It takes more than two to tango: how nodule microbiomes shape legume performance

Symbiotic nitrogen fixation by legume-rhizobia associations is a cornerstone of terrestrial nitrogen cycling, yet the role of the broader nodule microbiome, particularly non-rhizobial endophytes (NREs), in modulating this symbiosis remains poorly understood. While NREs are consistently found within legume nodules, their functional significance has been studied almost exclusively in agricultural crops and through limited methodologies (culture-based assays or 16S rRNA amplicon sequencing), leaving a blind spot regarding wild legumes under natural conditions and the specific metabolic functions that NREs contribute to or detract from plant performance. Moreover, a critical but unresolved question concerns whether and how NREs influence the efficiency of the primary rhizobial symbiont, and whether microbial "cheating", i.e. where bacteria consume plant carbon without delivering proportional nitrogen fixation benefits, operates under natural field conditions. This study addresses these gaps using Calicotome villosa, a Mediterranean legume shrub that relies exclusively on Bradyrhizobium as its primary symbiont and exhibits tightly regulated nodule biomass allocation, as a model system. Through a controlled inoculation experiment using soils collected from multiple natural field sites, the authors examined how variation in nodule microbiome composition and functional gene repertoire affects key plant performance traits, including total biomass, leaf nitrogen content, nitrogen fixation per nodule, and nodule mass fraction. The study employed shotgun metagenomics and metagenome-assembled genomes (MAGs), combined with KEGG Orthology-based functional annotation, targeted nitrogen metabolism gene analysis, and machine-learning approaches (Boruta feature selection, WGCNA co-expression networks), to deliver a comprehensive, culture-independent view of this multipartite symbiosis at functional resolution. By explicitly partitioning microbial functions between the primary rhizobial symbiont and NREs, the work aims to determine which bacterial genes and pathways are predictive of plant performance, and to reveal how accessory microbiome members beyond the canonical symbiont shape legume nitrogen nutrition -- with broader implications for understanding the ecology and evolution of symbiotic mutualisms and for the potential manipulation of nodule microbiomes in sustainable agriculture.