Seed endophytes from invasive alien plants influence plant performance: a global meta‐analysis with implications for propagule pressure theory

The seed microbiome, encompassing bacteria, fungi, archaea and viruses residing within or on seeds, represents an underexplored but potentially critical determinant of plant establishment and spread. In invasive alien plants, these co‐dispersed microbial communities may function as portable adaptive toolkits that enhance nutrient uptake, pathogen defence, and host plant performance. Here, we specifically evaluate the effects of seed endophytes of invasive alien species on plant performance. To evaluate this perspective, we conducted a systematic review and meta‐analysis following PRISMA‐EcoEvo guidelines, screening Web of Science, Google Scholar and Citation Chaser. Seventeen studies representing 287 effect sizes met the inclusion criteria. We calculated effect sizes as log response ratios (lnRR) for mean plant performance and log coefficient of variation ratios (lnCVR) for variability in plant performance using multilevel meta‐analytic models and meta‐regressions. Across studies, alien invasive seed endophyte inoculation significantly increased mean plant performance, though effects varied widely. In contrast, the responses in performance variability were non‐significant, with a prediction interval ranging from −1.32 to 1.00, indicating high uncertainty and variability across studies. Stronger positive effects emerged under field and laboratory conditions compared to greenhouse or growth chamber experiments, with both bacterial and fungal inocula enhancing mean performance, and bacterial inoculation additionally reducing variability. These findings highlight seed‐associated microbiomes as potentially important contributors to invasion success. By enhancing plant performance, these microbial assemblages can directly shape establishment success and competitive outcomes. This perspective supports a shift to a holobiont view, where seeds and their microbiota function as units of dispersal and establishment, and highlights the need to integrate microbial symbionts into propagule pressure, risk assessment, and management frameworks.