TY - JOUR AB - Advanced transcriptome sequencing has revealed that the majority of eukaryotic genes undergo alternative splicing (AS). Nonetheless, little effort has been dedicated to investigating the functional relevance of particular splicing events, even those in the key developmental and hormonal regulators. Combining approaches of genetics, biochemistry and advanced confocal microscopy, we describe the impact of alternative splicing on the PIN7 gene in the model plant Arabidopsis thaliana. PIN7 encodes a polarly localized transporter for the phytohormone auxin and produces two evolutionarily conserved transcripts, PIN7a and PIN7b. PIN7a and PIN7b, differing in a four amino acid stretch, exhibit almost identical expression patterns and subcellular localization. We reveal that they are closely associated and mutually influence each other's mobility within the plasma membrane. Phenotypic complementation tests indicate that the functional contribution of PIN7b per se is minor, but it markedly reduces the prominent PIN7a activity, which is required for correct seedling apical hook formation and auxin-mediated tropic responses. Our results establish alternative splicing of the PIN family as a conserved, functionally relevant mechanism, revealing an additional regulatory level of auxin-mediated plant development. AU - Kashkan, Ivan AU - Hrtyan, Mónika AU - Retzer, Katarzyna AU - Humpolíčková, Jana AU - Jayasree, Aswathy AU - Filepová, Roberta AU - Vondráková, Zuzana AU - Simon, Sibu AU - Rombaut, Debbie AU - Jacobs, Thomas B. AU - Frilander, Mikko J. AU - Hejátko, Jan AU - Friml, Jiří AU - Petrášek, Jan AU - Růžička, Kamil ID - 10282 JF - New Phytologist SN - 0028-646X TI - Mutually opposing activity of PIN7 splicing isoforms is required for auxin-mediated tropic responses in Arabidopsis thaliana VL - 233 ER - TY - JOUR AB - Small RNAs (smRNA, 19–25 nucleotides long), which are transcribed by RNA polymerase II, regulate the expression of genes involved in a multitude of processes in eukaryotes. miRNA biogenesis and the proteins involved in the biogenesis pathway differ across plant and animal lineages. The major proteins constituting the biogenesis pathway, namely, the Dicers (DCL/DCR) and Argonautes (AGOs), have been extensively studied. However, the accessory proteins (DAWDLE (DDL), SERRATE (SE), and TOUGH (TGH)) of the pathway that differs across the two lineages remain largely uncharacterized. We present the first detailed report on the molecular evolution and divergence of these proteins across eukaryotes. Although DDL is present in eukaryotes and prokaryotes, SE and TGH appear to be specific to eukaryotes. The addition/deletion of specific domains and/or domain-specific sequence divergence in the three proteins points to the observed functional divergence of these proteins across the two lineages, which correlates with the differences in miRNA length across the two lineages. Our data enhance the current understanding of the structure–function relationship of these proteins and reveals previous unexplored crucial residues in the three proteins that can be used as a basis for further functional characterization. The data presented here on the number of miRNAs in crown eukaryotic lineages are consistent with the notion of the expansion of the number of miRNA-coding genes in animal and plant lineages correlating with organismal complexity. Whether this difference in functionally correlates with the diversification (or presence/absence) of the three proteins studied here or the miRNA signaling in the plant and animal lineages is unclear. Based on our results of the three proteins studied here and previously available data concerning the evolution of miRNA genes in the plant and animal lineages, we believe that miRNAs probably evolved once in the ancestor to crown eukaryotes and have diversified independently in the eukaryotes. AU - Moturu, Taraka Ramji AU - Sinha, Sansrity AU - Salava, Hymavathi AU - Thula, Sravankumar AU - Nodzyński, Tomasz AU - Vařeková, Radka Svobodová AU - Friml, Jiří AU - Simon, Sibu ID - 7582 IS - 3 JF - Plants TI - Molecular evolution and diversification of proteins involved in miRNA maturation pathway VL - 9 ER - TY - JOUR AB - AtNHX5 and AtNHX6 are endosomal Na+,K+/H+ antiporters that are critical for growth and development in Arabidopsis, but the mechanism behind their action remains unknown. Here, we report that AtNHX5 and AtNHX6, functioning as H+ leak, control auxin homeostasis and auxin-mediated development. We found that nhx5 nhx6 exhibited growth variations of auxin-related defects. We further showed that nhx5 nhx6 was affected in auxin homeostasis. Genetic analysis showed that AtNHX5 and AtNHX6 were required for the function of the ER-localized auxin transporter PIN5. Although AtNHX5 and AtNHX6 were co-localized with PIN5 at ER, they did not interact directly. Instead, the conserved acidic residues in AtNHX5 and AtNHX6, which are essential for exchange activity, were required for PIN5 function. AtNHX5 and AtNHX6 regulated the pH in ER. Overall, AtNHX5 and AtNHX6 may regulate auxin transport across the ER via the pH gradient created by their transport activity. H+-leak pathway provides a fine-tuning mechanism that controls cellular auxin fluxes. AU - Fan, Ligang AU - Zhao, Lei AU - Hu, Wei AU - Li, Weina AU - Novák, Ondřej AU - Strnad, Miroslav AU - Simon, Sibu AU - Friml, Jirí AU - Shen, Jinbo AU - Jiang, Liwen AU - Qiu, Quan ID - 462 JF - Plant, Cell and Environment TI - NHX antiporters regulate the pH of endoplasmic reticulum and auxin-mediated development VL - 41 ER - TY - JOUR AB - Plant development mediated by the phytohormone auxin depends on tightly controlled cellular auxin levels at its target tissue that are largely established by intercellular and intracellular auxin transport mediated by PIN auxin transporters. Among the eight members of the Arabidopsis PIN family, PIN6 is the least characterized candidate. In this study we generated functional, fluorescent protein-tagged PIN6 proteins and performed comprehensive analysis of their subcellular localization and also performed a detailed functional characterization of PIN6 and its developmental roles. The localization study of PIN6 revealed a dual localization at the plasma membrane (PM) and endoplasmic reticulum (ER). Transport and metabolic profiling assays in cultured cells and Arabidopsis strongly suggest that PIN6 mediates both auxin transport across the PM and intracellular auxin homeostasis, including the regulation of free auxin and auxin conjugates levels. As evidenced by the loss- and gain-of-function analysis, the complex function of PIN6 in auxin transport and homeostasis is required for auxin distribution during lateral and adventitious root organogenesis and for progression of these developmental processes. These results illustrate a unique position of PIN6 within the family of PIN auxin transporters and further add complexity to the developmentally crucial process of auxin transport. AU - Simon, Sibu AU - Skůpa, Petr AU - Viaene, Tom AU - Zwiewka, Marta AU - Tejos, Ricardo AU - Klíma, Petr AU - Čarná, Mária AU - Rolčík, Jakub AU - De Rycke, Riet AU - Moreno, Ignacio AU - Dobrev, Petre AU - Orellana, Ariel AU - Zažímalová, Eva AU - Friml, Jirí ID - 1417 IS - 1 JF - New Phytologist TI - PIN6 auxin transporter at endoplasmic reticulum and plasma membrane mediates auxin homeostasis and organogenesis in Arabidopsis VL - 211 ER - TY - JOUR AB - Strigolactones, first discovered as germination stimulants for parasitic weeds [1], are carotenoid-derived phytohormones that play major roles in inhibiting lateral bud outgrowth and promoting plant-mycorrhizal symbiosis [2-4]. Furthermore, strigolactones are involved in the regulation of lateral and adventitious root development, root cell division [5, 6], secondary growth [7], and leaf senescence [8]. Recently, we discovered the strigolactone transporter Petunia axillaris PLEIOTROPIC DRUG RESISTANCE 1 (PaPDR1), which is required for efficient mycorrhizal colonization and inhibition of lateral bud outgrowth [9]. However, how strigolactones are transported through the plant remained unknown. Here we show that PaPDR1 exhibits a cell-type-specific asymmetric localization in different root tissues. In root tips, PaPDR1 is co-expressed with the strigolactone biosynthetic gene DAD1 (CCD8), and it is localized at the apical membrane of root hypodermal cells, presumably mediating the shootward transport of strigolactone. Above the root tip, in the hypodermal passage cells that form gates for the entry of mycorrhizal fungi, PaPDR1 is present in the outer-lateral membrane, compatible with its postulated function as strigolactone exporter from root to soil. Transport studies are in line with our localization studies since (1) a papdr1 mutant displays impaired transport of strigolactones out of the root tip to the shoot as well as into the rhizosphere and (2) DAD1 expression and PIN1/PIN2 levels change in plants deregulated for PDR1 expression, suggestive of variations in endogenous strigolactone contents. In conclusion, our results indicate that the polar localizations of PaPDR1 mediate directional shootward strigolactone transport as well as localized exudation into the soil. AU - Sasse, Joëlle AU - Simon, Sibu AU - Gübeli, Christian AU - Liu, Guowei AU - Cheng, Xi AU - Friml, Jirí AU - Bouwmeester, Harro AU - Martinoia, Enrico AU - Borghi, Lorenzo ID - 1536 IS - 5 JF - Current Biology TI - Asymmetric localizations of the ABC transporter PaPDR1 trace paths of directional strigolactone transport VL - 25 ER - TY - JOUR AB - The plant hormone auxin is a key regulator of plant growth and development. Auxin levels are sensed and interpreted by distinct receptor systems that activate a broad range of cellular responses. The Auxin-Binding Protein1 (ABP1) that has been identified based on its ability to bind auxin with high affinity is a prime candidate for the extracellular receptor responsible for mediating a range of auxin effects, in particular, the fast non-transcriptional ones. Contradictory genetic studies suggested prominent or no importance of ABP1 in many developmental processes. However, how crucial the role of auxin binding to ABP1 is for its functions has not been addressed. Here, we show that the auxin-binding pocket of ABP1 is essential for its gain-of-function cellular and developmental roles. In total, 16 different abp1 mutants were prepared that possessed substitutions in the metal core or in the hydrophobic amino acids of the auxin-binding pocket as well as neutral mutations. Their analysis revealed that an intact auxin-binding pocket is a prerequisite for ABP1 to activate downstream components of the ABP1 signalling pathway, such as Rho of Plants (ROPs) and to mediate the clathrin association with membranes for endocytosis regulation. In planta analyses demonstrated the importance of the auxin binding pocket for all known ABP1-mediated postembryonic developmental processes, including morphology of leaf epidermal cells, root growth and root meristem activity, and vascular tissue differentiation. Taken together, these findings suggest that auxin binding to ABP1 is central to its function, supporting the role of ABP1 as auxin receptor. AU - Grones, Peter AU - Chen, Xu AU - Simon, Sibu AU - Kaufmann, Walter AU - De Rycke, Riet AU - Nodzyński, Tomasz AU - Zažímalová, Eva AU - Friml, Jirí ID - 1562 IS - 16 JF - Journal of Experimental Botany TI - Auxin-binding pocket of ABP1 is crucial for its gain-of-function cellular and developmental roles VL - 66 ER - TY - CHAP AB - Exogenous application of biologically important molecules for plant growth promotion and/or regulation is very common both in plant research and horticulture. Plant hormones such as auxins and cytokinins are classes of compounds which are often applied exogenously. Nevertheless, plants possess a well-established machinery to regulate the active pool of exogenously applied compounds by converting them to metabolites and conjugates. Consequently, it is often very useful to know the in vivo status of applied compounds to connect them with some of the regulatory events in plant developmental processes. The in vivo status of applied compounds can be measured by incubating plants with radiolabeled compounds, followed by extraction, purification, and HPLC metabolic profiling of plant extracts. Recently we have used this method to characterize the intracellularly localized PIN protein, PIN5. Here we explain the method in detail, with a focus on general application. AU - Simon, Sibu AU - Skůpa, Petr AU - Dobrev, Petre AU - Petrášek, Jan AU - Zažímalová, Eva AU - Friml, Jirí ED - Hicks, Glenn ED - Robert, Stéphanie ID - 2245 SN - 10643745 T2 - Plant Chemical Genomics TI - Analyzing the in vivo status of exogenously applied auxins: A HPLC-based method to characterize the intracellularly localized auxin transporters VL - 1056 ER - TY - JOUR AB - The mode of action of auxin is based on its non-uniform distribution within tissues and organs. Despite the wide use of several auxin analogues in research and agriculture, little is known about the specificity of different auxin-related transport and signalling processes towards these compounds. Using seedlings of Arabidopsis thaliana and suspension-cultured cells of Nicotiana tabacum (BY-2), the physiological activity of several auxin analogues was investigated, together with their capacity to induce auxin-dependent gene expression, to inhibit endocytosis and to be transported across the plasma membrane. This study shows that the specificity criteria for different auxin-related processes vary widely. Notably, the special behaviour of some synthetic auxin analogues suggests that they might be useful tools in investigations of the molecular mechanism of auxin action. Thus, due to their differential stimulatory effects on DR5 expression, indole-3-propionic (IPA) and 2,4,5-trichlorophenoxy acetic (2,4,5-T) acids can serve in studies of TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALLING F-BOX (TIR1/AFB)-mediated auxin signalling, and 5-fluoroindole-3-acetic acid (5-F-IAA) can help to discriminate between transcriptional and non-transcriptional pathways of auxin signalling. The results demonstrate that the major determinants for the auxin-like physiological potential of a particular compound are very complex and involve its chemical and metabolic stability, its ability to distribute in tissues in a polar manner and its activity towards auxin signalling machinery. AU - Simon, Sibu AU - Kubeš, Martin AU - Baster, Pawel AU - Robert, Stéphanie AU - Dobrev, Petre AU - Friml, Jirí AU - Petrášek, Jan AU - Zažímalová, Eva ID - 2443 IS - 4 JF - New Phytologist TI - Defining the selectivity of processes along the auxin response chain: A study using auxin analogues VL - 200 ER - TY - JOUR AB - Plant-specific PIN-formed (PIN) efflux transporters for the plant hormone auxin are required for tissue-specific directional auxin transport and cellular auxin homeostasis. The Arabidopsis PIN protein family has been shown to play important roles in developmental processes such as embryogenesis, organogenesis, vascular tissue differentiation, root meristem patterning and tropic growth. Here we analyzed roles of the less characterised Arabidopsis PIN6 auxin transporter. PIN6 is auxin-inducible and is expressed during multiple auxin-regulated developmental processes. Loss of pin6 function interfered with primary root growth and lateral root development. Misexpression of PIN6 affected auxin transport and interfered with auxin homeostasis in other growth processes such as shoot apical dominance, lateral root primordia development, adventitious root formation, root hair outgrowth and root waving. These changes in auxin-regulated growth correlated with a reduction in total auxin transport as well as with an altered activity of DR5-GUS auxin response reporter. Overall, the data indicate that PIN6 regulates auxin homeostasis during plant development. AU - Cazzonelli, Christopher AU - Vanstraelen, Marleen AU - Simon, Sibu AU - Yin, Kuide AU - Carron Arthur, Ashley AU - Nisar, Nazia AU - Tarle, Gauri AU - Cuttriss, Abby AU - Searle, Iain AU - Benková, Eva AU - Mathesius, Ulrike AU - Masle, Josette AU - Friml, Jirí AU - Pogson, Barry ID - 2472 IS - 7 JF - PLoS One TI - Role of the Arabidopsis PIN6 auxin transporter in auxin homeostasis and auxin-mediated development VL - 8 ER - TY - JOUR AB - Clathrin-mediated endocytosis (CME) regulates many aspects of plant development, including hormone signaling and responses to environmental stresses. Despite the importance of this process, the machinery that regulates CME in plants is largely unknown. In mammals, the heterotetrameric ADAPTOR PROTEIN COMPLEX-2 (AP-2) is required for the formation of clathrin-coated vesicles at the plasma membrane (PM). Although the existence of AP-2 has been predicted in Arabidopsis thaliana, the biochemistry and functionality of the complex is still uncharacterized. Here, we identified all the subunits of the Arabidopsis AP-2 by tandem affinity purification and found that one of the large AP-2 subunits, AP2A1, localized at the PM and interacted with clathrin. Furthermore, endocytosis of the leucine-rich repeat receptor kinase, BRASSINOSTEROID INSENSITIVE1 (BRI1), was shown to depend on AP-2. Knockdown of the two Arabidopsis AP2A genes or overexpression of a dominant-negative version of the medium AP-2 subunit, AP2M, impaired BRI1 endocytosis and enhanced the brassinosteroid signaling. Our data reveal that the CME machinery in Arabidopsis is evolutionarily conserved and that AP-2 functions in receptormediated endocytosis. AU - Di Rubbo, Simone AU - Irani, Niloufer AU - Kim, Soo AU - Xu, Zheng AU - Gadeyne, Astrid AU - Dejonghe, Wim AU - Vanhoutte, Isabelle AU - Persiau, Geert AU - Eeckhout, Dominique AU - Simon, Sibu AU - Song, Kyungyoung AU - Kleine Vehn, Jürgen AU - Friml, Jirí AU - De Jaeger, Geert AU - Van Damme, Daniël AU - Hwang, Inhwan AU - Russinova, Eugenia ID - 509 IS - 8 JF - Plant Cell TI - The clathrin adaptor complex AP-2 mediates endocytosis of brassinosteroid INSENSITIVE1 in arabidopsis VL - 25 ER - TY - JOUR AB - The native auxin, indole-3-acetic acid (IAA), is a major regulator of plant growth and development. Its nonuniform distribution between cells and tissues underlies the spatiotemporal coordination of many developmental events and responses to environmental stimuli. The regulation of auxin gradients and the formation of auxin maxima/minima most likely involve the regulation of both metabolic and transport processes. In this article, we have demonstrated that 2-oxindole-3-acetic acid (oxIAA) is a major primary IAA catabolite formed in Arabidopsis thaliana root tissues. OxIAA had little biological activity and was formed rapidly and irreversibly in response to increases in auxin levels. We further showed that there is cell type-specific regulation of oxIAA levels in the Arabidopsis root apex. We propose that oxIAA is an important element in the regulation of output from auxin gradients and, therefore, in the regulation of auxin homeostasis and response mechanisms. AU - Pěnčík, Aleš AU - Simonovik, Biljana AU - Petersson, Sara AU - Henyková, Eva AU - Simon, Sibu AU - Greenham, Kathleen AU - Zhang, Yi AU - Kowalczyk, Mariusz AU - Estelle, Mark AU - Zažímalová, Eva AU - Novák, Ondřej AU - Sandberg, Göran AU - Ljung, Karin ID - 511 IS - 10 JF - Plant Cell TI - Regulation of auxin homeostasis and gradients in Arabidopsis roots through the formation of the indole-3-acetic acid catabolite 2-oxindole-3-acetic acid VL - 25 ER - TY - JOUR AB - Auxin is a key coordinative signal required for many aspects of plant development and its levels are controlled by auxin metabolism and intercellular auxin transport. Here we find that a member of PIN auxin transporter family, PIN8 is expressed in male gametophyte of Arabidopsis thaliana and has a crucial role in pollen development and functionality. Ectopic expression in sporophytic tissues establishes a role of PIN8 in regulating auxin homoeostasis and metabolism. PIN8 co-localizes with PIN5 to the endoplasmic reticulum (ER) where it acts as an auxin transporter. Genetic analyses reveal an antagonistic action of PIN5 and PIN8 in the regulation of intracellular auxin homoeostasis and gametophyte as well as sporophyte development. Our results reveal a role of the auxin transport in male gametophyte development in which the distinct actions of ER-localized PIN transporters regulate cellular auxin homoeostasis and maintain the auxin levels optimal for pollen development and pollen tube growth. AU - Ding, Zhaojun AU - Wang, Bangjun AU - Moreno, Ignacio AU - Dupláková, Nikoleta AU - Sibu Simon AU - Carraro, Nicola AU - Reemmer, Jesica AU - Pěnčík, Aleš AU - Xu Chen AU - Tejos, Ricardo I AU - Skůpa, Petr AU - Pollmann, Stephan AU - Mravec, Jozef AU - Petrášek, Jan AU - Zažímalová, Eva AU - Honys, David AU - Rolčík, Jakub AU - Murphy, Angus S AU - Orellana, Ariel AU - Geisler, Markus AU - Jirí Friml ID - 3114 IS - AN 941 JF - Nature Communications TI - ER-localized auxin transporter PIN8 regulates auxin homeostasis and male gametophyte development in Arabidopsis VL - 3 ER - TY - JOUR AB - Spatial distribution of the plant hormone auxin regulates multiple aspects of plant development. These self-regulating auxin gradients are established by the action of PIN auxin transporters, whose activity is regulated by their constitutive cycling between the plasma membrane and endosomes. Here, we show that auxin signaling by the auxin receptor AUXIN-BINDING PROTEIN 1 (ABP1) inhibits the clathrin-mediated internalization of PIN proteins. ABP1 acts as a positive factor in clathrin recruitment to the plasma membrane, thereby promoting endocytosis. Auxin binding to ABP1 interferes with this action and leads to the inhibition of clathrin-mediated endocytosis. Our study demonstrates that ABP1 mediates a nontranscriptional auxin signaling that regulates the evolutionarily conserved process of clathrin-mediated endocytosis and suggests that this signaling may be essential for the developmentally important feedback of auxin on its own transport. AU - Robert, Stéphanie AU - Kleine-Vehn, Jürgen AU - Barbez, Elke AU - Sauer, Michael AU - Paciorek, Tomasz AU - Pawel Baster AU - Vanneste, Steffen AU - Zhang, Jing AU - Sibu Simon AU - Čovanová, Milada AU - Hayashi, Kenichiro AU - Dhonukshe, Pankaj AU - Yang, Zhenbiao AU - Bednarek, Sebastian Y AU - Jones, Alan M AU - Luschnig, Christian AU - Aniento, Fernando AU - Zažímalová, Eva AU - Jirí Friml ID - 3075 IS - 1 JF - Cell TI - ABP1 mediates auxin inhibition of clathrin-dependent endocytosis in Arabidopsis VL - 143 ER - TY - JOUR AB - Remarkable progress in various techniques of in vivo fluorescence microscopy has brought an urgent need for reliable markers for tracking cellular structures and processes. The goal of this manuscript is to describe unexplored effects of the FM (Fei Mao) styryl dyes, which are widely used probes that label processes of endocytosis and vesicle trafficking in eukaryotic cells. Although there are few reports on the effect of styryl dyes on membrane fluidity and the activity of mammalian receptors, FM dyes have been considered as reliable tools for tracking of plant endocytosis. Using plasma membrane-localized transporters for the plant hormone auxin in tobacco BY-2 and Arabidopsis thaliana cell suspensions, we show that routinely used concentrations of FM 4-64 and FM 5-95 trigger transient re-localization of these proteins, and FM 1-43 affects their activity. The active process of re-localization is blocked neither by inhibitors of endocytosis nor by cytoskeletal drugs. It does not occur in A. thaliana roots and depends on the degree of hydrophobicity (lipophilicity) of a particular FM dye. Our results emphasize the need for circumspection during in vivo studies of membrane proteins performed using simultaneous labelling with FM dyes. AU - Jelínková, Adriana AU - Malínská, Kateřina AU - Sibu Simon AU - Kleine-Vehn, Jürgen AU - Pařezová, Markéta AU - Pejchar, Přemysl AU - Kubeš, Martin AU - Martinec, Jan AU - Jirí Friml AU - Zažímalová, Eva AU - Petrášek, Jan ID - 3067 IS - 5 JF - Plant Journal TI - Probing plant membranes with FM dyes: Tracking dragging or blocking? VL - 61 ER - TY - JOUR AB - Plant development is governed by signaling molecules called phytohormones. Typically, in certain developmental processes more than 1 hormone is implicated and, thus, coordination of their overlapping activities is crucial for correct plant development. However, molecular mechanisms underlying the hormonal crosstalk are only poorly understood. Multiple hormones including cytokinin and auxin have been implicated in the regulation of root development. Here we dissect the roles of cytokinin in modulating growth of the primary root. We show that cytokinin effect on root elongation occurs through ethylene signaling whereas cytokinin effect on the root meristem size involves ethylene-independent modulation of transport-dependent asymmetric auxin distribution. Exogenous or endogenous modification of cytokinin levels and cytokinin signaling lead to specific changes in transcription of several auxin efflux carrier genes from the PIN family having a direct impact on auxin efflux from cultured cells and on auxin distribution in the root apex. We propose a novel model for cytokinin action in regulating root growth: Cytokinin influences cell-to-cell auxin transport by modification of expression of several auxin transport components and thus modulates auxin distribution important for regulation of activity and size of the root meristem. AU - Růžička, Kamil AU - Šimášková, Mária AU - Duclercq, Jérôme AU - Petrášek, Jan AU - Zažímalová, Eva AU - Sibu Simon AU - Jirí Friml AU - Van Montagu, Marc C AU - Eva Benková ID - 3050 IS - 11 JF - PNAS TI - Cytokinin regulates root meristem activity via modulation of the polar auxin transport VL - 106 ER -