P-374 - Pseudomonas intra-genus competition determines protective function of SynComs in Arabidopsis thaliana

Plant roots accommodate host species-specific bacterial communities. In addition, commensal bacteria display host preference when competing for colonization of their host of origin. Which factors control this preferential colonization, and how the host benefits, is unknown. We tested if different bacterial synthetic communities (SynComs) provide differential protection against detrimental root-colonizing Pseudomonas isolate R401 in Arabidopsis thaliana (At). Using SynComs derived from At- and Lotus japonicus (Lj), we show that Lj-SynComs are fully protective, whereas At-SynComs display full protection only towards a R401 mutant impaired in the production of inhibitory exometabolites. Protection was associated with a reduced R401 titer. In vitro antagonist assays, in planta and in vitro bacterial community profiling, plus strain-swapping and -dropout experiments revealed that competition among commensal Pseudomonas strains and R401 determines the success of the opportunist, independent of the original host of the commensals. We determined the carbon utilization potential of these isolates, which may explain the competition with R401 and the role of host-secreted compounds. Our results provide evidence that intra-genus interactions within SynComs modulate plant health and disease, and that an individual beneficial strain can be sufficient to outcompete an opportunistic relative. This has implications for the development of beneficial microbial consortia for agriculture.