Trichomes form genotype-specifically composed microbial hotspots in the phyllosphere of tomato

The plant phyllosphere is a well-studied habitat characterized by low nutrient availability and high community dynamics. In contrast, plant trichomes, known for their production of a large number of metabolites, are a yet unexplored habitat for microbes. We analyzed the phyllo- sphere as well as trichomes of two tomato genotypes (Solanum lycopersicum LA4024, S. habrochaites LA1777) by targeting bacterial 16S rRNA gene fragments. Leaves, leaves without trichomes, and trichomes alone harbored similar abundances of bacteria (108- 109 16S rRNA gene copy numbers per gram of sample). In contrast, bacterial diversity was found significantly increased in trichome samples (Shannon index: 4.4 vs. 2.5). Moreover, the community composition was significantly different when assessed with beta diversity analysis and corresponding statistical tests. At the bacterial class level, Alphaproteobacteria (23.6%) were significantly increased, whereas Bacilli (8.6%) were decreased in trichomes. The bacterial family Sphingomonadacea (8.4%) was identified as the most prominent, trichome-specific taxa; Burkholderiaceae and Actinobacteria showed similar pattern. Moreover, Sphingomonas was identified as a central element in the core microbiome of trichome samples, while distinct low-abundant bacterial families including Hymenobacteraceae and Alicyclobacillaceae were exclusively found in trichome samples. Niche preferences were statistically significant for both genotypes and genotype-specific enrichments were further observed. Our results provide first evidence of a highly specific trichome microbiome in tomato and show the importance of micro-niches for the structure of bacterial communities on leaves. These findings provide further clues for breeding, plant protection as well as so far unexplored natural pathogen defense strategies.