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 ).