Nitrate- and inoculation-enhanced ethylene biosynthesis in soybean roots as a possible mediator of nodulation control

Root ethylene biosynthesis has been studied in soybean (Glycine max [L] Mer r.) cv. Bragg plants and its supernodulating (nts382 and nts1007) and non-n odulating (nod49 and nod139) mutants. Regardless of NO3- treatment, inocula tion with Bradyrhizobium japanicum significantly increased root ethylene ev olution rate, reaching plateau between 24 and 48 h after inoculation, with the rates being significantly higher in 8 mmol L-1 fed roots (high) than in those given I mmol L-1 (low) during the time of experiment. This Inoculati on Stimulated Ethylene Release (ISER) response appears to be related to the infection process and nodule development, as treatment with Ag+ (an inhibi tor of ethylene action) at the moment of inoculation markedly increased nod ule number of Bragg plants under both high and low NO3- concentrations. Com pared with the parental Bragg, the near-isogenic nodulating mutants used in this study showed normal ethylene biosynthesis ability (ethylene evolution and ACC oxidase activity), although significant quantitative differences w ere detected among them. Whether these differences are causally related to the nodulation phenotype is not known. Our previous observations on the inv olvement of endogenous ethylene in the control of nodule number in alfalfa are therefore also applicable to soybean, a determinate nodule type legume. The results further suggest that effects other than an alteration of ethyl ene biosynthesis might have also been caused by the nts mutations.