Relationship between growth promotion and root colonization
In the assays described above, root colonization was assessed when plants were 11 days old. For Arabidopsis, Broccoli and Artemisia,Pf SS101 established population densities ranging from 1.5 ± 0.1 x 105 to 9.4 ± 0.7 x 107 CFU mg-1 root fresh weight. In contrast, MB colonization of roots varied greatly between different plant species: MB established population densities on Arabidopsis roots of 7.5 ± 0.5 x 106 CFU mg-1, whereas MB densities on Artemisia roots were below the detection limit. Pbg colonized roots of all three plant species at relatively high densities ranging from 8.1 ± 0.3 x 107 to 1.0 ± 0.1 x 109 CFU mg-1 (Supplementary Material, Table S2 ). To determine correlations, if any, between the rhizosphere population densities and specific plant phenotypes (i.e., biomass), we plotted the rhizobacterial densities against various plant parameters (Supplementary Material, Figure S2 ). Colonization of the root at higher density showed a positive, negative or no association with shoot and root biomass and depended on the host-rhizobacterium combination. For example, Pbg reached higher population densities than Pf SS101 and MB on the root of Arabidopsis and Artemisia. In Arabidopsis, high population densities ofPbg were associated with a reduction in shoot biomass while in Artemisia it resulted in significant enhancement of shoot biomass. In Broccoli, all three rhizobacterial strains showed a similar level of root colonization, yet the plant phenotype varied greatly: Pbginduced a significant increase in shoot biomass while Pf SS101 resulted in a significant reduction in shoot biomass and MB showed no significant effect on shoot biomass. Hence, there was no clear overall correlation between rhizosphere population densities of the introduced rhizobacterial strains and shoot biomass (Supplementary Material,Figure S2a ). Also for root biomass, no consistent association was found with rhizosphere population densities of the introduced strains (Supplementary Material, Figure S2b ).