Discovery and quantitative structure-activity relationship study of lepidopteran HMG-CoA reductase inhibitors as selective insecticides

Molecular targets of insecticides and herbicides - Are there useful overlaps?

New insecticide modes of action are needed for insecticide resistance management strategies. The number of molecular targets of commercial herbicides and insecticides are fewer than 35 for both. Few commercial insecticide targets are found in plants, but ten targets of commercial herbicides are found in insects. For several of these commonly held targets, some compounds kill both plants and insects. For example, herbicidal inhibitors of p-hydroxyphenylpyruvate dioxygenase are effective insecticides on blood-fed insects. The glutamine synthetase-inhibiting herbicide glufosinate is insecticidal by the same mechanism of action, inhibition of glutamine synthetase. These and other examples of shared activities of commercial herbicides with insecticides through the same target site are discussed. Compounds with novel herbicide targets shared by insects that are not commercialized as pesticides (such as statins) are also discussed. Compounds that are both herbicidal and insecticidal can be used for insect pests not associated with crops or with crops made resistant to the compounds.

A fungal tolerance trait and selective inhibitors proffer HMG-CoA reductase as a herbicide mode-of-action

Decades of intense herbicide use has led to resistance in weeds. Without innovative weed management practices and new herbicidal modes of action, the unabated rise of herbicide resistance will undoubtedly place further stress upon food security. HMGR (3-hydroxy-3-methylglutaryl-coenzyme A reductase) is the rate limiting enzyme of the eukaryotic mevalonate pathway successfully targeted by statins to treat hypercholesterolemia in humans. As HMGR inhibitors have been shown to be herbicidal, HMGR could represent a mode of action target for the development of herbicides. Here, we present the crystal structure of a HMGR from Arabidopsis thaliana (AtHMG1) which exhibits a wider active site than previously determined structures from different species. This plant conserved feature enables the rational design of specific HMGR inhibitors and we develop a tolerance trait through sequence analysis of fungal gene clusters. These results suggest HMGR to be a viable herbicide target modifiable to provide a tolerance trait.

Managing herbicide resistance problem needs the identification of new herbicidal modes of action. Here, the authors solve the crystal structures of Arabidopsis HMGR and show HMGR as a potential new herbicide target by identifying plant-specific HMGR inhibitors and engineering tolerant trait in Arabidopsis.

Synthesis, fungicidal activity, and 3D-QSAR of tetrazole derivatives containing phenyloxadiazole moieties

In an effort to discover new agents with good fungicidal activities against CDM (cucumber downy mildew), a series of tetrazole derivatives containing phenyloxadiazole moieties were designed and synthesized. The EC₅₀ values for fungicidal activities against CDM were determined. Bioassay results indicated that most synthesized compounds exhibited potential in vivo fungicidal activity against CDM. A CoMFA (comparative molecular field analysis) model based on the bioactivity was developed to identify some primary structural quality for the efficiency. The values of q² and r² for the established model were 0.791 and 0.982 respectively, which reliability and predict abilities were verified. Three analogues (q3, q4, q5) were designed and synthesized based on the model. All these compounds exhibited significant fungicidal activity on CDM with the EC₅₀ of 1.43, 1.52, 1.77 mg·L^-1. This work could provide a useful instruction for the further structure optimization.

Novel sulfone derivatives containing a 1,3,4-oxadiazole moiety: design and synthesis based on the 3D-QSAR model as potential antibacterial agent

BACKGROUND

The rice bacterial leaf blight (BLB) is one of the most serious bacterial diseases caused by Xanthomonas oryzae pv. oryzae (Xoo), which can cause yield loss of rice up to 50%. The three-dimensional quantitative structure-activity relationship (3D-QSAR) is an important auxiliary method to find potential high-efficient pesticides active structures.

RESULTS

A series of novel 1,3,4-oxadiazole compounds were designed and synthesized based on the 3D-QSAR model, and their antibacterial activities in vitro against Xoo were evaluated. The results indicated that all the target compounds showed excellent in vitro antibacterial activities. For example, the compounds 6, 12, 13, 20, 21, and 23 exhibited excellent antibacterial activities against Xoo, with half-maximal effective concentration (EC₅₀ ) values of 0.24, 0.31, 0.36, 0.29, 0.19, and 0.31 mg/L, respectively, which were superior to the antibacterial agents thiodiazole copper (127.44 mg/L) and bismerthiazol (91.08 mg/L). Meanwhile, compound 21 showed good antibacterial activity in vivo against BLB, with curative and protective activities of 46.7% and 56.4%, respectively, which were superior to thiodiazole copper (28.5% and 32.5%) and bismerthiazol (37.6% and 38.4%). Compound 21 can significantly reduce the extracellular polysaccharides production of Xoo, increase the permeability of the cell membranes, and also can cause cell surface wrinkles, deformation and dryness.

CONCLUSION

The 3D-QSAR model can be used to find sulfone compounds containing a 1,3,4-oxadiazole moiety with higher antibacterial activity, and compound 21 can be used as a potential antibacterial agent in the future. © 2020 Society of Chemical Industry.

Enzymes in the juvenile hormone biosynthetic pathway can be potential targets for pest control

BACKGROUND

Discovery of novel insecticides and targets has received global attention in recent years. Ten genes coding for enzymes involved in the juvenile hormone biosynthetic pathway of Manduca sexta were studied as potential insecticide targets.

RESULTS

We determined the expression of genes encoding some critical enzymes in the JH biosynthetic pathway. Farnesol dehydrogenase (FOLD), Juvenile hormone acid O-methyltransferase (JHAMT) and Juvenile hormone epoxidase (CYP15C1) were selected as the candidate targets based on gene expression results. RNAi silencing and enzyme inhibitor tests were performed to validate whether these candidate genes could be the potential insecticide targets. The down-regulation of FOLD, JHAMT and CYP15C1 resulted in a 68%, 82% and 79% reduction in the rates of JH biosynthesis in vitro, respectively. In addition, RNA interference and inhibitor studies of these enzymes following oral administration demonstrated the potential application in pest management, with respect to high mortality and effects on growth.

CONCLUSION

Based on our study, FOLD, JHAMT and CYP15C1 could be potential targets for pest control as a consequence of their important roles in insect development. © 2019 Society of Chemical Industry.

Synthesis, stability and insecticidal activity of 2-arylstilbenes

BACKGROUND

A chemical scaffold-hopping approach from known 3-hydroxyl-3-methylglutaryl-CoA (HMG-CoA) reductase inhibitors identified (E/Z)-2-arylstilbenes as novel insecticidal hits against two lepidopteran species, Spodoptera exigua and Trichoplusia ni. A structure-activity relationship (SAR) study of the aryl substituents and the E/Z conformations was carried out in an effort to improve insecticidal potency.

RESULTS

A series of (E/Z)-2-arylstilbenes was prepared and separated to evaluate their insecticidal potency against lepidopterous species in diet-feeding assays. The results showed that the (Z)-2-arylstilbenes were more active than their corresponding (E)-isomers, and a stereoselective synthesis was utilized to expand the SAR of the (Z)-2-arylstilbenes. (Z)-4'-Fluoro-3'-methyl-2-(2,4-difluorostyryl)-4-fluoro-5-methoxy-1,1'-biphenyl was the most potent analog in this study with strong activity against S. exigua, T. ni, Helicoverpa zea, Plutella xylostella and Pseudoplusia includens.

CONCLUSION

The (Z)-2-arylstilbenes were found to have strong insecticidal potency against five lepidopteran species. Ultimately, synthetic efforts could not improve insecticidal potency to commercial levels, and a lack of UV stability led to efforts being discontinued. © 2019 Society of Chemical Industry.

MicroRNAs regulate the sesquiterpenoid hormonal pathway in Drosophila and other arthropods

Arthropods comprise the majority of all described animal species, and understanding their evolution is a central question in biology. Their developmental processes are under the precise control of distinct hormonal regulators, including the sesquiterpenoids juvenile hormone (JH) and methyl farnesoate. The control of the synthesis and mode of action of these hormones played important roles in the evolution of arthropods and their adaptation to diverse habitats. However, the precise roles of non-coding RNAs, such as microRNAs (miRNAs), controlling arthropod hormonal pathways are unknown. Here, we investigated the miRNA regulation of the expression of the juvenile hormone acid methyltransferase gene (JHAMT), which encodes a rate-determining sesquiterpenoid biosynthetic enzyme. Loss of function of the miRNA bantam in the fly Drosophila melanogaster increased JHAMT expression, while overexpression of the bantam repressed JHAMT expression and resulted in pupal lethality. The male genital organs of the pupae were malformed, and exogenous sesquiterpenoid application partially rescued the genital deformities. The role of the bantam in the regulation of sesquiterpenoid biosynthesis was validated by transcriptomic, qPCR and hormone titre (JHB3 and JH III) analyses. In addition, we found a conserved set of miRNAs that interacted with JHAMT, and the sesquiterpenoid receptor methoprene-tolerant (Met) in different arthropod lineages, including insects (fly, mosquito and beetle), crustaceans (water flea and shrimp), myriapod (centipede) and chelicerate (horseshoe crab). This suggests that these miRNAs might have conserved roles in the post-transcriptional regulation of genes in sesquiterpenoid pathways across the Panarthropoda. Some of the identified lineage-specific miRNAs are potential targets for the development of new strategies in aquaculture and agricultural pest control.

Discovery of a Manduca sexta Allatotropin Antagonist from a Manduca sexta Allatotropin Receptor Homology Model

Insect G protein coupled receptors (GPCRs) have important roles in modulating biology, physiology and behavior. They have been identified as candidate targets for next-generation insecticides, yet these targets have been relatively poorly exploited for insect control. In this study, we present a pipeline of novel Manduca sexta allatotropin (Manse-AT) antagonist discovery with homology modeling, docking, molecular dynamics simulation and structure-activity relationship. A series of truncated and alanine-replacement analogs of Manse-AT were assayed for the stimulation of juvenile hormone biosynthesis. The minimum sequence required to retain potent biological activity is the C-terminal amidated octapeptide Manse-AT (6–13). We identified three residues essential for bioactivity (Thr⁴, Arg6 and Phe⁸) by assaying alanine-replacement analogs of Manse-AT (6–13). Alanine replacement of other residues resulted in reduced potency but bioactivity was retained. The 3D structure of the receptor (Manse-ATR) was built and the binding pocket was identified. The binding affinities of all the analogs were estimated by calculating the free energy of binding. The calculated binding affinities corresponded to the biological activities of the analogs, which supporting our localization of the binding pocket. Then, based on the docking and molecular dynamics studies of Manse-AT (10–13), we described it can act as a potent Manse-AT antagonist. The antagonistic effect on JH biosynthesis of Manse-AT (10–13) validated our hypothesis. The IC₅₀ value of antagonist Manse-AT (10–13) is 0.9 nM. The structure-activity relationship of antagonist Manse-AT (10–13) was also studied for the further purpose of investigating theoretically the structure factors influencing activity. These data will be useful for the design of new Manse-AT agonist and antagonist as potential pest control agents.