In plants, the balance between auxin nuclear activity and polar auxin transport (PAT) between cells determines each cell's transcriptional response to auxin. Mutations in BREVIS RADIX (BRX), a root growth regulator, disturb auxin-responsive gene expression, but what is the function of BRX? Now, Christian Hardtke and colleagues determine that BRX is a plasma membrane-associated protein that translocates to the nucleus in response to auxin exposure or auxin flux (see p. 2059). Using BRX-GFP fusion proteins, the researchers demonstrate that BRX is membrane-associated and translocates to the nucleus after auxin stimulation,where it might associate with transcription factors to regulate gene expression. BRX localization is polarized in vascular cells, as is that of the PIN family of auxin efflux carriers; expressing BRX under the control of a PIN promoter rescues brx root phenotypes, whereas inhibiting PAT mimics brx mutants. Auxin also induces BRX degradation. Together, these findings highlight how BRX might function in a novel auxin signalling pathway and how gene expression could be modulated in response to an auxin gradient.

2009