Mature Nodules and Root Tips Control Nodulation in Soybean

Nodule formation in soybean (Glycine max (L.) Merr.) is suppressed in younger regions of the root system as a feedback response to development of first nodules. Removal of functional nodules caused already initiated infections to develop into mature nodules, in the same regions of the roots where the initial nodules were formed. This was true for soybean cv. Bragg and the supernodulating mutant derivative nts382. In experiments where the tips of the lateral roots of Bragg were removed before or at inoculation, nodulation on the primary root was considerably increased. In contrast, removal of the lateral root tips from nts382 had no effect. Our results suggest that nodulation in soybean is preferentially controlled by the arrest rather than the abortion of early subepidermal cell division during nodule development, and that both mature nodules and root apical meristems elicit feedback regulation of nodule formation.