Seed endophytic bacteria from invasive Lactuca serriola increase soil available phosphorus under phosphorus deficiency.

Invasion by alien plant species can alter soil biogeochemical processes, including phosphorus (P) cycling. Plant litter and root exudates have been proposed to influence soil chemistry either directly through the release of diverse metabolites or indirectly by modifying the rhizosphere microbiome. Notably, some seed endophytic bacteria co-dispersed with seeds possess phosphate-solubilizing activity (PSA), suggesting their potential contribution to soil P dynamics. However, this possibility has rarely been tested. In this study, we conducted in vitro PSA assays on bacterial strains isolated from seeds of the invasive Lactuca serriola. To comprehensively assess their capacity, both individual isolates and their synthetic consortia were examined. Individual isolates exhibited variable PSA, and two isolates showed synergistic PSA when combined with other isolates. Based on these results, we constructed dual-strain consortia containing either of the synergistic strains with another isolate and inoculated them onto L. serriola seeds. Plants were then grown under P-deficient conditions, and both plant and soil traits were measured. Seed inoculation with specific dual-strain consortia significantly increased soil P, and these effects exceeded those of individual strains, indicating synergistic interactions between bacterial partners. The plant root-to-shoot ratio was negatively associated with soil P. Our results imply that plants harboring specific seed endophytic bacteria can enhance soil P under P-limiting conditions. In addition, they suggest the importance of bacterial interactions when evaluating the effects of bacteria on plant and soil traits.