Abscisic acid deficiency increases defence responses against Myzus persicae in Arabidopsis

Spider mite herbivory induces an ABA-driven stomatal defense

Arthropod herbivory poses a serious threat to crop yield, prompting plants to employ intricate defense mechanisms against pest feeding. The generalist pest 2-spotted spider mite (Tetranychus urticae) inflicts rapid damage and remains challenging due to its broad target range. In this study, we explored the Arabidopsis (Arabidopsis thaliana) response to T. urticae infestation, revealing the induction of abscisic acid (ABA), a hormone typically associated with abiotic stress adaptation, and stomatal closure during water stress. Leveraging a Forster resonance energy transfer (FRET)-based ABA biosensor (nlsABACUS2-400n), we observed elevated ABA levels in various leaf cell types postmite feeding. While ABA's role in pest resistance or susceptibility has been debated, an ABA-deficient mutant exhibited increased mite infestation alongside intact canonical biotic stress signaling, indicating an independent function of ABA in mite defense. We established that ABA-triggered stomatal closure effectively hinders mite feeding and minimizes leaf cell damage through genetic and pharmacological interventions targeting ABA levels, ABA signaling, stomatal aperture, and density. This study underscores the critical interplay between biotic and abiotic stresses in plants, highlighting how the vulnerability to mite infestation arising from open stomata, crucial for transpiration and photosynthesis, reinforces the intricate relationship between these stress types.

Harnessing Phytohormones: Advancing Plant Growth and Defence Strategies for Sustainable Agriculture

Phytohormones, pivotal regulators of plant growth and development, are increasingly recognized for their multifaceted roles in enhancing crop resilience against environmental stresses. In this review, we provide a comprehensive synthesis of current research on utilizing phytohormones to enhance crop productivity and fortify their defence mechanisms. Initially, we introduce the significance of phytohormones in orchestrating plant growth, followed by their potential utilization in bolstering crop defences against diverse environmental stressors. Our focus then shifts to an in-depth exploration of phytohormones and their pivotal roles in mediating plant defence responses against biotic stressors, particularly insect pests. Furthermore, we highlight the potential impact of phytohormones on agricultural production while underscoring the existing research gaps and limitations hindering their widespread implementation in agricultural practices. Despite the accumulating body of research in this field, the integration of phytohormones into agriculture remains limited. To address this discrepancy, we propose a comprehensive framework for investigating the intricate interplay between phytohormones and sustainable agriculture. This framework advocates for the adoption of novel technologies and methodologies to facilitate the effective deployment of phytohormones in agricultural settings and also emphasizes the need to address existing research limitations through rigorous field studies. By outlining a roadmap for advancing the utilization of phytohormones in agriculture, this review aims to catalyse transformative changes in agricultural practices, fostering sustainability and resilience in agricultural settings.

Induction of resistance, enzyme activity, and phytochemicals in canola plants treated with abscisic acid elevated based on nutrient availability: a case study on Brevicoryne brassicae L. (Hemiptera: Aphididae)

The cabbage aphid, Brevicoryne brassicae L. (Hemiptera: Aphididae), is one of the important pests of cruciferous plants throughout the world including Iran. In the present study, we grew cultivated canola plants under different fertilizers or distilled water and sprayed them with 100 µM abscisic acid (ABA) or a control solution (NaOH dissolved in water) to study (i) the antibiosis parameters of B. brassicae on these plants; (ii) the antixenosis of B. brassicae adults on these plants; (iii) the plant's peroxidase (POD), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) activity; and (iv) the plant's total phenolic and glucosinolate content. The results of antibiosis experiments showed that ABA and fertilizers have a profound and negative effect on the performance of B. brassicae. In the antixenosis experiment, control plants attracted a significantly higher number of adult females in comparison to treated plants. Also, B. brassicae had lower performance and preference when they were reared on the ABA-treated fertilized plants with higher levels of phenolic and glucosinolate content. These results prompted us to hypothesize that fertilizers enable canola plants to trigger a higher level of secondary metabolites. Our findings reveal that the type and level of nutrient availability may have different impacts on how the plant regulates its defense mechanisms.

Water availability and plant-herbivore interactions

Water is essential to plant growth and drives plant evolution and interactions with other organisms such as herbivores. However, water availability fluctuates, and these fluctuations are intensified by climate change. How plant water availability influences plant-herbivore interactions in the future is an important question in basic and applied ecology. Here we summarize and synthesize the recent discoveries on the impact of water availability on plant antiherbivore defense ecology and the underlying physiological processes. Water deficit tends to enhance plant resistance and escape traits (i.e. early phenology) against herbivory but negatively affects other defense strategies, including indirect defense and tolerance. However, exceptions are sometimes observed in specific plant-herbivore species pairs. We discuss the effect of water availability on species interactions associated with plants and herbivores from individual to community levels and how these interactions drive plant evolution. Although water stress and many other abiotic stresses are predicted to increase in intensity and frequency due to climate change, we identify a significant lack of study on the interactive impact of additional abiotic stressors on water-plant-herbivore interactions. This review summarizes critical knowledge gaps and informs possible future research directions in water-plant-herbivore interactions.

Interplay of phytohormones facilitate sorghum tolerance to aphids

Interactions among phytohormones are essential for providing tolerance of sorghum plants to aphids. Plant's encounter with insect herbivores trigger defense signaling networks that fine-tune plant resistance to insect pests. Although it is well established that phytohormones contribute to antixenotic- and antibiotic-mediated resistance to insect pests, their role in conditioning plant tolerance, the most durable and promising category of host plant resistance, is largely unknown. Here, we screened a panel of sorghum (Sorghum bicolor) inbred lines to identify and characterize sorghum tolerance to sugarcane aphids (SCA; Melanaphis sacchari Zehntner), a relatively new and devastating pest of sorghum in the United States. Our results suggest that the sorghum genotype SC35, the aphid-tolerant line identified among the sorghum genotypes, displayed minimal plant biomass loss and a robust photosynthetic machinery, despite supporting higher aphid population. Phytohormone analysis revealed significantly higher basal levels of 12-oxo-phytodienoic acid, a precursor in the jasmonic acid biosynthesis pathway, in the sorghum SCA-tolerant SC35 plants. Salicylic acid accumulation appeared as a generalized plant response to aphids in sorghum plants, however, SCA feeding-induced salicylic acid levels were unaltered in the sorghum tolerant genotype. Conversely, basal levels of abscisic acid and aphid feeding-induced cytokinins were accumulated in the SCA-tolerant sorghum genotype. Our findings imply that the aphid-tolerant sorghum genotype tightly controls the relationship among phytohormones, as well as provide significant insights into the underlying mechanisms that contribute to plant tolerance to sap-sucking aphids.

Reprogramming of sorghum proteome in response to sugarcane aphid infestation

Sugarcane aphid (SCA; Melanaphis sacchari Zehntner) is a key piercing-sucking pest of sorghum (Sorghum bicolor) that cause significant yield losses. While feeding on host plants, complex signaling networks are invoked from recognition of insect attack to induction of plant defenses. Consequently, these signaling networks lead to the production of insecticidal compounds or limited access of nutrients to insects. Previously, several studies were published on the transcriptomics analysis of sorghum in response to SCA infestation, but no information is available on the physiological changes of sorghum at the proteome level. We used the SCA resistant sorghum genotype SC265 for the global proteomics analysis after 1 and 7 days of SCA infestation using the TMT-plex technique. Peptides matching a total of 4211 proteins were identified and 158 proteins were differentially expressed at day 1 and 7. Overall, proteome profiling of SC265 after SCA infestation at days 1 and 7 revealed the suppression of plant defense-related proteins and upregulation of plant defense and signaling-related proteins, respectively. The plant defense responses based on proteome data were validated using electrical penetration graph (EPG) technique to observe changes in aphid feeding. Feeding behavior analyses revealed that SCA spent significantly longer time in phloem phase on SCA infested plants for day 1 and lesser time in day 7 SCA infested sorghum plants, compared to their respective control plants. Overall, our study provides insights into underlying mechanisms that contribute to sorghum resistance to SCA.

Transcriptome Analysis Reveals Crosstalk between the Abscisic Acid and Jasmonic Acid Signaling Pathways in Rice-Mediated Defense against Nilaparvata lugens

The brown planthopper (BPH) impacts both rice yield and quality. The exogenous application of abscisic acid (ABA) and jasmonic acid (JA) has been previously shown to induce rice resistance to BPH; however, the regulation of rice-mediated defense by these plant growth regulators is unclear. We applied exogenous JA and ABA to rice and analyzed molecular responses to BPH infestation. Nine RNA libraries were sequenced, and 6218 differentially expressed genes (DEGs) were generated and annotated. After ABA + BPH and JA + BPH treatments, 3491 and 2727 DEGs, respectively, were identified when compared with the control (BPH alone). GO enrichment and KEGG pathway analysis showed that the expression of several JA pathway genes (OsAOS2, encoding allene oxide synthase; OsOPR, 12-oxo-phytodienoic acid reductase; and OsACOX, acy1-CoA oxidase) were significantly up-regulated after ABA + BPH treatment. Furthermore, exogenous JA increased the expression of genes involved in ABA synthesis. Meanwhile, the expression levels of genes encoding WRKY transcription factors, myelocytomatosis protein 2 (MYC2) and basic leucine zippers (bZIPs) were up-regulated significantly, indicating that ABA and JA might function together to increase the expression of transcription factors during the rice defense response. The DEGs identified in this study provide vital insights into the synergism between ABA and JA and further contribute to the mechanistic basis of rice resistance to BPH.

Effects of Plant Hormones, Metal Ions, Salinity, Sugar, and Chemicals Pollution on Glucosinolate Biosynthesis in Cruciferous Plant

Cruciferous vegetable crops are grown widely around the world, which supply a multitude of health-related micronutrients, phytochemicals, and antioxidant compounds. Glucosinolates (GSLs) are specialized metabolites found widely in cruciferous vegetables, which are not only related to flavor formation but also have anti-cancer, disease-resistance, and insect-resistance properties. The content and components of GSLs in the Cruciferae are not only related to genotypes and environmental factors but also are influenced by hormones, plant growth regulators, and mineral elements. This review discusses the effects of different exogenous substances on the GSL content and composition, and analyzes the molecular mechanism by which these substances regulate the biosynthesis of GSLs. Based on the current research status, future research directions are also proposed.

Terpene synthases in cucumber (Cucumis sativus) and their contribution to herbivore-induced volatile terpenoid emission

Terpenoids play important roles in flavour, pollinator attraction and defence of plants. In cucumber (Cucumis sativus) they are important components of the herbivore-induced plant volatile blend that attracts natural enemies of herbivores. We annotated the cucumber TERPENE SYNTHASE gene (CsTPS) family and characterized their involvement in the response towards herbivores with different feeding guilds using a combined molecular and biochemical approach. Transcripts of multiple CsTPS genes were upregulated in leaves upon herbivory and the products generated by the expressed proteins match the terpenoids recorded in the volatile blend released by herbivore-damaged leaves. Spatial and temporal analysis of the promoter activity of CsTPS genes showed that cell content-feeding spider mites (Tetranychus urticae) and thrips (Frankliniella occidentalis) induced promoter activity of CsTPS9 and CsTPS19 within hours after initiation of infestation, while phloem-feeding aphids (Myzus persicae) induced CsTPS2 promoter activity. Our findings offer detailed insights into the involvement of the TPS gene family in the dynamics and fine-tuning of the emission of herbivore-induced plant volatiles in cucumber, and open a new avenue to understand molecular mechanisms that affect plant-herbivore interactions.

Changes in plant responses induced by an arthropod influence the colonization behavior of a subsequent herbivore

BACKGROUND

Plants in nature can be sequentially attacked by different arthropod herbivores. Feeding by one arthropod species may induce plant-defense responses that might affect the performance of a later-arriving herbivorous species. Understanding these interactions can help in developing pest-management strategies. In tomato, the sweet-potato whitefly Bemisia tabaci and the two-spotted spider mite Tetranychus urticae are key pests that frequently cohabit on the same plant. We studied whether colonization by one species can either facilitate or impede later colonization of tomato plants by conspecific or heterospecific individuals.

RESULTS

B. tabaci females showed a strong preference for and increased oviposition on plants previously colonized by conspecifics. In contrast, plants infested with T. urticae repelled B. tabaci females and reduced their oviposition rate by 86%. Although females of T. urticae showed no preference between conspecific-infested or uninfested plants, we observed a 50% reduction in the number of eggs laid on conspecific-infested plants. Both herbivorous arthropods up-regulated the expression of genes involving the jasmonic acid and abscisic acid pathways, increasing emissions of fatty-acid derivatives, but only B. tabaci increased the expression of genes related to the salicylic acid pathway and the total amount of phenylpropanoids released. Terpenoids were the most abundant compounds in the volatile blends; many terpenoids were emitted at different rates, which might have influenced the arthropods' host selection.

CONCLUSION

Our results indicate that B. tabaci infestation facilitated subsequent infestations by conspecifics and mites, while T. urticae infestation promoted herbivore-induced resistance. Based on both the molecular and behavioral findings, a novel sustainable pest-management strategy is discussed.

Nitrogen and Sulphur Fertilisation for Marketable Yields of Cabbage (Brassica oleracea L. var. Capitata), Leaf Nitrate and Glucosinolates and Nitrogen Losses Studied in a Field Experiment in Central Slovenia

A field trial of white cabbage (Brassica oleracea var. Capitata L.) was carried out under the humid temperate climate conditions in Central Slovenia to investigate the effects of calcium ammonium nitrate (0, 180 and 240 kg N ha^-1) and gypsum (0 and 40 kg S ha^-1) fertilisation on yield, yield quality (nitrate, glucosinolate levels and glucosinolate profile) and nitrogen use efficiency. The highest marketable yield, dry matter yield and nitrogen uptake were obtained at the highest nitrogen fertilisation rate when in combination with sulphur. For this treatment, the nitrogen surplus in the soil after harvesting was lower than for the same nitrogen fertilisation without sulphur application. For the combination N₂₄₀S₄₀, the sulphur addition significantly increased nitrogen use efficiency, which resulted in reduced nitrate content in the cabbage heads. The chemical forms of glucosinolates showed that 80-85% were aliphatic glucosinolates with the remainder as the indole group. For the aliphatic glucosinolates, significant interactions between nitrogen and sulphur fertilisations were reflected in increased levels of progoitrin and glucoiberin when sulphur was applied at the lower nitrogen fertilisation rates. For the indole group, the levels of glucobrassicin and the indole group itself decreased at higher nitrogen fertilisation rates, independent of sulphur fertilisation.

Insights Into the Mechanisms Implicated in Pinus pinaster Resistance to Pinewood Nematode

Pine wilt disease (PWD), caused by the plant-parasitic nematode Bursaphelenchus xylophilus, has become a severe environmental problem in the Iberian Peninsula with devastating effects in Pinus pinaster forests. Despite the high levels of this species' susceptibility, previous studies reported heritable resistance in P. pinaster trees. Understanding the basis of this resistance can be of extreme relevance for future programs aiming at reducing the disease impact on P. pinaster forests. In this study, we highlighted the mechanisms possibly involved in P. pinaster resistance to PWD, by comparing the transcriptional changes between resistant and susceptible plants after infection. Our analysis revealed a higher number of differentially expressed genes (DEGs) in resistant plants (1,916) when compared with susceptible plants (1,226). Resistance to PWN is mediated by the induction of the jasmonic acid (JA) defense pathway, secondary metabolism pathways, lignin synthesis, oxidative stress response genes, and resistance genes. Quantification of the acetyl bromide-soluble lignin confirmed a significant increase of cell wall lignification of stem tissues around the inoculation zone in resistant plants. In addition to less lignified cell walls, susceptibility to the pine wood nematode seems associated with the activation of the salicylic acid (SA) defense pathway at 72 hpi, as revealed by the higher SA levels in the tissues of susceptible plants. Cell wall reinforcement and hormone signaling mechanisms seem therefore essential for a resistance response.

Plant Defense Networks against Insect-Borne Pathogens

Upon infection with insect-borne microbial pathogens, plants are exposed to two types of damage simultaneously. Over the past decade, numerous molecular studies have been conducted to understand how plants respond to pathogens or herbivores. However, investigations of host responses typically focus on a single stress and are performed under static laboratory conditions. In this review, we highlight research that sheds light on how plants deploy broad-spectrum mechanisms against both vector-borne pathogens and insect vectors. Among the host genes involved in multistress resistance, many are involved in innate immunity and phytohormone signaling (especially jasmonate and salicylic acid). The potential for genome editing or chemical modulators to fine-tune crop defensive signaling, to develop sustainable methods to control insect-borne diseases, is discussed.

Genome-wide association analysis of Russian wheat aphid (Diuraphis noxia) resistance in Dn4 derived wheat lines evaluated in South Africa

Russian wheat aphid (RWA; Diuraphis noxia Kurdjumov) resistance on the 1D chromosome of wheat has been the subject of intensive research. Conversely, the deployment of the Dn4 derived RWA resistant varieties diminished in recent years due to the overcoming of the resistance it imparts in the United States of America. However, this resistance has not been deployed in South Africa despite reports that Dn4 containing genotypes exhibited varying levels of resistance against the South African RWA biotypes. It is possible that there may be certain genetic differences within breeding lines or cultivars that influence the expression of resistance. The aim of this study was to identify single nucleotide polymorphism (SNP) markers associated with resistance to South African RWA biotypes. A panel of thirty-two wheat lines were phenotyped for RWA resistance using four South African RWA biotypes and a total of 181 samples were genotyped using the Illumina 9K SNP wheat chip. A genome wide association study using 7598 polymorphic SNPs showed that the population was clustered into two distinct subpopulations. Twenty-seven marker trait associations (MTA) were identified with an average linkage disequilibrium of 0.38 at 10 Mbp. Four of these markers were highly significant and three correlated with previously reported quantitative trait loci linked to RWA resistance in wheat. Twenty putative genes were annotated using the IWGSC RefSeq, three of which are linked to plant defence responses. This study identified novel chromosomal regions that contribute to RWA resistance and contributes to unravelling the complex genetics that control RWA resistance in wheat.

Phytohormone responses in pepper (Capsicum annuum L.) leaves under a high density of aphid infestation

The time course response of selected phytohormones has been evaluated in sweet pepper plants (Capsicum annuum L.) submitted to a high density (200 aphids/plant) of aphid (Myzus persicae Sulzer) infestation. Abscisic acid (ABA), salicylic acid (SA), indole-3-acetic acid (IAA), and jasmonates (JAs), including jasmonic acid (JA), jasmonoyl-l-isoleucine (JA-Ile), and cis-OPDA have been simultaneously identified and quantitated by UHPLC-MS/MS in pepper leaf tissue harvested at 3, 8 hours post-infestation (hpi), 1, 2, 4 and 7 days post-infestation (dpi). Infested plants showed a reduction in stem length at 7 dpi and in the number of leaves and leaf width from 4 dpi onwards. JA and JA-Ile significantly increased very early (from 3 hpi) while SA only accumulated at 7 dpi. Despite the high density of infestation, the aphid-induced accumulation of JAs was much lower than the burst typically induced by chewing herbivores. On the other side, ABA peaked in aphid-infested plants at 2 and 4 dpi, while IAA content did not change significantly at any time point. Growth inhibition may be partially explained by the high levels of JAs found in aphid-infested plants. The possibility that the obtained results support the hypothesis of the aphid manipulation of plant metabolism is discussed.

Glucosinolate Biosynthesis and the Glucosinolate–Myrosinase System in Plant Defense

Insect pests represent a major global challenge to important agricultural crops. Insecticides are often applied to combat such pests, but their use has caused additional challenges such as environmental contamination and human health issues. Over millions of years, plants have evolved natural defense mechanisms to overcome insect pests and pathogens. One such mechanism is the production of natural repellents or specialized metabolites like glucosinolates. There are three types of glucosinolates produced in the order Brassicales: aliphatic, indole, and benzenic glucosinolates. Upon insect herbivory, a “mustard oil bomb” consisting of glucosinolates and their hydrolyzing enzymes (myrosinases) is triggered to release toxic degradation products that act as insect deterrents. This review aims to provide a comprehensive summary of glucosinolate biosynthesis, the “mustard oil bomb”, and how these metabolites function in plant defense against pathogens and insects. Understanding these defense mechanisms will not only allow us to harness the benefits of this group of natural metabolites for enhancing pest control in Brassicales crops but also to transfer the “mustard oil bomb” to non-glucosinolate producing crops to boost their defense and thereby reduce the use of chemical pesticides.

Differential Defense Responses of Upland and Lowland Switchgrass Cultivars to a Cereal Aphid Pest

Yellow sugarcane aphid (YSA) (Sipha flava, Forbes) is a damaging pest on many grasses. Switchgrass (Panicum virgatum L.), a perennial C4 grass, has been selected as a bioenergy feedstock because of its perceived resilience to abiotic and biotic stresses. Aphid infestation on switchgrass has the potential to reduce the yields and biomass quantity. Here, the global defense response of switchgrass cultivars Summer and Kanlow to YSA feeding was analyzed by RNA-seq and metabolite analysis at 5, 10, and 15 days after infestation. Genes upregulated by infestation were more common in both cultivars compared to downregulated genes. In total, a higher number of differentially expressed genes (DEGs) were found in the YSA susceptible cultivar (Summer), and fewer DEGs were observed in the YSA resistant cultivar (Kanlow). Interestingly, no downregulated genes were found in common between each time point or between the two switchgrass cultivars. Gene co-expression analysis revealed upregulated genes in Kanlow were associated with functions such as flavonoid, oxidation-response to chemical, or wax composition. Downregulated genes for the cultivar Summer were found in co-expression modules with gene functions related to plant defense mechanisms or cell wall composition. Global analysis of defense networks of the two cultivars uncovered differential mechanisms associated with resistance or susceptibility of switchgrass in response to YSA infestation. Several gene co-expression modules and transcription factors correlated with these differential defense responses. Overall, the YSA-resistant Kanlow plants have an enhanced defense even under aphid uninfested conditions.

Coulter J, Liu C. Phytohormone Cross-Talk Synthesizes Glycoalkaloids in Potato (Solanum tuberosum L.) in Response to Aphid (Myzus persicae Sulzer) Infestation under Drought Stress

Simple Summary

Potato (Solanum tuberosum L.) is a vegetable crop that plays a major role in global food security. However, its production and sustainability are adversely affected due to aphid infestation. The green peach aphid, Myzus persicae (Sulzer), poses a significant threat to potato plants globally due to its high production of honeydew and transmission of viruses. Other researchers reported that drought severity could result in an outbreak of insect pests such as aphids. Accordingly, understanding the mechanism of host plant defense against aphids under drought stress is a major concern for insect pest management. This study was conducted to examine the cross-talk of phytohormones in potato glycoalkaloids’ defense against green peach aphids under greenhouse conditions. The results showed that under drought conditions, the cross-talks of phytohormones do not only function as signal hormones, but also modify host plant secondary metabolites to defend against sap-sucking insects. Many potato cultivars may activate phytohormones under drought stress; however, only host plants with a greater level of secondary metabolites may be able to defend against aphid attack. This research will provide a scientific basis for the development of potato varieties with high yield, improved quality, and drought and pest resistance.

Abstract

Potato production is adversely affected by aphid infestation across the globe. Understanding the mechanism of host plant defense against aphids under drought stress is paramount for insect pest management. This study was conducted to examine the cross-talk of phytohormones in potato glycoalkaloids’ defense against green peach aphids under greenhouse conditions. A 3 × 2 × 2 factorial experiment comprising three potato cultivars (Qingshu 9, Longshu 3, and Atlantic) and two levels each of water availability and aphid infestation was conducted. The results show that under drought stress, green peach aphids thrive well on host plants, which contain a relatively high water content. The resistant cultivar DXY, which exhibited a higher level of phytohormones, also demonstrated higher α-chaconine and α-solanine contents in both leaf and root, under drought and aphid stress. Conversely, the susceptible cultivar QS9, which exhibited a lower level of phytohormones, also demonstrated low α-chaconine and α-solanine contents in both leaf and root, under drought and aphid stress. The DXY cultivar, which possessed high resistant traits such as α-chaconine and α-solanine, can be used in areas where green peach aphid infestation is a major setback.

Early Molecular Responses of Tomato to Combined Moderate Water Stress and Tomato Red Spider Mite Tetranychus evansi Attack

Interaction between plants and their environment is changing as a consequence of the climate change and global warming, increasing the performance and dispersal of some pest species which become invasive species. Tetranychus evansi also known as the tomato red spider mite, is an invasive species which has been reported to increase its performance when feeding in the tomato cultivar Moneymaker (MM) under water deficit conditions. In order to clarify the underlying molecular events involved, we examined early plant molecular changes occurring on MM during T. evansi infestation alone or in combination with moderate drought stress. Hormonal profiling of MM plants showed an increase in abscisic acid (ABA) levels in drought-stressed plants while salicylic acid (SA) levels were higher in drought-stressed plants infested with T. evansi, indicating that SA is involved in the regulation of plant responses to this stress combination. Changes in the expression of ABA-dependent DREB2, NCED1, and RAB18 genes confirmed the presence of drought-dependent molecular responses in tomato plants and indicated that these responses could be modulated by the tomato red spider mite. Tomato metabolic profiling identified 42 differentially altered compounds produced by T. evansi attack, moderate drought stress, and/or their combination, reinforcing the idea of putative manipulation of tomato plant responses by tomato red spider mite. Altogether, these results indicate that the tomato red spider mite acts modulating plant responses to moderate drought stress by interfering with the ABA and SA hormonal responses, providing new insights into the early events occurring on plant biotic and abiotic stress interaction.

Aphid-Responsive Defense Networks in Hybrid Switchgrass

Aphid herbivory elicits plant defense-related networks that are influenced by host genetics. Plants of the upland switchgrass (Panicum virgatum) cultivar Summer can be a suitable host for greenbug aphids (Schizaphis graminum; GB), and yellow sugarcane aphids (Sipha flava, YSA), whereas the lowland cultivar Kanlow exhibited multi-species resistance that curtails aphid reproduction. However, stabilized hybrids of Summer (♀) x Kanlow (♂) (SxK) with improved agronomics can be damaged by both aphids. Here, hormone and metabolite analyses, coupled with RNA-Seq analysis of plant transcriptomes, were utilized to delineate defense networks induced by aphid feeding in SxK switchgrass and pinpoint plant transcription factors (TFs), such as WRKYs that potentially regulate these responses. Abscisic acid (ABA) levels were significantly higher in GB infested plants at 5 and 10 days after infestation (DAI). ABA levels were highest at 15DAI in YSA infested plants. Jasmonic acid levels were significantly elevated under GB infestation, while salicylic acid levels were signifi40cantly elevated only at 15 DAI in YSA infested plants. Similarly, levels of several metabolites were altered in common or specifically to each aphid. YSA infestation induced a significant enrichment of flavonoids consistent with an upregulation of many genes associated with flavonoid biosynthesis at 15DAI. Gene co-expression modules that responded singly to either aphid or in common to both aphids were differentiated and linked to specific TFs. Together, these data provide important clues into the interplay of metabolism and transcriptional remodeling accompanying defense responses to aphid herbivory in hybrid switchgrass.

O3-Induced Priming Defense Associated With the Abscisic Acid Signaling Pathway Enhances Plant Resistance to Bemisia tabaci

Elevated ozone (O₃) modulates phytohormone signals, which subsequently alters the interaction between plants and herbivorous insects. It has been reported that elevated O₃ activates the plant abscisic acid (ABA) signaling pathway, but its cascading effect on the performance of herbivorous insects remains unclear. Here, we used the ABA-deficient tomato mutant notabilis (not) and its wild type, Ailsa Craig (AC), to determine the role of ABA signaling in mediating the effects of elevated O₃ on Bemisia tabaci in field open-top chambers (OTCs). Our results showed that the population abundance and the total phloem-feeding duration of B. tabaci were decreased by O₃ exposure in AC plants compared with not plants. Moreover, elevated O₃ and B. tabaci infestation activated the ABA signaling pathway and enhanced callose deposition in AC plants but had little effect on those in not plants. The exogenous application of a callose synthesis inhibitor (2-DDG) neutralized O₃-induced resistance to B. tabaci, and the application of ABA enhanced callose deposition and exacerbated the negative effects of elevated O₃ on B. tabaci. However, the application of 2-DDG counteracted the negative effects of O₃ exposure on B. tabaci in ABA-treated AC plants. Collectively, this study revealed that callose deposition, which relied on the ABA signaling pathway, was an effective O₃-induced priming defense of tomato plants against B. tabaci infestation.

CIRCADIAN CLOCK-ASSOCIATED1 Controls Resistance to Aphids by Altering Indole Glucosinolate Production

CIRCADIAN CLOCK-ASSOCIATED1 (CCA1), a well-known central circadian clock regulator, coordinates plant responses to environmental challenges. Its daily rhythmic expression in Arabidopsis (Arabidopsis thaliana) confers host resistance to the caterpillar Trichoplusia ni However, it is unclear whether CCA1 plays a role in defense against phloem sap-feeding aphids. In this study, we showed that green peach aphid (Myzus persicae) displayed an intrinsic circadian feeding rhythm. Under constant light, wild-type Columbia-0 (Col-0) Arabidopsis plants coentrained with aphids in the same light/dark cycles exhibited greater antixenotic activity than plants preentrained in the opposite cycle from the aphids. Consistently, circadian mutants cca1-1, cca1-11, lhy-21, ztl-1, ztl-4, and lux-2 suffered more severe damage than Col-0 plants when infested by aphids, suggesting that the Arabidopsis circadian clock plays a defensive role. However, the arrhythmic CCA1 overexpression line (CCA1-OX) displayed strong antixenotic and antibiotic activities despite its loss of circadian regulation. Aphids feeding on CCA1-OX plants exhibited lower reproduction and smaller body size and weight than those on Col-0. Apparently, CCA1 regulates both clock-dependent and -independent defense responses. Systematic investigation based on bioinformatics analyses indicated that resistance to aphids in CCA1-OX plants was due primarily to heightened basal indole glucosinolate levels. Interestingly, aphid feeding induced alternatively spliced intron-retaining CCA1a/b transcripts, which are normally expressed at low levels, whereas expression of the major fully spliced CCA1 transcript remained largely unchanged. We hypothesize that posttranscriptional modulation of CCA1 expression upon aphid infestation maximizes the potential of circadian-mediated defense and stress tolerance while ensuring normal plant development.

Defence gene expression and phloem quality contribute to mesophyll and phloem resistance to aphids in wild barley

Aphids, including the bird cherry-oat aphid (Rhopalosiphum padi), are significant agricultural pests. The wild relative of barley, Hordeum spontaneum 5 (Hsp5), has been described to be partially resistant to R. padi, with this resistance proposed to involve higher thionin and lipoxygenase gene expression. However, the specificity of this resistance to aphids and its underlying mechanistic processes are unknown. In this study, we assessed the specificity of Hsp5 resistance to aphids and analysed differences in aphid probing and feeding behaviour on Hsp5 and a susceptible barley cultivar (Concerto). We found that partial resistance in Hsp5 to R. padi extends to two other aphid pests of grasses. Using the electrical penetration graph technique, we show that partial resistance is mediated by phloem- and mesophyll-based resistance factors that limit aphid phloem ingestion. To gain insight into plant traits responsible for partial resistance, we compared non-glandular trichome density, defence gene expression, and phloem composition of Hsp5 with those of the susceptible barley cultivar Concerto. We show that Hsp5 partial resistance involves elevated basal expression of thionin and phytohormone signalling genes, and a reduction in phloem quality. This study highlights plant traits that may contribute to broad-spectrum partial resistance to aphids in barley.

Molecular Interactions Between Plants and Insect Herbivores

Diverse molecular processes regulate the interactions between plants and insect herbivores. Here, we review genes and proteins that are involved in plant-herbivore interactions and discuss how their discovery has structured the current standard model of plant-herbivore interactions. Plants perceive damage-associated and, possibly, herbivore-associated molecular patterns via receptors that activate early signaling components such as Ca²⁺, reactive oxygen species, and MAP kinases. Specific defense reprogramming proceeds via signaling networks that include phytohormones, secondary metabolites, and transcription factors. Local and systemic regulation of toxins, defense proteins, physical barriers, and tolerance traits protect plants against herbivores. Herbivores counteract plant defenses through biochemical defense deactivation, effector-mediated suppression of defense signaling, and chemically controlled behavioral changes. The molecular basis of plant-herbivore interactions is now well established for model systems. Expanding molecular approaches to unexplored dimensions of plant-insect interactions should be a future priority.

Transcriptional and Chemical Changes in Soybean Leaves in Response to Long-Term Aphid Colonization

Soybean aphids (Aphis glycines Matsumura) are specialized insects that feed on soybean (Glycine max) phloem sap. Transcriptome analyses have shown that resistant soybean plants mount a fast response that limits aphid feeding and population growth. Conversely, defense responses in susceptible plants are slower and it is hypothesized that aphids block effective defenses in the compatible interaction. Unlike other pests, aphids can colonize plants for long periods of time; yet the effect on the plant transcriptome after long-term aphid feeding has not been analyzed for any plant-aphid interaction. We analyzed the susceptible and resistant (Rag1) transcriptome response to aphid feeding in soybean plants colonized by aphids (biotype 1) for 21 days. We found a reduced resistant response and a low level of aphid growth on Rag1 plants, while susceptible plants showed a strong response consistent with pattern-triggered immunity. GO-term analyses identified chitin regulation as one of the most overrepresented classes of genes, suggesting that chitin could be one of the hemipteran-associated molecular pattern that triggers this defense response. Transcriptome analyses also indicated the phenylpropanoid pathway, specifically isoflavonoid biosynthesis, was induced in susceptible plants in response to long-term aphid feeding. Metabolite analyses corroborated this finding. Aphid-treated susceptible plants accumulated daidzein, formononetin, and genistein, although glyceollins were present at low levels in these plants. Choice experiments indicated that daidzein may have a deterrent effect on aphid feeding. Mass spectrometry imaging showed these isoflavones accumulate likely in the mesophyll cells or epidermis and are absent from the vasculature, suggesting that isoflavones are part of a non-phloem defense response that can reduce aphid feeding. While it is likely that aphid can initially block defense responses in compatible interactions, it appears that susceptible soybean plants can eventually mount an effective defense in response to long-term soybean aphid colonization.

Plant Genes Benefitting Aphids-Potential for Exploitation in Resistance Breeding

Aphids are phloem sap-feeding insects common as pests in various crops. Here we review 62 omics studies of aphid/plant interactions to search for indications of how aphids may manipulate the plants to make them more suitable as hosts, i.e. more susceptible. Our aim is to try to reveal host plant susceptibility (S) genes, knowledge which can be exploited for making a plant more resistant to its pest by using new plant breeding techniques to knock out or down such S genes. S genes may be of two types, those that are involved in reducing functional plant defense and those involved in further increasing plant factors that are positive to the aphid, such as facilitated access to food or improved nutritional quality. Approximately 40% of the omics studies we have reviewed indicate how aphids may modify their host to their advantage. To exploit knowledge obtained so far, we suggest knocking out/down candidate aphid S genes using CRISPR/Cas9 or RNAi techniques in crops to evaluate if this will be sufficient to keep the aphid pest at economically viable levels without severe pleiotropic effects. As a complement, we also propose functional studies of recessively inherited resistance previously discovered in some aphid-crop combinations, to potentially identify new types of S genes that later could be knocked out or down also in other crops to improve their resistance to aphids.

Local and systemic hormonal responses in pepper (Capsicum annuum L.) leaves under green peach aphid (Myzus persicae Sulzer) infestation

This study examined the temporal changes in the leaf content of defence-involved phytohormones in pepper (Capsicum annuum L.) plants responding to the green peach aphid (Myzus persicae Sulzer) infestation, at both local and systemic level. Aphid infestation did not alter the content of cis-12-oxo-phytodienoic acid, the jasmonic acid (JA) precursor, even though endogenous levels of JA and its bioactive isoleucine-conjugated form (JA-Ile) significantly increased from 8 to 96 h in local infested leaves. Systemic effects in jasmonates were only showed at 48 h for JA, and 8 and 48 h in the case of JA-Ile. SA accumulated only in local infested leaves after 96 h of infestation, when the level of JA-Ile decreased in these leaves. This suggests a possible antagonistic interaction between JA and SA pathways, although other pathways may be also involved. Endogenous level of indole-3-acetic acid was higher in systemic relative to local infested leaves at 3 and 24 h, although no significant changes in its content were found compared to control leaves. Abscisic acid content was lower in local infested relative to control leaves at 24 h, but was higher at 48 h when it also increased systemically. The possible roles of the studied phytohormones in plant defence responses against aphids are discussed.

Effects of sugarcane aphid herbivory on transcriptional responses of resistant and susceptible sorghum

BACKGROUND

Sugarcane aphid (Melanaphis sacchari) outbreaks in sorghum that were first reported in 2013 are now the most significant threat to this crop in all major sorghum production areas in the U.S. The outcomes of interactions between sugarcane aphid and sorghum and thus the severity of the outbreaks depend on sorghum genotype and potentially also on the phenology of sorghum. Mechanisms underlying these interactions are not known, however. Thus, the goal of this research was to characterize transcriptional changes in a commercially available resistant and a susceptible genotype of sorghum at 2- and 6-wk post-emergence exposed to M. sacchari herbivory. The effects of sorghum age and genotype on the daily change in aphid densities were also evaluated in separate greenhouse experiments.

RESULTS

A higher number of diffentially expressed genes (DEGs) was recovered from the 2-wk plants exposed to aphid herbivory compared to the 6-wk plants across genotypes. Further, gene ontology and pathway analysis indicated a suite of transcriptional changes in the resistant genotype that were weak or absent in the susceptible sorghum. Specifically, the aphid-resistant genotype exposed to M. sacchari up-regulated several genes involved in defense, which was particularly evident in the 2-wk plants that showed the most robust transcriptional responses. These transcriptional changes in the younger resistant sorghum were characterized by induction of hormone-signaling pathways, pathways coding for secondary metabolites, glutathion metabolism, and plant-pathogen interaction. Furthermore, the 2-wk resistant plants appeared to compensate for the effects of oxidative stress induced by sugarcane aphid herbivory with elevated expression of genes involved in detoxification. These transcriptional responses were reflected in the aphid population growth, which was significantly faster in the susceptible and older sorghum than in the resistant and younger plants.

CONCLUSION

This experiment provided the first insights into molecular mechanisms underlying lower population growth of M. sacchari on the resistant sorghum genotype. Further, it appears that the younger resistant sorghum was able to mount a robust defense response following aphid herbivory, which was much weaker in the older sorghum. Several pathways and specific genes provide specific clues into the mechanisms underlying host plant resistance to this invasive insect.

Reciprocal plant-mediated interactions between a virus and a non-vector herbivore

Vector-borne viruses alter many physical and chemical traits of their plant hosts, indirectly affecting the fitness and behavior of vectors in ways that promote virus transmission. However, it is unclear whether viruses induce plant-mediated shifts in the behavior and fitness of non-vector herbivores, or if non-vectors affect the dynamics of vector-borne viruses. Here we evaluated reciprocal interactions between Pea enation mosaic virus (PEMV), a pathogen transmitted by the aphid Acrythosiphon pisum, and a non-vector weevil, Sitona lineatus. In the field, PEMV-infected plants experienced more defoliation from S. lineatus than uninfected plants; behavioral assays similarly showed S. lineatus adults preferred to feed on infected plants. In turn, infectious A. pisum preferred plants damaged by S. lineatus, and S. lineatus herbivory led to increased PEMV titer. These interactions may be mediated by plant phytohormone levels, as S. lineatus induced jasmonic acid, while PEMV induced salicylic acid. Levels of abscisic acid were not affected by attack from either PEMV or S. lineatus alone, but plants challenged by both had elevated levels of this phytohormone. As plant viruses and their vectors often exist in diverse communities, our study highlights the importance of non-vector species in influencing plant pathogens and their vectors through host-mediated effects.

Arabidopsis Transcription Factor MYB102 Increases Plant Susceptibility to Aphids by Substantial Activation of Ethylene Biosynthesis

Induction of ethylene biosynthesis by aphids increases the susceptibility of several plant species to aphids. Recent studies have indicated that some MYB transcription factors regulate the phloem-based defense against aphid infestation by modulating ethylene (ET) signaling. Arabidopsis MYB102 has previously been shown to be induced by wound signaling and regulate defense response against chewing insects. However, it remains unclear whether ArabidopsisMYB102 takes part in the defense response of plants to aphids. Here, we investigated the function of MYB102 in the response of Arabidopsis to aphid infestation. ArabidopsisMYB102 was primarily expressed in vascular tissues, and its transcription was remarkably induced by green peach aphids (GPA; Myzus persicae). The results of RNA-Sequencing revealed that overexpression of MYB102 in Arabidopsis promoted ET biosynthesis by upregulation of some 1-aminocyclopropane-1-carboxylate synthase (ACS) genes, which are rate-limiting enzymes of the ET-synthetic pathway. Enhanced ET levels led to reduced Arabidopsis resistance to GPA. Furthermore, dominant suppression of MYB102 inhibited aphid-induced increase of ET levels in Arabidopsis. In agreement with a negative regulatory role for ET in aphid defense responses, the MYB102-overexpressing lines were more susceptible to GPA than wild-type (WT) plants. Overexpression of MYB102 in Arabidopsis obviously repressed aphid-induced callose deposition. Conversely, overexpression of MYB102 failed to increase aphid susceptibility in both the ET-insensitive mutants and plants treated with inhibitors of ET signaling pathways, demonstrating that the ET was critical for promoting aphid performance conferred by overexpression of MYB102. Collectively, our findings indicate that the Arabidopsis MYB102 increases host susceptibility to GPA through the ET-dependent signaling pathways.

Changes in the free amino acid composition of Capsicum annuum (pepper) leaves in response to Myzus persicae (green peach aphid) infestation. A comparison with water stress

Amino acids play a central role in aphid-plant interactions. They are essential components of plant primary metabolism, function as precursors for the synthesis of defense-related specialized metabolites, and are major growth-limiting nutrients for aphids. To quantify changes in the free amino acid content of pepper (Capsicum annuum L.) leaves in response to green peach aphid (Myzus persicae Sulzer) feeding, plants were infested with a low (20 aphids/plant) or a high (200 aphids/plant) aphid density in time-course experiments ranging from 3 hours to 7 days. A parallel experiment was conducted with pepper plants that had been subjected to water stress. Factor Analysis of Mixed Data revealed a significant interaction of time x density in the free amino acid response of aphid-infested leaves. At low aphid density, M. persicae did not trigger a strong response in pepper leaves. Conversely, at high density, a large increase in total free amino acid content was observed and specific amino acids peaked at different times post-infestation. Comparing aphid-infested with water-stressed plants, most of the observed differences were quantitative. In particular, proline and hydroxyproline accumulated dramatically in response to water stress, but not in response to aphid infestation. Some additional differences and commonalities between the two stress treatments are discussed.

Two invasive herbivores on a shared host: patterns and consequences of phytohormone induction

Herbivore-induced changes in host quality mediate indirect interactions between herbivores. The nature of these indirect interactions can vary depending on the identity of herbivores involved, species-specific induction of defense-signaling pathways, and sequence of attack. However, our understanding of the role of these signaling pathways in the success of multiple exotic herbivores is less known. Eastern hemlock (Tsuga canadensis) is attacked by two invasive herbivores [elongate hemlock scale (EHS; Fiorinia externa) and hemlock woolly adelgid (HWA; Adelges tsugae)] throughout much of its range, but prior attack by EHS is known to deter HWA. The potential role of phytohormones in this interaction is poorly understood. We measured endogenous levels of phytohormones in eastern hemlock in response to attack by these invasive herbivores. We also used exogenous application of methyl jasmonate (MJ) and acibenzolar-S-methyl (ASM), a salicylic acid (SA) pathway elicitor, to test the hypothesis that defense-signaling phytohormones typically induced by herbivores could deter HWA. Resistance to adelgid attack was assessed using a behavioral assay. Adelgid feeding significantly elevated both abscisic acid (ABA) and SA in local tissues, while EHS feeding had no detectable effect on either phytohormone. HWA progrediens and sistens crawlers preferred to settle on ASM-treated foliage. In contrast, HWA crawlers actively avoided settlement on MJ-treated foliage. We suggest that induction of ABA- and SA-signaling pathways, in concert with defense-signaling interference, may aid HWA invasion success, and that defense-signaling interference, induced by exotic competitors, may mediate resistance of native hosts.

Does drought stress modify the effects of plant-growth promoting rhizobacteria on an aboveground chewing herbivore?

Soil microbes have important effects on the interactions of plants with their environment, by promoting plant growth, inducing resistance to pests or by conferring tolerance to abiotic stress. However, their effects are variable and the factors responsible for this variation are mainly unknown. Our aim was to assess how drought stress modifies the effect of the nonpathogenic rhizobacterium Pseudomonas simiae WCS417r on plant growth and resistance against the generalist leaf-chewing caterpillar Mamestra brassicae. We studied Arabidopsis thaliana Col-0 plants, as well as mutants altered in the biosynthesis of the phytohormones jasmonic acid (JA) and abscisic acid (ABA). Caterpillars did not prefer rhizobacteria-treated plants, independently of drought stress. Rhizobacteria colonization had a variable effect on caterpillar performance, which ranged from positive in one experiment to neutral in a second one. Drought had a consistent negative effect on herbivore performance; however, it did not modify the effect of rhizobacteria on herbivore performance. The effect of drought on herbivore performance was JA-mediated (confirmed with the use of the dde2-2 mutant), but it was still present in the ABA-deficient mutant aba2-1. Plant biomass was reduced by both drought and herbivory but it was enhanced by rhizobacterial colonization. Pseudomonas simiae WCS417r is able to promote plant growth even when plants are suffering herbivory. Nevertheless, the microbial effect on the herbivore is variable, independently of drought stress. To get the best possible outcome from the rhizobacteria-plant mutualism it is important to understand which other factors may be responsible for its context-dependency.

MATI, a Novel Protein Involved in the Regulation of Herbivore-Associated Signaling Pathways

The defense response of the plants against herbivores relies on a complex network of interconnected signaling pathways. In this work, we characterized a new key player in the response of Arabidopsis against the two-spotted spider mite Tetranychus urticae, the MATI (Mite Attack Triggered Immunity) gene. This gene was differentially induced in resistant Bla-2 strain relative to susceptible Kon Arabidopsis accessions after mite attack, suggesting a potential role in the control of spider mites. To study the MATI gene function, it has been performed a deep molecular characterization of the gene combined with feeding bioassays using modified Arabidopsis lines and phytophagous arthropods. The MATI gene belongs to a new gene family that had not been previously characterized. Biotic assays showed that it confers a high tolerance not only to T. urticae, but also to the chewing lepidopteran Spodoptera exigua. Biochemical analyses suggest that MATI encodes a protein involved in the accumulation of reducing agents upon herbivore attack to control plant redox homeostasis avoiding oxidative damage and cell death. Besides, molecular analyses demonstrated that MATI is involved in the modulation of different hormonal signaling pathways, affecting the expression of genes involved in biosynthesis and signaling of the jasmonic acid and salicylic acid hormones. The fact that MATI is also involved in defense through the modulation of the levels of photosynthetic pigments highlights the potential of MATI proteins to be exploited as biotechnological tools for pest control.

Modulation of Legume Defense Signaling Pathways by Native and Non-native Pea Aphid Clones

The pea aphid (Acyrthosiphon pisum) is a complex of at least 15 genetically different host races that are native to specific legume plants, but can all develop on the universal host plant Vicia faba. Despite much research, it is still unclear why pea aphid host races (biotypes) are able to colonize their native hosts while other host races are not. All aphids penetrate the plant and salivate into plant cells when they test plant suitability. Thus plants might react differently to the various pea aphid host races. To find out whether legume species vary in their defense responses to different pea aphid host races, we measured the amounts of salicylic acid (SA), the jasmonic acid-isoleucine conjugate (JA-Ile), other jasmonate precursors and derivatives, and abscisic acid (ABA) in four different species (Medicago sativa, Trifolium pratense, Pisum sativum, V. faba) after infestation by native and non-native pea aphid clones of various host races. Additionally, we assessed the performance of the clones on the four plant species. On M. sativa and T. pratense, non-native clones that were barely able to survive or reproduce, triggered a strong SA and JA-Ile response, whereas infestation with native clones led to lower levels of both phytohormones. On P. sativum, non-native clones, which survived or reproduced to a certain extent, induced fluctuating SA and JA-Ile levels, whereas the native clone triggered only a weak SA and JA-Ile response. On the universal host V. faba all aphid clones triggered only low SA levels initially, but induced clone-specific patterns of SA and JA-Ile later on. The levels of the active JA-Ile conjugate and of the other JA-pathway metabolites measured showed in many cases similar patterns, suggesting that the reduction in JA signaling was due to an effect upstream of OPDA. ABA levels were downregulated in all aphid clone-plant combinations and were therefore probably not decisive factors for aphid-plant compatibility. Our results suggest that A. pisum clones manipulate plant-defense signaling to their own advantage, and perform better on their native hosts due to their ability to modulate the SA- and JA-defense signaling pathways.

Brevicoryne brassicae aphids interfere with transcriptome responses of Arabidopsis thaliana to feeding by Plutella xylostella caterpillars in a density-dependent manner

Plants are commonly attacked by multiple herbivorous species. Yet, little is known about transcriptional patterns underlying plant responses to multiple insect attackers feeding simultaneously. Here, we assessed transcriptomic responses of Arabidopsis thaliana plants to simultaneous feeding by Plutella xylostella caterpillars and Brevicoryne brassicae aphids in comparison to plants infested by P. xylostella caterpillars alone, using microarray analysis. We particularly investigated how aphid feeding interferes with the transcriptomic response to P. xylostella caterpillars and whether this interference is dependent on aphid density and time since aphid attack. Various JA-responsive genes were up-regulated in response to feeding by P. xylostella caterpillars. The additional presence of aphids, both at low and high densities, clearly affected the transcriptional plant response to caterpillars. Interestingly, some important modulators of plant defense signalling, including WRKY transcription factor genes and ABA-dependent genes, were differentially induced in response to simultaneous aphid feeding at low or high density compared with responses to P. xylostella caterpillars feeding alone. Furthermore, aphids affected the P. xylostella-induced transcriptomic response in a density-dependent manner, which caused an acceleration in plant response against dual insect attack at high aphid density compared to dual insect attack at low aphid density. In conclusion, our study provides evidence that aphids influence the caterpillar-induced transcriptional response of A. thaliana in a density-dependent manner. It highlights the importance of addressing insect density to understand how plant responses to single attackers interfere with responses to other attackers and thus underlines the importance of the dynamics of transcriptional plant responses to multiple herbivory.

Metabolite Profiling Reveals a Specific Response in Tomato to Predaceous Chrysoperla carnea Larvae and Herbivore(s)-Predator Interactions with the Generalist Pests Tetranychus urticae and Myzus persicae

The spider mite Tetranychus urticae Koch and the aphid Myzus persicae (Sulzer) both infest a number of economically significant crops, including tomato (Solanum lycopersicum). Although used for decades to control pests, the impact of green lacewing larvae Chrysoperla carnea (Stephens) on plant biochemistry was not investigated. Here, we used profiling methods and targeted analyses to explore the impact of the predator and herbivore(s)-predator interactions on tomato biochemistry. Each pest and pest-predator combination induced a characteristic metabolite signature in the leaf and the fruit thus, the plant exhibited a systemic response. The treatments had a stronger impact on non-volatile metabolites including abscisic acid and amino acids in the leaves in comparison with the fruits. In contrast, the various biotic factors had a greater impact on the carotenoids in the fruits. We identified volatiles such as myrcene and α-terpinene which were induced by pest-predator interactions but not by single species, and we demonstrated the involvement of the phytohormone abscisic acid in tritrophic interactions for the first time. More importantly, C. carnea larvae alone impacted the plant metabolome, but the predator did not appear to elicit particular defense pathways on its own. Since the presence of both C. carnea larvae and pest individuals elicited volatiles which were shown to contribute to plant defense, C. carnea larvae could therefore contribute to the reduction of pest infestation, not only by its preying activity, but also by priming responses to generalist herbivores such as T. urticae and M. persicae. On the other hand, the use of C. carnea larvae alone did not impact carotenoids thus, was not prejudicial to the fruit quality. The present piece of research highlights the specific impact of predator and tritrophic interactions with green lacewing larvae, spider mites, and aphids on different components of the tomato primary and secondary metabolism for the first time, and provides cues for further in-depth studies aiming to integrate entomological approaches and plant biochemistry.

Cross-tolerance to biotic and abiotic stresses in plants: a focus on resistance to aphid infestation

Plants co-evolved with an enormous variety of microbial pathogens and insect herbivores under daily and seasonal variations in abiotic environmental conditions. Hence, plant cells display a high capacity to respond to diverse stresses through a flexible and finely balanced response network that involves components such as reduction-oxidation (redox) signalling pathways, stress hormones and growth regulators, as well as calcium and protein kinase cascades. Biotic and abiotic stress responses use common signals, pathways and triggers leading to cross-tolerance phenomena, whereby exposure to one type of stress can activate plant responses that facilitate tolerance to several different types of stress. While the acclimation mechanisms and adaptive responses that facilitate responses to single biotic and abiotic stresses have been extensively characterized, relatively little information is available on the dynamic aspects of combined biotic/abiotic stress response. In this review, we consider how the abiotic environment influences plant responses to attack by phloem-feeding aphids. Unravelling the signalling cascades that underpin cross-tolerance to biotic and abiotic stresses will allow the identification of new targets for increasing environmental resilience in crops.

Up-regulation of abscisic acid signaling pathway facilitates aphid xylem absorption and osmoregulation under drought stress

The activation of the abscisic acid (ABA) signaling pathway reduces water loss from plants challenged by drought stress. The effect of drought-induced ABA signaling on the defense and nutrition allocation of plants is largely unknown. We postulated that these changes can affect herbivorous insects. We studied the effects of drought on different feeding stages of pea aphids in the wild-type A17 of Medicago truncatula and ABA signaling pathway mutant sta-1. We examined the impact of drought on plant water status, induced plant defense signaling via the abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA) pathways, and on the host nutritional quality in terms of leaf free amino acid content. During the penetration phase of aphid feeding, drought decreased epidermis/mesophyll resistance but increased mesophyll/phloem resistance of A17 but not sta-1 plants. Quantification of transcripts associated with ABA, JA and SA signaling indicated that the drought-induced up-regulation of ABA signaling decreased the SA-dependent defense but increased the JA-dependent defense in A17 plants. During the phloem-feeding phase, drought had little effect on the amino acid concentrations and the associated aphid phloem-feeding parameters in both plant genotypes. In the xylem absorption stage, drought decreased xylem absorption time of aphids in both genotypes because of decreased water potential. Nevertheless, the activation of the ABA signaling pathway increased water-use efficiency of A17 plants by decreasing the stomatal aperture and transpiration rate. In contrast, the water potential of sta-1 plants (unable to close stomata) was too low to support xylem absorption activity of aphids; the aphids on sta-1 plants had the highest hemolymph osmolarity and lowest abundance under drought conditions. Taken together this study illustrates the significance of cross-talk between biotic-abiotic signaling pathways in plant-aphid interaction, and reveals the mechanisms leading to alter aphid fecundity in water stresses plants.

Dynamic Maize Responses to Aphid Feeding Are Revealed by a Time Series of Transcriptomic and Metabolomic Assays

As a response to insect attack, maize (Zea mays) has inducible defenses that involve large changes in gene expression and metabolism. Piercing/sucking insects such as corn leaf aphid (Rhopalosiphum maidis) cause direct damage by acquiring phloem nutrients as well as indirect damage through the transmission of plant viruses. To elucidate the metabolic processes and gene expression changes involved in maize responses to aphid attack, leaves of inbred line B73 were infested with corn leaf aphids for 2 to 96 h. Analysis of infested maize leaves showed two distinct response phases, with the most significant transcriptional and metabolic changes occurring in the first few hours after the initiation of aphid feeding. After 4 d, both gene expression and metabolite profiles of aphid-infested maize reverted to being more similar to those of control plants. Although there was a predominant effect of salicylic acid regulation, gene expression changes also indicated prolonged induction of oxylipins, although not necessarily jasmonic acid, in aphid-infested maize. The role of specific metabolic pathways was confirmed using Dissociator transposon insertions in maize inbred line W22. Mutations in three benzoxazinoid biosynthesis genes, Bx1, Bx2, and Bx6, increased aphid reproduction. In contrast, progeny production was greatly decreased by a transposon insertion in the single W22 homolog of the previously uncharacterized B73 terpene synthases TPS2 and TPS3. Together, these results show that maize leaves shift to implementation of physical and chemical defenses within hours after the initiation of aphid feeding and that the production of specific metabolites can have major effects in maize-aphid interactions.