Nodulation Gene Regulation and Quorum Sensing Control Density-Dependent Suppression and Restriction of Nodulation in the Bradyrhizobium japonicum-Soybean Symbiosis

Department of Soil, Water, and Climate; BioTechnology Institute; and Microbial and Plant Genomics Institute, University of Minnesota, St. Paul, MN

^* To whom correspondence should be addressed. Email: sadowsky{at}umn.edu.

	Abstract

Nodulation of Glycine max cv. Lambert and the nodulation-restricting plant introduction genotype PI 417566 by wild-type Bradyrhizobium japonicum USDA110 is regulated in a population density-dependent manner. Nodulation on both plant genotypes was suppressed (inhibited) when plants received a high density inoculum (10⁹ cells/ml) of strain USDA110 grown in complex medium, and more nodules were produced on plants receiving a low cell density inoculum (10⁵ cells/ml). Since cell-free supernatants from strain USDA110 grown to high cell density in complex medium decreased expression of a nodY-lacZ fusion, this phenomenon was attributed to bradyoxetin-induced repression of nod gene expression. Inoculation of either the permissive soybean genotype (cv. Lambert) or PI 417566 with 10⁹ cells of the nodD2^-, nolA^-, nodW^-, and nwsB^- mutants of USDA 110 enhanced nodulation (up to 24%) relative to that seen with inoculations done with 10⁵ cells/ml of the mutants or the wild-type strain, indicating that these genes are involved in population density-dependent nodulation of soybean. In contrast, the number of nodules produced by a nodD1^- mutant on either soybean genotype was less than that seen with the wild-type strain inoculated at low inoculum density. The nodD2^- mutant outcompeted B. japonicum strain USDA123 for nodulation of G. max cv. Lambert at high or low inoculum densities, and root tip marking and time-to-nodulate studies indicated that the nolA^- and nodD2^- mutants nodulated this soybean genotype faster than wild-type USDA 110. Taken together, results from these studies indicate that the nodD2^- mutant of B. japonicum may be useful to enhance soybean nodulation at high inoculum densities and that NodD2 is a key repressor influencing host-controlled restriction of nodulation, density-dependent suppression of nodulation, perception of bradyoxetin, and competitiveness in the soybean-B. japonicum symbiosis.