Alleviation of nickel toxicity in plants by a rhizobacterium strain is not dependent on its siderophore production

To examine the role of rhizobacteria for plants in a heavy- metal nickel ( Ni)contaminated soil, a rhizobacterium, Pseudomonas putida strain ARB86, was isolated from the rhizosphere of a soil with Arabidopsis growing. The strain ARB86 was resistant to Ni at 1 mM, which showed toxicity to Arabidopsis plants. A suspension of the bacterial strain was inoculated into the Arabidopsis rhizosphere before treatment to the soil with 1 mM Ni. Plant fresh and dry weights, chlorophyll content, anthocyanin accumulation, and Ni concentration in the plant shoots were monitored. Seven days after Ni treatment, both growth and chlorophyll content of plants treated with the bacterium were higher than that of the controls, and the anthocyanin accumulation in the bacterial treatment was lower than that of control. To examine the role of siderophore in an alleviative mechanism by the bacterium, two derivatives ( side-rophore-deficient and siderophore- overproducing mutants) of strain ARB86 were obtained by transposon mutagenesis. However, both mutants, as well as the wild type, alleviated Ni toxicity in plants to the same degree. Nickel influx into plants was decreased by bacterial absorption in the rhizosphere. It is suggested that alleviation of the Ni toxicity in plant by the strain ARB86 is not dependent on its siderophore production.