Critical protective role of bacterial superoxide dismutase in rhizobium-legume symbiosis.

In nitrogen-poor soils, rhizobia elicit nodule formation on legume roots, within which they differentiate into bacteroids that fix atmospheric nitrogen. Protection against reactive oxygen species (ROS) was anticipated to play an important role in Rhizobium-legume symbiosis because nitrogenase is extremely oxygen sensitive. We deleted the sodA gene encoding the sole cytoplasmic superoxide dismutase (SOD) of Sinorhizobium meliloti. The resulting mutant, deficient in superoxide dismutase, grew almost normally and was only moderately sensitive to oxidative stress when free living. In contrast, its symbiotic properties in alfalfa were drastically affected. Nitrogen-fixing ability was severely impaired. More strikingly, most SOD-deficient bacteria did not reach the differentiation stage of nitrogen-fixing bacteroids. The SOD-deficient mutant nodulated poorly and displayed abnormal infection. After release into plant cells, a large number of bacteria failed to differentiate into bacteroids and rapidly underwent senescence. Thus, bacterial SOD plays a key protective role in the symbiotic process.