J. PlantPbysiol. Vol. 137. pp. 389-396 (1991) Mature Nodules and Root Tips Control Nodulation in Soybean GUSTAVO CAETANO-ANOLLES\ ELLEN T. PAPAROZZI 2 , and PETER M. GRESSHOFF Plant Molecular Genetics (OHLD), Institute of Agriculture and Center for Legume Research, The University of Tennessee, Knoxville, Tennessee 37901-1071 Received May 25, 1990 . Accepted August IS, 1990 Summary 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 al- ready initiated to develop into mature nodules, in the same regions of the roots where the in- itial 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. Key words: Bradyrhizobium japonicum; feedback regulation; nodulation; Glycine max; symbiosis; root me- ristems. Abbreviations: EH = smallest emergent root hair; RT = root tip; RDU = relative distance unit. Introduction Soybean (Glycine max (L.) Merr.) plants are able to form nitrogen-fixing root nodules in close association with bac- teria of the genus Bradyrhizobium. The establishment and function of the nodule results from the coordinate exchange of information between the symbionts in the way of signal molecules, which initially results on root hair curling and in- fection thread formation (Long, 1989). Prior to these host re- sponses, cortical cells beyond the site of the incipient infec- tion are induced to redifferentiate and divide to form a nodule primordium (Turgeon and Bauer, 1982; 1985). These cell division foci occur even in the absence of local root hair curling and infection (Calvert et aI., 1984; Mathews et aI., 1989). Several tranposon-induced Rhizobium and Bradyrhizo- bium mutants as well as Agrobacterium strains harboring cloned nodulation genes were found to induce nodules with- out infection threads and intracellular bacteria (Truchet et I Corresponding author. 2 Present address: Department of Horticulture, University of Nebraska, Lincoln, Nebraska 68583-0724. © 1991 by Gustav Fischer Verlag, Stuttgm al., 1984; Hirsch et aI., 1984; Finan et aI., 1985; Stanley et aI., 1986). Based on these and other observations, infection and cell division were considered to be uncoupled events. The host plant controls the timing, frequency and success of infection. In soybean, the region most susceptible to infec- tion and nodulation by rhizobia coincides with the zone of elongation of the root (Bhuvaneswari et aI., 1980; 1981), where epidermal cells with immature or as yet unformed root hairs are the usual sites of bacterial penetration (Tur- geon and Bauer, 1982). This susceptible region moves along with root development defining a developmental time window in which root cells are transiently susceptible to in- fection. However, the roots are unable to support nodula- tion once first infections are established (Pierce and Bauer, 1983). Many more cell division foci are formed than ever be- come infected or result in an emergent nodule (Calvert et aI., 1984; Mathews et aI., 1989). The mechanism that controls the progress of infection has been termed autoregulation or feedback suppression of nodule formation and is elicited systemically (Kosslak and Bohlool, 1984; Olsson et aI., 1989) when cortical cell division foci are induced in the roots (Caetano-Anolles and Gresshoff, 1990). The systemic nature