Turning on the microscope turret: a new view for the study of brassinosteroid signaling in plant development

Brassinosteroid (BR) hormones are essential for plant growth and development. In Arabidopsis, the general understanding of BR signaling has been greatly attained by genetic and biochemical approaches that led to the identification of central BR signaling components, from the BRI1 receptor at the plasma membrane to downstream acting BR-regulated BRZ1 and BES1 transcription factors in the nuclei. Recently, an emerging trend is being established to further advance our understanding of the BR signaling pathway in plant development. Scientists have turned on the microscope lens turret to revisit the pleiotropic phenotypes of the BR mutants at a higher magnification, uncovering novel and specific cellular defects in the plant. In-depth phenotypic analysis in combination with the search for cell-specific signaling components that are responsible for those particular defects in the mutants are leading to: (1) definition of novel roles for BRs in vascular development, (2) unraveling BR function in cell division through quantitative analysis of Arabidopsis root growth, (3) establishment of a molecular connection between known patterning and BR-signaling components in organ boundary and stomata development and (4) development of novel strategies toward the identification of BR signaling components with spatiotemporal resolution. In this review, we highlight the importance of these emerging studies to investigate the spatiotemporal control of BR pathways in plant development.