Down to the roots: plant-mediated interactions between shoot- and root-feeding insect herbivores

Plants are preyed upon by a diverse group of insect herbivores above- and belowground. Over the course of millions of years, plants have evolved an intricate immune system. When a plant perceives an attacker, responses are rapidly triggered, leading to the activation of defence mechanisms aimed at deterring or resisting the attacker. Activation of plant defence does not only occur locally but throughout the plant; this is in part to prepare for attack in systemic tissues, and in part because plant secondary metabolites may be produced in systemic tissues and transported to the site where they are needed. As such, defence responses triggered by one insect herbivore can affect another insect herbivore feeding on the same plant, even if they are separated in space and time. Such plant-mediated interactions between insects are common in natural settings, as plants are rarely visited by only a single herbivore. The outcome of these interactions between insect herbivores can range from facilitation to antagonism, depending on many factors such as the identity and feeding site of the herbivores involved. Most studies on plant defence and plant-mediated species interactions have focussed on aboveground tissues. As a result, there is a gap in our understanding of regulation of plant defence in the roots, and how these defence responses may be modulated in plant-mediated interactions.The aim of this thesis was to identify and understand the effects of aboveground herbivory, via plant-mediated interactions, on belowground herbivory. My study system consisted of cabbage plants and three of the most important pest species in the field; foliar feeding Brevicoryne brassicae aphids and Plutella xylostella caterpillars, and root feeding Delia radicum maggots. I focussed on the effects induction by the aboveground feeders on defence in the roots and consequences for D. radicum. To understand how plant-mediated interactions may occur, I extensively studied plant defence in the primary roots of cabbage plants against D. radicum alone or in combination with the leaf herbivores.Chapter 2 addresses the plant-mediated effects of P. xylostella and B. brassicae on the performance of D. radicum. To unravel the molecular mechanisms underlying these interactions, I included measurements of phytohormones and gene expression within these phytohormonal pathways. I discuss the plant responses to D. radicum in roots, and how aboveground herbivores may modulate them. This work was a stepping stone for chapter 3, in which I used RNA-sequencing to investigate how the plant root transcriptome changes in response to herbivory above- and/or belowground. The transcriptomic analysis led to novel hypotheses on plant responses to root herbivory and plant-mediated effects. I tested two of these hypotheses in follow-up experiments. First, I used mutant B. oleracea plants to study whether aliphatic glucosinolates confer resistance to D. radicum, and second, I studied whether P. xylostella primes plant roots to respond faster to D. radicum.Female D. radicum flies integrate cues from leaves in their oviposition choice behaviour as described above. As such, plant-mediated effects of aboveground herbivores may have additional effects on D. radicum through their host-searching behavior. Furthermore, while plant-mediated effects of aboveground chewers on root chewers have been targeted in many studies, they usually include only a single species combination. In chapter 4, I studied the plant-mediated effects of six different foliage-chewing herbivores on D. radicum preference and performance and induction of plant defence. The species of foliar herbivores spanned three insect orders and included both specialist and generalist herbivores. The combination of measurements and inclusion of multiple inducing herbivores allowed me to address two hypotheses in the field of insect-plant interactions: that “mother knows best”, i.e. that oviposition preference is linked to higher larval performance, and that generalist and specialist herbivores induce distinct plant responses. Plants influence the rhizosphere microbiome through root exudation, and this is altered upon herbivory. This way, plant-mediated species interactions may be mediated by changes in the soil microbiome. In chapter 5, I performed a plant-soil feedback experiment. In this experiment, I analysed the rhizosphere microbiome after treating plants with above- and belowground herbivores or beneficial microbes. Furthermore, the soil conditioned by these treated plants was used to grow a new set of plants on which plant defence against D. radicum was tested.Plant-mediated effects found in greenhouse studies are not necessarily translatable to a field setting. Therefore, in chapter 6, I studied D. radicum oviposition and abundance in the field. To connect the field data to results from the previous greenhouse experiments, I assessed the aboveground herbivore community prior to measurements of D. radicum oviposition and abundance. Moreover, when searching the roots and surrounding soil for D. radicum larvae and pupae, other belowground macrofauna was also recorded. Plants assessed for these experiments were part of a large intercropping trial, where different cropping systems were compared. In this chapter, I discuss how different cropping systems affect D. radicum oviposition and abundance, and how these measurements were connected to the abundance of above- and belowground macrofauna.Finally, in chapter 7, I discuss my findings and place them in the framework of plant defence in roots and plant-mediated interactions. This general discussion provides an extensive outline of plant defence against D. radicum, and the potential mechanisms that can cause plant-mediated species interactions. The results of this thesis advance the knowledge on plant defence against root feeding insects and provide new insights on how plants interact with their complex environment.