1. Plants interact with various organisms, aboveground as well as belowground. Such interactions result in changes in plant traits with consequences for members of the plant-associated community at different trophic levels. Research thus far focussed on interactions of plants with individual species. However, studying such interactions in a community context is needed to gain a better understanding.2. Members of the aboveground insect community induce defences that systemically influence plant interactions with herbivorous as well as carnivorous insects. Plant roots are associated with a community of plant-growth promoting rhizobacteria (PGPR). This PGPR community modulates insect-induced defences of plants. Thus, PGPR and insects interact indirectly via plant-mediated interactions.3. Such plant-mediated interactions between belowground PGPR and aboveground insects have usually been addressed unidirectionally from belowground to aboveground. Here, we take a bidirectional approach to these cross-compartment plant-mediated interactions.4. Recent studies show that upon aboveground attack by insect herbivores, plants may recruit rhizobacteria that enhance plant defence against the attackers. This rearranging of the PGPR community in the rhizosphere has consequences for members of the aboveground insect community. This review focusses on the bidirectional nature of plant-mediated interactions between the PGPR and insect communities associated with plants, including (a) effects of beneficial rhizobacteria via modification of plant defence traits on insects and (b) effects of plant defence against insects on the PGPR community in the rhizosphere. We discuss how such knowledge can be used in the development of sustainable crop-protection strategies.

Plant-soil feedback (PSF) may influence plant-insect interactions. Although plant defense differs between shoot and root tissues, few studies have examined root-feeding insect herbivores in a PSF context. We examined here how plant growth and resistance against rootfeeding Delia radicum larvae was influenced by PSF.We conditioned soil with cabbage plants that were infested with herbivores that affect D. radicum through plant-mediated effects: leaf-feeding Plutella xylostella caterpillars and Brevicoryne brassicae aphids, root-feeding D. radicum larvae, and/or added rhizobacterium Pseudomonas simiae WCS417r. We analyzed the rhizosphere microbial community, and in a second set of conspecific plants exposed to conditioned soil, we assessed growth, expression of defense-related genes, and D. radicum performance.The rhizosphere microbiome differed mainly between shoot and root herbivory treatments. Addition of Pseudomonas simiae did not influence rhizosphere microbiome composition. Plant shoot biomass, gene expression, and plant resistance against D. radicum larvae was affected by PSF in a treatment-specific manner. Soil conditioning overall reduced plant shoot biomass, Pseudomonas simiae-amended soil causing the largest growth reduction.In conclusion, shoot and root insect herbivores alter the rhizosphere microbiome differently, with consequences for growth and resistance of plants subsequently exposed to conditioned soil.

Plant growth-promoting rhizobacteria (PGPR) can enhance plant growth and defence. Via plant-mediated effects, PGPR have been reported to impact the performance of generalist leaf-chewing insects either negatively or positively. However, only a few insect species, mainly feeding on aboveground tissues, have thus far been investigated. Here, we investigated how addition of rhizobacteria to the soil in which cabbage plants are growing affects the performance of three chewing insect herbivores, two leaf chewers and one root feeder. In a greenhouse experiment, we grew white cabbage (Brassica oleracea) plants in soil supplemented with and without the rhizobacterium Pseudomonas simiae WCS417r. We investigated the consequences for three important insect pests of Brassica species, larvae of the cabbage moth Mamestra brassicae, the diamondback moth Plutella xylostella, or the cabbage root fly Delia radicum after 5 weeks of plant growth. We recorded aboveground plant biomass, insect biomass, plant defence marker gene expression levels and plant defence-related hormone concentrations. Rhizobacterial inoculation increased aboveground plant biomass in non-infested plants but not in infested plants. Rhizobacterial inoculation affected insect performance differently: on PGPR-inoculated plants biomass of Plutella xylostella was lower, while biomass of Delia radicum was higher than on control plants. However, no effect was found on Mamestra brassicae biomass. Rhizobacterial inoculation increased the expression of the defence marker gene LOX2 in P. xylostella-infested plants and MYC2 in M. brassicae-infested plants. Transcription levels of the plant defence marker gene PAL1 showed upregulation between inoculated and non-inoculated insect-free plants. Levels of the phytohormones jasmonic acid, salicylic acid and abscisic acid were similar in inoculated and non-inoculated plants. We conclude that rhizobacterial inoculation has potential to be applied in the protection of cabbage crops against the diamondback moth whereas this does not apply to reducing damage caused by the cabbage moth or cabbage root fly. K E Y W O R D S cabbage moth (Mamestra brassicae), cabbage root fly (Delia radicum), diamondback moth (Plutella xylostella), plant resistance, rhizobacteria (Pseudomonas simiae WCS417r), white cabbage (Brassica oleracea) This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Soil microbes have important effects on the interactions of plants with their environment, by promoting plant growth, inducing resistance to pests or by conferring tolerance to abiotic stress. However, their effects are variable and the factors responsible for this variation are mainly unknown. Our aim was to assess how drought stress modifies the effect of the nonpathogenic rhizobacterium Pseudomonas simiae WCS417r on plant growth and resistance against the generalist leaf-chewing caterpillar Mamestra brassicae. We studied Arabidopsis thaliana Col-0 plants, as well as mutants altered in the biosynthesis of the phytohormones jasmonic acid (JA) and abscisic acid (ABA). Caterpillars did not prefer rhizobacteria-treated plants, independently of drought stress. Rhizobacteria colonization had a variable effect on caterpillar performance, which ranged from positive in one experiment to neutral in a second one. Drought had a consistent negative effect on herbivore performance; however, it did not modify the effect of rhizobacteria on herbivore performance. The effect of drought on herbivore performance was JA-mediated (confirmed with the use of the dde2-2 mutant), but it was still present in the ABA-deficient mutant aba2-1. Plant biomass was reduced by both drought and herbivory but it was enhanced by rhizobacterial colonization. Pseudomonas simiae WCS417r is able to promote plant growth even when plants are suffering herbivory. Nevertheless, the microbial effect on the herbivore is variable, independently of drought stress. To get the best possible outcome from the rhizobacteria-plant mutualism it is important to understand which other factors may be responsible for its context-dependency.

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.