Glutathione plays a fundamental role in growth and symbiotic capacity of Sinorhizobium meliloti

Abstract

Rhizobia form a symbiotic relationship with plants of the legume family to produce nitrogen-fixing root nodules under nitrogen-limiting conditions. We have examined the importance of glutathione (GSH) during free-living growth and symbiosis of <i>Sinorhizobium meliloti</i>. An <i>S. meliloti</i> mutant strain (SmgshA) which is unable to synthesize GSH due to a gene disruption in <i>gshA</i>, encoding the enzyme for the first step in the biosynthesis of GSH, was unable to grow under nonstress conditions, precluding any nodulation. In contrast, an <i>S. meliloti</i> strain (SmgshB) with <i>gshB</i>, encoding the enzyme involved in the second step in GSH synthesis, deleted was able to grow, indicating that γ- glutamylcysteine, the dipeptide intermediate, can partially substitute for GSH. However, the SmgshB strain showed a delayed-nodulation phenotype coupled to a 75% reduction in the nitrogen fixation capacity. This phenotype was linked to abnormal nodule development. Both the SmgshA and SmgshB mutant strains exhibited higher catalase activity than the wild-type <i>S. meliloti</i> strain, suggesting that both mutant strains are under oxidative stress. Taken together, these results show that GSH plays a critical role in the growth of <i>S. meliloti</i> and during its interaction with the plant partner.