Leaf-Position-Dependent Remodeling of Phyllosphere Microbiome Mediated by Foliar Zinc Sulfide Nanoparticles Drives Tomato Growth

Foliar nano-fertilization is emerging as a promising strategy to improve crop performance, yet the microbiome-mediated mechanisms and their dependence on canopy microhabitats remain poorly understood. Here, we prepared ball-milled ZnS nanoparticles (11.6 ± 2.8 nm) and conducted an equimolar Zn foliar experiment (ZnS at 0.6/1.2/2.4 mM; ZnO and ZnSO4 at matched molarities). ZnS with recommend concentration (1.2 mM) produced the most robust overall plant improvement. 16S rRNA profiling showed that ZnS-induced microbiome restructuring was stronger in epiphytic than endophytic communities. Among epiphytic communities, ZnS induced a pronounced leaf-position-dependent response across top, middle, and bottom leaves, with the strongest remodeling occurring in middle leaves. Middle-leaf epiphytes showed marked dominant-genus turnover, including enrichment of Brevundimonas and elevation of Sphingobacterium with concurrent declines of control-associated Flavobacterium, together with the most pronounced remodeling of predicted functions (e.g., increased pyruvate metabolism and the TCA cycle). This response matched the growth pattern, suggesting that middle-leaf microbiome remodeling may link ZnS nanoparticles application to improved tomato performance. These results firstly identify middle leaves as a key microecological locus of ZnS action and provide a mechanistic basis for selecting an optimal spraying leaf position.