The inoculation of plants with plant-growth-promoting rhizobacteria has become a priority in the phytoremediation of heavy-metal-contaminated soils. A total of 82 bacteria were isolated from Sulla coronaria root nodules cultivated on four soil samples differently contaminated by heavy metals. The phenotypic characterization of these isolates demonstrated an increased tolerance to cadmium reaching 4.1mM, and to other metals, including Zn, Cu and Ni. Polymerase Chain Reaction/Restriction Fragment Length Polymorphism (PCR/RFLP) analysis showed a large diversity represented by genera related to Agrobacterium sp., R. leguminosarum, Sinorhizobium sp., Pseudomonas sp., and Rhizobium sp. Their symbiotic effectiveness was evaluated by nodulation tests. Taking into consideration efficiency and cadmium tolerance, four isolates were chosen; their 16SrRNA gene sequence showed that they belonged to Pseudomonas sp. and the Rhizobium sullae. The selected consortium of soil bacteria had the ability to produce plant-growth-promoting substances such as indole acetic acid and siderophore. The intracellular Cd accumulation was enhanced by increasing the time of incubation of the four soil bacteria cultivated in a medium supplemented with 0.1mM Cd. The existence of a cadmium-resistant gene was confirmed by PCR. These results suggested that Sulla coronaria in symbiosis with the consortium of plant-growth-promoting rhizobacteria (PGPR) could be useful in the phytoremediation of cadmium-contaminated soils.

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In search of efficient and resistant plant growth-promoting rhizobacteria (PGPR) strains with multiple activities, a total of twelve bacterial belonging to R. leguminosarum, S. meliloti, Pseudomonas sp., P. fluorescens, Luteibacter sp., Variovorax sp., B. simplex, and B. megaterium were isolated from root nodules of grass pea (Lathyrus sativus L.) grown in contaminated soils. Upon screening, all test strains were able to synthesize indoleacetic acid; more than 90% were siderophore producers and 75% showed varying levels of phosphate solubilizing ability. The gaseous metabolite biosynthesis showed that 42% of strains were cyanogenic. The lead (Pb) bioaccumulation differs with incubation times between cell wall and cytoplasm. Indeed, the most part of Pb was adsorbed to cell surface. A pot experiment was conducted for investigating the capability of combined bacteria to promote plant growth of Lathyrus sativus under controlled conditions. Subsequently, the performance of symbiosis Lathyrus sativus-PGPR (I4: R. leguminosarum (M5) + B. simplex + Luteibacter sp. + Variovorax sp.) was investigated under lead stress using hydroponic culture to elucidate the effect of bacterial inoculation on Pb uptake as well as plant growth. Results showed that under 0.5 mM Pb, inoculation with I4 significantly increased shoots and roots biomass by 59% and 56%, respectively, and improved Pb uptake in both shoots and roots by 39% and 47%, respectively, as compared to uninoculated plants. The inoculation of Lathyrus sativus with efficient and Pb resistant PGPR is a promising symbiosis that having significant potential to improve phytoremediation of Pb-polluted soils.

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The ability of plant growth-promoting rhizobacteria (PGPR) to enhance Lathyrus sativus tolerance to lead (Pb) stress was investigated. Ten consortia formed by mixing four efficient and Pb-resistant PGPR strains were assessed for their beneficial effect in improving Pb (0.5 mM) uptake and in inducing the host defence system of L. sativus under hydroponic conditions based on various physiological and biochemical parameters. Lead stress significantly decreased shoot (SDW) and root (RDW) dry weight, but PGPR inoculation improved both dry weights, with highest increases in SDW and RDW of plants inoculated with I5 (R. leguminosarum (M5) + P. fluorescens (K23) + Luteibacter sp. + Variovorax sp.) and I9 (R. leguminosarum (M5) + Variovorax sp. + Luteibacter sp. + S. meliloti) by 151% and 94%, respectively. Additionally, inoculation significantly enhanced both chlorophyll and soluble sugar content, mainly in I5 inoculated leaves by 238% and 71%, respectively, despite the fact that Pb decreased these parameters. We also found that PGPR inoculation helps to reduce oxidative damage and enhances antioxidant enzyme activity, phenolic compound biosynthesis, carotenoids and proline content. PGPR inoculation increased Pb uptake in L. sativus, with highest increase in shoots of plants inoculated with I5 and I7, and in roots and nodules of plants inoculated with I1. Moreover, PGPR inoculation enhanced mineral homeostasis for Ca, Cu and Zn under Pb stress, mainly in plants inoculated with I1, I5, I7 and I9. Results of our study suggest the potential of efficient and Pb-resistant PGPR in alleviating harmful effects of metal stress via activation of various defence mechanisms and enhancing Pb uptake that promotes tolerance of L. sativus to Pb stress.

Phytoremediation comprises a set of plant and microbe-based technologies for remediation of soil heavy metal contamination. In this work, four Pb-resistant bacteria (Agrobacterium tumefaciens, Rahnella aquatilis, and two Pseudomonas sp.) were selected among a collection of isolates from root nodule of Lens culinaris. They had a high degree of bioaccumulation ability in nutrient medium containing 2 mM Pb, and the maximum Pb accumulation of whole cell was found after 48-h incubation. These Pb-resistant bacteria synthesized plant growth promoting substances such as indole acetic acid and siderophore. The presence of the Pb resistance genes (pbrA) in these bacteria has been confirmed by PCR. L. culinaris cultivated in two experimental soils with different levels of contamination showed that Pb contamination affected plant growth; therefore, it's co-inoculation with the consortium of Pb-resistant bacteria improved plant biomass. The present study demonstrated that lentil accumulated Pb primarily in their roots and poorly in their shoots; in addition, it's co-inoculation in moderately Pb-contaminated soil induced a reduction in Pb accumulation in roots and shoots by 22 and 80 %, respectively. Whereas in highly Pb-contaminated soil, we registered a diminution in concentration of Pb in shoots (66 %) and an augmentation in roots (21 %). The contamination of soil by Pb caused an oxidative stress in lentil plant, inducing modulation in antioxidant enzymes activities, essentially in superoxide dismutase (SOD) and peroxidase (GPOX) activities which were more pronounced in lentil cultivated in highly Pb-contaminated soil, in addition, co-inoculation enhanced these activities, suggesting the protective role of enzymatic antioxidant against Pb-induced plant stress.Thus, the present study demonstrated that co-inoculation of lentil with A. tumefaciens, R. aquatilis, and Pseudomonas sp. formed a symbiotic system useful for phytostabilization of highly and moderately Pb-contaminated soils.