Dynamic, auxin-responsive plasma membrane-to-nucleus movement ofArabidopsisBRX

Abstract

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.

Molecular genetic framework for protophloem formation

“…A similar phenotype is observed in loss-of-function mutants of BREVIS RADIX (BRX) (11), another polar localized plasma membrane-associated protein that is also thought to act in the nucleus (11,12). The brx short root phenotype is rescued by second site mutation in BARELY ANY MERISTEM 3 (BAM3), which encodes a receptor-like kinase that is required for suppression of protophloem differentiation on treatment with the peptide ligand, CLAVATA3/EMBRYO SURROUNDING REGION 45 (CLE45) (13).…”

Do Trees Grow on Money? Auxin as the Currency of the Cellular Economy

“…In roots, auxin regulates both the expression and intracellular localization of the putative transcription factor BREVIS RADIX (BRX) (Scacchi et al 2009). brx mutants have short roots and impaired auxin responses (Mouchel et al 2006).…”

“…brx mutants have short roots and impaired auxin responses (Mouchel et al 2006). Both of these phenotypes can be rescued by application of brassinosteroids or by constitutively elevating expression of auxin-responsive genes through introduction of a loss-of-function mutation in hy5 (Mouchel et al 2006;Scacchi et al 2009). Nuclearlocalized BRX is required for cells to maintain normal levels of CONSTITUTIVE PHOTO-MORPHOGENIC DWARF (CPD), a brassinosteroid biosynthetic enzyme.…”

Primary root protophloem differentiation requires balanced phosphatidylinositol-4,5-biphosphate levels and systemically affects root branching

“…Genetic studies in Arabidopsis have identified two genes that promote the commitment of precursor cells to the sieve element differentiation program (Rodriguez-Villalon et al, 2014;Truernit et al, 2012): BREVIS RADIX (BRX) and OCTOPUS (OPS). Both encode different plantspecific plasma membrane-associated proteins, which are expressed in the developing protophloem and polar localized at the root-ward (BRX) or shoot-ward (OPS) end of cells (Scacchi et al, 2009;Truernit et al, 2012). In both brx-and ops-null mutants, root meristem protophloem cells frequently fail to differentiate properly.…”