Topic 1: Identification of molecular switches for the fungal colonization of arbuscular mycorrhizal roots
Based on cell- and development-specific gene expression profiles, we try to identify key factors mediating the pre-symbiotic communication as well as the subsequent infection and colonization of Medicago truncatula roots by arbuscular mycorrhizal fungi of the order Glomeromycota. Functional analyses of selected transcriptional regulators and other candidates for symbiont recognition and function will be performed in transgenic knock-down roots or mutant lines, via protein-protein interaction assays, and by subcellular localization studies using confocal laser scanning microscopy. By combining molecular genetic, histological, microscopic, and physiological approaches, insights into key molecular switches governing important early and late reprogramming steps during the successful establishment of an arbuscular mycorrhizal symbiosis will be obtained.
Topic 2: Impact of nodulation on plant mitochondrial metabolism
Mitochondria perform key reactions of primary metabolism in non-green plant organs, such as roots. Legume root nodules contain nitrogen-fixing bacteroids, which compete with mitochondria for essential metabolites such as malate, oxygen, and amino acid precursors. In an experimental system consisting of Vicia faba and Rhizobium leguminosarum bv. viciae, we investigate the impact of nodulation on plant mitochondrial metabolism. A comparative proteomic approach will initially provide data to formulate hypotheses, which are subsequently tested by functional assays. Our research aims at understanding the intricate metabolic interplay between bacteroids and mitochondria in different symbiotic conditions, with a long-term goal to improve plant yield.
Topic 3: Control of photosynthetic performance and respiration in arbuscular mycorrhizal symbioses
This topic addresses the impact of arbuscular mycorrhizal symbioses on plant carbon metabolism. Mycorrhizae provide a major carbon sink for plants, but this is not reflected in lower plant biomass production. We therefore hypothesize that mycorrhizae actively regulate plant photosynthesis and respiration in order to provide sufficient carbon for both plant and fungal growth. This hypothesis shall be investigated with a combination of molecular and physiological techniques.
Topic 4: Mechanisms of carbon-nutrient trading in mycorrhizal symbioses and consequences for carbon sequestration in soils
This topic is based on the new concept that storage and turnover of soil organic carbon is strongly affected by the carbon economy of mycorrhizal symbioses. Our basic hypothesis is that the performance of nutrient acquisition by the mycorrhiza controls the carbon flux belowground. Regulation of carbon flux, depending on nutritional status, shall be studied in microcosm experiments. While strigolactone analysis will inform about the plant's stimulation of hyphal growth, the allocation of carbon in the soil-wide-web will be investigated by IRMS and nanoSIMS following a 13C labeling.
Topic 5: Microbial contributions to plant growth promotion
Plant growth promoting rhizobacteria (PGPRs) support plants by stimulating root development, improving nutrient supply, or by suppressing pathogenic organisms. PGPRs are highly important for growth yields and therefore the understanding of the interaction between these bacteria and plant roots is of fundamental importance. As proteins on the bacterial cell surface are involved in signaling pathways and colonization of root surfaces, we here address the role of the various protein translocation systems and bacterial surface structures in the processes that result in plant growth promotion. A second approach of this topic analyzes the contribution of the various microbiological conversions of nitrogen compounds on plant growth promotion. This specifically includes the competition of denitrification and nitrate ammonification that impact nitrogen retention in the rhizosphere.
Topic 6: Elucidation of the biosynthesis of symbiotic ureides in root nodules
Legumes such as soybean can embark on a symbiotic interaction with nitrogen (N) fixing bradyrhizobia in root nodules to ensure the plant’s supply with N in the exchange for photosynthates. This symbiosis is conditional, signaled by the availability of inorganic N from the soil. For export of N into the shoot, some legumes like soybean synthesize purine nucleotide derivatives (ureides) from the ammonium released by the symbiont, whereas others generate amides. The biosynthetic route of ureide production in root nodules is in many aspects still unclear to date and will be analyzed by using bioinformatic tools for comparing genome and gene expression data from ureide and amide exporting legumes to identify candidate genes involved in ureide biosynthesis. Candidates will be tested for functionality using (1) protein biochemistry, (2) subcellular localization studies using confocal laser scanning microscopy, and (3) genetic tools including gene knock-outs in transgenic soybean roots by CRISPR-Cas technology.
Topic 7: Transcriptomic analysis of plant reactions to soils with replant disease
Replant disease or soil sickness are serious problems in nurseries, especially those working with Rosaceae species. Plants grown in these soils show severe growth inhibition and reduced yields. The complex phenomenon is not well understood, but recent studies show clear changes in rhizosphere microbial community profiles. In this topic, we will compare transcriptomic data collected on plants grown in soil with replant disease and plants grown in disinfected soil in order to identify changes in gene expression. Differentially expressed genes will be further analyzed, first in different genotypes of apple and later in other members of the Rosaceae.
Topic 8: Interplay of abiotic stress in roots with resistance- and defence-associated responses in leaves
This topic deals with the dependence of resistance to biotrophic and hemibiotrophic leaf pathogens and the nutritional status of the plant. We will focus on P and S nutrition of roses and the interaction of plants with powdery mildew (biotrophic) and black spot (hemibiotrophic) pathogens. We already identified plants resistant and susceptible to both pathogens in earlier studies and analysed the rose leaf transcriptome. By combining phytopathological screens with transcriptomic, proteomic and metabolomic approaches, we will analyse the influence of nutrition on resistance to these pathogens as well as underlying signal transduction pathways.