The mechanisms whereby lateral organ initial cells are organized from the peripheral zone of the shoot apical meristem (SAM) are poorly understood. The maize gene NARROWSHEATH1(NS1)/ WOX3is expressed at the marginal boundary of leaf founder cells in the SAM and in young leaf primordia, where it mediates mediolateral outgrowth. To investigate the mechanisms of NS1 function, we used ChIP-seq of NS1 followed by laser-microdissection RNAseq of nsmutant and wild type primordial margins to identify gene targets that are bound and modulated by NS1, and used single-cell RNAseq analyses to further investigate genes co-expressed in NS1-expressing cells. In a comparative approach, ChIP-seq was also performed on the Arabidopsis WOX3 paralog PRESSED FLOWER1 (PRS1), to identify conserved mechanisms of founder cell recruitment and primordial outgrowth in maize and Arabidopsis. These data, combined with microscopic analyses of cell division dynamics, reverse genetic analyses of homologous NS1/WOX3 target genes in Arabidopsis, and NS1overexpressing plants in maize suggest that NS1/WOX3 controls mediolateral outgrowth by direct repression of growth inhibitory genes, and indirect promotion of cell division in primordial leaf margins. Intriguingly, the homologous WOXgenes WUS1and WOX5are expressed in the organizing centers of the Arabidopsis SAM and root meristem respectively, whereupon these protein products traffic to adjoining cells to activate stem cell identity non-autonomously. In contrast, our previous data revealed that PRS1/WOX3 does not traffic, and these latest data suggest that NS1/WOX3stimulates primordial cell division in the same margin initial cells where it is transcribed.
