Shading Shapes Phyllosphere and Rhizosphere Bacterial Communities in Seedlings of the Karst Endangered Plant Malania oleifera

Malania oleifera is an endangered woody oil tree species endemic to China, where light conditions play a key role in seedling establishment. However, responses of seedling-associated bacterial communities to shading remain poorly characterized. This study investigated phyllosphere and rhizosphere bacterial communities under different shading regimes using 16S rRNA amplicon sequencing in two-year-old seedlings. Seedlings were subjected to 75%, 50%, and 25% shading, full-light conditions, and a field-grown reference group. The phyllosphere bacterial community showed lower alpha diversity and stronger compositional variation across shading treatments than the rhizosphere community, indicating higher sensitivity of leaf-associated bacteria to changes in the light environment. The rhizosphere maintained a larger shared bacterial pool and more stable community composition, with treatment-related differences mainly reflected in shifts in relative abundance. Functional prediction using PICRUSt2 indicated that phyllosphere bacterial functions were more responsive to shading than those in the rhizosphere, particularly pathways associated with genetic information processing and metabolism. PICRUSt2-based KEGG predictions suggested that both the phyllosphere and rhizosphere communities in the less-shaded and unshaded treatments were enriched for pathways related to aromatic compound degradation. Co-occurrence network analysis further revealed that phyllosphere association patterns were more sensitive to shading variation, whereas rhizosphere network complexity remained relatively stable. Shading exerted compartment-specific effects on the bacterial communities of M. oleifera seedlings, with the phyllosphere microbiota showing higher sensitivity to light variation than the rhizosphere communities.