Nitrogen fixation in soybean takes place in root nodules that arise from de novo cell divisions in the root cortex. Although several early nodulin genes have been identified, the mechanism behind the stimulation of cortical cell division during nodulation has not been fully resolved. Here we provide evidence that two paralogs of soybean SHORT-ROOT (GmSHR) play vital roles in soybean nodulation. Expression of GmSHR4 and GmSHR5 (GmSHR4/5) is induced in cortical cells at the beginning of nodulation, when the first cell divisions occur. The expression level of GmSHR4/5 is positively associated with cortical cell division and nodulation. Knockdown of GmSHR5 inhibits cell division in outer cortical layers during nodulation. Knockdown of both paralogs disrupts the cell division throughout the cortex, resulting in poorly organized nodule primordia with delayed vascular tissue formation. GmSHR4/5 function by enhancing cytokinin signaling and activating early nodulin genes. Interestingly, D-type cyclins act downstream of GmSHR4/5, and GmSHR4/5 form a feedforward loop regulating D-type cyclins. Overexpression of D-type cyclins in soybean roots also enhanced nodulation. Collectively, we conclude that the GmSHR4/5-mediated pathway represents a vital module that triggers cytokinin signaling and activates D-type cyclins during nodulation in soybean.

CRISPR/Cas9 (hereafter Cas9)-mediated gene knockout is one of the most important tools for studying gene function. However, many genes in plants play distinct roles in different cell types. Engineering the currently used Cas9 system to achieve cell-type-specific knockout of functional genes is useful for addressing the cell-specific functions of genes. Here we employed the cell-specific promoters of the WUSCHEL RELATED HOMEOBOX 5 (WOX5), CYCLIND6;1 (CYCD6;1), and ENDODERMIS7 (EN7) genes to drive the Cas9 element, allowing tissue-specific targeting of the genes of interest. We designed the reporters to verify the tissue-specific gene knockout in vivo. Our observation of the developmental phenotypes provides strong evidence for the involvement of SCARECROW (SCR) and GIBBERELLIC ACID INSENSITIVE (GAI) in the development of quiescent center (QC) and endodermal cells. This system overcomes the limitations of traditional plant mutagenesis techniques, which often result in embryonic lethality or pleiotropic phenotypes. By allowing cell-type-specific manipulation, this system has great potential to help us better understand the spatiotemporal functions of genes during plant development.