Breaking through the dark side: microbial help for invading shady forests

Plant–microbe interactions are increasingly recognised as drivers of plant invasion and adaptation, via their capacity to modulate phenotypic plasticity. Although soil biota and plant–soil feedbacks are well studied in open or disturbed systems, their role in naturally resistant habitats – such as closed‐canopy forest interiors – remains unclear. In such environments, microbial mediation of phenotypic plasticity could help overcome strong abiotic constraints on establishment. We hypothesise that the seed‐borne endomicrobiome would be associated with increased phenotypic plasticity, potentially influencing survival across heterogeneous environments. We tested this hypothesis using the invasive plant Taraxacum officinale across the forest mosaic (core, edge, matrix) of the Maulino Coastal Forest in central Chile, combining field and greenhouse experiments. We characterised the environmental gradient, revealing severe light limitation in forest cores, and transplanted plants with an intact endomicrobiome (E+) and a reduced endomicrobiome (E−) into core, edge, and matrix microsites to assess survival. Further, by growing plants under simulated core and matrix conditions, we quantified phenotypic plasticity for functional traits (height, specific leaf area, and BBX24 gene expression) and performance traits (net CO ₂ assimilation rate, biomass and flower production). In the field experiment, E+ plants sustained higher survival in shaded forest cores, whereas E− plants declined sharply. E+ plants showed greater plasticity in functional traits while maintaining stable performance across environments in the greenhouse experiment. Functional‐trait plasticity co‐–varied with field survival across endomicrobiome treatments. Our results show that the endomicrobiome is associated with enhanced functional‐trait plasticity and improved establishment in invasion‐resistant habitats, a microbiome‐informed ‘jack–and–master' strategy. We advocate integrating plant–microbiome interactions into invasion ecology frameworks, particularly where microbial mediation may erode forest biotic resistance.