Interactions between endophytic Metarhizium brunneum and oilseed rape plants: effects on plant health and possible mechanismsDissertation
Datum der mündl. Prüfung:2023-10-04
Betreuer:Prof. Dr. Michael Rostás
Gutachter:PD Dr. Jürgen Gros
Gutachter:Prof. Dr. Andreas von Tiedemann
Gutachter:Prof. Dr. Stefan Vidal
EnglischFungi of the genus Metarhizium, belonging to the order Hypocreales, are well-known entomopathogens that primarily infect insects. However, recent research has revealed their multifunctional lifestyles, including their ability to live as saprophytes in the soil and proliferate in the rhizosphere of various plant species. Metarhizium species have been found to be associated with plant roots, grow endophytically, and have shown beneficial effects on plant health, such as improved growth under stress conditions and protection against insect pests and plant diseases. Despite these findings, there is a lack of information on the mechanisms responsible for these interactions. Therefore, the objective of this thesis was to study the mechanisms behind the effects of fungal-plant associations on insects and plant pathogens. This thesis comprises five chapters, each focussing on different aspects of the interaction of Metarhizium brunneum with plants, insects, and plant pathogens. Chapter 1 addresses the challenge of distinguishing among isolates of the same species and the importance of identifying specific fungal isolates to track their efficacy as biocontrol agents in the field. This study characterised a collection of M. brunneum isolates and developed a strain-specific PCR assay using random amplified polymorphic DNA (RAPD) markers and direct sequencing. The results demonstrate the usefulness of RAPD markers in developing sequence-characterised amplified regions (SCAR) to differentiate M. brunneum strains, providing a valuable tool to track specific isolates in field studies. Chapter 2 focusses on the ability of entomopathogenic fungi to associate with two crop plants, winter oilseed rape and potato. This study evaluated the colonization of these plants by five M. brunneum isolates using quantitative polymerase chain reaction (qPCR) providing insights into the associations between the fungus and different crop species. These findings contribute to the selection of oilseed rape as a crop for further investigation of the fungus-plant association and its effects on insect pests, plant diseases, and potential mechanisms. Chapter 3 examines the effects of M. brunneum on herbivores, specifically the chewing specialist Psylliodes chrysocephala, the generalist aphid Myzus persicae and the specialist aphid Brevicoryne brassicae using Brassica napus plants. This study revealed a feeding stimulant effect of M. brunneum on a specialist herbivore and a variable impact on aphid fecundity. The results are discussed in the context of plant-fungal interactions and their influence on nutritional quality, defence metabolites, and insect parameters. Chapter 4 focusses on evaluating the direct and plant-mediated effects of M. brunneum on the cabbage root fly Delia radicum (CRF). This study demonstrates CRF infection by different fungal isolates and uses a split-root design to separate direct from systemic effects. It assesses their impact on herbivore damage, insect mortality, and the modulation of phytohormones, through measurements of plant hormone levels using HPLC-MS and examination of gene expression via qPCR. These findings show that root-associated M. brunneum controls the CRF by direct insect infection, local and systemic priming of defence pathways, and modulation of phytohormones. Chapter 5 explores the ability of M. brunneum to protect oilseed rape plants against the soil-borne pathogen Verticillium longisporum. The study investigates the mechanisms involved, employing in vitro and greenhouse experiments, gene expression analysis, and split-root setup to study direct competition, antibiosis, and plant-mediated effects. The results indicate that M. brunneum delays pathogen colonisation, exhibits competitive growth, and activates defence hormone pathways, resulting in decreased disease symptoms and improved plant growth. In conclusion, this Ph.D. thesis advances our understanding of the interactions of M. brunneum with insects and plant pathogens, separating direct effects from plant-mediated effects as endophytes of plants. Characterisation of M. brunneum isolates, development of strain-specific assays, and evaluation of colonisation capabilities contribute to the tracking and utilisation of specific isolates effectively. The study highlights the potential of M. brunneum to reduce herbivore damage and protect plants against soil-borne pathogens. The findings contribute to advancing our understanding of the ecological roles and applications of M. brunneum in agricultural systems.
Keywords: Metarhizium brunneum; Endophytic entomopathogenic fungi; Oilseed rape; Verticillium longisporum; Priming; Cabbage root fly; Cabbage stem flea beetle