In this work, we evaluated the ability of rhizobacteria
isolated from a metal-contaminated site to produce
plant growth-promoting substances, such as indole-3-
acetic acid (IAA), ammonia, hydrogen cyanide, siderophores
and extracellular enzymes, to solubilize phosphate
and for 1-aminocyclopropane-1-carboxylate (ACC)-deaminase
activity, while assessing their capacity to enhance
Trifolium repens
growth. The rhizobacteria that better
performed were used as bioinoculants for plants grown in
zinc- and cadmium-spiked soils.
Rhizobacteria were inoculated
onto white clover seedlings growing in 250 and
500 mg kg
-1 of zinc and in 10 and 30 mg kg
-1 of cadmium-
spiked soils and plant traits determined. The available
and exchangeable rhizosphere metal concentrations
were analyzed. Twenty rhizobacteria stimulated white
clover growth under axenic conditions. Overall, strains
Rhodococcus erythropolis
EC 34,
Achromobacter
sp. 1AP2
and
Microbacterium
sp. 3ZP2 increased clover biomass in
the absence and in presence of metals; however, in soils
with 250 mg kg
-1 of zinc, bacterial inoculation did not
enhance plant biomass. Biomass enhancement may be
related to the multiple bacterial promoting traits, especially
the production of high levels of IAA and siderophores and
the activity of ACC-deaminase. To the best of our
knowledge, this is the first study reporting the potential of
R. erythropolis to increase plant growth in metal-contaminated
soils. In general, strains
Microbacterium sp. 3ZP2
and
Arthrobacter sp. EC 10 enhanced the available and
exchangeable metal concentrations in rhizosphere showing
their potential to improve phytoremediation efficiency.
This work clearly indicates that rhizobacteria are promising
for the development of bioinoculants useful for phytoremediation
of polluted soils.