AUTOREGULATION AND NITRATE INHIBITION OF NODULE FORMATION IN SOYBEAN CV ENREI AND ITS NODULATION MUTANTS

The regulation and nitrate inhibition of nodule formation in soybean, Glycine max (L.) Merr., was further examined using the nodulation mutants of cv. Enrei. The non-nodulating mutants En115, En1282, and En 1314 produced extremely few markedly-curled root hairs which were all devoid of infection threads, and invariably failed to initiate sub-epidermal cell divisions (SCDs) in the root cortex. A considerable number of arrested SCDs was found before nodule emergence in Enrei, but not in En6500 which had significantly more SCDs that progressively increased at more advanced stages of nodule ontogeny. These observations indicate that autoregulation acts by blocking the developmental stage before nodule emergence. In both Enrei and En6500, the maturation of emerged nodules was restricted by a late-acting nodulation control mechanism that is apparently unrelated to autoregulation. Reciprocal wedge-grafts of plants inoculated at sowing showed that the control of the supernodulating phenotype resides in the shoot, while the non-nodulating phenotype is strictly root-controlled. The nodulation phenotype of the current non-nodulating mutants results not from an alteration of the autoregulatory mechanism, but from mutation that exerts a root-localized effect that blocks SCDs which trigger the autoregulatory mechanism. Reciprocal grafting experiments on Enrei and En6500 seedlings grown under various nitrate levels suggest that nitrate inhibition of nodulation, like autoregulation, is shoot-controlled. Since these two processes are invariably expressed together, they are probably causally related, acting synergistically to regulate nodule formation in soybean. These results indicate that the regulation and nitrate inhibition of nodulation in the nodulation mutants of cv. Enrei are similar to those of cv. Bragg nodulation mutants.