Brassinosteroid gene regulatory networks at cellular resolution in the Arabidopsis root

The Arabidopsis root is a tractable model to address the fundamental question of how the progeny of stem cells develop into differentiated tissues. To explore these processes, we constructed an integrated atlas of the Arabidopsis root using single-cell RNA-seq, which highlighted the continuous nature of development and provided new insights into cell identity acquisition . To investigate how cell identity and hormone signaling affect one another, we investigated responses to Brassinosteroids (BRs), a group of plant steroid hormones. Brassinosteroids regulate diverse processes such as cell division and cell elongation through gene regulatory networks that vary in space and time. By using time-series single-cell RNA-sequencing to profile brassinosteroid-responsive gene expression specific to different cell types and developmental stages of the Arabidopsis root, we identified the elongating cortex as a site where brassinosteroids trigger a shift from proliferation to elongation associated with increased expression of cell wall-related genes. Our analysis revealed HAT7 and GTL1 as brassinosteroid-responsive transcription factors that regulate cortex cell elongation. These results establish the cortex as a site of brassinosteroid-mediated growth and unveil a brassinosteroid signaling network regulating the transition from proliferation to elongation, illuminating aspects of spatiotemporal hormone response.