Heavy metal accumulation in Lathyrus sativus growing in contaminated soils and identification of symbiotic resistant bacteria

In this study, two populations of leguminous plants Lathyrus sativus were grown in four soils that were collected from sites differently contaminated by heavy metals. Evaluations included basic soil properties, concentrations of major nutrients and four metals (copper, zinc, lead and cadmium) in these soils. Investigation of Lathyrus sativus response to contamination showed that the increase of heavy metal concentration in soils affected biomass of plant, number of nodules and plant metal uptake. Heavy metal tolerance of 46 isolated bacteria from the root nodules was evaluated and demonstrated that the maximum concentration of Cd, Pb, Cu and Zn tolerated by strains were 0.8, 2.5, 0.2, and 0.5 mM, respectively. Twenty-two isolates were tested for their effects on plant biomass production and nodule formation and showed that only R. leguminosarum nodulated Lathyrus sativus, while some bacteria improved the shoot and root dry biomass. Sequences of their 16S rDNA gene fragments were also obtained and evaluated for tentative identification of the isolates which revealed different bacterial genera represented by Rhizobium sp, Rhizobium leguminosarum, Sinorhizobium meliloti, Pseudomonas sp, Pseudomonas fluorescens, Luteibacter sp, Variovorax sp, Bacillus simplex and Bacillus megaterium. The existence of Pb- and Cd-resistant genes (PbrA and CadA) in these bacteria was determined by PCR, and it showed high homology with PbrA and CadA genes from other bacteria. The tested resistant population was able to accumulate high concentrations of Pb and Cd in all plant parts and, therefore, can be classified as a strong metal accumulator with suitable potential for phytoremediation of Pb and Cd polluted sites. Heavy metal resistant and efficient bacteria isolated from root nodules were chosen with Lathyrus sativus to form symbiotic associations for eventual bioremediation program, which could be tested to remove pollutants from contaminated sites.