Abstract
Brassinosteroid (BR) hormones are important regulators of plant growth and development. Recent studies revealed the cell-specific role of BRs in vascular and stem cell development by the action of cell-specific BR receptor complexes and downstream signaling components in Arabidopsis thaliana Despite the importance of spatiotemporal regulation of hormone signaling in the control of plant vascular development, the mechanisms that confer cellular specificity to BR receptors within the vascular cells are not yet understood. The present work shows that BRI1-like receptor genes 1 and 3 (BRL1 and BRL3) are differently regulated by BRs. By using promoter deletion constructs of BRL1 and BRL3 fused to GFP/GUS (green fluorescent protein/β-glucuronidase) reporters in Arabidopsis, analysis of their cell-specific expression and regulation by BRs in the root apex has been carried out. We found that BRL3 expression is finely modulated by BRs in different root cell types, whereas the location of BRL1 appears to be independent of this hormone. Physiological and genetic analysis show a BR-dependent expression of BRL3 in the root meristem. In particular, BRL3 expression requires active BES1, a central transcriptional effector within the BRI1 pathway. ChIP analysis showed that BES1 directly binds to the BRRE present in the BRL3 promoter region, modulating its transcription in different subsets of cells of the root apex. Overall our study reveals the existence of a cell-specific negative feedback loop from BRI1-mediated BES1 transcription factor to BRL3 in phloem cells, while contributing to a general understanding of the spatial control of steroid signaling in plant development.
Highlights
Brassinosteroids (BRs) are polyhydroxylated plant steroid Mandava, 1988)
These results indicate that the BRL3 promoter requires a minimal promoter length of 755 bp for proper BRL3 expression in the root vascular tissue, the expression pattern differs slightly from the one detected in ProBRL3-1719::GUS transgenic lines
The BRL3 and BRL1 genes have been described as BRASSINOSTEROID INSENSITIVE 1 (BRI1) homologs capable of binding BRs with high affinity and are expressed in the plant vasculature (Caño-Delgado et al, 2004)
Summary
Brassinosteroids (BRs) are polyhydroxylated plant steroid Mandava, 1988). The identification of mutants affected in hormones that were first identified by their ability to elongate BR synthesis or signal transduction has revealed an essential plant stems when applied exogenously (Grove et al, 1979; role for this hormone in cell elongation and differentiation4952 | Salazar-Henao et al.(Fukuda, 1997; Yamamoto et al, 1997). The BRASSINOSTEROID INSENSITIVE 1 (BRI1) gene was identified by a genetic screening of a BR-insensitive loss-offunction mutant performed in the model plant Arabidopsis (Arabidopsis thaliana) (Clouse, 1996). Genetic and biochemical assays have demonstrated that the BRI1 receptor complex directly bind BRs with high affinity (Wang et al, 2001) This occurs via direct binding of BRs to the extracellular domain of the LRR-RLK proteins at the cell membrane (Wang et al, 2001; Kinoshita et al, 2005)
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