Root colonization by rhizobacteria showed no clear association with growth promotion
It is widely known that plant growth-promoting rhizobacteria must colonize the rhizosphere of the host plant to a specific threshold density to be beneficial (Benizri et al. 2001). The establishment of high population densities of PGPR on roots has been proposed as a prerequisite of plant growth promotion. This is exemplified in our results by the lack of phenotypic responses of Artemisia to MB inoculation (Figure 3a1 and Supplementary material,Figure S2 ). However, our results also indicated that high rhizosphere population densities of the introduced strains can be associated with either growth promotion or growth reduction as was seen for the combination of Pbg with Artemisia (growth promotion) andPbg with Arabidopsis (growth reduction) (Supplementary Material,Figures S2a2 and a1 , respectively).