The plant hormone brassinosteroid (BR), which signals through its receptors BR INSENSITIVE 1 (BRI1), BRI1-LIKE 1 (BRL1) and BRL3, is known to regulate hypocotyl elongation but how it functions in the root is less clear. In this issue, Christian Hardtke and colleagues assess the role of BR during cell differentiation in the Arabidopsis root (p. 272). They first show that bri1 brl1 brl3 triple mutants display protophloem differentiation defects. These defects cannot be rescued by activating BR signalling in adjacent cell files, suggesting that BR acts in a cell-autonomous manner to control protophloem differentiation. Triple mutants also exhibit a small meristem, and the authors show that this can be explained by reduced cell elongation that, together with increased formative divisions in the radial dimension, contributes to the overall reduction in root growth observed in these mutants. Finally, the researchers demonstrate that the protophloem-specific activation of BR signalling can rescue all major aspects of the triple mutant phenotype, thus uncovering a new facet of the non-cell-autonomous effects of BR signalling. Based on these and other findings, the authors propose that BR perception in the protophloem is sufficient to systemically convey BR action within the root meristem.
