Time-course of lipoxygenase, antioxidant enzyme activities and H2O2 accumulation during the early stages of Rhizobium-legume symbiosis

The involvement of lipoxygenase and antioxidant enzyme activities as well a s hydrogen peroxide (H2O2) accumulation are reported during early infection steps in alfalfa (Medicago sativa) roots inoculated either with a wild typ e 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 enzy me activities assayed, and an increase in H2O2 accumulation. Enhancement of scavenging activities for several reactive oxygen species co rrelated with compatibility of the S. meliloti-alfalfa symbiosis, whereas t he Nod C- strain triggered a defence response. Nod factors were essential t o suppress this response. Increase in lipoxygenase and lipid hydroperoxide decomposing activities, ob served during the first hours after inoculation with a compatible strain, c ould be related to tissue differentiation and/or the production of signal m olecules involved in autoregulation of nodulation by the plant.