Background Bacterial viruses, phages, play a key role in nutrient turnover and lysis of bacteria in terrestrial ecosystems. While phages are abundant in soils, their effects on plant pathogens and rhizosphere bacterial communities are poorly understood. Here, we used metagenomics and direct experiments to causally test if differences in rhizosphere phage communities could explain variation in soil suppressiveness and bacterial wilt plant disease outcomes by plant-pathogenic Ralstonia solanacearum bacterium. Specifically, we tested two hypotheses: (1) that healthy plants are associated with stronger top-down pathogen control by R. solanacearum-specific phages (i.e. ‘primary phages’) and (2) that ‘secondary phages’ that target pathogen-inhibiting bacteria play a stronger role in diseased plant rhizosphere microbiomes by indirectly ‘helping’ the pathogen. Results Using a repeated sampling of tomato rhizosphere soil in the field, we show that healthy plants are associated with distinct phage communities that contain relatively higher abundances of R. solanacearum-specific phages that exert strong top-down pathogen density control. Moreover, ‘secondary phages’ that targeted pathogen-inhibiting bacteria were more abundant in the diseased plant microbiomes. The roles of R. solanacearum-specific and ‘secondary phages’ were directly validated in separate greenhouse experiments where we causally show that phages can reduce soil suppressiveness, both directly and indirectly, via top-down control of pathogen densities and by alleviating interference competition between pathogen-inhibiting bacteria and the pathogen. Conclusions Together, our findings demonstrate that soil suppressiveness, which is most often attributed to bacteria, could be driven by rhizosphere phage communities that regulate R. solanacearum densities and strength of interference competition with pathogen-suppressing bacteria. Rhizosphere phage communities are hence likely to be important in determining bacterial wilt disease outcomes and soil suppressiveness in agricultural fields.

1 Background: Sequencing and PCR errors are a major challenge when characterising genetic 2 diversity using high-throughput amplicon sequencing (HTAS). 3 4 Results: We have developed a multiplexed HTAS method, MAUI-seq, which uses unique 5 molecular identifiers (UMIs) to improve error correction by exploiting variation among 6 sequences associated with a single UMI. We show that two main advantages of this approach 7 are efficient elimination of chimeric and other erroneous reads, outperforming DADA2 and 8 UNOISE3, and the ability to confidently recognise genuine alleles that are present at low 9 abundance or resemble chimeras. 10 11 Conclusions:The method provides sensitive and flexible profiling of diversity and is readily 12 adaptable to most HTAS applications, including microbial 16S rRNA profiling and 13 metabarcoding of environmental DNA. 14

Bacteria secrete siderophores whose function is to acquire iron. In recent years, the siderophores of several Chryseobacterium species were shown to promote the health and growth of various plants such as tomato or rice. However, the chemical nature of Chryseobacterium siderophores remained unexplored despite great interest. In this work, we present the purification and structure elucidation by NMR and MS/MS of chryseochelin A, a novel citrate-based siderophore secreted by three Chryseobacterium strains involved in plant protection. It contains the unusual building blocks 3-hydroxycadaverine and fumaric acid. Furthermore, the unstable structural isomer chryseochelin B and its stable derivative containing fatty acid chains, named chryseochelin C, were identified by mass spectrometric methods. The latter two incorporate an unusual ester connectivity to the citrate moiety showing similarities to achromobactin from the plant pathogen Dickeya dadantii. Finally, we show that chryseochelin A acts in a concentration-dependent manner against the plant-pathogenic Ralstonia solanacearum strain by reducing its access to iron. Thus, our study provides valuable knowledge about the siderophores of Chryseobacterium strains, which have great potential in various applications.