Mercury-resistant rhizobial bacteria isolated from nodules of leguminous plants growing in high Hg-contaminated soils

Abstract

A survey of symbiotic bacteria from legumes grown in high mercury-contaminated soils (Almadén, Spain) was performed to produce a collection of rhizobia which could be well adapted to the environmental conditions of this region and be used for restoration practices. Nineteen Hg-tolerant rhizobia were isolated from nodules of 11 legume species (of the genera Medicago, Trifolium, Vicia, Lupinus, Phaseolus, and Retama) and characterized. Based on their growth on Hg-supplemented media, the isolates were classified into three susceptibility groups. The minimum inhibitory concentrations (MICs) and the effective concentrations that produce 50% mortality identified the patterns of mercury tolerance and showed that 15 isolates were tolerant. The dynamics of cell growth during incubation with mercury showed that five isolates were unaffected by exposure to Hg concentrations under the MICs. Genetic analyses of the 16S rRNA gene assigned ten strains to Rhizobium leguminosarum, six to Ensifer medicae, two to Bradyrhizobium canariense, and one to Rhizobium radiobacter. Inoculation of host plants and analysis of the nodC genes revealed that most of them were symbiotically effective. Finally, three isolates were selected for bioremediation processes with restoration purposes on the basis of their levels of Hg tolerance, their response to high concentrations of this heavy metal, and their genetic affiliation and nodulation capacity.

Genes Conferring Copper Resistance in Sinorhizobium meliloti CCNWSX0020 Also Promote the Growth of Medicago lupulina in Copper-Contaminated Soil

“…However, many environmental factors, including heavy and transition metals, often limit the potential of symbiotic systems and have negative effects on both growth and nitrogen fixation of rhizobia (13). Recently, many rhizobial strains with enhanced ability to survive in the presence of high concentrations of heavy and transition metals have been isolated from the root nodules of various legumes (14,15). Further studies revealed that the nodulation and nitrogen-fixing ability of metal-resistant rhizobia were not affected by metal ions.…”

RAPD-inferred genetic variability of some indigenousRhizobium leguminosarumisolates from red clover (Trifolium pratenseL.) nodules

“…The isolation and characterization of native rhizobia could lead to improvement in the manufacture of commercial inoculants and crop production. Rhizobial strains can also be used as co-inoculants with plant growth rhizobacteria [8,9] and for bioremediation purposes [18,22,25].…”

Possible reasons for tolerance to mercury of Lupinus albus cv. G1 inoculated with Hg-resistant and sensitive Bradyrhizobium canariense strains

“…Environmental abiotic stresses not only have an injurious effect on metabolism, growth and plant development but also affect the organ of the symbiosis, the nodule (de María et al 2007). The effect of Hg on the Bradyrhizobium-Lupinus symbiosis has been previously investigated (Ruiz-Díez et al 2012a;Quiñones et al 2013 growing in Hg-contaminated soils. Among these isolates, one of them, L7AH, was very tolerant of Hg growing in a liquid culture medium containing 12.5 μM Hg.…”

“…Seeds were sown in 1 L autoclaved pots filled with vermiculite moistened with water containing the corresponding Hg concentration (0-200 μM HgCl 2 ). Seeds were inoculated twice (first at sowing and then 1 week later), according to de Lorenzo et al (1994) with 1 ml of the Hgresistant strain L7AH (Ruiz-Díez et al 2012a) or the Hgsensitive strain L3 (Ruiz-Díez et al 2012b) of Bradyrhizobium canariense. Increasing concentrations of HgCl 2 (0-200 μM) were added together with the nutrient solution three times a week and until the end of the experiment (6 weeks), according to Quiñones et al (2013).…”

“…In relation to the Lupinus-Bradyrhizobium symbiosis Hg has been previously studied with respect to its effect on nodulation, nitrogenase activity, photosynthetic efficiency and chlorophyll and carotenoid contents (Quiñones et al 2013), and it has also been demonstrated that inoculation with a mercury-resistant bradyrhizobial strain (Ruiz-Díez et al 2012a) confers on L. albus the ability to grow under high concentrations of Hg and to accumulate it. Plant growth, photosynthetic efficiency and nitrogenase activity of plants inoculated with the resistant strain were similar to control plants without Hg, while in those inoculated with the sensitive strain all of them decreased significantly.…”