Unraveling molecular mechanisms underlying plant defense in response to dual insect attack : studying density-dependent effects

In the field, plants suffer from attack by herbivorous insects. Plants have numerous adaptations to defend against herbivory. Not only do these defense responses reduce performance of the feeding herbivore, they also result in the attraction of natural enemies of herbivores. The majority of studies investigating plant-insect interactions addressed mainly the effects of attack by a single herbivore species on induced plant defenses. However, because plants are members of complex communities, plants are exposed to different insect attackers at the same time. Moreover, attacks by different herbivores interact at different levels of biological organization, ranging from the level of gene expression, phytohormone production and biochemical changes up to the individual level. Effects of plant responses to feeding by two or more herbivore species simultaneously might cascade through the community and thereby affect insect community composition. The induction of plant defense responses is regulated by a network of signaling pathways that mainly involve the phytohormones jasmonic acid (JA), salicylic acid (SA) and ethylene (ET). The signaling pathways of the two phytohormones SA and JA interact antagonistically, whereas JA and ET signaling pathways can interact both synergistically and antagonistically in regulating plant defense responses. In general, JA-mediated signaling underlies defense responses against leaf-chewing herbivores, such as caterpillars, whereas phloem-feeding insects, such as aphids, mainly induce SA-regulated defenses. When caterpillars and aphids simultaneously feed on the same host plant, crosstalk between phytohormonal signaling pathways may affect the regulation of plant defenses. Consequently, multiple insect herbivores feeding on plants interact indirectly through plant-mediated effects. Studies investigating molecular mechanisms underlying interference by multiple attacking insects with induced plant defenses will benefit studies on the ecological consequences of induced plant responses. The aim of this thesis was to elucidate molecular mechanisms that underlie plant-mediated interactions between attacking herbivores from different feeding guilds, namely <em>Brevicoryne brassicae </em>aphids and <em>Plutella xylostella </em>caterpillars. Because herbivore density affects the regulation of plant defense responses, it may also influence the outcome of multiple insect-plant interactions. To study if modulation of induced plant defenses in response to dual insect attack depends on insect density, plants were infested with two densities of aphids. Responses of <em>Arabidopsis thaliana </em>plants to simultaneous feeding by aphids and caterpillars were investigated by combining analyses of phytohormone levels, defense gene expression, volatile emission, insect performance and behavioral responses of parasitoids. To better predict consequences of interactions between plants and multiple insect attackers for herbivore communities, the regulation of defense responses against aphids and caterpillars was also studied in the ecological model plant wild <em>Brassica oleracea</em>. Transcriptomic changes of plants during multiple insect attack and their consequences for the plant’s interactions with members of the associated insect community take place at different time scales. Direct correlation of transcriptomic responses with community development is, therefore, challenging. However, detailed knowledge of subcellular mechanisms can provide tools to address this challenge. One of the objectives of this thesis, therefore, was to investigate the involvement of phytohormonal signaling pathways and their interactions during defense responses against caterpillars or aphids at different densities, when feeding alone or simultaneously on the model plant <em>A. thaliana</em>. The studies show that aphids at different densities interfere in contrasting ways with caterpillar-induced defenses, which required both SA- and JA-signal-transduction pathways. Transcriptional analysis revealed that expression of the SA transcription factor gene <em>WRKY70 </em>was differentially affected upon infestation by aphids at low or high densities. Interestingly, the expression data indicated that a lower expression level of <em>WRKY70</em> led to significantly higher <em>MYC2</em> expression through SA-JA crosstalk. Based on these findings, it is proposed that by down-regulating <em>WRKY70</em> expression, the plant activates JA-dependent defenses which could lead to a higher resistance against aphids and caterpillars. <em>Plutella xylostella</em> caterpillars also influenced plant defense responses when feeding simultaneously with aphids. Caterpillar feeding affected aphid-induced defenses which had negative consequences for aphid performance. Induction of both ET- and JA-mediated defense responses is required for this interference. Moreover, aphid density also played an important role in the modulation by <em>P. xylostella </em>of aphid-induced defenses: <em>P. xylostella </em>caterpillars induced changes in levels of JA and its biologically active from, JA-Ile, only when feeding simultaneously with aphids at a high density. To study the overall effect of dual herbivory on induced plant defenses, not only interference with induced direct defense, but also with induced indirect defenses was addressed in <em>A. thaliana</em>. We found a significant preference of the aphid parasitoid <em>Diaeretiella rapae </em>for volatiles from aphid-infested <em>A. thaliana</em> wild-type plants and <em>ein2-1 </em>(ET-insensitive) mutants. Interestingly, simultaneous feeding by <em>P. xylostella </em>caterpillars on wild-type plants increased <em>D. rapae</em>’s preference for odors from aphid-infested plants. However, upon disruption of the ET-signaling pathway, <em>D. rapae </em>did not distinguish between <em>ein2-1 </em>mutants infested by aphids or by both aphids and caterpillars. This showed that intact ET signaling is needed for caterpillar modulation of the attraction of <em>D. rapae </em>parasitoids. On the other hand, attraction of the caterpillar parasitoid <em>Diadegma semiclausum </em>to volatiles emitted by <em>A. thaliana</em> plants simultaneously infested by caterpillars and aphids was influenced by the density of the feeding aphids. Biosynthesis and emission of the terpene (<em>E</em>,<em>E</em>)-α-farnesene could be linked to the observed preference of <em>D. semiclausum</em> parasitoids for the HIPV blend emitted by plants dually infested by caterpillars and aphids at a high density, compared to dually infested plants with a low aphid density. Transcriptomic changes in the response of <em>A. thaliana</em> wild-type plants to simultaneous feeding by <em>P. xylostella</em> caterpillars and <em>B. brassicae</em> aphids compared to plants infested by <em>P. xylostella </em>caterpillars alone were assessed using a microarray analysis. I particularly addressed the question whether the transcriptomic response to simultaneously attacking aphids and caterpillars was dependent on aphid density and time since initiation of herbivory. The data show that in response to simultaneous feeding by <em>P. xylostella </em>caterpillars and <em>B. brassicae </em>aphids the number of differentially expressed genes was higher compared to plants on which caterpillars had been feeding alone. Additionally, specific genes were differentially expressed in response to aphids feeding at low or high density. Cluster analysis showed that the pattern of gene expression over the different time points in response to dual infestation was also affected by the density of the attacking aphids. These results suggest that insects attacking at a high density cause an acceleration in plant responses compared to insects attacking at low density. As a next step in the study of multiple interacting herbivores, I studied whether plant responses to dual herbivory have consequences for the performance of a subsequently arriving herbivore, <em>Mamestra brassicae </em>caterpillars. The ecological consequences of plant responses to dual herbivory cascading into a chain of interactions affecting other community members have remained unstudied so far. We used wild <em>B. oleracea </em>plants to evaluate dual herbivore-induced plant adaptations for subsequent herbivory. We found that simultaneous feeding by <em>P. xylostella</em> and <em>B. brassicae</em> resulted in different plant defense-related gene expression and differences in plant hormone levels compared to single herbivory, and this had a negative effect on subsequently arriving <em>M. brassicae</em> caterpillars. Differential induction of JA-regulated transcriptional responses to dual insect attack was observed which could have mediated a decrease in <em>M. brassicae</em> performance. The induction of plant defense signaling also affected both <em>P. xylostella</em> and <em>B. brassicae</em> performance. This study further helps to understand herbivore community build-up in the context of plant-mediated species interactions. Altogether, findings from this thesis reveal a molecular basis underlying plant responses against multiple herbivory and provide insight in plant-mediated interactions between aphids and caterpillars feeding on plants growing in the field or used in agriculture.