Brassinosteroids define the stem elongation zone along the apical-basal axis to coordinate gravitropism and self-standability

To achieve self-standability, the apical region of the stem must remain extensible and bendable, whereas the basal region must form a rigid supporting tissue. However, the mechanisms regulating stem growth along the apical-basal axis remain poorly understood. Here, we show that brassinosteroid (BR) biosynthesis and signaling are elevated in the apical, actively elongating regions of Arabidopsis and mung bean stems. Exogenous brassinolide promoted stem elongation, concomitant with increased cell wall extensibility, but impaired stem self-standability. In contrast, BR deficiency reduced the elongation/bendable zone and inhibited gravitropic curvature. Through spatiotemporal transcriptome analysis along the apical-basal axis of Arabidopsis inflorescence stems, combined with analysis of BR-responsive genes in the BR-dependent transition zone, we identified a set of genes, termed BR Down-regulated in Stem (BRDS) genes. BRDS gene expression was repressed by BR and predominantly expressed in the non-elongating region, indicating that BR plays a central role in spatial gene expression along the apical-basal axis. These genes were enriched for functions related to secondary cell wall formation, suggesting that BR differentially regulates primary and secondary cell wall formation. While auxin governs growth direction along the abaxial-adaxial axis of the stem, BR defines the spatial domains of elongation along the apical-basal axis of the stem. The synergistic action of these hormones determines both the direction and spatial domain of gravitropic bending, thereby enabling stem elongation, gravitropism, and standability in three-dimensional space.