The influence of glutathione on the tolerance of Rhizobium leguminosarum to cadmium

Rhizobia play an important role in agriculture and crop production as they induce nitrogen-fixing nodules on the roots of leguminous plants. Due to the injudicious use of fertilizers and industrial and domestic sludges, the heavy-metal contamination of soils is becoming one of the most concerning environmental problems, which negatively affects the soil microbial communities and consequently the crop productivity. Among the nonessential metals, cadmium (Cd) poses a major threat due to its high mobility and bioavailability. Cadmium affects the survival and the ability of rhizobia to form nitrogen-fixing nodules. The identification of mechanisms that improve rhizobial tolerance to Cd, its persistence in soil, and its ability to improve nodulation efficiency of rhizobia in Cd-contaminated soils is an important issue that requires urgent attention for maintaining fertility of soils polluted with metals. Here we discuss the influence of glutathione (GSH) on Cd tolerance of Rhizobium leguminosarum and have tried to establish the chronology of Cd tolerance mechanism. To understand this, several strains were screened for their Cd tolerance, and the effect of bacterial pregrowth in the presence of extracellular GSH was determined. Cadmium and GSH levels were also monitored over 72 h. The importance of GSH in Cd tolerance was confirmed by the intracellular levels of this tripeptide: GSH intracellular levels remained unaffected in the sensitive strain, yet it increased significantly in the tolerant strain. Moreover, GSH synthesis was induced by intracellular Cd levels; the addition of extracellular GSH had a protective effect toward Cd, particularly in the sensitive strains. These results lead to a better understanding of the metal tolerance mechanisms in free-living bacteria and are likely to improve the Rhizobium-plant symbiosis in heavy-metal-contaminated soils.