Influence of seed-borne endophytic bacteria and their consortia from the invasive Lactuca serriola on soil phosphorus availability and plant competitive ability

Abstract

Seed-borne endophytes represent vertically transmitted microbial assemblages that can influence early plant development and adaptation to nutrient stress. Despite their ecological relevance, their roles in regulating soil nutrient dynamics and plant competitiveness remain underexplored, particularly in invasive species. This dissertation investigates the contributions of seed endophytic bacteria associated with the invasive plant Lactuca serriola L., focusing on their ability to mobilize soil phosphorus (P) under deficiency and to mediate soil–plant interactions across ecological contexts. By integrating trait-based screening, in planta validation, and competition experiments, this study provides a mechanistic framework linking microbial inheritance with invasion-related nutrient cycling. Nineteen representative bacterial isolates were obtained from surface-sterilized seeds of wild L. serriola and characterized for key plant growth-promoting (PGP) traits, including indole-3-acetic acid (IAA) production, phosphate-solubilizing activity (PSA), siderophore production (SPP), and 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. All isolates exhibited at least one PGP trait, and consortium-level assembly revealed emergent interactions beyond single-strain performance. In particular, consortia containing Kosakonia cowanii SD1 and Xanthomonas spp. SD2 displayed strong synergistic enhancement of phosphate solubilization, indicating functional complementarity among seed-derived strains. Under phosphate-deficient conditions, dual-strain inoculations constructed around K. cowanii SD1 or Xanthomonas spp. SD2 significantly increased soil available phosphorus relative to single and uninoculated controls. The consortia of K. cowanii SD1 with Pantoea dispersa SD25 and Xanthomonas spp. SD2 with Stenotrophomonas maltophilia SD8 showed the highest soil available phosphorus. These findings demonstrate that cooperative metabolism among vertically inherited endophytes can enhance soil P mobilization under strong nutrient limitation. However, these soil-level improvements were not consistently accompanied by increased plant biomass or leaf P concentration, suggesting a decoupling between microbial nutrient transformation and immediate host benefit. In competition experiments under varying phosphate conditions, bacterial inoculation consistently elevated soil available phosphorus across monoculture and interspecific settings, yet plant responses remained limited and varied across phosphate and competition conditions. Endophyte-mediated nutrient mobilization thus functioned more as an environmental modifier than a direct growth promoter, subtly altering soil nutrient balance rather than competitive hierarchies. Collectively, these findings reveal that seed-borne bacterial consortia in L. serriola act as inherited nutrient regulators capable of altering soil phosphorus dynamics through cooperative metabolism. Such vertically transmitted consortia represent a heritable microbial mechanism that enhances environmental adaptability and may contribute to the persistence of invasive plants in nutrient-impoverished habitats. This study advances an integrative perspective linking microbial inheritance, synergism of endophytes, and ecological outcomes, thereby expanding the conceptual scope of invasion ecology to include inherited microbial processes driving nutrient cycling.