A morpho-transcriptomic map of brassinosteroid action in the Arabidopsis root

The effects of brassinosteroid signaling on shoot and root development have been characterized in great detail but did not identify a simple consistent positive or negative impact on a basic cellular parameter that would comprehensively explain the phenotype of brassinosteroid-related mutants. Here we combined digital 3D single-cell shape analysis and single-cell mRNA sequencing to characterize root meristems and mature root segments of brassinosteroid-blind mutants and wildtype. These data demonstrate that brassinosteroid signaling neither affects cell volume nor cell proliferation capacity. Instead, brassinosteroid signaling is essential for the precise orientation of cell division planes and the extent and timing of anisotropic cell expansion. Moreover, we found that the cell-aligning effects of brassinosteroid signaling can propagate to normalize the anatomy of both adjacent and distant brassinosteroid-blind cells through non-cell-autonomous functions, which are sufficient to restore overall root growth vigor. Finally, single-cell transcriptome data discern directly brassinosteroid-responsive genes from genes that can react to non-cell-autonomous brassinosteroid-dependent signals and highlight arabinogalactans as sentinels of brassinosteroid-dependent anisotropic cell expansion.