The microbes within: what happens inside Acacia longifolia nodules during development

Plant-microbe interactions are important for plant development, particularly mutualisms where host promiscuity allows associations with diverse microbial partners. This flexibility is crucial for adaptation, especially in invasive species. Understanding microbial community dynamics is therefore key to explain invasion success. Acacia longifolia, a member of the Fabaceae family, is an aggressive invader capable of establishing symbioses with several microorganisms including rhizobia within root nodules. These interactions promote growth and play a role in its invasive capacity. However, the nodulation process remains poorly understood, regarding microbial succession and the dynamics of these communities, following nodule development. We assessed microbial profiles in root nodules from 1-year-old saplings in two habitats using Next-Generation Sequencing, targeting 16 S and 25-28 S rRNA genes. Nodules were classified by size as a proxy for developmental stage. Our findings show that (i) different developmental stages have a characteristic microbial community; (ii) there is a shift in dominance (i.e., abundance) from early to fully developed stage, with nodules containing respectively more microbes from seed or from soil; (iii) microbial partners change in each habitat. The microbial succession indicates a shift in abundance over time, highlighting mostly the changes in recruitment: while several genera that dominate early-stage nodules are mostly found in seeds, fully developed nodules have a community mostly acquired from the surrounding soils and showed a much more specialized fungal community. Our study shows a dynamic assembly of root nodules communities within invasive range that might contribute to the plasticity and adaptative strategy of A. longifolia in these new habitats.