Research: Plant Microbe Interactions

Roots constitute important plant organs for water and nutrient uptake. They, however, also release a wide range of carbon compounds of low molecular weight. These can amount between 10 and 20% of total net fixed carbon and form the basis for an environment rich in diversified microbiological populations, the rhizosphere. Its specific conditions lead to the selection of distinct microbial communities, where fungi play an important role. Symbiotic fungi form specific structures with roots (mycorrhizas) where both partners exchange solutes: the plant delivers photoassimilates, the fungus nutrients and water via soil exploring hyphae. An important focus of our research is thus to understand, how the exchange take place, and how it is regulated on a molecular level.

Hyphae of mycorrhiza-forming fungi also release a substantial amount of the acquired plant carbon to the soil. These energy-rich plant compounds promote growth and survival of soil bacteria, and the area rich in mycorrhizal fungal hyphae is therefore often depicted as the mycorrhizosphere. The mycorrhizosphere effect leads to the enrichment of micro-organisms that improve plant fitness. Some of the mycorrhizosphere organisms directly influence the development and physiology of the plant through the production of plant growth regulators, by increasing the root branching rate or root permeability. Others may interact in a more indirect way that supports plant growth. For example, the interacting organisms may improve nitrogen or phosphate availability, lead to better survival of the hyphae and the plants in a contaminated soil, assist the plant resistance against pathogens by biological control, or show direct effects on soil quality.

The most investigated group of micro-organisms interacting with mycorrhizal fungi and plants are the mycorrhizosphere bacteria. These include intrahyphal bacteria in ectomycorrhizal and intra-spore bacteria in arbuscular mycorrhizal fungi, as well as bacterial species that colonise the surfaces of fungal hyphae and mycorrhizal roots. Some of the mycorrhizosphere bacteria, classified as Mycorrhiza Helper Bacteria, can promote mycorrhiza formation. These include a variety of gram-negative and gram-positive species.

We work on a collection of actinomycetes (gram-positive) that were isolated from the rhizosphere of a Norway spruce (Picea abies) stand. Some isolates significantly promote the mycelial growth and mycorrhization rate of the symbiotic fungi such as Amanita muscaria (Fliegenpilz), and at the same time suppress the growth of the plant pathogenic fungi. Finally, such streptomycetes induce plant resistance towards plant pathogens. In collaboration with members of the microbiology department we try to identify effector substances released by such bacteria, and find out the molecular basis for these multilevel interactions.