Summary• Alyssum murale has a remarkable ability to hyperaccumulate Ni from soils containing mostly nonlabile Ni. Here, rhizobacteria are shown to play an important role in increasing the availability of Ni in soil, thus enhancing Ni accumulation by A. murale .• Three bacteria, originally isolated from the rhizosphere of A. murale , were examined for their ability to solubilize Ni in soil and for their effect on Ni uptake into Alyssum . Sphingomonas macrogoltabidus , Microbacterium liquefaciens , and Microbacterium arabinogalactanolyticum were added to both sterile and nonsterile Ni-rich Serpentine soil from OR, USA.• Sphingomonas macrogoltabidus significantly reduced Ni extraction by 10 mM Sr(NO 3 ) 2 from soil, M. arabinogalactanolyticum significantly increased Ni extraction, whereas M. liquefaciens had no effect. Extractability of few other metals was affected by inoculation. When these bacteria were added to surface-sterilized seeds of A. murale grown in nonsterile soil, they increased Ni uptake into the shoot by 17% ( S. macrogoltabidus ), 24% ( M. liquefaciens ), and 32.4% ( M. arabinogalactanolyticum ), compared with uninoculated controls.• These results show that bacteria are important for Ni uptake, which from a commercial perspective, could significantly increase revenue generated during phytomining of Ni from soil.

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Four bacterial isolates were examined for their ability to increase the availability of water soluble Cu, Cr, Pb and Zn in soils and for their effect on metals uptake by Zea mays and Sorghum bicolor. Random Amplified Polymorphic DNA (RAPD) analysis was used to show that the bacterial cultures were genetically diverse. Bacterial isolates S3, S28, S22 and S29 had 16S rRNA gene sequences that were most similar to Bacillus subtilis, Bacillus pumilus, Pseudomonas pseudoalcaligenes and Brevibacterium halotolerans based on 100% similarity in their 16S rDNA gene sequence, respectively. Filtrate liquid media that had supported B. pumilus and B. subtilis growth significantly increased Cr and Cu extraction from soil polluted with tannery effluent and from Cu-rich soil, respectively, compared to axenic media. The highest concentrations of Pb (0.2 g kg -1 ), Zn (4 g kg -1 ) and Cu (2 g kg -1 ) were accumulated in shoots of Z. mays grown on Cu-rich soil inoculated with Br. halotolerans. The highest concentration of Cr (5 g kg -1 ) was accumulated in S. bicolor roots grown in tannery-effluent-polluted soil inoculated with a mixed inoculum of bacterial strains. These results show that bacteria play an important role in increasing metal availability in soil, thus enhancing Cr, Pb, Zn and Cu accumulation by Z. mays and S. bicolor.

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