Sabine Ravnskov

The microbial community in the rhizosphere plays a key role in plant growth and -health, either directly by
influencing plant nutrient uptake and by causing disease, or indirectly via microbial interactions in the rhizosphere. The
majority of field grown crops (70-80 %) naturally form symbiosis with Arbuscular Mycorrhizal Fungi (AMF); thus the
relation between root pathogens and most plants under field conditions is an interaction between AM and pathogens. The AM
symbiosis has functionally been characterised as the reciprocal exchange of nutrients between the symbionts: the fungus is
obligate biotrophic, whereas the plant receives inorganic nutrients from the AMF. However, the antagonistic potential of
AMF against a range of soil-borne pathogens has also been demonstrated, but the underlying mechanisms are unknown. Both
direct competition for nutrients/space or antibiosis have been suggested, as indirect competition by AM fungi, changes of
plant root structure, root-exudations, nutrient uptake and growth as well as AMF induction of plant defence have been
investigated. Moreover, a bacterial community structure associated to AM structures has been demonstrated and some of
these bacteria have shown antagonistic potential against pathogens. This raises the question whether it is the AMF or the
associated bacteria, which control the pathogens. So far, a general mechanism for AMF control with soil-borne pathogens has
not been identified, but investigations of more mechanisms and interplays between them might be the answer.