What If Root Nodules Are a Guesthouse for a Microbiome? The Case Study of Acacia longifolia

SIMPLE SUMMARY: Acacia longifolia is an invasive plant highly dispersed in the Mediterranean region. Being a legume, the association with nitrogen-fixing bacteria (i.e., rhizobia) allows access to readily fixed nitrogen, facilitating its growth, development, and dispersal. The extent to which this relation is restricted to bacteria was investigated in this study. We aimed to (i) characterize the microbial community inside root nodules, i.e., structures where nitrogen fixation occurs, and (ii) understand how these partners change following a fire event. In fact, fire potentiates A. longifolia invasion, so it is of remarkable interest to understand the role of nodulation in dispersal. We found that root nodules are a guesthouse for microorganisms and, while some are common between unburnt and burnt sites, others shift, particularly considering fungal community. ABSTRACT: Acacia longifolia is one of the most aggressive invaders worldwide whose invasion is potentiated after a fire, a common perturbation in Mediterranean climates. As a legume, this species establishes symbioses with nitrogen-fixing bacteria inside root nodules; however, the overall microbial diversity is still unclear. In this study, we addressed root nodules’ structure and biodiversity through histology and Next-Generation Sequencing, targeting 16S and 25S-28S rDNA genes for bacteria and fungi, respectively. We wanted to evaluate the effect of fire in root nodules from 1-year-old saplings, by comparing unburnt and burnt sites. We found that although having the same general structure, after a fire event, nodules had a higher number of infected cells and greater starch accumulation. Starch accumulated in uninfected cells can be a possible carbon source for the microbiota. Regarding diversity, Bradyrhizobium was dominant in both sites (ca. 77%), suggesting it is the preferential partner, followed by Tardiphaga (ca. 9%), a non-rhizobial Alphaproteobacteria, and Synechococcus, a cyanobacteria (ca. 5%). However, at the burnt site, additional N-fixing bacteria were included in the top 10 genera, highlighting the importance of this process. Major differences were found in the mycobiome, which was diverse in both sites and included genera mostly described as plant endophytes. Coniochaeta was dominant in nodules from the burnt site (69%), suggesting its role as a facilitator of symbiotic associations. We highlight the presence of a large bacterial and fungal community in nodules, suggesting nodulation is not restricted to nitrogen fixation. Thus, this microbiome can be involved in facilitating A. longifolia invasive success.