies have investigated its immunomodulatory effects on herbivorous insects. Here, the innate immunity of gypsy moth (Lymantria dispar) larvae under Cd stress was evaluated by integrating cellular and humoral immunity, and the immunomodulation mechanism of Cd stress was further understood by the proteomics analysis of larval hemolymph. Our results showed that the total hemocyte count, as well as phagocytic, encapsulation and bacteriostatic activity, of hemolymph in gypsy moth larvae exposed to Cd stress was significantly lower than that in un-treated larvae. Further proteomic analysis revealed that Cd exposure may reduce the total hemocyte count in larval hemolymph by inducing endoplasmic reticulum pathway-mediated hemocyte apoptosis, thereby causing the collapse of cellular immunity in gypsy moth larvae. In addition, the transcriptional level of signal transduction genes (IMD, Toll, Relish, JAK and STAT) and antimicrobial peptide genes (cecropin and lebocin), as well as the protein abundance of pattern recognition receptors (PGRP and GNBP3) in the Toll, IMD and JAK/STAT signaling pathways was significantly decreased in Cd-treated larvae, clearly implying an immunosuppresive effect of Cd stress on pathogen recognition, signal transduction and effector synthesis of humoral immunity in gypsy moth larvae. Taken together, these results suggest that Cd exposure decreases both cellular immunity and humoral immunity of gypsy moth larvae, and provides a new entry point for systematically and comprehensively unraveling the heavy metal pollutants-caused immunotoxicity.

Cd exposure-triggered metabolic disruption increases the susceptibility of Lymantria dispar (Lepidoptera: Erebidae) larvae to Mamestra brassicae nuclear polyhedrosis virus: A multi-omics study

Biological control is an environmentally friendly and effective pest control strategy, but it is often affected by a variety of abiotic factors in the pest control area. Here, the susceptibility of gypsy moth larvae to Mamestra brassicae nuclear polyhedrosis virus (MbNPV) under Cd treatment at the low and high dosages was investigated, and the mechanism of Cd stress affecting virus susceptibility of gypsy moth larvae was analyzed from a metabolic perspective by combining transcriptome and metabolome of the larval fat body. Our results showed that the mortality of MBNPV infection on gypsy moth larvae pre-exposed to Cd was significantly higher than that of larvae without Cd pre-exposure, and the joint effects of Cd exposure and virus infection on larval mortality were demonstrated to be synergistic. Transcriptome analysis revealed that amino acid and carbohydrate transport and metabolism accounted for most of the differently expressed genes in the low Cd and high Cd treatment groups. Consistent with the transcriptome results, metabolome analysis also showed that most metabolites affected by Cd exposure were involved in amino acid and carbohydrate metabolism. Function analysis showed that the contents of several amino acids (e.g. tryptophan and tyrosine) with antioxidant properties were significantly increased in Cd-treated gypsy moth larvae. Taken together, Cd exposure as an environmental factor, promotes the susceptibility of gypsy moth larvae to MbNPV, and metabolic disruption, especially amino acids and carbohydrates-related metabolism, is responsible for the increased susceptibility of gypsy moth larvae to virus under Cd stress.

Transcriptomic analysis of interactions between Lymantria dispar larvae and carvacrol

Due to its biological activity, carvacrol (CAR) is widely used in medicine, agriculture, and forestry. Our previous studies showed that in Lymantria dispar larvae, CAR treatment can induce the production of antifeedants and lead to growth inhibition and death of larvae. However, the effect CAR exerts on RNA levels in L. dispar larvae remains unclear. In this study, the Illumina HiSeq4000 sequencing platform was used to sequence the total RNA of L. dispar larvae. A total of six cDNA libraries (three treatments and three controls) were established and 39,807 genes were generated. Compared with the control group, 296 differentially expressed genes (DEGs) (142 up-regulated and 154 down-regulated) were identified after CAR treatment. GO and KEGG enrichment analyses showed that these DEGs mainly clustered in the metabolism of xenobiotics, carbohydrates, and lipids. Furthermore, 12 DEGs were found to be involved in detoxification, including six cytochrome P450s, two esterases, one glutathione peroxidase, one UDP-glycosyltransferase gene, and two genes encoding heat shock proteins. The expression levels of detoxification genes changed under CAR treatment (especially P450s), which further yielded candidate genes for explorations of the insecticidal mechanism of CAR. The reliability of transcriptome data was verified by qRT-PCR. The enzyme activities of CYP450 and acid phosphatase significantly increased (by 38.52 U/mg·prot and 0.12 μmol/min·mg, respectively) 72 h after CAR treatment. However, the activity of alkaline phosphatase did not change significantly. These changes in enzyme activity corroborated the reliability of the transcriptome data at the protein level. The results of GO enrichment analysis of DEGs indicated that CAR influenced the oxidation-reduction process in L. dispar larvae. Furthermore, CAR can cause oxidative stress in L. dispar larvae, identified through the determination of peroxidase and polyphenol oxidase activities, total antioxidant capacity, and hydrogen peroxide content. This study provides useful insight into the insecticidal mechanism of CAR.

The susceptibility of Lymantria dispar larvae to Beauveria bassiana under Cd stress: A multi-omics study

Insect susceptibility to entomopathogenic microorganisms under heavy metal stress, as well as its regulatory mechanism is still poorly understood. This study aims to investigate the susceptibility of gypsy moth larvae to Beauveria bassiana under cadmium (Cd) stress (at 3.248 or 44.473 mg Cd/kg fresh food), and reveal the potential molecular mechanisms underlying the Cd effect on the larval susceptibility to B. bassiana via combined transcriptome and proteome analyses. Our results showed that pre-exposure to Cd increased the susceptibility of gypsy moth larvae to B. bassiana, and there was an additive effect between Cd exposure and B. bassiana infection on the larval mortality. Under the Cd stress at low and high concentrations, 138 and 899 differentially expressed genes (DEGs), as well as 514 and 840 differentially expressed proteins (DEPs) were identified, respectively. Immunotoxic effects induced by Cd exposure at the transcription level increased in a negative dose-response manner, with no immunity-related DEGs obtained at the low Cd concentration and a high number of immunity-related DEGs down-regulated at the high Cd concentration. In contrast, a potentially suppressed or stimulated trend in the Toll and Imd signaling pathway at protein level was revealed under low or high concentration of Cd treatment. Analysis of xenobiotics biodegradation-related pathways at both transcription and translation levels revealed that the gypsy moth larvae possessed an efficient homeostasis regulatory mechanism to the low-level Cd exposure, but exhibited a reduced xenobiotics biodegradation capability to the Cd stress at high levels. Together, these findings demonstrate Cd contamination promote the microbial-based biocontrol efficacy, and unravel the molecular regulatory network of heavy metal exposures that affects susceptibility of insects to pathogenic diseases.

Transfer of heavy metal along food chain: a mini-review on insect susceptibility to entomopathogenic microorganisms under heavy metal stress

The development and physiological status of pest insects are important factors that affect the effectiveness of biological control. Current knowledge reveals that heavy metals can be transferred to phytophagous insects through food chains and cause various chronic toxicological effects on the growth and physiology of phytophagous insects. These findings potentially attribute heavy metal contamination to an environmental factor governing biocontrol efficiency against pest insects, pointing to an urgent demand to better understand the effects of heavy metal exposure on insect susceptibility to entomopathogenic microorganisms. Here we discuss the transfer characteristics of heavy metals along the food chains to phytophagous insects and conclude that heavy metal exposure may promote insect susceptibility to entomopathogenic microorganisms in the heavy metal-contaminated regions. Furthermore, we propose a 'combined effect' hypothesis that combination of entomopathogenic agent and heavy metal stress can cause a much higher overall insect mortality than does the entomopathogenic agent or the heavy metal stress alone. This is a new and relatively unexplored area in the microbial-based biocontrol research, which might have great potential for future optimization of biocontrol strategies against economically and ecologically important agricultural or forest pests in the heavy metal polluted areas. © 2020 Society of Chemical Industry.

Cd exposure-induced growth retardation involves in energy metabolism disorder of midgut tissues in the gypsy moth larvae

Cadmium, a common environmental contaminant in both terrestrial and aquatic ecosystems, presented a serious hazard to growth and development of phytophagous insects. For better understanding the toxicology of Cd exposure on phytophagous insects, the physiological and molecular mechanisms underlying the energy metabolism disorder in midgut tissue of gypsy moth larvae fed on Cd-amended artificial diets (3.248 or 44.473 mg Cd/kg fresh food) were investigated. Our results showed that compared with control, Cd exposure at both two levels triggered detriment effects on growth indexes, and with the increase of exposure concentrations, the adverse effects were significantly exacerbated. Larval growth and nutritional indexes (except approximate digestibility) showed a strong positive correlation, indicating that growth retardation in the gypsy moth larvae under Cd stress was tightly related to the food utilization. The key genes at mRNA level in glycolysis/gluconeogenesis, citrate cycle pathway and starch/sucrose metabolism pathway also presented a significant and positive correlation with growth indexes, once again demonstrating that energy metabolism was the key factor that controls the growth and development of the gypsy moth larvae under Cd stress. Antioxidant system collapse and oxidative damage, a chief cause of histopathological alterations in midgut tissue, consist of the physiological basis of energy metabolism disorder in Cd-treated gypsy moth larvae. Together, these results suggest that histopathological alterations or oxidative damage of tissue structure significant disturbed physiological functions of midgut tissue in gypsy moth larvae exposed to Cd stress, as reflected via food utilization or energy metabolism disorder, and eventually resulted in larval growth retardation.