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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.
Legume plants benefit from their symbiosis with rhizobial bacteria because the bacteria are able to fix molecular nitrogen and share it with the plant, allowing legumes to grow under nitrogen-limiting conditions. In exchange, the plant provides the rhizobia residing in root nodules with fixed carbon from photosynthesis. The interaction is complex and involves multiple layers of regulation by both partners. Genetic analysis of nodulation, initially begun because of the potential for agricultural improvement offered by understanding nitrogen-fixing symbioses, has revealed regulators relevant both to nodule formation and to nonleguminous plants (Kouchi et al., 2010).
The establishment of the symbiosis follows a similar pattern in most legumes. Legume roots secrete flavonoid signals into the rhizosphere. Rhizobia respond to flavonoids by producing a lipochitin oligosaccharide termed Nod factor. Perception of species-specific Nod factor by the compatible species of legume triggers Ca2+ spiking in root hair cells and induces changes in gene expression. Perception also results in a physical response; the plant root hair cell curls to sequester the bacteria. In indeterminate nodulators such as pea (Pisum sativum) and alfalfa (Medicago sativa) the inner cortical cells leave the G0 stage of the cell cycle and begin to divide. At the same time, the...