Characterization of bacterial communities associated with <i>Brassica napus</i> L. growing on a Zn-contaminated soil and their effects on root growth

The interaction between plant growth-promoting bacteria (PGPB) and plants can enhance biomass production and metal tolerance of the host plants. This work aimed at isolating and characterizing the cultivable bacterial community associated with <i>Brassica napus</i> growing on a Zn-contaminated site, for selecting cultivable PGPB that might enhance biomass production and metal tolerance of energy crops. The effects of some of these bacterial strains on root growth of <i>B. napus</i> exposed to increasing Zn and Cd concentrations were assessed. A total of 426 morphologically different bacterial strains were isolated from the soil, the rhizosphere, and the roots and stems of <i>B. napus</i>. The diversity of the isolated bacterial populations was similar in rhizosphere and roots, but lower in soil and stem compartments. <i>Burkoholderia, Alcaligenes, Agrococcus, Polaromonas, Stenotrophomonas, Serratia, Microbacterium</i>, and <i>Caulobacter</i> were found as root endophytes exclusively. The inoculation of seeds with <i>Pseudomonas</i> sp. strains 228 and 256, and <i>Serratia</i> sp. strain 246 facilitated the root development of <i>B. napus</i> at 1,000 µM Zn. <i>Arthrobacter</i> sp. strain 222, <i>Serratia</i> sp. strain 246, and <i>Pseudomonas</i> sp. 228 and 262 increased the root length at 300 µM Cd.