Optimal BR signalling is required for adequate cell wall orientation in the Arabidopsis root meristem

Plant brassinosteroid hormones (BRs) regulate growth in part through altering the properties of the cell wall, the extracellular matrix of plant cells. Conversely, feedback signalling from the wall connects the state of cell wall homeostasis to the BR receptor complex and modulates BR activity. Here, we report that both pectin-triggered cell wall signalling and impaired BR signalling result in altered cell wall orientation in the Arabidopsis root meristem. Furthermore, both depletion of endogenous BRs and exogenous supply of BRs triggered these defects. Cell wall signalling-induced alterations in the orientation of newly placed walls appear to occur late during cytokinesis, after initial positioning of the cortical division zone. Tissue-specific perturbations of BR signalling revealed that the cellular malfunction is unrelated to previously described whole organ growth defects. Thus, tissue type separates the pleiotropic effects of cell wall/BR signals and highlights their importance during cell wall placement.