The ROOT DETERMINED NODULATION1 Gene Regulates Nodule Number in Roots of Medicago truncatula and Defines a Highly Conserved, Uncharacterized Plant Gene Family

The formation of nitrogen-fixing nodules in legumes is tightly controlled by a long-distance signaling system in which nodulating roots signal to shoot tissues to suppress further nodulation. A screen for supernodulating Medicago truncatula mutants defective in this regulatory behavior yielded loss-of-function alleles of a gene designated ROOT DETERMINED NODULATION1 (RDN1). Grafting experiments demonstrated that RDN1 regulatory function occurs in the roots, not the shoots, and is essential for normal nodule number regulation. The RDN1 gene, Medtr5g089520, was identified by genetic mapping, transcript profiling, and phenotypic rescue by expression of the wild-type gene in rdn1 mutants. A mutation in a putative RDN1 ortholog was also identified in the supernodulating nod3 mutant of pea (Pisum sativum). RDN1 is predicted to encode a 357-amino acid protein of unknown function. The RDN1 promoter drives expression in the vascular cylinder, suggesting RDN1 may be involved in initiating, responding to, or transporting vascular signals. RDN1 is a member of a small, uncharacterized, highly conserved gene family unique to green plants, including algae, that we have named the RDN family.