Abstract: Biodiversity is known to promote ecosystem functions such as overall productivity, but most evidence comes from plant biodiversity × plant productivity studies. Studies exploring the effect of microbial diversity on productivity are less common, and the role of microbial diversity on plant productivity is particularly little understood. Here, we tested for microbiome diversity effects on plant growth and defence by using a collection of 92 leaf bacterial strains isolated from Arabidopsis thaliana. Arabidopsis plants were inoculated with bacterial communities, with four different categorical diversity levels. Our experiment had a full-factorial design with two levels of bacterial richness (three vs. nine strains), two levels of phylogenetic diversity (one vs. three clades), with 10 different bacterial communities in each diversity treatment, resulting in a total of 40 communities. Inoculated plants were then infected with four different strains of the leaf pathogen Pseudomonas viridiflava,as well as a mock treatment. We found the group composition was significantly different in both functional and phylogenetic diversity levels. Increases in phylogenetic, functional, or categorical diversity had positive impacts on plant growth and resistance. Of the three diversity indices, we found functional diversity was best at explaining our data. More detailed analyses of the observed diversity effects indicated that these were partly driven by clade identity - some bacterial clades appeared to be generally more effective at promoting plant growth and resistance than others – but also by the functional diversity of the bacterial communities. Our experiment provides proof-of-principle that microbiome diversity plays a role in the functioning of plant hosts, and that it should therefore receive greater attention in basic and applied plant research.
