Selection and Initial Characterization of Partially Nitrate Tolerant Nodulation Mutants of Soybean

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Abstract
Since NO3− availability in the rooting medium seriously limits symbiotic N2 fixation by soybean (Glycine max [L.] Merr.), studies were initiated to select nodulation mutants which were more tolerant to NO3− and were adapted to the Midwest area of the United States. Three independent mutants were selected in the M2 generation from ethyl methanesulfonate or N-nitroso-N-methylurea mutagenized Williams seed. All three mutants (designated NOD1-3, NOD2-4, and NOD3-7) were more extensively nodulated (427 to 770 nodules plant−1) than the Williams parent (187 nodules plant−1) under zero-N growth conditions. This provided evidence that the mutational event(s) affected autoregulatory control of nodulation. Moreover, all three mutants were partially tolerant to NO3−; each retained greater acetylene reduction activity when grown hydroponically with 15 millimolar NO3− than did Williams at 1.5 millimolar NO3−. The NO3− tolerance did not appear to be related to an altered ability to take up or metabolize NO3−, based on solution NO3− depletion and on in vivo nitrate reductase assays. Enhanced nodulation appeared to be controlled by the host plant, being consistent across four Bradyrhizobium japonicum strains tested. In general, the mutant lines produced less dry weight than the control, with root dry weights being more affected than shoot dry weights. The nodulation trait has been stable through the M5 generation in all three mutants.