Soybean Nodule-Enhanced CLE Peptides in Roots Act as Signals in GmNARK-Mediated Nodulation Suppression

The number of nodules formed in the roots of leguminous plants is systemically controlled by autoregulation of nodulation (AON). This study characterized two of the CLAVATA3/endosperm-surrounding region (CLE) genes involved in AON signal transduction. The GmRIC1 and GmRIC2 genes initiated expression solely in the roots at approximately 3 days after inoculation (DAI) with Nod factor-producing rhizobia, corresponding to the time point of AON, and the expression was up-regulated by cytokinins. Levels of GmRIC1 and GmRIC2 gene expression were much higher in the supernodulation mutant, SS2-2, than in wild-type (WT) soybeans during nodule development, even after initiation of nitrogen fixation. At 3 DAI, GmRIC2 was induced in the cells of the pericycle and the outer cortex, which undergo cell division to form nodule primordia and spreads from the central region to the whole nodule as it develops. Overexpression of GmRIC1 and GmRIC2 strongly suppressed the nodulation of WT roots as well as transgenic hairy roots in a GmNARK-dependent manner. This systemic suppression of nodulation was caused by the secretion of two CLE proteins into the extracellular space. Double grafting between WT and SS2-2 soybeans showed that signal Q is larger in SS2-2 than in WT roots during nodulation. The results of this study suggest that GmRIC1 and GmRIC2 are good candidates for root-derived signal Q in AON signal transduction.