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First published online May 22, 2009
doi: 10.1242/10.1242/dev.035444

Development 136, 2059-2067 (2009)
Published by The Company of Biologists 2009
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Dynamic, auxin-responsive plasma membrane-to-nucleus movement of Arabidopsis BRX

Emanuele Scacchi1,*, Karen S. Osmont1,*, Julien Beuchat1, Paula Salinas1, Marisa Navarrete-Gómez2, Marina Trigueros2, Cristina Ferrándiz2 and Christian S. Hardtke1,{dagger}

1 Department of Plant Molecular Biology, University of Lausanne, Biophore Building, CH-1015 Lausanne, Switzerland.
2 Instituto de Biología Molecular y Celular de Plantas, UPV-CSIC, 46022 Valencia, Spain.

{dagger} Author for correspondence (e-mail: christian.hardtke{at}unil.ch)

Accepted 10 April 2009

In Arabidopsis, interplay between nuclear auxin perception and trans-cellular polar auxin transport determines the transcriptional auxin response. In brevis radix (brx) mutants, this response is impaired, probably indirectly because of disturbed crosstalk between the auxin and brassinosteroid pathways. Here we provide evidence that BRX protein is plasma membrane-associated, but translocates to the nucleus upon auxin treatment to modulate cellular growth, possibly in conjunction with NGATHA class B3 domain-type transcription factors. Application of the polar auxin transport inhibitor naphthalene phthalamic acid (NPA) resulted in increased BRX abundance at the plasma membrane. Thus, nuclear translocation of BRX could depend on cellular auxin concentration or on auxin flux. Supporting this idea, NPA treatment of wild-type roots phenocopied the brx root meristem phenotype. Moreover, BRX is constitutively turned over by the proteasome pathway in the nucleus. However, a stabilized C-terminal BRX fragment significantly rescued the brx root growth phenotype and triggered a hypocotyl gain-of-function phenotype, similar to strong overexpressors of full length BRX. Therefore, although BRX activity is required in the nucleus, excess activity interferes with normal development. Finally, similar to the PIN-FORMED 1 (PIN1) auxin efflux carrier, BRX is polarly localized in vascular cells and subject to endocytic recycling. Expression of BRX under control of the PIN1 promoter fully rescued the brx short root phenotype, suggesting that the two genes act in the same tissues. Collectively, our results suggest that BRX might provide a contextual readout to synchronize cellular growth with the auxin concentration gradient across the root tip.

Key words: Plants, Hormones, Auxin, Arabidopsis, BRX, PIN1, Root meristem, Endocytosis


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