Identification and Characterization of the Detoxification Genes Based on the Transcriptome of Tomicus yunnanensis

Bark beetle, as a trunk borer, has caused a large number of tree deaths and seriously damaged the mountain forest ecosystem. Bark beetles oxidize the secondary metabolites of plants, degrade them, and excrete them from the body or convert them into components needed by the body. This process is completed by the cooperation of CYPs, GSTs, and CCEs and occurs in different tissues of the insects, including the gut (i.e., the part where beetle pheromone is produced and accumulated) and antennae (i.e., the olfactory organ used to sense defensive monoterpenes and other plant-related compounds and pheromones in the air). In this study, we identified and characterized three gene superfamilies of CYPs, GSTs, and CCEs involved in the detoxification of endobiotics (e.g., hormones and steroids) and xenobiotics (e.g., insecticides, sex pheromones, and plant allelochemicals) through a combination approach of bioinformatics, phylogenetics, and expression profiles. Transcriptome analyses led to the identification of 113 transcripts encoding 51 P450s, 33 GSTs, and 29 CCEs from Tomicus yunnanensis Kirkendall and Faccoli, 2008 (Coleoptera, Scolytinae). The P450s of T. yunnanensis were phylogenetically classified into four clades, representing the majority of the genes in the CYP3 clan. The CCEs from T. yunnanensis were separately grouped into five clades, and the GST superfamily was assigned to five clades. Expression profiles revealed that the detoxification genes were broadly expressed in various tissues as an implication of functional diversities. Our current study has complemented the resources for the detoxification genes in the family Coleoptera and allows for functional experiments to identify candidate molecular targets involved in degrading plants’ secondary metabolites, providing a theoretical basis for insect resistance in mixed forests.

RNAi Targets in Agricultural Pest Insects: Advancements, Knowledge Gaps, and IPM

The unprecedented target-specificity of double-stranded RNA (dsRNA), due to its sequence-specific mode of action, puts dsRNA at the forefront of biosafe insecticide technology. Since 2007, sensitive target genes have been identified in numerous crop pest insects, with an end goal of applying RNA interference (RNAi) in pest management. Key RNAi targets identified include genes involved in (1) feeding and digestion, (2) production of dsRNases, (3) resistance to insecticides and plant allelochemicals, (4) reproductive fitness, and (5) transmission of plant viruses. Despite the advances, there remain critical knowledge gaps in each of these areas. Particular emphasis must be placed on ensuring RNAi's compatibility with integrated pest management (IPM), via further identification of molecular targets that reduce crop damage while sustaining pest (host) populations for highly specialized biocontrol agents, the latter representing a core pillar of IPM.

Why are there not more herbivorous insect species?

Insect species richness is estimated to exceed three million species, of which roughly half is herbivorous. Despite the vast number of species and varied life histories, the proportion of herbivorous species among plant-consuming organisms is lower than it could be due to constraints that impose limits to their diversification. These include ecological factors, such as vague interspecific competition; anatomical and physiological limits, such as neural limits and inability of handling a wide range of plant allelochemicals; phylogenetic constraints, like niche conservatism; and most importantly, a low level of concerted genetic variation necessary to a phyletic conversion. It is suggested that diversification ultimately depends on what we call the intrinsic trend of diversification of the insect genome. In support of the above, we survey the major types of host-specificity, the mechanisms and constraints of host specialization, possible pathways of speciation, and hypotheses concerning insect diversification.

Initiation of compatible wheat-Hessian fly interactions triggers the expression of a novel UDP-glycosyltransferase, MdesUGT1, in virulent Hessian fly larvae

Insect UDP-glycosyltransferases (UGTs) play an important role in detoxification of substrates such as plant allelochemicals, and cuticle formation by the process of glucosidation. Hessian fly (Mayetiola destructor), belonging to the order Diptera (Family: Cecidomyiidae), is a destructive pest of host wheat causing significant economic losses. In the current study, using the assembled genome, we identified thirteen genes in M. destructor that belong to the family of UGTs (MdesUGT). Expression profiling revealed differential expression of MdesUGT genes in Hessian fly feeding instars. Further, we report the molecular cloning of MdesUGT1, designated as UGT301F1, from M. destructor. Characterization of the MdesUGT1 amino acid sequence revealed a conserved signature motif and sugar donor-binding domains characteristic of UGT proteins. Further expression analysis revealed dramatic increase in transcript accumulation of MdesUGT1 in the first and second feeding instars during compatible interactions (susceptible wheat, virulent larvae) but lacked significant upregulation during incompatible wheat Hessian fly interactions. Similar increase in MdesUGT1 transcripts was also observed during interactions of Hessian fly with nonhost, Brachypodium distachyon. These findings suggest the possible early involvement of MdesUGT1 in detoxification of plant toxins, and subsequent role in cuticular formation, thus contributing to the growth and development of this dipteran insect pest. Identification and characterization of insect UGTs could provide valuable insights into the detoxification and growth inhibitory mechanisms and facilitate future plant pest management strategies.

Plant Allelochemicals as Sources of Insecticides

In this review, we describe the role of plant-derived biochemicals that are toxic to insect pests. Biotic stress in plants caused by insect pests is one of the most significant problems, leading to yield losses. Synthetic pesticides still play a significant role in crop protection. However, the environmental side effects and health issues caused by the overuse or inappropriate application of synthetic pesticides forced authorities to ban some problematic ones. Consequently, there is a strong necessity for novel and alternative insect pest control methods. An interesting source of ecological pesticides are biocidal compounds, naturally occurring in plants as allelochemicals (secondary metabolites), helping plants to resist, tolerate or compensate the stress caused by insect pests. The abovementioned bioactive natural products are the first line of defense in plants against insect herbivores. The large group of secondary plant metabolites, including alkaloids, saponins, phenols and terpenes, are the most promising compounds in the management of insect pests. Secondary metabolites offer sustainable pest control, therefore we can conclude that certain plant species provide numerous promising possibilities for discovering novel and ecologically friendly methods for the control of numerous insect pests.

Identification of Wild-Type CYP321A2 and Comparison of Allelochemical-Induced Expression Profiles of CYP321A2 with Its Paralog CYP321A1 in Helicoverpa zea

One possible way to overcome the diversity of toxic plant allelochemicals idiosyncratically distributed among potential host plants is to have more counterdefense genes via gene duplication or fewer gene losses. Cytochrome P450 is the most important gene family responsible for detoxification of the diversity of plant allelochemicals. We have recently reported the identification and cloning of the transposon (HzSINE1)-disrupted non-functional CYP321A2, a duplicated paralog of the xenobiotic-metabolizing P450 CYP321A1 from a laboratory colony of Helicoverpa zea. Here we report the identification of the wild-type intact allele of CYP321A2 from another H. zea colony. This CYP321A2 allele encodes a deduced protein of 498 amino acids and has the P450 signature motifs. Quantitative RT-PCR experiments showed that this CYP321A2 allele was highly expressed in midgut and fat body and achieved the highest expression level in the developmental stage of 5th and 3rd instar larvae. CYP321A2 and CYP321A1 were constitutively expressed in low levels but can be differentially and significantly induced by a range of the plant allelochemicals and plant signal molecules, among which xanthotoxin, flavone, and coumarin were the most prominent inducers of CYP321A2 both in midgut and fat body, whereas flavone, coumarin, and indole-3-carbinol were the prominent inducers of CYP321A1 in midgut and fat body. Moreover, xanthotoxin- and flavone-responsive regulatory elements of CYP321A1 were also detected in the promoter region of CYP321A2. Our results enrich the P450 inventory by identifying an allelochemical broadly induced CYP321A2, a paralog of CYP321A1 in H. zea. Our data also suggest that the CYP321A2/CYP321A1 paralogs are a pair of duplicated genes of multigene families and CYP321A2 could potentially be involved in the detoxification of plant allelochemicals and adaptation of H. zea to its chemical environment.

Phytotoxic Potential and Phenolic Profile of Extracts from Scrophularia striata

A large number of plants produce secondary metabolites known as allelochemicals that are capable of inhibiting the germination of competitive species. This process is known as allelopathy and is mediated by several classes of chemicals, among which phenolic compounds are the most frequent. Thus, plant allelochemicals can be used to control weeds in agricultural systems. In the present work, we analyzed the phenolic profile and phytotoxic potential of different extracts (pure water or water: ethanol 50:50) from Scrophulariastriata plants that were collected from two ecological regions in Iran (Pahleh and Lizan). The total polyphenolic content (TPC), as evaluated by the Folin-Ciocolteau method, ranged from 28.3 mg/g in the aqueous extract obtained from the Lizan ecotype to 39.6 mg/g in the hydroalcoholic extract obtained from the Pahleh ecotype. Moreover, HPLC analysis was aimed at determining the content of eight phenolic compounds, namely eugenol, rosmarinic acid, hesperetin, hesperedin, trans-ferulic acid, vanillin, and caffeic acid. According to the results, rosmarinic acid appeared to be the most abundant component. The phytotoxic activities of S.striata extracts were examined on the seed germination of a crop species, Lepidium sativum, and two weeds, Chenopodium album and Malva sylvestris. All extracts showed inhibitory effects on these species. The efficiency of these inhibitory effects depended on the type of plant species, origin, and concentration of extract. The highest phytotoxic activity was caused by approximately 1% concentration of extract. The most susceptible weed was M. sylvestris. The extracts that were obtained from the Pahleh ecotype, notably the hydroalcoholic ones, showed higher phytotoxicity against L. sativum, C. album and M. sylvestris. These results encourage further studies to support the use of S. striata as a source of bioherbicides.

The indole-alkaloid gramine shapes the bacterial communities thriving at the barley root-soil interface

The biosynthesis of plant allelochemicals underpinning inter-organismal relationships has been moulded by domestication and breeding selection. The indole-alkaloid gramine, whose occurrence in barley (Hordeum vulgare L.) is widespread among wild genotypes but virtually absent from modern varieties, is a paradigmatic example of this phenomenon. This prompted us to investigate how the exogenous application of gramine impacted on the rhizosphere bacterial microbiota of two, gramine-free, elite barley varieties grown in a reference agricultural soil. Our investigation revealed that the application of the indole-alkaloid gramine modulates the proliferation of a subset of soil bacteria with a relatively broad phylogenetic assignment. This effect is two-pronged: a limited, but significant, component of the barley microbiota responds to gramine application in a genotype- and dosage-independent manner while at the highest dosage this secondary metabolite attenuates the host recruitment cues of the barley microbiota. Interestingly, this latter effect displayed a bias for members of the phyla Proteobacteria. These initial observations indicate that gramine can act as a determinant of the bacterial communities inhabiting the root-soil interface.

Chemical composition of essential oils from Pinus caribaea Morelet needles

Plant allelochemicals from essential oils have recently received considerable attention in pharmaceutical, cosmetic and agricultural sectors due to their biodegradability and low toxicity. This study analyzed the composition of essential oils of Pinus caribaea Morelet var. hondurensis needles. Thirty-nine compounds were identified using gas chromatography/mass chromatography and gas chromatography, and the most abundant components were limonene (38.6%), α-pinene (27.6%), borneol (6.7%) and myrcene (3.5%). Aristolene, ledol and guaiol were reported for the first time in P. caribeae needles. Composition of the needles was dominated by monoterpene hydrocarbons (77.2%) followed by oxygenated monoterpenes (12.0%), sesquiterpene hydrocarbons (4.7%) and oxygenated sesquiterpenes (1.7%).