Cadmium accumulation and tolerance in Bradyrhizobium sp. (peanut microsymbiont)

ELIANA BIANUCCI, ADRIANA FABRA, STELLA CASTRO

Rosario

Congreso; V Congreso Argentino de Microbiologia General SAMIGE; 2008

SAMIGE

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:595.3pt 841.9pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:35.4pt; mso-footer-margin:35.4pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Cadmium (Cd) is incorporated into agricultural soils through phosphate fertilizers, sewage sludge, and atmospheric fallout from industrial and urban activities. Because of its high toxicity, at low concentrations, Cd imposes a serious threat to organisms and the food transfer of metals ions has become a major public concern. Mechanisms of Cd tolerance in microorganisms are diverse, such as immobilization within the cell wall and biochemical transformation of the metal which involves the participation of the glutathione (GSH). We have recently informed that GSH plays a crucial role in the survival of Bradyrhizobium sp. SEMIA6144 against the toxic effects of acid, saline and oxidative stresses using a GSH-deficient mutant (Bradyrhizobium SEMIA 6144-S7Z) obtained by disruption of gshA gene which encodes the enzyme γ-glutamylcysteine synthetase (γECS). The objectives of this work were 1) to investigate Cd accumulation and its effect on growth of two strains recommended as peanut inoculants (Bradyrhizobium sp. SEMIA 6144 and Bradyrhizobium sp. C145) and two native peanut isolates (NLH25 and NOD31) obtained from Córdoba soils;  2) to elucidate the role of GSH in the tolerance to Cd. The microorganisms were grown in YEM medium supplemented with CdCl2 concentrations ranging from 0 to 100 μM. B. sp. SEMIA 6144 and B. sp. C145 strains grew up to 15 μM Cd meanwhile B. sp 6144-S7Z mutant strain was unable to grow at this concentration. NLH25 and NOD31 isolates grew up to 50 µM Cd.  The Cd uptake by cells growing in presence of 15 µM Cd was assayed by inductively coupled plasma-atomic emission spectrometry (ICP-AES).  In B. sp. SEMIA 6144 was found the highest Cd accumulation in relation to other strains. At 15 μM Cd, the intracellular GSH content in B. sp. C145 and NLH25 showed a significant increase in comparison to B. sp. SEMIA 6144 and NOD31 whose content was not modified. In conclusion, the differences in Cd tolerance among Bradyrhizobium sp. genotypes suggests that beyond the basic metabolic machinery of this species, there are also variations that allow strains to display distinct responses. Thus, the native peanut isolates tolerated higher Cd concentration than the reference strains, possibly the GSH level found could act as a detoxifying agent. This approach is currently being investigated.