Tjeerd A. L. Snoeren

Jasmonate and ethylene signaling mediate whitefly‐induced interference with indirect plant defense in Arabidopsis thaliana

Upon herbivore attack, plants activate an indirect defense, that is, the release of a complex mixture of volatiles that attract natural enemies of the herbivore. When plants are simultaneously exposed to two herbivore species belonging to different feeding guilds, one herbivore may interfere with the indirect plant defense induced by the other herbivore. However, little is understood about the mechanisms underlying such interference. Here, we address the effect of herbivory by the phloem-feeding whitefly Bemisia tabaci on the induced indirect defense of Arabidopsis thaliana plants to Plutella xylostella caterpillars, that is, the attraction of the parasitoid wasp Diadegma semiclausum. Assays with various Arabidopsis mutants reveal that B. tabaci infestation interferes with indirect plant defense induced by P. xylostella, and that intact jasmonic acid and ethylene signaling are required for such interference caused by B. tabaci. Chemical analysis of plant volatiles showed that the composition of the blend emitted in response to the caterpillars was significantly altered by co-infestation with whiteflies. Moreover, whitefly infestation also had a considerable effect on the transcriptomic response of the plant to the caterpillars. Understanding the mechanisms underlying a plants responses to multiple attackers will be important for the development of crop protection strategies in a multi-attacker context.

Natural variation in herbivore-induced volatiles in Arabidopsis thaliana

To study whether natural variation in Arabidopsis thaliana could be used to dissect the genetic basis of responses to herbivory in terms of induced volatile emissions, nine accessions were characterized upon herbivory by biting-chewing Pieris rapae caterpillars or after treatment with the phytohormone jasmonic acid (JA). Analysis of 73 compounds in the headspace showed quantitative differences in the emission rates of several individual compounds among the accessions. Moreover, variation in the emission of volatile compounds after JA treatment was reflected in the behaviour of the parasitoid Diadegma semiclausum when they were offered the headspace volatiles of several combinations of accessions in two-choice experiments. Accessions also differ in transcript levels of genes that are associated with the emission of plant volatiles. The genes BSMT1 and Cyp72A13 could be connected to the emission of methyl salicylate and (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT), respectively. Overall, Arabidopsis showed interesting phenotypic variations with respect to the volatile blend emitted in response to herbivory that can be exploited to identify genes and alleles that underlie this important plant trait.

Predatory mites learn to discriminate between plant volatiles induced by prey and nonprey herbivores

Many carnivorous arthropods can use chemical information from plants to locate their herbivorous prey. The composition of blends of herbivore-induced plant volatiles can vary with plant and herbivore species and thus carnivores are confronted with variable information about the presence of their prey. Such environmental variation is expected to favour learning. We investigated the learning ability of the predatory mite Phytoseiulus persimilis, a specialized natural enemy of polyphagous spider mites. We reared mites on different plant species, and subsequently tested their preference for volatiles from lima bean plants infested with either the prey herbivore Tetranychus urticae or the nonprey caterpillar Spodoptera exigua in a Y-tube olfactometer. Predators reared on lima bean preferred the volatiles induced by T. urticae, whereas predators reared on cucumber did not. We also investigated the foraging behaviour of mites after a nonrewarding experience during the adult phase (i.e. food deprivation in the presence of S. exigua-induced volatiles from lima bean) or after a rewarding experience (i.e. feeding in the presence of T. urticae-induced volatiles). The rewarding experience had a much larger impact on the foraging responses. Predatory mites with multiple experiences (i.e. a nonrewarding experience followed by a rewarding experience) had the strongest preference for T. urticae-induced versus S. exigua-induced volatiles. We conclude that these learning abilities enable the predatory mites to forage in an environment where their prey can feed on a different plant species than the one on which the predator developed, and where nonprey caterpillars are also present.

Exploiting natural variation to identify insect‐resistance genes

Herbivorous insects are widespread and often serious constraints to crop production. The use of insect-resistant crops is a very effective way to control insect pests in agriculture, and the development of such crops can be greatly enhanced by knowledge on plant resistance mechanisms and the genes involved. Plants have evolved diverse ways to cope with insect attack that has resulted in natural variation for resistance towards herbivorous insects. Studying the molecular genetics and transcriptional background of this variation has facilitated the identification of resistance genes and processes that lead to resistance against insects. With the development of new technologies, molecular studies are not restricted to model plants anymore. This review addresses the need to exploit natural variation in resistance towards insects to increase our knowledge on resistance mechanisms and the genes involved. We will discuss how this knowledge can be exploited in breeding programmes to provide sustainable crop protection against insect pests. Additionally, we discuss the current status of genetic research on insect-resistance genes. We conclude that insect-resistance mechanisms are still unclear at the molecular level and that exploiting natural variation with novel technologies will contribute greatly to the development of insect-resistant crop varieties.

The Herbivore-Induced Plant Volatile Methyl Salicylate Negatively Affects Attraction of the Parasitoid Diadegma semiclausum

The indirect defense mechanisms of plants comprise the production of herbivore-induced plant volatiles that can attract natural enemies of plant attackers. One of the often emitted compounds after herbivory is methyl salicylate (MeSA). Here, we studied the importance of this caterpillar-induced compound in the attraction of the parasitoid wasp Diadegma semiclausum by using a mutant Arabidopsis line. Pieris rapae infested AtBSMT1-KO mutant Arabidopsis plants, compromised in the biosynthesis of MeSA, were more attractive to parasitoids than infested wild-type plants. This suggests that the presence of MeSA has negative effects on parasitoid host-finding behavior when exposed to wild-type production of herbivore-induced Arabidopsis volatiles. Furthermore, in line with this, we recorded a positive correlation between MeSA dose and repellence of D. semiclausum when supplementing the headspace of caterpillar-infested AtBSMT1-KO plants with synthetic MeSA.

Indirect plant-mediated interactions among parasitoid larvae

Communities are riddled with indirect species interactions and these interactions can be modified by organisms that are parasitic or symbiotic with one of the indirectly interacting species. By inducing plant responses, herbivores are well known to alter the plant quality for subsequent feeders. The reduced performance of herbivores on induced plants cascades into effects on the performance of higher trophic level organisms such as parasitoids that develop inside herbivores. Parasitoids themselves may also, indirectly, interact with the host plant by affecting the behaviour and physiology of their herbivorous host. Here, we show that, through their herbivorous host, larvae of two parasitoid species differentially affect plant phenotypes leading to asymmetric interactions among parasitoid larvae developing in different hosts that feed on the same plant. Our results show that temporally separated parasitoid larvae are involved in indirect plant-mediated interactions by a network of trophic and non-trophic relationships.

Disruption of plant carotenoid biosynthesis through virus-induced gene silencing affects oviposition behaviour of the butterfly Pieris rapae

Optical plant characteristics are important cues to plant-feeding insects. In this article, we demonstrate for the first time that silencing the phytoene desaturase (PDS) gene, encoding a key enzyme in plant carotenoid biosynthesis, affects insect oviposition site selection behaviour. Virus-induced gene silencing employing tobacco rattle virus was used to knock down endogenous PDS expression in three plant species (Arabidopsis thaliana, Brassica nigra and Nicotiana benthamiana) by its heterologous gene sequence from Brassica oleracea. We investigated the consequences of the silencing of PDS on oviposition behaviour by Pieris rapae butterflies on Arabidopsis and Brassica plants; first landing of the butterflies on Arabidopsis plants (to eliminate an effect of contact cues); first landing on Arabidopsis plants enclosed in containers (to eliminate an effect of volatiles); and caterpillar growth on Arabidopsis plants. Our results show unambiguously that P. rapae has an innate ability to visually discriminate between green and variegated green-whitish plants. Caterpillar growth was significantly lower on PDS-silenced than on empty vector control plants. This study presents the first analysis of PDS function in the interaction with an herbivorous insect. We conclude that virus-induced gene silencing is a powerful tool for investigating insect-plant interactions in model and nonmodel plants.

Multidisciplinary approach to unravelling the relative contribution of different oxylipins in indirect defense of Arabidopsis thaliana.

The oxylipin pathway is commonly involved in induced plant defenses, and is the main signal-transduction pathway induced by insect folivory. Herbivory induces the production of several oxylipins, and consequently alters the so-called ‘oxylipin signature’ in the plant. Jasmonic acid (JA), as well as pathway intermediates are known to induce plant defenses. Indirect defense against herbivorous insects comprises the production of herbivore-induced plant volatiles (HIPVs). To unravel the precise oxylipin signal-transduction underlying the production of HIPVs in Arabidopsis thaliana and the resulting attraction of parasitoid wasps, we used a multidisciplinary approach that includes molecular genetics, metabolite analysis, and behavioral analysis. Mutant plants affected in the jasmonate pathway (18:0 and/or 16:0 -oxylipin routes; mutants dde2-2, fad5, opr3) were studied to assess the effects of JA and its oxylipin intermediates 12-oxo-phytodienoate (OPDA) and dinor-OPDA (dnOPDA) on HIPV emission and parasitoid (Diadegma semiclausum) attraction. Interference with the production of the oxylipins JA and OPDA altered the emission of HIPVs, in particular terpenoids and the phenylpropanoid methyl salicylate, which affected parasitoid attraction. Our data show that the herbivore-induced attraction of parasitoid wasps to Arabidopsis plants depends on HIPVs that are induced through the 18:0 oxylipin-derivative JA. Furthermore, our study shows that the 16:0-oxylipin route towards dnOPDA does not play a role in HIPV induction, and that the role of 18:0 derived oxylipin-intermediates, such as OPDA, is either absent or limited.

Caught between Parasitoids and Predators – Survival of a Specialist Herbivore on Leaves and Flowers of Mustard Plants

The survival of insect herbivores typically is constrained by food choice and predation risk. Here, we explored whether movement from leaves to flowers increases survival of herbivores that prefer to feed on floral tissues. Combining field and greenhouse experiments, we investigated whether flowering influences the behavior of Pieris brassicae butterflies and caterpillars and, consequently, herbivore survival in the field. In this context, we investigated also if flowers of Brassica nigra can provide caterpillars refuge from the specialist parasitoid Cotesia glomerata and from predatory social wasps. By moving to flowers, caterpillars escaped from the parasitoid. Flowers are nutritionally superior when compared with leaves, and caterpillars develop faster when feeding on flowers. However, late-stage caterpillars can be preyed upon intensively by social wasps, irrespective of whether they feed on leaves or flowers. We conclude that flower preference by P. brassicae is more likely driven by nutritional advantages and reduced parasitism on flowers, than by risks of being killed by generalist predators.

Jasmonates differentially affect interconnected signal-transduction pathways of Pieris rapae-induced defenses in Arabidopsis thaliana

Abstract  The jasmonic acid (JA) pathway is the main signal‐transduction pathway induced by insect folivory. Mutant plants affected in the jasmonate pathway (18:0 and/or 16:0‐oxylipin routes) were studied to assess the effects of JA and its oxylipin intermediates 12‐oxophytodienoic acid (OPDA) and dinor‐OPDA (dnOPDA) on interconnected signal‐transduction pathways that underlie induced defenses in Arabidopsis. Our data show that the oxylipin jasmonates dnOPDA, OPDA and JA have different roles in defense signaling induced after feeding by the chewing‐biting caterpillar Pieris rapae. Jasmonic acid, and not OPDA or dnOPDA, is the major signaling compound required for the induction of the defense‐related genes LOX2 (Lipoxygenase 2), OPR3 (12‐Oxophytodienoate reductase 3), ACX1 (Acyl‐CoA oxidase 1) and PAL1 (Phenylalanine ammonia‐lyase 1). Monitoring PAL1 transcript levels clearly showed that accumulation of JA upon P. rapae feeding results in the induction of the salicylic acid pathway. Furthermore, JA is the major signaling compound required for the P. rapae‐induced expression of the defense‐related gene HPL1 (Hydroperoxide lyase 1). The jasmonate dnOPDA influences the induction of the HPL‐branch as well, yet its effect is antagonistic to the effect of JA. Our data show that these jasmonates may be used to fine‐tune Arabidopsis’ herbivore‐induced responses in terms of the HPL‐branch from the oxylipin pathway.