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<div>Plant-associated microbes are crucial for the health of their hosts. However, the high complexity of plant microbiomes challenges detailed studies to define experimentally the mechanisms underlying the beneficial effects of such microbiota on plant hosts and the dynamics of community assembly. Using host-mediated selection, we obtained a greatly simplified synthetic bacterial community consisting of seven strains (<i>Enterobacter cloacae</i>, <i>Stenotrophomonas maltophilia, Ochrobactrum pituitosum, Herbaspirillum frisingense, Pseudomonas putida, Curtobacterium pusillum</i> and<i> </i><i>Chryseobacterium </i><i>indologenes</i>) representing three of the four most dominant phyla found in maize roots<b>. </b><i>In planta </i>and <i>in vitro</i>, this model community inhibited the phytopathogenic fungus <i>Fusarium </i><i>verticillioides</i> indicating a clear benefit to the host. By utilizing a selective culture-dependent method to track the abundance of each strain we investigated the role that each plays in community assembly on roots of axenic maize seedlings. Only the removal of <i>E. cloacae </i>led to the complete loss of the community and <i>C. pusillum </i>took over. This suggests that <i>E. cloacae</i> plays the role of keystone species in this model ecosystem. Thus, our synthetic seven-species community has the potential to serve as a useful system to dissect the beneficial effects of root microbiota on hosts and explore how bacterial interspecies interactions affect root microbiome assembly under laboratory conditions in future.</div>