N BODENHAUSEN¹, M VAN DER HEIJDEN¹, K SCHLAEPPI <sup>1

1</sup>Institute for Sustainability Science, Agroscope, Switzerland, Reckenholzstr. 191, 8046 Zurich

Plants require phosphorus (P) for growth. A small fraction of soil P is directly available for plant growth while the larger fraction is complexed to minerals and inaccessible for plants. Farmers optimize their yields by fertilizing their fields with plant-available phosphate. This P pool, however, is a limited resource and the world reserves will be depleted within the next 50 to 100 years. Therefore, understanding how plants tap the complex P fraction is crucial. Microbes - mainly bacteria - can support plants to access this P source in soil through their capacity to solubilize complex soil P. In addition, symbiotic arbuscular mycorrhizal fungi (AMF) connect to plant roots and through their large hyphal network they allow plants to access a larger soil volume and distant sources of P. The P availability in soils determines the extent of the symbiosis between AMF and their host: in soils with low P, plants are heavily colonized by the fungus and little in soils with high P. If the P availability in soil is low, plants secrete a hormone called strigolactone, which attracts and promotes the symbiotic relationship with AMF. In this study, we investigated the interplay between bacteria and fungi that are associated with the roots of Petunia plants as a function of the P availability and strigolactone exudation using 16S rRNA gene profiling. We found differentially abundant OTUs both under the high P and low P conditions; interestingly, we found little overlap of P-dependent OTUs between Petunia, which is colonised by AMF, and Arabidopsis, a plant that does not form symbiosis with AMF.