RT Journal Article SR Electronic T1 The relationship between microbial population size and disease in the Arabidopsis thaliana phyllosphere JF bioRxiv FD Cold Spring Harbor Laboratory SP 828814 DO 10.1101/828814 A1 Talia L. Karasov A1 Manuela Neumann A1 Alejandra Duque-Jaramillo A1 Sonja Kersten A1 Ilja Bezrukov A1 Birgit Schröppel A1 Efthymia Symeonidi A1 Derek S. Lundberg A1 Julian Regalado A1 Gautam Shirsekar A1 Joy Bergelson A1 Detlef Weigel YR 2020 UL http://biorxiv.org/content/early/2020/04/08/828814.abstract AB A central goal in microbiome research is to learn what distinguishes a healthy from a dysbiotic microbial community. Shifts in diversity and taxonomic composition are important indicators of dysbiosis, but a full understanding also requires knowledge of absolute microbial population sizes. In addition to the number of microbial cells, information on taxonomic composition can provide important insight into microbiome function and disease state. Here we use shotgun metagenomics to simultaneously assess microbiome composition and microbial load in the phyllosphere of wild populations of the plant Arabidopsis thaliana. We find that wild plants vary substantially in the load of colonizing microbes, and that high loads are typically associated with the proliferation of single taxa, with only a few putatively pathogenic taxa achieving high abundances in the field. Our results suggest (i) that the inside of a plant leaf is on average sparsely colonized with an estimated two bacterial genomes per plant genome and an order of magnitude fewer eukaryotic microbial genomes, and (ii) that higher levels of microbial cells often indicate successful colonization by pathogens. Lastly, our results show that load is a significant explanatory variable for loss of estimated Shannon diversity in phyllosphere microbiomes, implying that reduced diversity may be a significant predictor of microbial dysbiosis in a plant leaf.