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Abstract
Soils contain a multitude of microbes, including plant pathogens as well as microbes that benefit plant health. The beneficial microbes include root-colonizing bacteria that can promote plant growth. These bacteria also can trigger a resistance mechanism called induced systemic resistance, which provides the whole plant with a faster and stronger response when the plant is attacked. This thesis explores the effects of adding a specific rhizobacterium on plant growth and plant resistance against herbivorous insects. By using cabbage, Brassica oleracea, this thesis aims to investigate (1) plant responses to addition of the rhizobacterium Pseudomonas simiae WCS417r, and (2) how common cabbage-associated insect herbivores respond to the bacterial addition via plant-mediated effects.
Chapter 2 presents a literature review on the bi-directional effects of plant-insect-rhizobacteria interactions. Few previous literature reviews explicitly focuses on plant resistance against insects. This review explored how herbivorous insects and their communities are affected by rhizobacterial addition through modification of plant resistance. In addition, the literature study includes a novel focus to the field on how insect feeding affects root-associated bacterial communities.
In chapter 3, I expand the knowledge on plant responses and plant resistance against herbivorous insects after soil rhizobacterial inoculated. After growing cabbage (B. oleracea) in sterilized or P. simiae-inoculated soil, I infested the plants with larvae of the cabbage moth Mamestra brassicae, the diamondback moth Plutella xylostella, or the cabbage root fly Delia radicum. The rhizobacterial soil inoculation resulted in an increased aboveground plant biomass compared to biomass of plants growing in non-inoculated soil. Furthermore, the inoculation affected insect performance. Inoculation decreased P. xylostella biomass and increased D. radicum biomass, when insects fed on plants grown on rhizobacterially inoculated soil, compared to plants grown on non-inoculated soil. Mamestra brassicae biomass was similar on plants grown on inoculated and non-inoculated soil. Taken together, these results indicate that insect herbivores have species-specific responses when feeding on plants growing in rhizobacteria-inoculated soil.
Instead of mixing rhizobacteria into the soil, or pouring a rhizobacterial solution on the soil, the bacteria can be disseminated together with the seed. This can be done through biopriming: the seeds are hydrated together with a bacterium in liquid, but removed from the liquid before the seed radicle appears. This method has rarely been considered with respect to plant-insect interactions. I assessed B. oleracea’s growth and resistance against herbivorous insects, when grown from seeds bacterized with P. simiae (Chapter 4). Aboveground plant biomass was similar between plants grown from bacterized seeds and water-soaked seeds. The performance of larvae of M. brassicae and P. xylostella was similar when feeding on plants grown from either bacterized seeds or water-soaked seeds. No conclusions could be drawn on the performance of D. radicum, when the insects were feeding on plants grown from bacterized seeds or water-soaked seeds. These results indicate that the choice of method for rhizobacterial dissemination is influencing the effect on cabbage plant growth. Furthermore, compared to soil inoculation, rhizobacterial bacterization of seeds may affect plant-insect interactions to a lesser extent.
In chapter 5, I explored the rhizobacterial plant resistance modification of B. oleracea using co-feeding by two insect herbivores and soil inoculation. Here, the shoot feeder P. xylostella and the root feeder D. radicum were simultaneously attacking the same host plant. The plants were grown in sterilized soil with inoculated P. simiae. The inoculation increased plant aboveground biomass compared to biomass of plants grown in non-inoculated soil. Furthermore, inoculation decreased the measured consumed leaf area by P. xylostella compared to consumed leaf area from plants grown on non-inoculated soil. However, insect biomass was similar when feeding on inoculated or non-inoculated plants. Plutella xylostella biomass decreased when co-feeding with D. radicum, regardless of rhizobacterial soil inoculation. The extent of root damage by D. radicum feeding singly was similar as after co-feeding with P. xylostella. I conclude that rhizobacterial soil inoculation can influence the consumed leaf area by the herbivore.
Chapter 6 focused on rhizosphere microbial alterations after herbivore infestation and rhizobacterium inoculation, and how these community alterations affect the growth and resistance of plants growing consecutively in the conditioned soil. Brassica oleracea plants were infested with the herbivores Brevicoryne brassicae, P. xylostella, D. radicum, or D. radicum plus P. simiae soil inoculation, or only with P. simiae soil inoculation. The rhizosphere was sampled after two weeks of plant growth and soil conditioning. After the plants and insects were removed, the soil was used to grow a new set of B. oleracea plants. The second set of plants were assessed for growth and resistance against D. radicum. A principle component analysis clustered the microbiome mainly into groups according to herbivore feeding location, i.e. the shoot- and root feeders, but did not separate the inoculated plants’ microbiome from the control plants’ microbiome. Assessment of the second set of plants showed that soil conditioning affected plant growth and resistance in a treatment-specific way, but not according to previous herbivore shoot- or root feeding location. The results suggest that specific members, and not the complete microbial community, are responsible for alterations in the plant defense.
This thesis contributes to fundamental as well as applied knowledge of how rhizobacterium addition to plants affects growth and defense against herbivorous insects, with a focus on B. oleracea. Taken together, the research presented shows a context dependency of the effects of rhizobacterium addition on plant growth promotion, as well as rhizobacterial increase in plant resistance to insects: the method of rhizobacterial dissemination and the identity of the herbivorous insect affect the rhizobacterial effects on insects via plant-mediated effects. Generally, measuring the outcomes of insect-plant-microbe interactions is complex and time-consuming, calling for simplifications such as utilizing specialized plant growth media. Yet for a successful rhizobacterium application to promote crop health within agriculture, more knowledge is required from field studies. Many answers to questions on how and when to add microbes are still unknown. However, such answers are needed to optimize bacterial plant growth promotion for many species and environments, particularly answers regarding soil type and soil nutrient status will be instrumental to optimize bacteria-assisted agriculture.
Original language | English |
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Qualification | Doctor of Philosophy |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 28 Jun 2021 |
Place of Publication | Wageningen |
Publisher | |
Print ISBNs | 9789463958028 |
DOIs | |
Publication status | Published - 28 Jun 2021 |
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- 1 Finished
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Use of beneficial rhizobacteria against herbivore pests in Brassica crops: modulating factors and underlying mechanisms
Friman, J. (PhD candidate), Dicke, M. (Promotor) & van Loon, J. (Promotor)
1/06/16 → 28/06/21
Project: PhD