A model of the genetic components and developmental relations of the autoregulation of nodulation in soybean and medics (Medicago). SN1, NOD3 and RDN1 have not yet been cloned and characterized. Distans (now called Klavier) is cloned but not yet published (Masayoshi Kawaguchi, personal communication). Arrest of nodulation occurs at different stages in soybean and medics as indicated by the white boxes. This may reflect a difference of determinate (spherical) and indeterminate (cylindrical) nodulating species. Cells are shown as boxes with nuclei indicated as circles. LCO, lipo-chito-oligosaccharide, the nodulation factor synthesized by the Rhizobium. Q, the 'cue' from the root to the shoot to indicate that Rhizobium attack has commenced; SDI, the shoot-derived inhibitor signal from the leaf to facilitate blockage of further nodulation advance. SDI may attenuate the response of the root to the nodulation factor. For other abbreviations, see text.