Early induction of feedback regulatory responses governing nodulation in soybean

Separately grown soybean (Glycine max (L.) Merr. cv. Bragg) plants of identical or different nodulation genotype were approach-grafted just below the cotyledons. Five days later, their roots were inoculated at different times to study the effects of prior inoculation of one root system on nodulation of the other. Time-separated inoculation of isografts showed that nodulation in parental Bragg was strongly suppressed, while no suppression was observed in the supernodulating mutant nts382. In Bragg isografts, inoculation of one root system resulted in 50-59% decrease in nodulation on the other if inucolation of the latter was delayed by 3 days. Upon grafting the non-nodulating soybean derivative nod139, which fails to induce cortical cell divisions, did not elicit feedback suppressive responses in Bragg. Instead, nodules emerged 4-6 days earlier in grafted Bragg and nodulation was clearly stimulated in Bragg and nts382. In contrast the non-nodulating mutant nod49, which induces some cortical cell divisions that do not develop beyond the very early stages of nodule ontogeny, systemically suppressed nodule formation in Bragg, but did not have stimulatory effects on nodulation of nts382. These results suggest that: (a) cortical cell division foci of early ontogeny elicit feedback suppressive responses that control nodule number systemically; (b) feedback regulation is present in wild-type soybean but is defective in the supernodulating mutant; (c) a counteracting stimulatory response is induced early during pre-infection before the formation of a nodule primordia. A model for regulation of nodule formation in soybean is proposed, in which cortical cell division foci induce negative regulatory responses in the shoot through systemic signaling that ultimately results in blockage of further nodule development in the roots.