Development of Sustainable Insecticide Candidates for Protecting Pollinators: Insight into the Bioactivities, Selective Mechanism of Action and QSAR of Natural Coumarin Derivatives against Aphids

Plants employ abundant toxic secondary metabolites to withstand insect attack, while pollinators can tolerate some natural defensive compounds. Coumarins, as promising green alternatives to chemical insecticides, possess wide application prospects in the crop protection field. Herein, the bioactivities of 30 natural coumarin derivatives against Aphis gossypii were assessed and revealed that 6-methylcoumarin exhibited potent aphicidal activity against aphids but displayed no toxicity to honeybees. Additionally, using biochemical, bioinformatic, and molecular assays, we confirmed that the action mode of 6-methylcoumarin against aphids was by inhibiting acetylcholinesterase (AChE). Meanwhile, functional assays revealed that the difference in action site, which located in Lys585 in aphid AChE (equivalent to Val548 in honeybee AChE), was the principal reason for 6-methylcoumarin being toxic to aphids but safe to pollinators. This action site was further validated by mutagenesis data, which uncovered how 6-methylcoumarin was unique selective to the aphid over honeybee or mammalian AChE. Furthermore, a 2D-QSAR model was established, revealing that the central structural feature was H3m, which offers guidance for the future design of more potent coumarin compounds. This work provides a sustainable strategy to take advantage of coumarin analogues for pest management while protecting nontarget pollinators.

Additive Effect of the Composition of Endophytic Bacteria Bacillus subtilis on Systemic Resistance of Wheat against Greenbug Aphid Schizaphis graminum Due to Lipopeptides

The use of biocontrol agents based on endophytic bacteria against phloem-feeding insects is limited by a lack of knowledge and understanding of the mechanism of action of the endophyte community that makes up the plant microbiome. In this work, the mechanisms of the additive action of endophytic strains B. subtilis 26D and B. subtilis 11VM on the resistance of bread spring wheat against greenbug aphid Schizaphis graminum, was studied. It was shown that B. subtilis 26D secreted lipopeptide surfactin and phytohormones cytokinins, and B. subtilis 11VM produced iturin and auxins into the cultivation medium. Both strains and their lipopeptide-rich fractions showed direct aphicidal activity against greenbug aphid. For the first time, it was shown that B. subtilis 26D and B. subtilis 11VM in the same manner, as well as their lipopeptide-rich fractions, activated the expression of salicylate- and ethylene-dependent PR genes, and influenced plant redox metabolism, which led to an increase in plant endurance against aphids. The composition of endophytic strains B. subtilis 26D + B. subtilis 11VM had an additive effect on plant resistance to aphids due to an increase in the number of endophytic bacterial cells, and, as well as due to the synergistic effect of their mixture of lipopeptides - surfactin + iturin, both on the aphid mortality and on the expression of PR1 and PR3 genes. All these factors can be the reason for the observed increase in the growth of plants affected by aphids under the influence of B. subtilis 26D and B. subtilis 11VM, individually and in composition. The study demonstrates the possibility of creating in the future an artificial composition to enhance plant microbiome with endophytic bacteria, which combines growth-promoting and plant immunity stimulating properties against phloem-feeding insects. This direction is one of the most promising approaches to green pesticide discovery in the future.

Identification of azukisapogenol triterpenoid saponins from Oxytropis hirta Bunge and their aphicidal activities against pea aphid Acyrthosiphon pisum Harris

BACKGROUND

Acyrthosiphon pisum Harris is the most destructive pest worldwide because of its ability to feed on plants directly and transmit plant viruses as a vector. This study aims to identify triterpenoid saponins from Oxytropis hirta Bunge as biopesticides to control aphids.

RESULTS

Three new azukisapogenol triterpenoid saponins (1-3), a new pinoresinol lignan glycoside (8), and four known saponins (4-7) were identified from the root of O. hirta. Compounds 4-7 displayed significant aphicidal activities against A. pisum with oral toxicities (LC₅₀  = 51.10-147.43 μg/mL, 72 h), deterrent effects (deterrence index = 1.00, 100-200 μg/mL, 24 h), and aphid reproduction inhibitory effects (inhibition rates = 75.91-86.73%, 400 μg/mL, 24 h), respectively. The carboxyl groups at C-3 GlcA and C-30 were functional groups for their aphicidal activities. The toxic symptoms caused by the optimal 5 involved insect body-color changes from light green to dark or gray-green, and then brown until death. The intestinal cavity, apical microvilli, nuclei, mitochondria, and electron dense granules in the midgut tissues of A. pisum were the target sites showing aphicidal activity. The suppression of pepsin and α-amylase, and the activation of lipase and trypsin could be the signs of organelle damage in the midgut tissues.

CONCLUSION

Azukisapogenol triterpenoid saponins from O. hirta could be used as biopesticides to control aphids for their multiple efficacies, including oral toxicity, deterrent activity, and reproduction inhibitory activity. The toxic symptoms involved insect body-color changes. Midgut tissues and their related enzymes were the targets for saponins showing aphicidal activities. © 2022 Society of Chemical Industry.

Insect kinin mimics act as potential control agents for aphids: Structural modifications of Trp4

Insect kinins are endogenous, biologically active peptides with various physiological functions. The use of insect kinins in plant protection is being evaluated by many groups. Some kinins have been chosen as lead compounds for pest control. We previously reported an insect kinin mimic IV-3 that had insecticidal activity. And by introducing a strong electron withdrawing group (-CF₃ ) on the benzene ring (Phe² ), we discovered a compound, L₇ , with better activity than lead IV-3. In this work, taking L₇ as the lead compound, we designed and synthesized 13 compounds to evaluate the influence of position 4 (Trp⁴ ) of insect kinin on insecticidal activity, by replacing the H atom on tryptophan with -CH₃ and -Cl or substituting the indole ring of tryptophan with the benzene, naphthalene, pyridine, imidazole, cyclohexane, and alkyl carboxamides. The aphid bioassay results showed that the compounds M₁ , M₃ , and M₅ were more active than the positive control, pymetrozine. Especially, replacing the side chain by an indole ring with 4-Cl substitution (M₁ , LC₅₀  = 0.0029 mmol/L) increased the aphicidal activity. The structure-activity relationships (SARs) indicated that the side chain benzene ring at this position may be important to the aphicidal activity. In addition, the toxicity prediction by Toxtree, and the toxicity experiments on Apis mellifera suggested that M₁ was no toxicity risk on a non-target organism. It could be used as a selective and bee-friendly insecticide to control aphids.

A novel bee-friendly peptidomimetic insecticide: Synthesis, aphicidal activity and 3D-QSAR study of insect kinin analogs at Phe2 modification

BACKGROUND

As one of the most abundant and destructive pests in agriculture, aphids cause significant damage to crops due to their sap-taking and as virus vectors. Chemical insecticides are the most effective method to control aphids, but they bring insecticide resistance problems and harm nontarget organisms, especially bees, therefore the search for novel eco-friendly aphid control agents with low bee toxicity is urgent. Insect kinins are a class of small neuropeptides that control important functions in insects. In our previous study, we found insect kinin analog IV-3 has good aphicidal activity and the location of the aromatic ring on the side chain of Phe² is the key to the formation of the β-turn resulting in the biological activity of insect kinin analogs. However, there are few studies on insect kinin Phe² substitution and modification, and its structure-activity relationship is still unclear.

RESULTS

In this project, 44 insect kinin analogs with the Phe² modification, replacing it with different natural or unnatural amino acids, were designed and synthesized based on the lead IV-3 to explore the role of the Phe² residues. Bioassays with soybean aphids of Aphis glycines indicated that nine analogs have better aphicidal activity than the lead IV-3. In particular, compound L₂₅ exhibits excellent aphicidal activity (LC₅₀  = 0.0047 mmol L^-1 ) and has low toxicity to bees. Furthermore, a reliable three-dimensional quantitative structure-activity relationship (3D-QSAR) was established to produce a helpful clue that introducing hydrophobic groups away from the backbone chain is beneficial to improve aphicidal activity.

CONCLUSION

The residue Phe² of insect kinin analogs is the key position and has a significant impact on the activity. L₂₅ has a high toxicity for aphids, while a low toxicity to bees, and therefore can be considered as a lead compound to develop new biosafe aphid control agents. Finally, we provide a useful 3D-QSAR model as theoretical guidance for further structural optimization. © 2022 Society of Chemical Industry.

Genomic, Antimicrobial, and Aphicidal Traits of Bacillus velezensis ATR2, and Its Biocontrol Potential against Ginger Rhizome Rot Disease Caused by Bacillus pumilus

Ginger rhizome rot disease, caused by the pathogen Bacilluspumilus GR8, could result in severe rot of ginger rhizomes and heavily threaten ginger production. In this study, we identified and characterized a new Bacillus velezensis strain, designated ATR2. Genome analysis revealed B. velezensis ATR2 harbored a series of genes closely related to promoting plant growth and triggering plant immunity. Meanwhile, ten gene clusters involved in the biosynthesis of various secondary metabolites (surfactin, bacillomycin, fengycin, bacillibactin, bacilysin, difficidin, macrolactin, bacillaene, plantazolicin, and amylocyclicin) and two clusters encoding a putative lipopeptide and a putative phosphonate which might be explored as novel bioactive compounds were also present in the ATR2 genome. Moreover, B. velezensis ATR2 showed excellent antagonistic activities against multiple plant pathogenic bacteria, plant pathogenic fungi, human pathogenic bacteria, and human pathogenic fungus. B. velezensis ATR2 was also efficacious in control of aphids. The antagonistic compound from B. velezensis ATR2 against B.pumilus GR8 was purified and identified as bacillomycin D. In addition, B. velezensis ATR2 exhibited excellent biocontrol efficacy against ginger rhizome rot disease on ginger slices. These findings showed the potential of further applications of B. velezensis ATR2 as a biocontrol agent in agricultural diseases and pests management.

Spirodiclofen ether derivatives: semisynthesis, structural elucidation, and pesticidal activities against Tetranychus cinnabarinus Boisduval, Aphis citricola Van der Goot and Mythimna separata Walker

BACKGROUND

Spirodiclofen is a spirocyclic tetronic acid-type acaricidal agent. Nowadays, serious pests resistance to spirodiclofen and cross-resistance to other acaricides has appeared. To overcome pests resistance and discover new potential agrochemicals, a series of ether derivatives were prepared based on spirodiclofen as a lead compound. Their pesticidal activities were investigated against three typically agricultural pests, Mythimna separata Walker, Aphis citricola Van der Goot and Tetranychus cinnabarinus Boisduval.

RESULTS

Four steric structures of compounds 5e, 5f, 5i and 5j were determined by single-crystal X-ray diffraction. Against T. cinnabarinus, compounds 5b, 5f and 5l exhibited potent acaricidal activity, and their good control effects in the glasshouse were observed when compared with spirodiclofen, especially the control efficiency of compound 5b was comparable to that of spirodiclofen; against M. separata, compound 5j showed > 1.8-fold potent insecticidal activity of spirodiclofen; against A. citricola, compounds 5d and 5j displayed > 2.0-fold potent aphicidal activity of spirodiclofen. The relationships between their structures and agricultural activities were also discussed.

CONCLUSION

Compounds 5b and 5d could be further studied as acaricidal and aphicidal agents, respectively; compound 5j can be considered as a lead compound for the insecticidal and aphicidal activities. This will pave the way for future application of these derivatives as pesticide substitutes for spirodiclofen. © 2021 Society of Chemical Industry.

Synthesis, aphicidal activity and conformation of novel insect kinin analogues as potential eco-friendly insecticides

BACKGROUND

The discovery of ecofriendly insecticides through a new strategy for aphid control is important because of the substantial resistance and unexpected eco-toxicity to honeybees caused by traditional insecticides. The insect kinins, a class of multifunctional insect neuropeptides, are considered for potential application in pest control. In our previous work we developed several series of insect kinin analogues and found a promising lead II-1 with good aphicidal activity. To seek further eco-friendly aphicides, the optimization of II-1 is carried out in this study.

RESULTS

Fifteen novel Yaa³ modified analogues based on the lead II-1 were synthesized. The aphicidal tests indicated that IV-3, IV-5 and IV-10 exhibited significant activity against the soybean aphid Aphis glycines with LC₅₀ values of 0.0029, 0.0072 and 0.0086 mmol L^-1 , respectively, higher than that of lead II-1 and the commercial Pymetrozine. The molecular modeling results showed that analogues II-1, IV-3, IV-5, IV-7 and IV-10 formed a β-turn-like conformation, while the conformation of analogues IV-1, IV-2 and IV-9 seemed to be linear. Some structural elements favorable for the activity were proposed based on the conformation-activity relationship of the analogues.

CONCLUSION

Insect kinin analogues derived from lead II-1 by modifying the hydrolysis site Yaa³ with natural, sterically hindered α- and β-amino acids showed great potential as eco-friendly insecticides. Inspiringly, the most active analogue IV-3 can be a candidate for further development. The β-turn-like conformation and the orientation of the aromatic rings of the side chain of Phe² and Trp⁴ may be critical factors beneficial to activity. © 2019 Society of Chemical Industry.

Bioassay-guided isolation of potent aphicidal Erythrina alkaloids against Aphis gossypii from the seed of Erythrina crista-galli L

BACKGROUND

The cotton aphid (Aphis gossypii Glover) is one of the most invasive pests of cotton. Many botanical phytochemicals have a long history as a source of insecticides, and as templates for new insecticides. This study was undertaken to isolate aphicidal compounds from the seeds of Erythrina crista-galli L. using the bioassay-guided isolation method.

RESULTS

Three novel and 11 known Erythrina alkaloids were isolated. Erysodine (9), erysovine (10), erysotrine (8) and erythraline (11) showed moderate to excellent aphicidal activity with LD₅₀ values of 7.48, 6.68, 5.13 and 4.67 ng aphid^-1 , respectively. The Potter spray tower bioassay gave corresponding LC₅₀ values of 186.81, 165.35, 163.74 and 112.78 µg ml^-1 . A unique substructure, which presents an sp³ methylene at C-8, a non-oxygenated site at N-9 and a conjugated dienes group (Δ^1,2 and Δ^6,7 ), plays a crucial role in the aphicidal activity. Application of erythraline (11) led to different increases in the activities of superoxide dismutase, catalase and glutathione S-transferase.

CONCLUSION

The study demonstrated that the Erythrina alkaloids erysodine (9), erysovine (10), erysotrine (8) and erythraline (11) have potential use as botanical aphicides for commercial application, or as templates for the development of new insecticides. © 2017 Society of Chemical Industry.

Design, Synthesis and Structure-Activity Relationship of Novel Aphicidal Mezzettiaside-Type Oligorhamnosides and Their Analogues

Oligosaccharides have been used for an environmentally friendly insect control in the agricultural industry. In order to discover novel eco-friendly pesticides, a series of partially acetylated oligorhamnoses mezzettiasides, 2-8, and their analogues, 9-14, with biosurfactant characteristics were designed and synthesized, some of which exhibited comparable to or even stronger aphicidal activity than pymetrozine. Preliminary SAR studies demonstrated that the aphicidal activity of mezzettiasides analogs is highly dependent on their structures, including both the sugar length and the substitutes on the sugar. Among them, trirhamnolipid 9 displayed the strongest aphicidal activity, with an LC50 of 0.019 mmol/L, indicating that the biosurfactant 9 may have potential for use as an environmentally friendly agricultural pesticide.

Eco-Friendly Insecticide Discovery via Peptidomimetics: Design, Synthesis, and Aphicidal Activity of Novel Insect Kinin Analogues

Insect kinin neuropeptides are pleiotropic peptides that are involved in the regulation of hindgut contraction, diuresis, and digestive enzyme release. They share a common C-terminal pentapeptide sequence of Phe(1)-Xaa(2)-Yaa(3)-Trp(4)-Gly(5)-NH2 (where Xaa(2) = His, Asn, Phe, Ser, or Tyr; Yaa(3) = Pro, Ser, or Ala). Recently, the aphicidal activity of insect kinin analogues has attracted the attention of researchers. Our previous work demonstrated that the sequence-simplified insect kinin pentapeptide analogue Phe-Phe-[Aib]-Trp-Gly-NH2 could retain good aphicidal activity and be the lead compound for the further discovery of eco-friendly insecticides which encompassed a broad array of biochemicals derived from micro-organisms and other natural sources. Using the peptidomimetics strategy, we chose Phe-Phe-[Aib]-Trp-Gly-NH2 as the lead compound, and we designed and synthesized three series, including 31 novel insect kinin analogues. The aphicidal activity of the new analogues against soybean aphid was determined. The results showed that all of the analogues exhibited aphicidal activity. Of particular interest was the analogue II-1, which exhibited improved aphicidal activity with an LC50 of 0.019 mmol/L compared with the lead compound (LC50 = 0.045 mmol/L) or the commercial insecticide pymetrozine (LC50 = 0.034 mmol/L). This suggests that the analogue II-1 could be used as a new lead for the discovery of potential eco-friendly insecticides.