Tripartite interactions between Brassica oleracea, soil microbes and shoot- and root feeding insects

Abstract

or positive depending on soil nutrient level (Pangesti et al., 2015a). Effects of P. simiae WCS417r inoculation via plant-mediated effects on P. xylostella and D. radicum performance are as yet unknown. Thesis outlineTo study the plant-mediated interactions between rhizobacteria and insect herbivores during different rhizobacterial inoculation methods in-depth, I start this thesis with a literature exploration in Chapter 2. In the following experimental chapters, I narrow the focus to the plant-insect interactions between Brassica oleracea and various associated insect herbivores as discussed above, to investigate plant growth and insect resistance after rhizobacterial colonization. Chapter 2. This chapter consists of a literature review of rhizobacterial effects on insect herbivores via plant-mediated effects. The chapter focusses on an update on the literature on rhizobacteria-plant-insect interactions. Further, I review bi-directional effects between rhizobacteria and insects via plant-mediated interactions. Chapter 3. Chapter 3 focuses on plant growth and resistance in B. oleracea plants grown in P. simiae WCS417r-inoculated soil, against associated insect herbivores. In a greenhouse study, I mixed the bacteria into sterilized soil, sowed sterilized seeds and when grown, the plants were infested with either D. radicum, P. xylostella or M. brassicae. I measured plant and insect growth, phytohormonal levels, plant gene expression and plant growth, and C/N concentration in leaves. Chapter 4. In this chapter, I added bacteria to the B. oleracea seeds according to the bio-priming method, instead of adding rhizobacteria to the soil prior to sowing. I then infested the grown plants with either D. radicum, P. xylostella or M. brassicae and measured plant and insect responses. Chapter 5. Previous studies on insect herbivore performance after feeding on a plant with added rhizobacteria, focused on one insect species feeding on a plant. In chapter 5 I included two insect species feeding simultaneously, to explore rhizobacterial modifications of plant resistance under dual attack after rhizobacterial soil inoculation.