Seed-Derived Synthetic Microbial Communities (SynComs) from Medicago Wild Relatives Modulate Early Plant Microbiome Assembly and Phenotypic Traits in Lucerne (Medicago sativa L.)

Seed-associated microbiomes represent an underexplored frontier in synthetic community (SynCom) design, particularly in forage legumes such as lucerne (<i>Medicago sativa</i> L.), where early microbial assembly can shape plant development. Crop wild relatives (CWRs) harbour more diverse seed microbiomes and may contain microbes with greater functional potential than domesticated lucerne. To test this, SynComs were constructed from seed-borne bacteria isolated from <i>M. laciniata</i> (drought-resilient) and <i>M. littoralis</i> (salt-tolerant). Two three-strain SynComs were assembled from taxa consistently shared across lucerne and its CWRs, and a third six-strain 'Mix' SynCom combined both sets. The aim of this study was to assess whether these SynComs exert phenotypic effects on lucerne growth when used as seed inocula alongside the native microbiome during early development and later vegetative stages under well-watered and drought conditions. Inoculation enhanced germination and early growth, with the Mix SynCom producing the strongest gains. Microbiome profiling at 24 days revealed treatment-specific restructuring, with enrichment of beneficial taxa and microbial coalescence. While early-stage benefits diminished at later stages, and drought ultimately reduced biomass across all treatments, the findings demonstrate that CWR-derived SynComs can enhance lucerne establishment and early growth while restructuring host microbiomes, providing a framework for seed-applied microbial solutions in sustainable agriculture.