Abstract

Much recent work has focused on characterising the mutualistic elements of the plant microbiome, often aiming to identify bacterial strains that can increase plant fitness. Although most work has focused on terrestrial plants, Lemna minor, a floating aquatic angiosperm, is increasingly used as a model in host-microbe interactions. Here we assess the fitness and phenotypic consequences of the full microbiome for L. minor by assaying plants from eight natural sites, with and without their microbiomes, over a range of environmental conditions. We find that the microbiome supresses plant fitness, for all genotypes and across all environmental conditions. This decrease in fitness was accompanied by phenotypic changes, with plants forming smaller colonies and producing smaller fronds and shorter roots with the microbiome present. Although the L. minor microbiome clearly includes important symbionts, our findings suggest that we cannot discount the important pathogenic, parasitic, and competitive interactions, whose influence can override that of mutualists.