Body of Abstract: Root architecture determines spatiotemporal control of resource exploration and acquisition, leading to crucial implications in plant productivity. Monocot roots, especially in the grasses, differ substantially from the well studied model Arabidopsis by producing embryonic seminal and non-embryonic crown roots that are essential for anchorage and nutrient acquisition. Little is known about cell type differences among these roots at the molecular level and how these affect cellular organization, development, and function. Here, we present a comparative study of single-cell transcriptomes from the embryonic primary and seminal roots, and post-embryonic lateral, and crown roots in maize. Integration of scRNAseq datasets combining more than 24 000 cells revealed conserved and unique transcriptional programs in cell type specification and allowed dissection of gene expression pattern changes within cell types. Interestingly, while primary and secondary roots morphologically differ in their number of cortical layers, we didn't see significant differences in the number of cells with cortical identity. However, a significantly higher number of cells showed endodermis identity in lateral roots, revealing a potential root-type specific transcriptional specialization. Post-embryonic crown roots showed overrepresentation of genes involved in water transport, epidermis differentiation, response to stress compared to primary and seminal roots. We also identified genes that switched cell-type specific expression patterns across root types. Grassy tillers1,a class I HD-Zip gene that controls lateral branching showed broad expression across all cell types in the lateral roots, while its expression was restricted to pericycle and less differentiated xylem, cortex, and epidermal layers in primary and seminal roots, and absent in crown roots. These results together show the potential of scRNAseq in unraveling functionally relevant molecular complexity of root architecture in maize.