Glutathione Is Involved in Environmental Stress Responses in Rhizobium tropici , Including Acid Tolerance

Abstract

The isolation of rhizobial strains which exhibit an intrinsic tolerance to acidic conditions has been reported and has facilitated studies on the basic mechanisms underlying acid tolerance. Rhizobium tropici strain CIAT899 displays a high intrinsic tolerance to acidity and therefore was used in this work to study the molecular basis of bacterial responses to acid conditions and other environmental stresses. We generated a collection of R. tropici CIAT899 mutants affected in acid tolerance using Tn5-luxAB mutagenesis, and one mutant strain (CIAT899-13T2), which fails to grow under acid conditions, was characterized in detail. Strain CIAT899-13T2 was found to contain a single Tn5-luxAB insertion in a gene showing a high degree of similarity with the Escherichia coli gshB gene, encoding the enzyme glutathione synthetase. Intracellular potassium pools and intracellular pH levels were found to be lower in the mutant than in the parent. The glutathione-deficient mutant was shown to be sensitive to weak organic acids, osmotic and oxidative stresses, and the presence of methylglyoxal. Glutathione restores responses to these stresses almost to wild-type levels. Our data show that in R. tropici the production of glutathione is essential for growth in extreme environmental conditions. The mutant strain CIAT899-13T2 induced effective nodules; however, it was found to be outcompeted by the wild-type strain in coinoculation experiments.

Glutathione Plays a Fundamental Role in Growth and Symbiotic Capacity of Sinorhizobium meliloti

“…The authors suggested that glutathione may play a role in the regulation of catalases, possibly through its regulation of intracellular H 2 O 2 or through its interaction with redox-sensitive sites in the OxyR regulatory protein. GSHdeficient yeast mutants, as well as R. tropici gshB mutants, showed dramatic sensitivity to H 2 O 2 (12,24). Altogether, these data suggest that GSH and catalases provide overlapping defenses for protection against H 2 O 2 in both prokaryotic and eukaryotic cells.…”

“…The emergence of nodules on M. sativa following inoculation with the mutant strain SmgshB was delayed in comparison to wt Rm1021, although the final nodule number was unaffected. Similarly, the number of nodules on Phaseolus vulgaris and Leucaena leucocephala 4 weeks postinoculation with the gshB-deficient strain was found to be similar to that on plants inoculated with the wild-type strain (24). A highly significant feature of our study was the large reduction in the nitrogen fixation capacity of nodules.…”

Biochemistry and Molecular Biology of Antioxidants in the Rhizobia-Legume Symbiosis

“…This mutant is sensitive to weak organic acids and to osmotic and oxidative stress, and the addition of GSH restores the responses to these stresses to wild-type levels. Interestingly, the mutant can form effective nodules on bean, but it is out competed by the wild-type strain, indicating that GSH is important for stress tolerance and the symbiotic process (Riccillo et al, 2000).…”

“…The GSH may be internally consumed by bacteroids in metabolic reactions and maintenance of cellular redox status rather than being exported to the plant (Iturbe-Ormaetxe et al, 2001). Recently, a mutant of Rhizobium tropici has been isolated which is deficient in glutathione synthetase and contains only 3% of the GSH present in the wild-type strain (Riccillo et al, 2000). This mutant is sensitive to weak organic acids and to osmotic and oxidative stress, and the addition of GSH restores the responses to these stresses to wild-type levels.…”

The Sinorhizobium medicae WSM419 lpiA gene is transcriptionally activated by FsrR and required to enhance survival in lethal acid conditions

“…A number of genes required for the growth of rhizobia in low pH conditions have been identified (Goss et al, 1990;O'Hara et al, 1989;Riccillo et al, 2000;Tiwari et al, 1992;Vinuesa et al, 2003). Construction of mutants from the acid-tolerant strain Sinorhizobium medicae WSM419 has allowed the identification of some of the genes required in acidic conditions (Dilworth et al, 2001).…”