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.