BRX mediates feedback between brassinosteroid levels and auxin signalling in root growth


Brassinosteroid and auxin decisively influence plant development, and overlapping transcriptional responses to these phytohormones suggest an interaction between the two pathways1,2,3. However, whether this reflects direct feedback or merely parallel inputs on common targets is unclear. Here we show that in Arabidopsis roots, this interaction is mediated by BREVIS RADIX (BRX), which is required for optimal root growth4. We demonstrate that the brx phenotype results from a root-specific deficiency of brassinosteroid and is due to reduced, BRX-dependent expression of a rate-limiting enzyme in brassinosteroid biosynthesis. Unexpectedly, this deficiency affects the root expression level of 15% of all Arabidopsis genes, but the transcriptome profile can be restored to wild type by brassinosteroid treatment. Thus, proper brassinosteroid levels are required for the correct expression of many more genes than previously suspected. Moreover, embryonic or post-embryonic brassinosteroid application fully or partially, respectively, rescues the brx phenotype. Further, auxin-responsive gene expression is globally impaired in brx, demonstrating that brassinosteroid levels are rate-limiting for auxin-responsive transcription. BRX expression is strongly induced by auxin and mildly repressed by brassinolide, which means that BRX acts at the nexus of a feedback loop that maintains threshold brassinosteroid levels to permit optimal auxin action.

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Figure 1: Microarray analyses.
Figure 2: Complementation of brx S root phenotypes by brassinolide.
Figure 3: Auxin-induced transcription in brxS.


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We thank T. Guilfoyle for the DR5::GUS reporter line; the DNA Array Facility of the University of Lausanne for assistance with microarray experiments; and J. MacDonald Chambers-Petetôt for assistance with sectioning and microscopy. C.F.M. was supported by the Canton de Vaud, Switzerland. This work was funded by a grant from the Swiss National Science Foundation awarded to C.S.H. Author Contributions C.F.M. contributed all data except those for Fig. 2c and 2h, and Supplementary Figs 1i and 3, contributed by K.S.O. All authors were involved in the design of the research and the writing of the manuscript.

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Correspondence to Christian S. Hardtke.

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Competing interests

The microarray experiments have been deposited in the Array Express Database under accession numbers E-MEXP-635 and M-MEXP-637. Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Figure 1

Expression and requirement of BRX in the root vasculature. (JPG 3232 kb)

Supplementary Figure 2

Expression level ratios for various brassinosteroid biosynthesis or signaling genes extracted from the various microarray experiments. (JPG 559 kb)

Supplementary Figure 3

Root growth in bri1 mutants. (JPG 482 kb)

Supplementary Figure 4

Predicted transcription factor binding sites in the BRX promoter (JPG 237 kb)

Supplementary Figure 5

Instability of BRX-GFP fusion protein. (JPG 472 kb)

Supplementary Table

Expression ratios of differentially expressed genes in microarray analyses. Original microarray data have been deposited at ArrayExpress database. (XLS 2748 kb)

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Mouchel, C., Osmont, K. & Hardtke, C. BRX mediates feedback between brassinosteroid levels and auxin signalling in root growth. Nature 443, 458–461 (2006).

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