Mechanical Considerations Underlying Context Specific Impact of Brassinosteroids in the Arabidopsis Root

Intracellular factors controlling cell proliferation, cell expansion and directionality are the underpinnings of plant morphogenesis. These factors respond to mechanical signals, interconnected hormonal pathways and corresponding transcriptional networks. Despite dramatic advances in identifying hormonal signaling components, how their spatiotemporal activities integrate to coordinate these processes are not fully understood. Using the steroid hormone brassinosteroid (BR) signaling pathway, we uncovered opposing tissue-specific effects of the BR signal that dictate the longitudinal (apical-basal) and the radial dimensions of the Arabidopsis root meristem. In agreement, our translatome mapping in response to the hormone suggested its context-specific effect on gene expression. This provides an entry point to explore the mechanisms underlying tissue-specific interpretation of the hormone, as a means of achieving balanced growth. Here we evaluate whether mechanical considerations could explain this differential growth output. Towards this end, we use imaging and dedicated software to quantify the volume of virtually all cells, in combination with readout of BR signaling strength, along the root meristem in wild type and BR mutants with tissue-specific perturbation of BR signaling. These data is applied in theoretical models to assist us in evaluating the relevance of mechanical input on growth decisions and hence, further bridge the gap between the established core signaling cascade and its less understood interpretation at the cell, tissue and organ levels.