20Factors that drive continental-scale variation in root microbiota and plant adaptation are poorly 21 understood. We monitored root-associated microbial communities in Arabidopsis thaliana and co-22 occurring grasses at 17 European sites across three years. Analysis of 5,625 microbial community 23 profiles demonstrated strong geographic structuring of the soil biome, but not of the root microbiota. 24Remarkable similarity in bacterial community composition in roots of A. thaliana and grasses was 25 explained by the presence of a few diverse and geographically widespread taxa that disproportionately 26 colonize roots across sites. In a reciprocal transplant between two A. thaliana populations in Sweden 27and Italy, we uncoupled soil from location effects and tested their respective contributions to root 28 2 microbiota variation and plant adaptation. The composition of the root microbiota was affected by 29 location and soil origin, and to a lesser degree by host genotype. The filamentous eukaryotes were 30 particularly strongly affected by location. Strong local adaptation between the two A. thaliana 31 populations was observed, with difference in soil properties and microbes of little importance for the 32 observed magnitude of adaptive differentiation. Our results suggest that, across large spatial scales, 33 climate is more important than are soil conditions for plant adaptation and variation in root-associated 34 filamentous eukaryotic communities. 35 36 differentiation among plant populations is still limited beyond classical examples of adaptation to 57 extreme soil conditions 25 . 58 59Here, we tested whether roots of A. thaliana and co-occurring grasses growing in various soils and 60 climatic environments establish stable associations with bacterial and filamentous eukaryotic 61 communities across a latitudinal gradient in Europe. In a reciprocal transplant between two A. thaliana 62 populations in Sweden and Italy, we uncoupled soil from location effects and experimentally tested the 63 hypothesis that soil properties and climate drive root microbiota assembly and adaptive differentiation 64 between the two A. thaliana populations. 65
66We found that a widespread set of bacteria, but not filamentous eukaryotes, establish stable associations 67 with roots of A. thaliana and grasses across 17 sites in Europe, despite strong geographical structuring 68 and variation in the surrounding soil communities. The reciprocal transplant of soil and plant genotypes 69 between two native A. thaliana populations in northern and southern Europe showed that the 70 composition of root microbiota was more affected by soil properties and location than by host genotype. 71The effect of soil was stronger than that of location for root-associated bacteria, whereas the effect of 72 location was stronger for root-associated fungi and oomycetes. Transplant location, rather than origin 73 of soil, also largely accounted for strong selection against the nonlocal A. thaliana genotype at each site. 74Our results suggest that climat...