Carbon flow from roots to rhizobacterial networks: Grafting effects

Plants recruit microorganisms from bulk soil by secreting easily available organic carbon into the rhizosphere. Grafting often increases the disease resistance of agricultural plants by modifying this carbon flow from roots into rhizosphere and by recruiting active microorganisms that suppress pathogens. Here, we continuously labeled grafted and ungrafted watermelon plants in a 13CO2 atmosphere to identify the active microorganisms assimilating root exudates. Multi-omics associated technologies (amplicon sequencing, metagenomics and metabolomics) combined with 13C tracing were used to examine the carbon flows, microbial utilization and transformation in the rhizosphere. The number of potentially active bacterial species recruited in the rhizosphere of grafted plants and utilizing root exudates was four times more than in ungrafted plants. These potentially active species matched to metagenome-assembled-genomes (MAGs) mainly belonging to Sphingomonas in the rhizosphere of ungrafted plants, and to Sphingomonas, Chitinophaga, Dyadobacter and Pseudoxanthomonas in the rhizosphere of grafted plants. Sphingomonas possesses the functional potential to metabolize a plant self-toxic substance, namely 4-hydroxybenzoic acid. Furthermore, grafting shaped the complex metabolic interactions and changed the original metabolic dependence between the potentially active bacterial species. Grafting plants diversified belowground carbon flows, activating a greater number of beneficial microbes.