Analysis of rhizobacterial communities in perennial Graminaceae from polluted water meadow soil, and screening of metal-resistant, potentially plant growth-promoting bacteria

This study investigates the impact of long-term heavy metal contamination on the culturable, heterotrophic, functional and genetic diversity of rhizobacterial communities of perennial grasses in water meadow soil. The culturable heterotrophic diversity was investigated by colony appearance on solid LB medium. Genetic diversity was measured as bands in denaturing gradient gel electrophoresis (DGGE) obtained directly from rhizosphere soil and rhizoplane DNA extracts, and from the corresponding culturable communities. In the two rhizospheric fractions the DGGE profiles of the direct DNA extracts were similar and stable among replicates, whereas in the enriched cultures the profiles of the fractions differed, but among the replicates they were similar. One hundred isolates were collected into 33 different operational taxonomic units by use of amplified internal transcribed spacers and into 19 heavy metal-resistant phenotypes. The phylogenetic position of strains belonging to 18 operational taxonomic units, representing more than 80% of the isolates, was determined by 16S rRNA gene sequencing. Several heavy metal-resistant strains were isolated from rhizoplane. Finally, metal-resistant rhizobacteria were tested for plant growth-promoting characteristics; some were found to contain 1-aminocyclopropane-1-carboxylic acid deaminase and/or to produce indole acetic acid and siderophores. Two strains resistant to cadmium and zinc, Pseudomonas tolaasii RP23 and Pseudomonas fluorescens RS9, had all three plant growth-promoting characteristics. Our findings suggest that bacteria can respond to soil metal contamination, and the described methodological approach appears promising for targeting potential plant growth-promoting rhizobacteria.