Brassinosteroids regulate organ boundary formation in the shoot apical meristem of Arabidopsis

Abstract

Spatiotemporal control of the formation of organ primordia and organ boundaries from the stem cell niche in the shoot apical meristem (SAM) determines the patterning and architecture of plants, but the underlying signaling mechanisms remain poorly understood. Here we show that brassinosteroids (BRs) play a key role in organ boundary formation by repressing organ boundary identity genes. BR-hypersensitive mutants display organ-fusion phenotypes, whereas BR-insensitive mutants show enhanced organ boundaries. The BR-activated transcription factor BZR1 directly represses the CUP-SHAPED COTYLEDON (CUC) family of organ boundary identity genes. In WT plants, BZR1 accumulates at high levels in the nuclei of central meristem and organ primordia but at a low level in organ boundary cells to allow CUC gene expression. Activation of BR signaling represses CUC gene expression and causes organ fusion phenotypes. This study uncovers a role for BR in the spatiotemporal control of organ boundary formation and morphogenesis in the SAM.hormone | steroid

Arabidopsis LATERAL ORGAN BOUNDARIES negatively regulates brassinosteroid accumulation to limit growth in organ boundaries

“…Similar fusion defects have been observed in other BR signaling mutants such as bzr1-d (26). In addition, Gendron et al (26) reported that a BZR1-YFP fusion protein accumulated to low levels in the boundary region compared with the adjacent meristem and primordia, consistent with reduced BR levels in these cells. Moreover, 60% (106) of the 175 genes that are regulated by both LOB and BR are putative BZR1 targets (Dataset S1), raising the possibility that the combined action of LOB and BZR1 influence the expression of a subset of BRregulated genes.…”

The BRASSINOSTEROID INSENSITIVE1–LIKE3 Signalosome Complex Regulates Arabidopsis Root Development

“…Alternatively, the analysis of stomata patterning defects in BR-deficient mutants has set a role for mitogen-activated protein kinases as cell type-specific regulators of BR-mediated responses independently of BES1/BRZ1 via BIN2 (Gudesblat et al, 2012 ;Kim et al, 2012). Another example is the contribution of BRs to the lateral organ boundary at the plant shoot by reducing BR levels in the boundary domain, while repressing BZR transcription of CUP-SHAPED COTYLEDON genes outside the boundary (Bell et al, 2012;Gendron et al, 2012). Future studies tracing back the control of cell typespecific targets to specific BR receptor complexes will help delineate the different BR signaling pathways that control plant development.…”

Lateral organ boundaries 1 is a disease susceptibility gene for citrus bacterial canker disease

“…The plant-specific LOB domain family is composed of a conserved DNA-binding Cys repeat motif (CX 2 CX 6 CX 3 C), an invariant glycine residue, and a coiled-coil Leu zipper-like motif (LX 6 LX 3 LX 6 L), the latter of which often functions in protein-protein interactions (35). Although the specific functions of CsLOB1 are unknown, previous studies have revealed that LOB domain proteins are involved in the regulation of lateral organ development, anthocyanin and nitrogen metabolism, and are responsive to phytohormones and environmental stimuli such as auxin, cytokinin, gibberellin, brassinosteroid, and salinity or glucose (36,37). One member of the LOB domain family, AtLBD18, was reported to bind with the promoter of EXPANSIN14, a gene involved in cell wall loosening (38).…”