Effect of Soy Leaf Flavonoids on Pea Aphid Probing Behavior

Aphid-Resistant Plant Secondary Metabolites: Types, Insecticidal Mechanisms, and Prospects for Utilization

Aphids pose a significant threat to global agricultural crop production, leading to widespread pesticide use and resistance. This necessitates the use of alternative substances, like plant secondary metabolites (PSMs). Plants have developed protective compounds known as alkaloids, terpenoids, phenolics, sulfur- and nitrogen-containing metabolites. These compounds exhibit promising characteristics against aphids, such as antifeedant, aphicidal, and disrupting survival fitness. This review highlights the importance and application of secondary metabolites in combating aphid populations. Different insect-resistant substances have different mechanisms for managing aphids and other pests, including defensive signaling, inhibiting growth, and attracting natural predators by releasing herbivore-induced volatiles (HIPV). The application of plant secondary metabolites as biopesticides has proven to be an effective, economical, and eco-friendly alternative to synthetic pesticide chemicals. Furthermore, this review comprehensively discusses the principle role of plant secondary metabolites, encouraging sustainable agricultural practices and emphasizing the integrated management of the aphid population.

Hesperidin as a Species-Specific Modifier of Aphid Behavior

Hesperidin is a highly bioactive natural flavonoid whose role in ecological interactions is poorly known. In particular, the effects of hesperidin on herbivores are rarely reported. Flavonoids have been considered as prospective biopesticides; therefore, the aim of the present study was to examine the influence of hesperidin on the host plant selection behavior of three aphid (Hemiptera: Aphididae) species: Acyrthosiphon pisum Harrris, Rhopalosiphum padi (L.), and Myzus persicae (Sulz.). The aphid host plants were treated with 0.1% and 0.5% ethanolic solutions of hesperidin. Aphid probing behavior in the no-choice experiment was monitored using electropenetrography and aphid settling on plants in the choice experiment was recorded. The results demonstrated that hesperidin can be applied as a pre-ingestive, ingestive, and post-ingestive deterrent against A. pisum, as an ingestive deterrent against R. padi, and as a post-ingestive deterrent against M. persicae using the relatively low 0.1% concentration. While in A. pisum the deterrent effects of hesperidin were manifested as early as during aphid probing in peripheral plant tissues, in M. persicae, the avoidance of plants was probably the consequence of consuming the hesperidin-containing phloem sap.

Effects of Seven Plant Essential Oils on the Growth, Development and Feeding Behavior of the Wingless Aphis gossypii Glover

Cotton aphid Aphis gossypii Glover damages plants such as cotton directly by feeding on leaves and indirectly by transmitting viruses and excreting honeydew, which interferes with photosynthesis. The control of A. gossypii is still dominated by the frequent use of insecticides, which leads to a gradual increase in pesticide resistance in A. gossypii. Research is therefore needed on non-pesticide controls. In this study, seven plant essential oils (EOs) of Ocimum sanctum L., Ocimum basilicum L., Ocimum gratissimum L., Mentha piperita L., Mentha arvensis L., Tagetes erecta L., and Lavandula angustifolia Mill. were examined as potential controls for A. gossypii. We used life tables and electrical penetration graphs (EPG) to explore the effects of these EOs on the growth, development, and feeding behavior of A. gossypii, followed by a study of effects of the EOs on honeydew secretion by A. gossypii as a measure of their antifeedant activity. We found that the EOs of O. sanctum, M. piperita, M. arvensis and T. erecta significantly extended the pre-adult developmental period. Also, adult longevity, number of oviposition days, and total fecundity of A. gossypii treated with the EOs of M. arvensis or T. erecta were all significantly reduced. Aphids treated with the EOs of O. sanctum, M. piperita, or L. angustifolia showed significant reductions in their net reproductive rate (R0), intrinsic rate of increase (rm), and finite rate of increase (λ), and significant increases in mean generation time (T). In terms of their effects on the feeding behavior of A. gossypii, all seven EOs significantly reduced the total duration of phloem feeding (E2 waves), the number of phloem-feeding bouts, and the proportion of time spent in secretion of saliva into phloem sieve elements (E1 waves) and phloem feeding (E2). The total duration and number of E1 waves (saliva secretion) were significantly reduced by the EOs of O. sanctum, O. gratissimum, and M. arvensis. For C waves (probing in non-vascular tissues), the total duration spent in this behavior was significantly increased by the EOs of O. gratissimum, M. piperita, and L. angustifolia, but the number of such probing events was increased only by L. angustifolia EO. The EOs of O. basilicum, M. arvensis, and T. erecta significantly increased the total duration of ingestion of xylem sap (G waves), while the total time of mechanical difficulty in stylet penetration (F waves) was increased by M. arvensis. The total duration and number of the non-probing events (Np waves) were significantly increased by EOs of O. sanctum and O. basilicum. After treatment with all seven of these EOs, the area covered by honeydew was significantly reduced compared with the control. Studies have analyzed that EOs of O. sanctum, M. piperita, and T. erecta were most effective, followed by the EOs of M. arvensis and L. angustifolia, and finally the EOs of O. basilicum and O. gratissimum. In the present study, the EOs of O. sanctum, M. piperita, and T. erecta were found to have potential for the development as antifeedants of A. gossypii, and these data provide a basis for future research on non-pesticide chemical control of A. gossypii.

Flavonoids as Insecticides in Crop Protection-A Review of Current Research and Future Prospects

Pesticide overuse in agricultural systems has resulted in the development of pest resistance, the impoverishment of soil microbiota, water pollution, and several human health issues. Nonetheless, farmers still depend heavily on these agrochemicals for economically viable production, given the high frequency at which crops are affected by pests. Phytopathogenic insects are considered the most destructive pests on crops. Botanical pesticides have gained attention as potential biopesticides and complements to traditional pesticides, owing to their biodegradability and low toxicity. Plant-based extracts are abundant in a wide variety of bioactive compounds, such as flavonoids, a class of polyphenols that have been extensively studied for this purpose because of their involvement in plant defense responses. The present review offers a comprehensive review of current research on the potential of flavonoids as insecticides for crop protection, addressing the modes and possible mechanisms of action underlying their bioactivity. The structure-activity relationship is also discussed. It also addresses challenges associated with their application in pest and disease management and suggests alternatives to overcome these issues.

Chrysanthemum (Chrysanthemum morifolium) CmHRE2-like negatively regulates the resistance of chrysanthemum to the aphid (Macrosiphoniella sanborni)

BACKGROUND

The growth and ornamental value of chrysanthemums are frequently hindered by aphid attacks. The ethylene-responsive factor (ERF) gene family is pivotal in responding to biotic stress, including insect stress. However, to date, little is known regarding the involvement of ERF transcription factors (TFs) in the response of chrysanthemum to aphids.

RESULTS

In the present study, CmHRE2-like from chrysanthemum (Chrysanthemum morifolium), a transcription activator that localizes mainly to the nucleus, was cloned. Expression is induced by aphid infestation. Overexpression of CmHRE2-like in chrysanthemum mediated its susceptibility to aphids, whereas CmHRE2-like-SRDX dominant repressor transgenic plants enhanced the resistance of chrysanthemum to aphids, suggesting that CmHRE2-like contributes to the susceptibility of chrysanthemum to aphids. The flavonoids in CmHRE2-like-overexpression plants were decreased by 29% and 28% in two different lines, whereas they were increased by 42% and 29% in CmHRE2-like-SRDX dominant repressor transgenic plants. The expression of Chrysanthemum-chalcone-synthase gene(CmCHS), chalcone isomerase gene (CmCHI), and flavonoid 3'-hydroxylase gene(CmF3'H) was downregulated in CmHRE2-like overexpression plants and upregulated in CmHRE2-like-SRDX dominant repressor transgenic plants, suggesting that CmHRE2-like regulates the resistance of chrysanthemum to aphids partially through the regulation of flavonoid biosynthesis.

CONCLUSION

CmHRE2-like was a key gene regulating the vulnerability of chrysanthemum to aphids. This study offers fresh perspectives on the molecular mechanisms of chrysanthemum-aphid interactions and may bear practical significance for developing new strategies to manage aphid infestation in chrysanthemums.

Seed priming with rutin enhances tomato resistance against the whitefly Bemisia tabaci

Flavonoids are ubiquitously distributed in plants, showing pleiotropic effects in defense against abiotic and biotic stresses. Although it has been shown that seed priming with flavonoids can enhance plant resistance to abiotic stress, little is known about its potential to enhance plant tolerance to biotic stresses, especially for herbivorous insects. Here, we investigated whether treatment of tomato (Solanum lycopersicum) seeds with rutin improves plant resistance against the whitefly (Bemisia tabaci). Specifically, we measured the effect of rutin seed treatment on tomato seedling vigour, plant growth, feeding behavior and performance of B. tabaci on plants grown from control and rutin-treated seeds, and plant defense responses to B. tabaci attack. We found that seed treatment with different concentrations of rutin (viz 1, 2, 5, 10, and 20 mM) had minimal impact on shoot growth. Furthermore, seed treatment of rutin reduced the developmental rate of nymphs, the fecundity and feeding efficiency of adult females on plants grown from these seeds. The enhanced resistance of tomato against B. tabaci is closely associated with increased flavonoids accumulation, callose deposition and the expression of jasmonic acid (JA)-dependent defense genes. Additionally, callose deposition and expression of JA-dependent genes in tomato plants grown from rutin-treated seeds significantly increased upon B. tabaci infestation. These results suggest that seed treatment with rutin primes tomato resistance against B. tabaci, and are not accompanied by reductions in shoot growth. Defense priming by seed treatments may therefore be suitable for commercial exploitation.

The role of flavonols in insect resistance and stress response

Plants are sessile organisms and must adapt to various environmental changes, especially from stress conditions. Synthesis of secondary metabolites by the plant is one of the adaptive mechanisms against stress to provide resistance. Among several secondary metabolites, flavonols, a subgroup of flavonoids, are one of the most widely distributed in the plant kingdom. These molecules work as antioxidants, reduce reactive oxygen species (ROS) in plants, and cause detrimental effects on insect growth on feeding. Despite the great interest in flavonol function leading to insect tolerance and stress response, the detailed mechanisms related to these specific functions have yet to be studied. In this review, we have summarized the role of flavonols in plant defense against insects and different abiotic stresses and possible mechanisms involved in these functions.

Advances in the study of the function and mechanism of the action of flavonoids in plants under environmental stresses

MAIN CONCLUSION

This review summarizes the anti-stress effects of flavonoids in plants and highlights its role in the regulation of polar auxin transport and free radical scavenging mechanism. As secondary metabolites widely present in plants, flavonoids play a vital function in plant growth, but also in resistance to stresses. This review introduces the classification, structure and synthetic pathways of flavonoids. The effects of flavonoids in plant stress resistance were enumerated, and the mechanism of flavonoids in plant stress resistance was discussed in detail. It is clarified that plants under stress accumulate flavonoids by regulating the expression of flavonoid synthase genes. It was also determined that the synthesized flavonoids are transported in plants through three pathways: membrane transport proteins, vesicles, and bound to glutathione S-transferase (GST). At the same time, the paper explores that flavonoids regulate polar auxin transport (PAT) by acting on the auxin export carrier PIN-FORMED (PIN) in the form of ATP-binding cassette subfamily B/P-glycoprotein (ABCB/PGP) transporter, which can help plants to respond in a more dominant form to stress. We have demonstrated that the number and location of hydroxyl groups in the structure of flavonoids can determine their free radical scavenging ability and also elucidated the mechanism by which flavonoids exert free radical removal in cells. We also identified flavonoids as signaling molecules to promote rhizobial nodulation and colonization of arbuscular mycorrhizal fungi (AMF) to enhance plant-microbial symbiosis in defense to stresses. Given all this knowledge, we can foresee that the in-depth study of flavonoids will be an essential way to reveal plant tolerance and enhance plant stress resistance.

Ingenious characterization and assessment of lentil germplasm collection to aphid Acyrthosiphon pisum stress unveils distinct responses

Lentil cultivation is often hampered by aphid population outspreads with detrimental impacts to crop development and production, challenging food safety and agriculture sustainability. The pea aphid (Acyrthosiphon pisum) is a significant threat to lentil in the temperate zone rainfed systems. A set of management practices including resilient cultivars and application of insecticides have effectively controlled aphid infestation. However, the plant defense against insect pests is scantily dissected and limited to the individual components including antibiosis, antixenosis and tolerance that constitute a combination of plant stress responses. Utilizing a lentil germplasm collection, we assessed the antixenosis and aphid tolerance mechanisms in association to important morphological parameters. Physiological parameters including relative water content (RWC) measured at 24h and 48h post-aphid infestation revealed genotype-specific responses. The contents of key plant hormones including salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA) and indoleacetic acid (IAA) implicated in defense signal-transduction pathways were also determined in lentil accessions after aphid herbivory infestation. In parallel, the expression of hallmark defense genes governed by SA- and JA-signaling pathways at 24h and 48h post aphid herbivory revealed significant differentiation patterns among the accessions. An interplay of hormone crosstalk is unveiled that possibly governs defense responses and aphid resistance. Besides the metabolomic profiling of accessions under aphid herbivory indicated the indispensable role of key secondary metabolites accumulation such as flavonoids, alkaloids, phenolics and fatty acids as a front line of plant defense and a potential integration of hormone signaling pathways in metabolome reprogramming. Overall, the study presents a panorama of distinct lentil responses to aphids and a critical view of the molecular mechanisms implicated in lentil insect defense to further our insight and advance crop protection and breeding approaches in a climate changing environment.

Antifeedant Potential of Geranylacetone and Nerylacetone and Their Epoxy-Derivatives against Myzus persicae (Sulz.)

Geranylacetone and nerylacetone are natural sesquiterpenoids, which play various roles in plant-insect interactions, including the deterrent and repellent effects on herbivores. The structural modifications of natural compounds often change their biological activities. The aim of the study was to evaluate the effect of geranylacetone, nerylacetone and their epoxy-derivatives on the probing and settling behavior of Myzus persicae (Sulz.) (Hemiptera: Aphididae). The no-choice test using the Electrical Penetration Graph (EPG) technique showed that the probes before the first phloem phase were usually shorter than 3 min, which means that they were terminated within the epidermis and/or outer layers of mesophyll. This resulted in a tendency to delay the initiation of the phloem phase in aphids, which reflects a weak preingestive deterrent activity of the studied compounds at the level of non-vascular tissues. Most M. persicae showed bouts of sustained phloem sap ingestion. However, the 24-h free-choice test demonstrated that aphids did not settle on the leaves treated with geranylacetone, nerylacetone, and their epoxy-derivatives. The refusal to settle after the consumption of phloem sap on treated plants indicated that the studied compounds had postingestive deterrent activity. The epoxidation of geranylacetone and nerylacetone did not evoke significant changes in their activity profiles.

Dynamic regulation of phenylpropanoid pathway metabolites in modulating sorghum defense against fall armyworm

Plants undergo dynamic metabolic changes at the cellular level upon insect infestation to better defend themselves. Phenylpropanoids, a hub of secondary plant metabolites, encompass a wide range of compounds that can contribute to insect resistance. Here, the role of sorghum (Sorghum bicolor) phenylpropanoids in providing defense against the chewing herbivore, fall armyworm (FAW), Spodoptera frugiperda, was explored. We screened a panel of nested association mapping (NAM) founder lines against FAW and identified SC1345 and Ajabsido as most resistant and susceptible lines to FAW, respectively, compared to reference parent, RTx430. Gene expression and metabolomic studies suggested that FAW feeding suppressed the expression level of genes involved in monolignol biosynthetic pathway and their associated phenolic intermediates at 10 days post infestation. Further, SC1345 genotype displayed elevated levels of flavonoid compounds after FAW feeding for 10 days, suggesting a diversion of precursors from lignin biosynthesis to the flavonoid pathway. Additionally, bioassays with sorghum lines having altered levels of flavonoids provided genetic evidence that flavonoids are crucial in providing resistance against FAW. Finally, the application of FAW regurgitant elevated the expression of genes associated with the flavonoid pathway in the FAW-resistant SC1345 genotype. Overall, our study indicates that a dynamic regulation of the phenylpropanoid pathway in sorghum plants imparts resistance against FAW.

Mycorrhizal Symbiosis Triggers Local Resistance in Citrus Plants Against Spider Mites

Citrus plants are a highly mycotrophic species with high levels of fungal colonization. Citrus aurantium rootstocks typically show abundant root colonization by Rhizophagus irregularis three weeks after inoculation. Mycorrhizal symbiosis protects plants against multiple biotic stressors, however, such protection against spider mites remains controversial. We examined mycorrhiza-induced resistance (MIR) in citrus against the two-spotted spider mite Tetranychus urticae. Mycorrhized C. aurantium displayed reduced levels of damage in leaves and lower mite oviposition rates, compared to non-mycorrhized controls. Mycorrhization did not affect host choice of mites in Y-tube assays; of note, C. aurantium has innate strong antixenotic resistance against this mite. Analysis of metabolism pathways in mycorrhized citrus plants showed upregulated expression of the oxylipin-related genes LOX-2 and PR-3 early after infestation. Accordingly, jasmonic acid (JA), 12-oxo phytodienoic acid (OPDA), and JA-Ile concentrations were increased by mycorrhization. Non-targeted metabolomic analysis revealed the amino acid, oxocarboxylic acid, and phenylpropanoid metabolism as the three major pathways with more hits at 24 h post infection (hpi) in mycorrhized plants. Interestingly, there was a transition to a priming profile of these pathways at 48 hpi following infestation. Three flavonoids (i.e., malic acid, coumaric acid, and diconiferyl alcohol) were among the priming compounds. A mixture containing all these compounds provided efficient protection against the mite. Unexpectedly, systemic resistance did not improve after 72 h of primary infestation, probably due to the innate strong systemic resistance of C. aurantium. This is the first study to show that MIR is functional against T. urticae in locally infested citrus leaves, which is mediated by a complex pool of secondary metabolites and is likely coordinated by priming of JA-dependent responses.