See online version for legend and references.1172 Cell 136, March 20, 2009 ©2009 Elsevier Inc. DOI 10.1016/j.cell.2009.03.009

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1172.e1 Cell 136, March 20, 2009 ©2009 Elsevier Inc. DOI 10.1016/j.cell.2009.03.009

SnapShot: Auxin Signaling and TransportDolf Weijers1 and Jirí Friml21Wageningen University, 6703 HA, Wageningen, The Netherlands; 2VIB and Ghent University, 9052 Gent, Belgium

Auxin controls numerous processes in plants by determining the rate of cell division and expansion, or a cell’s developmental fate. The differential distribution of auxin within plant tissues determines the spatio-temporal aspect of auxin signaling at the tissue level. Auxin action is predominantly mediated by a nuclear signaling pathway that controls the expression of auxin-responsive genes. The major form of auxin is indole-3-acetic acid (IAA).

(1) Intracellular IAA is synthesized from indole through tryptophan or through other intermediates. The YUC Monooxygenases catalyze the synthesis of the IAA precursor indole acetamide. The TAA1 aminotransferase converts Trp into the IAA precursor IPA. Expression of YUC and TAA1 genes is influenced by internal and environmental signals.The pool of free IAA is actively controlled by catabolic homeostasis through conjugation of IAA to sugars (by UGT84B1), methyl (by IAMT1), or amino acids (by enzymes of the GH3 family). Although some of these conjugates are inactive, others may have auxinic activity.(2) The major naturally occurring auxin (IAA) can freely diffuse across the plasma membrane in its protonated (IAAH) form that occurs in the relatively acidic cell wall (pH 5.5). Auxin entry into the cell is additionally facilitated by the AUX1/LAX permeases. In the cell interior (pH is 7.0) IAA becomes negatively charged (IAA−) and con-sequently becomes trapped in the cell.(3) IAA− can leave the cell through PGP1/19 ABC transporters, which typically have nonpolar localization, and by PIN proteins, which are typically polar. The subcellular localization of PIN proteins determines the direction of intercellular auxin transport. This mechanism allows each transporting cell to influence how much auxin will be transported and to which neighboring cells thereby connecting polarity of individual cells with the patterning of the whole tissues.(4) PIN proteins continuously cycle between the plasma membrane and endosomes. PIN internalization occurs by clathrin-coated vesicles and is inhibited by auxin through a mechanism that requires the callosin-like BIG1 protein. At the endosomes, the GNOM ARF-GEF (which is inhibited by the fungal toxin BFA) controls budding of the vesicles recycling PINs back to the basal plasma membrane. This contitutive cycling allows rapid and dynamic changes in PIN localization. The sorting of PIN pro-teins into the apical or basal trafficking pathway depends on the phosphorylation status of PIN, which is controlled by the PID protein kinase and the PP2A phosphatase. Entry into a vacuolar degradation pathway also controls PIN abundance. A yet unknown ARF-GEF regulates the delivery of PIN-containing vesicles into the prevacuolar compartment (PVC), and the SNX and VPS29 retromer proteins appear to control the retrograde transport of PIN1-containing vesicles from the PVC to the endosome.(5) Following its nuclear accumulation, IAA binds the SCFTIR1/AFB E3 ubiquitin ligase that targets transcriptional repressors of the Aux/IAA family for ubiquitination and subsequent degradation by the 26S proteasome. The activity of the SCFTIR1/AFB is regulated by the modification of its CUL1 subunit with the ubiquitin-like RUB1 protein through the action of RUB1-conjugation enzymes AXR1, RCE1, and ECR1 and the RUB1-deconjugation enzyme complex CSN. Auxin binds both the TIR1/AFB subunit of the SCFTIR1/AFB and the conserved domain II of the Aux/IAA protein directly, thereby functioning as “molecular glue” and strongly increasing the affinity of the ubiquitin ligase for its substrate.(6) Aux/IAA proteins interact with ARF family transcription factors through conserved domains III/IV that are shared by both types of proteins. ARFs bind to AuxRE sequence motifs in the promoters of auxin-responsive genes with their conserved DNA-binding domain (DBD). In the absence of auxin, Aux/IAA proteins bind to ARFs, presumably at gene promoters, and prevent gene expression in part by recruiting the transcriptional corepressor TPL through a conserved motif in domain I. Degrada-tion of Aux/IAA proteins releases the ARF transcription factors from inhibition and allows the ARFs to change gene expression through a nonconserved middle region (MR). ARFs can act either as monomers or as dimers, which are thought to allow higher-amplitude expression.The ARF family in Arabidopsis contains 23 members, of which 5 have glutamine-rich MRs. Many others act as repressors, rather than activators, of transcription. It is not known if and how these “nonactivating” ARFs are controlled by auxin. There are 29 Aux/IAA proteins in Arabidopsis. Hence, many specific Aux/IAA-ARF combinations are theoretically possible and could account for the wide variety of auxin effects.A pathway that controls auxin-dependent gene expression in parallel to the SCFTIR1/AFB complex has been proposed and involves the MPK12 as well as its inhibitor, the IBR5 phosphatase.(7) Although not fully explored, the genes that are up- or downregulated after auxin treatment include several types, including substantial feedback control of auxin action. Auxin treatment leads to upregulation of Aux/IAA genes, promoting repression of the ARF transcription factors. Furthermore, auxin levels are decreased by acti-vating the GH3 auxin-amino acid ligases and by increasing auxin efflux through activating transcription of PIN genes. A large gene family with unknown function (SAUR) is also upregulated, as are several transcription factors thought to mediate developmental responses.(8) In addition to the SCFTIR1/AFB receptor-mediated auxin response, another well-characterized auxin-binding protein (ABP1) presumably acts in the extracellular space to control cell elongation. Notably, a large portion of ABP1 is also found in the ER, but no function has been ascribed to this pool.

AbbreviationsABP1, Auxin-Binding Protein 1; ADP, Adenosine Diphosphate; AFB, Auxin F Box; ARF, Auxin Response Factor; ARF-GEF, Guanine Exchange Factor on ADP-Ribosylation Factors; ASK1, Arabidopsis SKP1; ATP, adenosine triphosphate; AUX1, Auxin 1; Aux/IAA, auxin/indole-3-acetic acid; AXR1, Auxin-Resistant 1; BFA, Brefeldin A; CL, Clathrin; CSN, COP9 Signalosome; CUL, Cullin; ECR1, E1 C-terminal Related 1; GN, GNOM; IBR5, indole-3-butyric acid resistant 5; IPA, indole-3-pyruvic acid; MPK12, MAP Kinase 12; P, phosphorylation of PIN protein; PGP, P-Glycoprotein; PID, PINOID; PIN, Pin-formed; PP2A, Protein Phosphatase 2A; PVC, prevacuolar compartment; RBX1, RING-Box 1; RCE1, RUB1-Conjugating Enzyme 1; RUB1, Related to Ubiquitin 1; SAUR, Small Auxin-Upregulated RNA; SCF, SKP1-CULLIN-F-Box; SNX, Sorting Nexin; TAA1, Tryptophan Aminotransferase 1; TIR1, Transport Inhibitor Resistant 1; TPL, TOPLESS; Ub, Ubiquitin; VPS29, Vacuolar Protein Sorting 29; YUC, YUCCA.

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