Nod factor inhibition of reactive oxygen efflux in a host legume

Hydrogen peroxide (H,02) efflux was measured from Medicago truncatula root segments exposed to purified Nod factor and to poly-GalUA (PGA) heptamers, Nod factor, at concentrations > 100 pm, reduced H2O2 efflux rates to 60% of baseline levels beginning 20 to 30 min after exposure, whereas the PGA elicitor, at > 75 nm, caused a rapid increase in H2O2 efflux to >200% of baseline rates. Pretreatment of plants with Nod factor alters the effect of PGA by limiting the maximum H2O2 efflux rate to 125% of that observed for untreated plants. Two Nod factor-related compounds showed no ability to modulate peroxide efflux, and tomato (Lycopersicon esculentum), a nonlegume, showed no response to 1 nm Nod factor. Seven M. truncatula mutants, lacking the ability to make nodules, were tested for Nod factor effects on H2O2 efflux. The nfp mutant was blocked for suppression of peroxide efflux, whereas the dmi1 and dmi2 mutants, previously shown to be blocked for early Nod factor responses, showed a wild-type peroxide efflux modulation. These data demonstrate that exposure to Nod factor suppresses the activity of the reactive oxygen-generating system used for plant defense responses.