Time-course of lipoxygenase, antioxidant enzyme activities and H O accumulation during the early stages of Rhizobium-legume symbiosis.

•  The involvement of lipoxygenase and antioxidant enzyme activities as well as hydrogen peroxide (H O ) accumulation are reported during early infection steps in alfalfa (Medicago sativa) roots inoculated either with a wild type Sinorhizobium meliloti or with a mutant defective in Nod-factor synthesis (Nod C ). •  Compatibility between M. sativa and Rhizobium correlates, at least in part, with an increase in the activities of these enzymes, particularly catalase and lipoxygenase, during the preinfection period (up to 12 h). The mutant strain, defective in Nod-factor biosynthesis, showed a decrease in all enzyme activities assayed, and an increase in H O accumulation. •  Enhancement of scavenging activities for several reactive oxygen species correlated with compatibility of the S. meliloti-alfalfa symbiosis, whereas the Nod C strain triggered a defence response. Nod factors were essential to suppress this response. •  Increase in lipoxygenase and lipid hydroperoxide decomposing activities, observed during the first hours after inoculation with a compatible strain, could be related to tissue differentiation and/or the production of signal molecules involved in autoregulation of nodulation by the plant.