Egg laying of cabbage white butterfly (Pieris brassicae) on Arabidopsis thaliana affects subsequent performance of the larvae

Butterfly eggs prime anti-herbivore defense in an annual but not perennial Arabidopsis species

MAIN CONCLUSION

Unlike Arabidopsis thaliana, defenses of Arabidopsis lyrata against Pieris brassicae larval feeding are not primable by P. brassicae eggs. Thus, egg primability of plant anti-herbivore defenses is not phylogenetically conserved in the genus Arabidopsis. While plant anti-herbivore defenses of the annual species Arabidopsis thaliana were shown to be primable by Pieris brassicae eggs, the primability of the phylogenetically closely related perennial Arabidopsis lyrata has not yet been investigated. Previous studies revealed that closely related wild Brassicaceae plant species, the annual Brassica nigra and the perennial B. oleracea, exhibit an egg-primable defense trait, even though they have different life spans. Here, we tested whether P. brassicae eggs prime anti-herbivore defenses of the perennial A. lyrata. We exposed A. lyrata to P. brassicae eggs and larval feeding and assessed their primability by (i) determining the biomass of P. brassicae larvae after feeding on plants with and without prior P. brassicae egg deposition and (ii) investigating the plant transcriptomic response after egg deposition and/or larval feeding. For comparison, these studies were also conducted with A. thaliana. Consistent with previous findings, A. thaliana's response to prior P. brassicae egg deposition negatively affected conspecific larvae feeding upon A. thaliana. However, this was not observed in A. lyrata. Arabidopsis thaliana responded to P. brassicae eggs with strong transcriptional reprogramming, whereas A. lyrata responses to eggs were negligible. In response to larval feeding, A. lyrata exhibited a greater transcriptome change compared to A. thaliana. Among the strongly feeding-induced A. lyrata genes were those that are egg-primed in feeding-induced A. thaliana, i.e., CAX3, PR1, PR5, and PDF1.4. These results suggest that A. lyrata has evolved a robust feeding response that is independent from prior egg exposure.

Unprecedented oviposition tactics avoid plant defences and reduce attack by parasitic wasps

Female insects oviposit in sites suitable for the development of their offspring. The Oriental armyworm, Mythimna separata is a serious pest of various crops including wheat and prefers to oviposit on withered leaves rather than on fresh plant material, which is surprisingly different from other insects. Studies here showed that this oviposition tactic enables avoidance of wheat defence against eggs and emerged larvae. Intact plants responded to M. separata egg deposition by releasing oviposition-induced plant volatiles including acetophenone, tetradecene and pentadecane after 24 h. Acetophenone was identified as quantitatively accounting for the attraction of the egg parasitoid wasp (Trichogramma chilonis). Leaf jasmonic acid levels significantly increased after M. separata laid eggs, and primed the plant against emerging larvae. In addition, newly emerged M. separata larvae adopted a fast crawling behaviour and starvation tolerance compared with other noctuid larvae, which enhanced the survival of larvae on the withered leaves. The elucidation of this complex and surprising plant-insect interaction provides the first explanation for a herbivore laying eggs on withered leaves to avoid natural enemies and live-plant defence against emerging larvae.

Sawfly egg deposition extends the insect life cycle and alters hormone and volatile emission profiles

Introduction

Insect oviposition can enhance plant defenses and decrease plant quality in response to future feeding damage by hatched larvae. Induced resistance triggered by egg deposition and its negative effect on insect herbivore performance is known for several annual plants but has been much less studied in woody perennials, such as species of the Salicaceae. Here we studied the response of the willow Salix babylonica to oviposition by the specialist willow sawfly Nematus oligospilus and its impact on insect performance.

Methods

We measured the effect of oviposition on larval feeding and pupa formation and evaluated its influence on plant phytohormones and volatile emission profile.

Results

We showed that oviposition reduced neonate larval growth and increased the proportion of prepupae that delayed their transition to pupae, thus extending the length of the sawfly cocoon phase. Oviposited willows increased jasmonic acid levels and changed their volatile profile through enhanced concentrations of the terpenoids, (E/E)-α-farnesene, (Z)- and (E)-β-ocimene. Volatile profiles were characteristic for each type of insect damage (oviposition vs. feeding), but no priming effect was found.

Discussion

We demonstrated that willows could perceive sawfly oviposition per se as a primary factor activating defense signaling via the jasmonic acid pathway. This induced response ultimately determined changes in pupation dynamics that may affect the whole insect population cycle.

Glucosinolates and Biotic Stress Tolerance in Brassicaceae with Emphasis on Cabbage: A Review

Glucosinolates (GSLs) and GSL-associated genes are receiving increasing attention from molecular biologists due to their multifunctional properties. GSLs are secondary metabolites considered to be highly active in most Brassica species. Their importance has motivated the discovery and functional analysis of the GSLs and GSL hydrolysis products involved in disease development in brassicas and other plants. Comprehensive knowledge of the GSL content of Brassica species and the molecular details of GSL-related genes will help elucidate the molecular control of this plant defense system. This report provides an overview of the current status of knowledge on GSLs, GSL biosynthesis, as well as hydrolysis related genes, and GSL hydrolysis products that regulate fungal, bacterial, and insect resistance in cabbage and other brassicas.

Priming of Arabidopsis resistance to herbivory by insect egg deposition depends on the plant's developmental stage

While traits of plant resistance to herbivory often change during ontogeny, it is unknown whether the primability of this resistance depends on the plant's developmental stage. Resistance in non-flowering Arabidopsis thaliana against Pieris brassicae larvae is known to be primable by prior egg deposition on leaves. We investigated whether this priming effect is maintained in plants at the flowering stage. Larval performance assays revealed that flowering plants' resistance to herbivory was not primable by egg deposition. Accordingly, transcriptomes of flowering plants showed almost no response to eggs. In contrast, egg deposition on non-flowering plants enhanced the expression of genes induced by subsequent larval feeding. Strikingly, flowering plants showed constitutively high expression levels of these genes. Larvae performed generally worse on flowering than on non-flowering plants, indicating that flowering plants constitutively resist herbivory. Furthermore, we determined the seed weight in regrown plants that had been exposed to eggs and larvae during the non-flowering or flowering stage. Non-flowering plants benefitted from egg priming with a smaller loss in seed yield. The seed yield of flowering plants was unaffected by the treatments, indicating tolerance towards the larvae. Our results show that the primability of anti-herbivore defences in Arabidopsis depends on the plant's developmental stage.

Sphingolipids are involved in insect egg-induced cell death in Arabidopsis

In Brassicaceae, hypersensitive-like programmed cell death (HR-like) is a central component of direct defenses triggered against eggs of the large white butterfly (Pieris brassicae). The signaling pathway leading to HR-like in Arabidopsis (Arabidopsis thaliana) is mainly dependent on salicylic acid (SA) accumulation, but downstream components are unclear. Here, we found that treatment with P. brassicae egg extract (EE) triggered changes in expression of sphingolipid metabolism genes in Arabidopsis and black mustard (Brassica nigra). Disruption of ceramide (Cer) synthase activity led to a significant decrease of EE-induced HR-like whereas SA signaling and reactive oxygen species levels were unchanged, suggesting that Cer are downstream activators of HR-like. Sphingolipid quantifications showed that Cer with C16:0 side chains accumulated in both plant species and this response was largely unchanged in the SA-induction deficient2 (sid2-1) mutant. Finally, we provide genetic evidence that the modification of fatty acyl chains of sphingolipids modulates HR-like. Altogether, these results show that sphingolipids play a key and specific role during insect egg-triggered HR-like.

Insect eggs trigger systemic acquired resistance against a fungal and an oomycete pathogen

Plants are able to detect insect eggs deposited on leaves. In Arabidopsis, eggs of the butterfly species Pieris brassicae (common name large white) induce plant defenses and activate the salicylic acid (SA) pathway. We previously discovered that oviposition triggers a systemic acquired resistance (SAR) against the bacterial hemibiotroph pathogen Pseudomonas syringae. Here, we show that insect eggs or treatment with egg extract (EE) induce SAR against the fungal necrotroph Botrytis cinerea BMM and the oomycete pathogen Hyaloperonospora arabidopsidis Noco2. This response is abolished in ics1, ald1 and fmo1, indicating that the SA pathway and the N-hydroxypipecolic acid (NHP) pathway are involved. Establishment of EE-induced SAR in distal leaves potentially involves tryptophan-derived metabolites, including camalexin. Indeed, SAR is abolished in the biosynthesis mutants cyp79B2 cyp79B3, cyp71a12 cyp71a13 and pad3-1, and camalexin is toxic to B. cinerea in vitro. This study reveals an interesting mechanism by which lepidopteran eggs interfere with plant-pathogen interactions.

Responses of Trichogramma pretiosum (Hymenoptera: Trichogrammatidae) to Rice and Corn Plants, Fed and Oviposited by Spodoptera frugiperda (Lepidoptera: Noctuidae)

The search behavior and parasitism of trichogrammatids can be affected by volatile compounds emitted by plants under herbivory and/or oviposition. Our aim was to evaluate the chemotactic behavior and parasitism rates of Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) females against two varieties of corn and one of rice that underwent herbivory or oviposition by Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae). In a glass Y-tube olfactometer, the parasitoids were given the choice between plants damaged by either herbivory or oviposition, with and without sentinel eggs, against those without damage. We also evaluated the average of parasitized eggs and the parasitoid emergence in sentinel eggs, which were next to plants that underwent herbivory contrasted with eggs next to undamaged plants. Trichogramma pretiosum was more attracted to rice and corn plants evaluated 24 h after herbivory compared to undamaged plants. Parasitoids preferred oviposited rice plants over control plants. Oviposited corn plants after 48 h were more attractive than non-oviposited plants without sentinel eggs. In the presence of sentinel eggs on the olfactometer tests, there was no difference in oviposition preference in corn. Parasitism was higher in sentinel eggs located near plants damaged by herbivory. This suggested that the egg parasitoid T. pretiosum not only uses chemical clues from rice and corn plants, damaged by herbivory, but also uses them as a strategy to search and increase parasitism in S. frugiperda eggs. However, the results of oviposition tests showed that plants of different species and varieties might respond differently to this type of damage.

Spider mite egg extract modifies Arabidopsis response to future infestations

Transcriptional plant responses are an important aspect of herbivore oviposition studies. However, most of our current knowledge is derived from studies using Lepidopteran models, where egg-laying and feeding are separate events in time. Little is known regarding plant response to pests where females feed and oviposit simultaneously. The present study characterized oviposition-induced transcriptomic response of Arabidopsis to Tetranychus urticae egg extracts. Transcriptional evidence indicates that early events in plant response to the egg extract involve responses typical to biotic stresses, which include the alteration in the levels of Ca²⁺ and ROS, the modification of pathways regulated by the phytohormones jasmonic acid and ethylene, and the production of volatiles and glucosinolates as defence mechanisms. These molecular changes affect female fertility, which was significantly reduced when mites fed on plants pre-exposed to the egg extract. However, longer periods of plant exposure to egg extract cause changes in the transcriptional response of the plant reveal a trend to a decrease in the activation of the defensive response. This alteration correlated with a shift at 72 h of exposition in the effect of the mite feeding. At that point, plants become more susceptible and suffer higher damage when challenged by the mite.

Neighbor GWAS: incorporating neighbor genotypic identity into genome-wide association studies of field herbivory

An increasing number of field studies have shown that the phenotype of an individual plant depends not only on its genotype but also on those of neighboring plants; however, this fact is not taken into consideration in genome-wide association studies (GWAS). Based on the Ising model of ferromagnetism, we incorporated neighbor genotypic identity into a regression model, named "Neighbor GWAS". Our simulations showed that the effective range of neighbor effects could be estimated using an observed phenotype when the proportion of phenotypic variation explained (PVE) by neighbor effects peaked. The spatial scale of the first nearest neighbors gave the maximum power to detect the causal variants responsible for neighbor effects, unless their effective range was too broad. However, if the effective range of the neighbor effects was broad and minor allele frequencies were low, there was collinearity between the self and neighbor effects. To suppress the false positive detection of neighbor effects, the fixed effect and variance components involved in the neighbor effects should be tested in comparison with a standard GWAS model. We applied neighbor GWAS to field herbivory data from 199 accessions of Arabidopsis thaliana and found that neighbor effects explained 8% more of the PVE of the observed damage than standard GWAS. The neighbor GWAS method provides a novel tool that could facilitate the analysis of complex traits in spatially structured environments and is available as an R package at CRAN ( https://cran.rproject.org/package=rNeighborGWAS ).

Priming by Timing: Arabidopsis thaliana Adjusts Its Priming Response to Lepidoptera Eggs to the Time of Larval Hatching

Plants can respond to eggs laid by herbivorous insects on their leaves by preparing (priming) their defense against the hatching larvae. Egg-mediated priming of defense is known for several plant species, including Brassicaceae. However, it is unknown yet for how long the eggs need to remain on a plant until a primed defense state is reached, which is ecologically manifested by reduced performance of the hatching larvae. To address this question, we used Arabidopsis thaliana, which carried eggs of the butterfly Pieris brassicae for 1-6 days prior to exposure to larval feeding. Our results show that larvae gained less biomass the longer the eggs had previously been on the plant. The strongest priming effect was obtained when eggs had been on the plant for 5 or 6 days, i.e., for (almost) the entire development time of the Pieris embryo inside the egg until larval hatching. Transcript levels of priming-responsive genes, levels of jasmonic acid-isoleucine (JA-Ile), and of the egg-inducible phytoalexin camalexin increased with the egg exposure time. Larval performance studies on mutant plants revealed that camalexin is dispensable for anti-herbivore defense against P. brassicae larvae, whereas JA-Ile - in concert with egg-induced salicylic acid (SA) - seems to be important for signaling egg-mediated primed defense. Thus, A. thaliana adjusts the kinetics of its egg-primed response to the time point of larval hatching. Hence, the plant is optimally prepared just in time prior to larval hatching.

Arabidopsis, tobacco, nightshade and elm take insect eggs as herbivore alarm and show similar transcriptomic alarm responses

Plants respond to insect eggs with transcriptional changes, resulting in enhanced defence against hatching larvae. However, it is unknown whether phylogenetically distant plant species show conserved transcriptomic responses to insect eggs and subsequent larval feeding. We used Generally Applicable Gene set Enrichment (GAGE) on gene ontology terms to answer this question and analysed transcriptome data from Arabidopsis thaliana, wild tobacco (Nicotiana attenuata), bittersweet nightshade (Solanum dulcamara) and elm trees (Ulmus minor) infested by different insect species. The different plant-insect species combinations showed considerable overlap in their transcriptomic responses to both eggs and larval feeding. Within these conformable responses across the plant-insect combinations, the responses to eggs and feeding were largely analogous, and about one-fifth of these analogous responses were further enhanced when egg deposition preceded larval feeding. This conserved transcriptomic response to eggs and larval feeding comprised gene sets related to several phytohormones and to the phenylpropanoid biosynthesis pathway, of which specific branches were activated in different plant-insect combinations. Since insect eggs and larval feeding activate conserved sets of biological processes in different plant species, we conclude that plants with different lifestyles share common transcriptomic alarm responses to insect eggs, which likely enhance their defence against hatching larvae.

Plant responses to insect eggs are not induced by egg-associated microbes, but by a secretion attached to the eggs

Plants can enhance their defence against herbivorous insects by responding to insect egg depositions preceding larval feeding. The similarity of plant responses to insect eggs with those to phytopathogens gave rise to the hypothesis that egg-associated microbes might act as elicitors. We tested this hypothesis by investigating first if elimination of microbes in the butterfly Pieris brassicae changes the responses of Brassica nigra and Arabidopsis thaliana to eggs and larvae of this insect species. An antibiotic treatment of butterflies mitigated the plant transcriptional response to the eggs and the egg-mediated enhancement of the plant's defence against larvae. However, application of cultivated microbial isolates from the eggs onto Arabidopsis thaliana did not enhance the plant's anti-herbivore defence. Instead, application of an egg-associated glandular secretion, which is attaching the eggs to the leaves, elicited the enhancing effect on the plant's defence against larvae. However, this effect was only achieved when the secretion was applied in similar quantities as released by control butterflies, but not when applied in the reduced quantity as released by antibiotic-treated butterflies. We conclude that glandular secretions rather than egg-associated microbes act in a dose-dependent manner as elicitor of the egg-mediated enhancement of the plant's defence against insect larvae.

Plant responses to butterfly oviposition partly explain preference-performance relationships on different brassicaceous species

The preference-performance hypothesis (PPH) states that herbivorous female insects prefer to oviposit on those host plants that are best for their offspring. Yet, past attempts to show the adaptiveness of host selection decisions by herbivores often failed. Here, we tested the PPH by including often neglected oviposition-induced plant responses, and how they may affect both egg survival and larval weight. We used seven Brassicaceae species of which most are common hosts of two cabbage white butterfly species, the solitary Pieris rapae and gregarious P. brassicae. Brassicaceous species can respond to Pieris eggs with leaf necrosis, which can lower egg survival. Moreover, plant-mediated responses to eggs can affect larval performance. We show a positive correlation between P. brassicae preference and performance only when including the egg phase: 7-day-old caterpillars gained higher weight on those plant species which had received most eggs. Pieris eggs frequently induced necrosis in the tested plant species. Survival of clustered P. brassicae eggs was unaffected by the necrosis in most tested species and no relationship between P. brassicae egg survival and oviposition preference was found. Pieris rapae preferred to oviposit on plant species most frequently expressing necrosis although egg survival was lower on those plants. In contrast to the lower egg survival on plants expressing necrosis, larval biomass on these plants was higher than on plants without a necrosis. We conclude that egg survival is not a crucial factor for oviposition choices but rather egg-mediated responses affecting larval performance explained the preference-performance relationship of the two butterfly species.

Changes in Content of Polyphenols and Ascorbic Acid in Leaves of White Cabbage after Pest Infestation

Crops, such as white cabbage (Brassica oleracea L. var. capitata (L.) f. alba), are often infested by herbivorous insects that consume the leaves directly or lay eggs with subsequent injury by caterpillars. The plants can produce various defensive metabolites or free radicals that repel the insects to avert further damage. To study the production and effects of these compounds, large white cabbage butterflies, Pieris brassicae and flea beetles, Phyllotreta nemorum, were captured in a cabbage field and applied to plants cultivated in the lab. After insect infestation, leaves were collected and UV/Vis spectrophotometry and HPLC used to determine the content of stress molecules (superoxide), primary metabolites (amino acids), and secondary metabolites (phenolic acids and flavonoids). The highest level of superoxide was measured in plants exposed to fifty flea beetles. These plants also manifested a higher content of phenylalanine, a substrate for the synthesis of phenolic compounds, and in activation of total phenolics and flavonoid production. The levels of specific phenolic acids and flavonoids had higher variability when the dominant increase was in the flavonoid, quercetin. The leaves after flea beetle attack also showed an increase in ascorbic acid which is an important nutrient of cabbage.

Insect egg deposition renders plant defence against hatching larvae more effective in a salicylic acid-dependent manner

Plants can improve their antiherbivore defence by taking insect egg deposition as cue of impending feeding damage. Previous studies showed that Pieris brassicae larvae feeding upon egg-deposited Brassicaceae perform worse and gain less weight than larvae on egg-free plants. We investigated how P. brassicae oviposition on Arabidopsis thaliana affects the plant's molecular and chemical responses to larvae. A transcriptome comparison of feeding-damaged leaves without and with prior oviposition revealed about 200 differently expressed genes, including enhanced expression of PR5, which is involved in salicylic acid (SA)-signalling. SA levels were induced by larval feeding to a slightly greater extent in egg-deposited than egg-free plants. The adverse effect of egg-deposited wild-type (WT) plants on larval weight was absent in an egg-deposited PR5-deficient mutant or other mutants impaired in SA-mediated signalling, that is, sid2/ics1, ald1, and pad4. In contrast, the adverse effect of egg-deposited WT plants on larvae was retained in egg-deposited npr1 and wrky70 mutants impaired further downstream in SA-signalling. Oviposition induced accumulation of flavonols in WT plants with and without feeding damage, but not in the PR5-deficient mutant. We demonstrated that egg-mediated improvement of A. thaliana's antiherbivore defence involves SA-signalling in an NPR1-independent manner and is associated with accumulation of flavonols.

The Arabidopsis Lectin Receptor Kinase LecRK-I.8 Is Involved in Insect Egg Perception

Plants induce defense responses after insect egg deposition, but very little is known about the perception mechanisms. In Arabidopsis thaliana, eggs of the specialist insect Pieris brassicae trigger accumulation of reactive oxygen species (ROS) and salicylic acid (SA), followed by induction of defense genes and localized necrosis. Here, the involvement of the clade I L-type lectin receptor kinase LecRK-I.8 in these responses was studied. Expression of LecRK-I.8 was upregulated at the site of P. brassicae oviposition and egg extract (EE) treatment. ROS, SA, cell death, and expression of PR1 were substantially reduced in the Arabidopsis knock-out mutant lecrk-I.8 after EE treatment. In addition, EE-induced systemic resistance against Pseudomonas syringae was abolished in lecrk-I.8. Expression of ten clade I homologs of LecRK-I.8 was also induced by EE treatment, but single mutants displayed only weak alteration of EE-induced PR1 expression. These results demonstrate that LecRK-I.8 is an early component of egg perception.

Oviposition by Spodoptera exigua on Solanum dulcamara Alters the Plant's Response to Herbivory and Impairs Larval Performance

Plant resistance traits against insect herbivores are extremely plastic. Plants respond not only to the herbivory itself, but also to oviposition by herbivorous insects. How prior oviposition affects plant responses to larval herbivory is largely unknown. Combining bioassays and defense protein activity assays with microarray analyses and metabolite profiling, we investigated the impact of preceding oviposition on the interaction of Solanum dulcamara with the generalist lepidopteran herbivore Spodoptera exigua at the levels of the plant’s resistance, transcriptome and metabolome. We found that oviposition increased plant resistance to the subsequent feeding larvae. While constitutive and feeding-induced levels of defensive protease inhibitor activity remained unaffected, pre-exposure to eggs altered S. dulcamara’s transcriptional and metabolic response to larval feeding in leaves local and systemic to oviposition. In particular, genes involved in phenylpropanoid metabolism were more strongly expressed in previously oviposited plants, which was reflected by reciprocal changes of primary metabolites upstream and within these pathways. Our data highlight that plants integrate signals from non-threatening life stages of their natural enemies to optimize their response when they become actually attacked. The observed transcriptional and metabolic reshaping of S. dulcamara’s response to S. exigua herbivory suggests a role of phenylpropanoids in oviposition-primed plant resistance.

Moth oviposition shapes the species-specific transcriptional and phytohormonal response of Nicotiana attenuata to larval feeding

Oviposition by lepidopteran herbivores on Nicotiana attenuata primes plant defence responses that are induced by the feeding larvae. While oviposition by both the generalist Spodoptera exigua and the specialist Manduca sexta primes the production of defensive phenylpropanoids, their larvae are differentially affected. We investigate here the impact of prior oviposition on the transcriptome and phytohormone levels of plants that were later attacked by larvae to find regulatory signals of this priming. In a full-factorial design, we evaluated the effects of oviposition and herbivory by both species. Oviposition alone had only subtle effects at the transcriptional level. Laval feeding alone induced species-specific plant responses. Larvae of the generalist regulated phytohormones and gene expression stronger than larvae of the specialist. A day after larvae started to feed, we detected no significant alterations of the plant's response to larval feeding due to prior oviposition by conspecific moths. Yet, oviposition by each of the species profoundly influenced the plant's transcriptional and phytohormonal response to feeding larvae of the other species. Remarkably, the species-specific plant responses to larval feeding shifted towards the response normally elicited by larvae of the ovipositing species. Thus, plants may already recognise an insect's identity upon its oviposition.

Combined biotic stresses trigger similar transcriptomic responses but contrasting resistance against a chewing herbivore in Brassica nigra

BACKGROUND

In nature, plants are frequently exposed to simultaneous biotic stresses that activate distinct and often antagonistic defense signaling pathways. How plants integrate this information and whether they prioritize one stress over the other is not well understood.

RESULTS

We investigated the transcriptome signature of the wild annual crucifer, Brassica nigra, in response to eggs and caterpillars of Pieris brassicae butterflies, Brevicoryne brassicae aphids and the bacterial phytopathogen Xanthomonas campestris pv. raphani (Xcr). Pretreatment with egg extract, aphids, or Xcr had a weak impact on the subsequent transcriptome profile of plants challenged with caterpillars, suggesting that the second stress dominates the transcriptional response. Nevertheless, P. brassicae larval performance was strongly affected by egg extract or Xcr pretreatment and depended on the site where the initial stress was applied. Although egg extract and Xcr pretreatments inhibited insect-induced defense gene expression, suggesting salicylic acid (SA)/jasmonic acid (JA) pathway cross talk, this was not strictly correlated with larval performance.

CONCLUSION

These results emphasize the need to better integrate plant responses at different levels of biological organization and to consider localized effects in order to predict the consequence of multiple stresses on plant resistance.

Bruchid egg induced transcript dynamics in developing seeds of black gram (Vigna mungo)

Black gram (Vigna mungo) seeds are a rich source of digestible proteins, however, during storage these seeds are severely damaged by bruchids (Callosobruchus spp.), reducing seed quality and yield losses. Most of the cultivated genotypes of black gram are susceptible to bruchids, however, few tolerant genotypes have also been identified but the mechanism of tolerance is poorly understood. We employed Suppression Subtractive Hybridization (SSH) to identify specifically, but rarely expressed bruchid egg induced genes in black gram. In this study, Suppression Subtractive Hybridization (SSH) library was constructed to study the genes involved in defense response in black gram against bruchid infestation. An EST library of 277 clones was obtained for further analyses. Based on CAP3 assembly, 134 unigenes were computationally annotated using Blast2GOPRO software. In all, 20 defense related genes were subject to quantitative PCR analysis (qPCR) out of which 12 genes showed up-regulation in developing seeds of the pods oviposited by bruchids. Few major defense genes like defensin, pathogenesis related protein (PR), lipoxygenase (LOX) showed high expression levels in the oviposited population when compared with the non-oviposited plants. This is the first report on defense related gene transcript dynamics during the bruchid-black gram interaction using SSH library. This library would be useful to clone defense related gene(s) such as defensin as represented in our library for crop improvement.

Prospects of herbivore egg-killing plant defenses for sustainable crop protection

Due to a growing demand of food production worldwide, new strategies are suggested to allow for sustainable production of food with minimal effects on natural resources. A promising alternative to the application of chemical pesticides is the implementation of crops resistant to insect pests. Plants produce compounds that are harmful to a wide range of attackers, including insect pests; thus, exploitation of their natural defense system can be the key for the development of pest-resistant crops. Interestingly, some plants possess a unique first line of defense that eliminates the enemy before it becomes destructive: egg-killing. Insect eggs can trigger (1) direct defenses, mostly including plant cell tissue growth or cell death that lead to eggs desiccating, being crushed or falling off the plant or (2) indirect defenses, plant chemical cues recruiting natural enemies that kill the egg or hatching larvae (parasitoids). The consequences of plant responses to eggs are that insect larvae do not hatch or that they are impeded in development, and damage to the plant is reduced. Here, we provide an overview on the ubiquity and evolutionary history of egg-killing traits within the plant kingdom including crops. Up to now, little is known on the mechanisms and on the genetic basis of egg-killing traits. Making use of egg-killing defense traits in crops is a promising new way to sustainably reduce losses of crop yield. We provide suggestions for new breeding strategies to grow egg-killing crops and improve biological control.

Resisting the onset of herbivore attack: plants perceive and respond to insect eggs

Plants can respond to attack by herbivorous insects very soon after herbivores start producing a new generation by depositing eggs onto their leaves. Egg-induced plant responses may result in killing the attacker in its egg stage. However, if the eggs do survive, they can also prime feeding-induced plant defenses against the larvae hatching from eggs. In this paper we focus first on egg-induced plant responses that resemble hypersensitive responses (HR) to phytopathogens and lead to egg desiccation or detachment from plants. We then summarize the current knowledge about egg-mediated effects on feeding-induced plant defenses against larvae. Finally, we discuss the insect species specificity of plant responses to eggs and the variability of insect susceptibility to these responses.

Pre-exposure of Arabidopsis to the abiotic or biotic environmental stimuli "chilling" or "insect eggs" exhibits different transcriptomic responses to herbivory

Plants can retain information about environmental stress and thus, prepare themselves for impending stress. In nature, it happens that environmental stimuli like ‘cold’ and ‘insect egg deposition’ precede insect herbivory. Both these stimuli are known to elicit transcriptomic changes in Arabidposis thaliana. It is unknown, however, whether they affect the plant’s anti-herbivore defence and feeding-induced transcriptome when they end prior to herbivory. Here we investigated the transcriptomic response of Arabidopsis to feeding by Pieris brassicae larvae after prior exposure to cold or oviposition. The transcriptome of plants that experienced a five-day-chilling period (4 °C) was not fully reset to the pre-chilling state after deacclimation (20 °C) for one day and responded differently to herbivory than that of chilling-inexperienced plants. In contrast, when after a five-day-lasting oviposition period the eggs were removed, one day later the transcriptome and, consistently, also its response to herbivory resembled that of egg-free plants. Larval performance was unaffected by previous exposure of plants to cold and to eggs, thus indicating P. brassicae tolerance to cold-mediated plant transcriptomic changes. Our results show strong differences in the persistence of the plant’s transcriptomic state after removal of different environmental cues, and consequently differential effects on the transcriptomic response to later herbivory.

Priming of anti-herbivore defence in Nicotiana attenuata by insect oviposition: herbivore-specific effects

Oviposition by Spodoptera exigua on Nicotiana attenuata primes plant defence against its larvae that consequently suffer reduced performance. To reveal whether this is a general response of tobacco to insect oviposition or species-specific, we investigated whether also Manduca sexta oviposition primes N. attenuata's anti-herbivore defence. The plant response to M. sexta and S. exigua oviposition overlapped in the egg-primed feeding-induced production of the phenylpropanoid caffeoylputrescine. While M. sexta larvae were unaffected in their performance, they showed a novel response to the oviposition-mediated plant changes: a reduced antimicrobial activity in their haemolymph. In a cross-resistance experiment, S. exigua larvae suffered reduced performance on M. sexta-oviposited plants like they did on S. exigua-oviposited plants. The M. sexta oviposition-mediated plant effects on the S. exigua larval performance and on M. sexta larval immunity required expression of the NaMyb8 transcription factor that is governing biosynthesis of phenylpropanoids such as caffeoylputrescine. Thus, NaMyb8-dependent defence traits mediate the effects that oviposition by both lepidopteran species exerts on the plant's anti-herbivore defence. These results suggest that oviposition by lepidopteran species on N. attenuata leaves may generally prime the feeding-induced production of certain plant defence compounds but that different herbivore species show different susceptibility to egg-primed plant effects.

Plant Quantity Affects Development and Survival of a Gregarious Insect Herbivore and Its Endoparasitoid Wasp

Virtually all studies of plant-herbivore-natural enemy interactions focus on plant quality as the major constraint on development and survival. However, for many gregarious feeding insect herbivores that feed on small or ephemeral plants, the quantity of resources is much more limiting, yet this area has received virtually no attention. Here, in both lab and semi-field experiments using tents containing variably sized clusters of food plants, we studied the effects of periodic food deprivation in a tri-trophic system where quantitative constraints are profoundly important on insect performance. The large cabbage white Pieris brassicae, is a specialist herbivore of relatively small wild brassicaceous plants that grow in variable densities, with black mustard (Brassica nigra) being one of the most important. Larvae of P. brassicae are in turn attacked by a specialist endoparasitoid wasp, Cotesia glomerata. Increasing the length of food deprivation of newly molted final instar caterpillars significantly decreased herbivore and parasitoid survival and biomass, but shortened their development time. Moreover, the ability of caterpillars to recover when provided with food again was correlated with the length of the food deprivation period. In outdoor tents with natural vegetation, we created conditions similar to those faced by P. brassicae in nature by manipulating plant density. Low densities of B. nigra lead to potential starvation of P. brassicae broods and their parasitoids, replicating nutritional conditions of the lab experiments. The ability of both unparasitized and parasitized caterpillars to find corner plants was similar but decreased with central plant density. Survival of both the herbivore and parasitoid increased with plant density and was higher for unparasitized than for parasitized caterpillars. Our results, in comparison with previous studies, reveal that quantitative constraints are far more important that qualitative constraints on the performance of gregarious insect herbivores and their gregarious parasitoids in nature.

Elm leaves 'warned' by insect egg deposition reduce survival of hatching larvae by a shift in their quantitative leaf metabolite pattern

Plants may take insect eggs on their leaves as a warning of future herbivory and intensify their defence against feeding larvae. Responsible agents are, however, largely unknown, and little knowledge is available on this phenomenon in perennial plants. We investigated how egg deposition affects the anti-herbivore defence of elm against the multivoltine elm leaf beetle. Prior egg deposition caused changes in the quality of feeding-damaged leaves that resulted in increased larval mortality and reduced reproductive capacity of the herbivore by harming especially female larvae. Chemical analyses of primary and secondary leaf metabolites in feeding-damaged, egg-free (F) and feeding-damaged, egg-deposited (EF)-leaves revealed only small differences in concentrations when comparing metabolites singly. However, a pattern-focused analysis showed clearly separable patterns of (F) and (EF)-leaves because of concentration differences in especially nitrogen and phenolics, of which robinin was consumed in greater amounts by larvae on (EF) than on (F)-leaves. Our study shows that insect egg deposition mediates a shift in the quantitative nutritional pattern of feeding-damaged leaves, and thus might limit the herbivore's population growth by reducing the number of especially female herbivores. This may be a strategy that pays off in a long run particularly in perennial plants against multivoltine herbivores.

A push-button: Spodoptera exigua oviposition on Nicotiana attenuata dose-independently primes the feeding-induced plant defense

Insect oviposition on a plant often precedes the attack by herbivorous larvae. We recently discovered that oviposition by Spodoptera exigua moths on the desert tobacco Nicotiana attenuata primes the induction of 2 defense traits, a phenylpropanoid and activity of protease inhibitors, in response to larval feeding. Oviposition-experienced plants suffer a reduced feeding damage by less and smaller larvae than unexperienced control plants. The increased resistance of oviposition-experienced plants requires the plant's ability to activate its biosynthesis of phenylpropanoids via a Myb transcription factor. Oviposition by S. exigua on N. attenuata is highly variable with respect to the amount, distribution and localization of the eggs on the plant. This raises the question, whether the plant's priming of herbivore defense depends on the egg number and localization. S. exigua moths prefer the oldest leaves for oviposition and yet prime defense-induction in the larval attacked young systemic leaves. Neither the levels of the primed defense traits, nor the affected larval mortality correlate with the number of eggs a plant previously received. This suggests that upon S. exigua oviposition, N. attenuata is shifted - independently of the egg-dose - into a primed state that is responding stronger to the feeding larvae than unprimed plants.

Pseudomonas syringae enhances herbivory by suppressing the reactive oxygen burst in Arabidopsis

Plant-herbivore interactions have evolved in the presence of plant-colonizing microbes. These microbes can have important third-party effects on herbivore ecology, as exemplified by drosophilid flies that evolved from ancestors feeding on plant-associated microbes. Leaf-mining flies in the genus Scaptomyza, which is nested within the paraphyletic genus Drosophila, show strong associations with bacteria in the genus Pseudomonas, including Pseudomonas syringae. Adult females are capable of vectoring these bacteria between plants and larvae show a preference for feeding on P. syringae-infected leaves. Here we show that Scaptomyza flava larvae can also vector P. syringae to and from feeding sites, and that they not only feed more, but also develop faster on plants previously infected with P. syringae. Our genetic and physiological data show that P. syringae enhances S. flava feeding on infected plants at least in part by suppressing anti-herbivore defenses mediated by reactive oxygen species.

Female butterflies adapt and allocate their progeny to the host-plant quality of their own larval experience

Recent studies with diverse taxa have shown that parents can utilize their experience of the environment to adapt their offspring's phenotype to the same environmental conditions. Thus, offspring would then perform best under environmental conditions experienced by their parents due to transgenerational phenotypic plasticity. Such an effect has been dubbed transgenerational acclimatization. However, evidence that parents can subsequently ensure the appropriate environmental conditions in order that offspring benefit from transgenerational acclimatization has never been demonstrated. We reared Pieris rapae larvae in the parental generation on high-nitrogen and low-nitrogen host plants, and reared the offspring (F1) of both treatments again on high- and low-nitrogen plants. Furthermore, we tested if females prefer to oviposit on high- or low-nitrogen host plants in two-way choice tests. We here show not only that females adapt their offspring's phenotype to the host-plant quality that they themselves experienced, but that females also mainly oviposit on the host quality to which they adapt their offspring. Moreover, effects of larval host plant on oviposition preference of females increased across two generations in F1-females acclimatized to low-nitrogen host plants, showing an adaptive host shift from one generation to the next. These findings may have profound implications for host-race formation and sympatric speciation.

Oviposition by Spodoptera exigua on Nicotiana attenuata primes induced plant defence against larval herbivory

Plants exhibit multifarious defence traits against herbivory that are constitutively expressed or induced upon attack. Insect egg deposition often precedes impending larval attack, and several plants can increase their resistance against larvae after experiencing the oviposition by an herbivore. The nature of such oviposition-mediated resistance remains unknown, and here we aim to determine plant traits that explain it. We test whether oviposition on a host plant can induce plant defence responses or enhance (prime) the induction of defence traits in response to larval herbivory. We exposed Nicotiana attenuata plants to oviposition by moths of a generalist herbivore, Spodoptera exigua. Its larvae suffered higher mortality, retarded development and inflicted less feeding damage on oviposition-experienced than on oviposition-unexperienced plants. While oviposition alone did not induce any of the examined defence traits, oviposited plants exhibited a stronger inducibility of known defence traits, i.e. caffeoylputrescine (CP) and trypsin protease inhibitors (TPIs). We found no effects of oviposition on phytohormone levels, but on the feeding-inducible accumulation of the transcription factor NaMyb8 that is governing biosynthesis of phenylpropanoid-polyamine conjugates, including CP. Comparison of larval performance on wild-type plants, CP-deficient plants (silenced NaMyb8 gene), and TPI-deficient plants (silenced NaPI gene) revealed that priming of plant resistance to larvae by prior oviposition required NaMyb8-mediated defence traits. Our results show that plants can use insect egg deposition as a warning signal to prime their feeding-induced defence.

New directions for improving crop resistance to insects by breeding for egg induced defence

Plant defence responses to insect oviposition, including tritrophic interactions with natural enemies of herbivores, have rarely been targeted in crop breeding programmes. Emission of herbivore induced plant volatiles (HIPVs) that attract natural enemies early on at the egg-laying stage of herbivore attack could provide timely biological control of pests and deter subsequent oviposition. This is needed in an agroecological context where the third trophic level often does not keep pace with the growth rate of pests. Our very recent data, using maize as an example, show that herbivore egg induced volatile emission is very rare in commercial hybrids but common in farmer selected landraces. Strategies for crop genetic improvement to enhance such responses to insect attack are considered.

Midgut serine proteases and alternative host plant utilization in Pieris brassicae L

Pieris brassicae L. is a serious pest of cultivated crucifers in several parts of the world. Larvae of P. brassicae also feed prolifically on garden nasturtium (Tropaeolum majus L., of the family Tropaeolaceae). Proteolytic digestion was studied in larvae feeding on multiple hosts. Fourth instars were collected from cauliflower fields before transfer onto detached, aerial tissues of selected host plants in the lab. Variable levels of midgut proteases were detected in larvae fed on different hosts using protein substrates (casein and recombinant RBCL cloned from cauliflower) and diagnostic, synthetic substrates. Qualitative changes in midgut trypsin activities and quantitative changes in midgut chymotrypsin activities were implicated in physiological adaptation of larvae transferred to T. majus. Midgut proteolytic activities were inhibited to different extents by serine protease inhibitors, including putative trypsin inhibitors isolated from herbivore-attacked and herbivore-free leaves of cauliflower (CfTI) and T. majus (TpTI). Transfer of larvae to T. majus significantly influenced feeding parameters but not necessarily when transferred to different tissues of the same host. Results obtained are relevant for devising sustainable pest management strategies, including transgenic approaches using genes encoding plant protease inhibitors.

Plant responses to insect egg deposition

Plants can respond to insect egg deposition and thus resist attack by herbivorous insects from the beginning of the attack, egg deposition. We review ecological effects of plant responses to insect eggs and differentiate between egg-induced plant defenses that directly harm the eggs and indirect defenses that involve egg parasitoids. Furthermore, we discuss the ability of plants to take insect eggs as warning signals; the eggs indicate future larval feeding damage and trigger plant changes that either directly impair larval performance or attract enemies of the larvae. We address the questions of how egg-associated cues elicit plant defenses, how the information that eggs have been laid is transmitted within a plant, and which molecular and chemical plant responses are induced by egg deposition. Finally, we highlight evolutionary aspects of the interactions between plants and insect eggs and ask how the herbivorous insect copes with egg-induced plant defenses and may avoid them by counteradaptations.

Insect eggs induce a systemic acquired resistance in Arabidopsis

Although they constitute an inert stage of the insect's life, eggs trigger plant defences that lead to egg mortality or attraction of egg parasitoids. We recently found that salicylic acid (SA) accumulates in response to oviposition by the Large White butterfly Pieris brassicae, both in local and systemic leaves, and that plants activate a response that is similar to the recognition of pathogen-associated molecular patterns (PAMPs), which are involved in PAMP-triggered immunity (PTI). Here we discovered that natural oviposition by P. brassicae or treatment with egg extract inhibit growth of different Pseudomonas syringae strains in Arabidopsis through the activation of a systemic acquired resistance (SAR). This egg-induced SAR involves the metabolic SAR signal pipecolic acid, depends on ALD1 and FMO1, and is accompanied by a stronger induction of defence genes upon secondary infection. Although P. brassicae larvae showed a reduced performance when feeding on Pseudomonas syringae-infected plants, this effect was less pronounced when infected plants had been previously oviposited. Altogether, our results indicate that egg-induced SAR might have evolved as a strategy to prevent the detrimental effect of bacterial pathogens on feeding larvae.

Perception, signaling and molecular basis of oviposition-mediated plant responses

Eggs deposited on plants by herbivorous insects represent a threat as they develop into feeding larvae. Plants are not a passive substrate and have evolved sophisticated mechanisms to detect eggs and induce direct and indirect defenses. Recent years have seen exciting development in molecular aspects of egg-induced responses. Some egg-associated elicitors have been identified, and signaling pathways and egg-induced expression profiles are being uncovered. Depending on the mode of oviposition, both the jasmonic acid and salicylic acid pathways seem to play a role in the induction of defense responses. An emerging concept is that eggs are recognized like microbial pathogens and innate immune responses are triggered. In addition, some eggs contain elicitors that induce highly specific defenses in plants. Examples of egg-induced suppression of defense or, on the contrary, egg-induced resistance highlight the complexity of plant-egg interactions in an on-going arms race between herbivores and their hosts. A major challenge is to identify plant receptors for egg-associated elicitors, to assess the specificity of these elicitors and to identify molecular components that underlie various responses to oviposition.