Fine scale sampling reveals spatial heterogeneity of rhizosphere microbiome in young Brachypodium plants

For a deeper and comprehensive understanding of the diversity, composition and function of rhizosphere microbiomes, we need to focus at the scale of individual roots in standardized growth containers. Root exudation patterns are known to vary across distinct parts of the root giving rise to spatially distinct microbial niches. To address this, we analyzed microbial community from two spatially distinct zones of the primary root (the tip vs. the base) in Brachypodium distachyon, grown in natural soil using standardized fabricated ecosystems known as EcoFABs as well as in more conventional pot and tubes. 16S rRNA based community analysis showed a stronger rhizosphere effect in the root base vs. bulk soil compared to the root tips vs. bulk soil, resulting in an enrichment of Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria, few OTUs belonging to less characterized lineages such as Verrucomicrobia and Acidobacteria. While the microbial community distributions are similar across growth containers, the EcoFAB displayed higher replicate reproducibility. Genome-resolved and bulk metagenomics revealed that genes associated with transcriptional regulation, transport of nutrients and catabolic enzymes indicating active metabolism, biofilm formation and root colonization were enriched in root tips. On the other hand, genes associated with nutrient-limitation and environmental stress were prominent in the bulk soil compared to the root tips, implying the presence of easily available, labile carbon and nutrients in the rhizosphere relative to bulk soil. Such insights into the relationships between root structure, exudation and microbial communities are critical for developing understanding of plant-microbe interactions.