ROP3 GTPase Contributes to Polar Auxin Transport and Auxin Responses and Is Important for Embryogenesis and Seedling Growth in Arabidopsis

ROP GTPases are crucial for the establishment of cell polarity and for controlling responses to hormones and environmental signals in plants. In this work, we show that ROP3 plays important roles in embryo development and auxin-dependent plant growth. Loss-of-function and dominant-negative (DN) mutations in ROP3 induced a spectrum of similar defects starting with altered cell division patterning during early embryogenesis to postembryonic auxin-regulated growth and developmental responses. These resulted in distorted embryo development, defective organ formation, retarded root gravitropism, and reduced auxin-dependent hypocotyl elongation. Our results showed that the expression of AUXIN RESPONSE FACTOR5/MONOPTEROS and root master regulators PLETHORA1 (PLT1) and PLT2 was reduced in DN-rop3 mutant embryos, accounting for some of the observed patterning defects. ROP3 mutations also altered polar localization of auxin efflux proteins (PINs) at the plasma membrane (PM), thus disrupting auxin maxima in the root. Notably, ROP3 is induced by auxin and prominently detected in root stele cells, an expression pattern similar to those of several stele-enriched PINs. Our results demonstrate that ROP3 is important for maintaining the polarity of PIN proteins at the PM, which in turn ensures polar auxin transport and distribution, thereby controlling plant patterning and auxin-regulated responses.     

植物生长素的极性运输载体研究进展

生长素极性运输在植物生长发育中起重要的调控作用.植物细胞间的生长素极性运输主要通过生长素运输载体进行调控.该文对近年来有关生长素极性运输载体,包括输入载体AUX/LAX、输出载体PIN、尤其是新近发现的兼有输入和输出载体功能的MDR/PGP等蛋白家族,以及生长素极性运输中PIN与MDR/PGP蛋白间相互作用关系进行综述.     

植物生长素极性运输调控机理的研究进展

生长素极性运输特异地调控植物器官发生、发育和向性反应等生理过程。本文综述和分析了生长素极性运输的调控机制。分子遗传和生理学研究证明极性运输这一过程是由生长素输入载体和输出载体活性控制的。小G蛋白ARF附属蛋白GEF和GAP分别调控输出载体(PIN1)和输入载体(AUX1)的定位和活性, 并影响高尔基体等介导的细胞囊泡运输系统, 小G蛋白ROP也参与输出载体PIN2活性的调节。本 文基于作者的研究工作提出小G蛋白在调控生长素极性运输中的可能作用模式。     

植物生长素极性运输调控机理的研究进展

生长素极性运输特异地调控植物器官发生、发育和向性反应等生理过程。本文综述和分析了生长素极性运输的调控机制。分子遗传和生理学研究证明极性运输这一过程是由生长素输入载体和输出载体活性控制的。小G蛋白ARF附属蛋白GEF和GAP分别调控输出载体（PINI）和输入载体（AUX1）的定位和活性。并影响高尔基体等介导的细胞囊泡运输系统,小G蛋白ROP也参与输出载体PIN2活性的调节。本文基于作者的研究工作提出小G蛋白在调控生长素极性运输中的可能作用模式。     

Polar Auxin Transport Is Essential for Medial versus Lateral Tissue Specification and Vascular-Mediated Valve Outgrowth in Arabidopsis Gynoecia

Although it is generally accepted that auxin is important for the patterning of the female reproductive organ, the gynoecium, the flow as well as the temporal and spatial actions of auxin have been difficult to show during early gynoecial development. The primordium of the Arabidopsis (Arabidopsis thaliana) gynoecium is composed of two congenitally fused, laterally positioned carpel primordia bisected by two medially positioned meristematic regions that give rise to apical and internal tissues, including the ovules. This organization makes the gynoecium one of the most complex plant structures, and as such, the regulation of its development has remained largely elusive. By determining the spatiotemporal expression of auxin response reporters and localization of PINFORMED (PIN) auxin efflux carriers, we have been able to create a map of the auxin flow during the earliest stages of gynoecial primordium initiation and outgrowth. We show that transient disruption of polar auxin transport (PAT) results in ectopic auxin responses, broadened expression domains of medial tissue markers, and disturbed lateral preprocambium initiation. Based on these results, we propose a new model of auxin-mediated gynoecial patterning, suggesting that valve outgrowth depends on PIN1-mediated lateral auxin maxima as well as subsequent internal auxin drainage and provascular formation, whereas the growth of the medial domains is less dependent on correct PAT. In addition, PAT is required to prevent the lateral domains, at least in the apical portion of the gynoecial primordium, from obtaining medial fates.The gynoecium is a highly complex assembly comprised of different tissues that work together to support female reproductive competence in angiosperms. As such, studies of the regulatory networks controlling gynoecial development are essential to not only understand plant reproduction, but also increase our knowledge about intertissue-specific cross talk and coordinated development. A gynoecium is composed of one or more carpels, which may have evolved by the invagination of an ancestral leaf-like structure carrying spores along its edges (for review, see Hawkins and Liu, 2014). The Arabidopsis (Arabidopsis thaliana) gynoecium is a bilateral structure composed of two congenitally fused carpels likely derived from the fusion of two leaf-like structures in which the central domains became the lateral valves and the peripheral meristematic margins carrying the ovules became the medial tissues (Hawkins and Liu, 2014). It has been suggested that the medial domains of the Arabidopsis gynoecial primordium are partially differentiated quasi-meristems with maintained meristematic characteristics allowing for prolonged proliferation (Girin et al., 2009). Accordingly, many lateral domain-specific genes are associated with leaf development, while several genes active in the medial domains are related to meristematic activity (Dinneny et al., 2005; Alonso-Cantabrana et al., 2007; González-Reig et al., 2012).Arabidopsis gynoecium development has been described and reviewed extensively (Sessions, 1997; Bowman et al., 1999; Ferrándiz et al., 1999; Balanzá et al., 2006; Østergaard, 2009; Sundberg and Ferrandíz, 2009) and is summarized in Figure 1. Briefly, at early floral stage 5 (stages according to Smyth et al., 1990), after the initiation of outer floral organs, the remaining floral meristem becomes dome shaped. Although the meristem still appears radially symmetric, it is considered to have a medial plane (black dashed lines in Fig. 1) facing the inflorescence meristem and a lateral plane (white dashed lines in Fig. 1) perpendicular to the medial plane. The terminal floral meristem subsequently broadens in the lateral plane, resulting in a bilateral flattened plate (late floral stage 5). At floral stage 6, differential growth has resulted in a central invagination positioned along the lateral plane, and differential gene expressions indicate that initial patterning events distinguishing medial and lateral domains as well as inner (adaxial) and outer (abaxial) tissues have initiated (Bowman et al., 1999). By the end of floral stage 7, the adaxial medial tissues grow toward each other, forming two medial ridges, also called carpel margin meristems (CMMs). The CMMs will give rise to placentae and subsequently ovule primordia at floral stage 8 (Schneitz et al., 1995). By stage 9, the major tissue types of the mature gynoecium become morphologically distinct as the style and stigmatic papillae start to differentiate. Cell differentiation and cell expansion continue during stages 10 to 12, and the gynoecium is fully mature and ready to accept pollen approximately 10 d after it started to initiate from the terminal floral meristem.Open in a separate windowFigure 1.Arabidopsis gynoecium development. Transmitted light confocal images of the remaining floral meristem (stage [st] 5) and the first stages of gynoecial primordia development (stages 6 and 7), and false-colored DIC images of floral stages 8 to 12 gynoecia along a developmental time scale showing the time in days after floral initiation at the end of each stage. Early and late stage 5 as well as upper images at floral stages 6 and 7 are viewed from above. Lower floral stage 6 image is viewed from the lateral side. Lower images of floral stages 7 to 12 gynoecia are viewed from the medial side. Upper images of floral stages 8 to 12 gynoecia show transverse sections. Stages and time scale are adapted after Smyth et al. (1990) and Sessions (1997). White dashed lines indicate lateral plane, black dashed lines indicate medial plane, arrowheads indicate lateral crease, and asterisks indicate CMM. Bars = 10 µm (stages 5–7), 25 µm (stages 8–10), and 50 µm (stages 11 and 12).It is commonly accepted that the plant hormone auxin is important for gynoecium development, and several models have been put forward to explain this on a mechanistic level (Nemhauser et al., 2000; Østergaard, 2009; Sundberg and Østergaard, 2009; Nole-Wilson et al., 2010; Marsch-Martínez et al., 2012; Hawkins and Liu, 2014). However, we still lack a clear picture of the auxin dynamics and response sites during the earliest developmental stages when the major patterning decisions are made. During lateral organ development, instructive auxin peaks or gradients are formed by site-specific auxin biosynthesis and polar auxin transport (PAT), which results in procambium formation, organ outgrowth, and tissue differentiation (Sachs, 1969; Benková et al., 2003; Mattsson et al., 2003; Heisler et al., 2005; Scarpella et al., 2006; Furutani et al., 2014). The plasma membrane-bound PINFORMED (PIN) proteins as well as at least four members of the ATP-binding cassette subfamily B (ABCB)/MULTI-DRUG RESISTANT/P-GLYCOPROTEIN (PGP) protein family show auxin efflux capacity (for review, see Habets and Offringa, 2014). The PIN proteins are often polarly localized at the plasma membrane, whereas the ABCB/PGP proteins are generally localized apolarly. Therefore, the PINs are largely responsible for the net directional flow of auxin, while the ABCB proteins most likely contribute to PAT by regulating the effective cellular auxin available for polar transport (Mravec et al., 2008; Wang et al., 2013). The phytotropin 1-N-naphtylphthalamic acid (NPA) is a well established and widely used PAT inhibitor, although its exact mode of action is obscure (Petrásek et al., 2003). NPA treatment mimics the pin-like shoot phenotype of pin1 loss-of-function mutants (Okada et al., 1991), and even though NPA appears not to directly interact with PIN proteins, it may influence subcellular dynamics and has been shown to bind to ABCB family members, thereby blocking their transport capacity (Noh et al., 2001; Murphy et al., 2002; Geisler et al., 2003; Nagashima et al., 2008; Kim et al., 2010). This suggests that NPA may reduce PAT in part by restricting the amount of auxin available for PIN-mediated polar transport.Although the pin1-1 knockout mutant rarely produces flowers (Okada et al., 1991), gynoecia of the hypomorphic pin1-5 mutant form elongated styles and reduced or even missing carpels (Bennett et al., 1995; Sohlberg et al., 2006). Auxin biosynthesis mutants also produce disproportionate gynoecial tissues (Cheng et al., 2006; Stepanova et al., 2008), suggesting that auxin peaks and fluxes are important for the coordinated development of gynoecial domains. However, because the gynoecium is the last organ to initiate from the floral meristem, the abnormal gynoecial development in auxin-related mutants may result from developmental defects that occurred prior to gynoecium formation. By transiently treating inflorescences with NPA, Nemhauser et al. (2000) showed that PAT in the gynoecial primordia is important for differential development. However, the whole gynoecium was regarded as one entity with apical-basal polarity, and the possibility that the lateral carpels and the medial meristematic tissues could respond differently to the treatment was never discussed. Thus, where and how NPA affects PAT-regulated development has remained elusive.To understand how local auxin activities influence the outgrowth and patterning events of young gynoecial primordia, we determined the localization of PIN and PGP auxin efflux proteins and the resulting auxin response domains. This allowed us to map the directional flow and auxin response peaks during the earliest stages of gynoecium development. In addition, we induced transient disruptions in PAT and assessed the response of auxin signaling and domain-specific markers to establish how auxin signaling and vascular, lateral, and medial domains are affected by alterations in PAT. Based on our data, we propose a new model for auxin-regulated gynoecial patterning in which the medial versus lateral identity is dependent on correct auxin localization, and subsequent carpel valve outgrowth is dependent on transport-mediated apical auxin drainage.     

myo-Inositol-1-phosphate Synthase Is Required for Polar Auxin Transport and Organ Development

myo-Inositol-1-phosphate synthase is a conserved enzyme that catalyzes the first committed and rate-limiting step in inositol biosynthesis. Despite its wide occurrence in all eukaryotes, the role of myo-inositol-1-phosphate synthase and de novo inositol biosynthesis in cell signaling and organism development has been unclear. In this study, we isolated loss-of-function mutants in the Arabidopsis MIPS1 gene from different ecotypes. It was found that all null mips1 mutants are defective in embryogenesis, cotyledon venation patterning, root growth, and root cap development. The mutant roots are also agravitropic and have reduced basipetal auxin transport. mips1 mutants have significantly reduced levels of major phosphatidylinositols and exhibit much slower rates of endocytosis. Treatment with brefeldin A induces slower PIN2 protein aggregation in mips1, indicating altered PIN2 trafficking. Our results demonstrate that MIPS1 is critical for maintaining phosphatidylinositol levels and affects pattern formation in plants likely through regulation of auxin distribution.     

拟南芥根中生长素极性运输的研究进展

生长素极性运输影响植物的生长和发育,在植物器官形态建成、发育等过程中发挥重要的调控作用.生长素极性运输是一个依赖于生长素运输载体来完成的复杂过程.近年来生长素极性运输载体AUX/LAX蛋白、PIN蛋白家族和MDR/PGP蛋白家族在生长素极性运输中作用的研究日益深入,并且在植物激素联系和转运方面取得了重大发现——PILC蛋白.     

A NuRD Complex from Xenopus laevis Eggs Is Essential for DNA Replication during Early Embryogenesis

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Inhibition of Polar Auxin Transport by Ethylene

Applied ethylene influences the growth of etiolated pea stem sections cut from untreated plants, but has no effect on (14)C-indoleacetic acid uptake, polar transport or destruction. However, the capacity of the polar auxin transport system is markedly reduced in sections cut from plants grown in ethylene, while the velocity of auxin transport is unchanged under these conditions. Inhibition of the polar transport system by ethylene could underlie certain responses in which the gas produces symptoms of auxin deficiency.     

生长素极性运输研究进展

高等植物的生长发育受激素的广泛调控,其中生长素的作用尤为独特,因为生长素在植物组织内的浓度梯度是由其极性运输维持的,而正是激素在植物组织的相对含量决定了该组织的发育命运。高等植物体内存在可运输的化学信使的概念首先由Ｄａｒｗｉｎ父子提出。通过对金丝鸟木亡草（Ｐｈａｌａｒｉｓｃａ ｎａｒｉｅｎｓｉｓ）幼苗的向光性的研究,他们认为植物的向光性受到一种可运输的物质的调控［1］。后来发现这一物质是生长素,在自然界中主要存在的形式是ＩＡＡ。到本世纪 30年代,禾谷类植物中的生长素的极性运输得到证实,后来发现所有…     

生长素极性运输研究进展

Recent advances in dissecting polar auxin transport, i.e., the physiological characteristics and regulation of polar auxin transport, the chemiosmotic hypothesis for polar auxin transport, and the role of polar auxin transport in plant growth and development were reviewed. The authors here focus on the progress of new supports-isolation and function analysis of the genes encoding putative auxin carriers, for the old model of polar auxin transport.     

Role for Apyrases in Polar Auxin Transport in Arabidopsis

Recent evidence indicates that extracellular nucleotides regulate plant growth. Exogenous ATP has been shown to block auxin transport and gravitropic growth in primary roots of Arabidopsis (Arabidopsis thaliana). Cells limit the concentration of extracellular ATP in part through the activity of ectoapyrases (ectonucleoside triphosphate diphosphohydrolases), and two nearly identical Arabidopsis apyrases, APY1 and APY2, appear to share this function. These findings, plus the fact that suppression of APY1 and APY2 blocks growth in Arabidopsis, suggested that the expression of these apyrases could influence auxin transport. This report tests that hypothesis. The polar movement of [³H]indole-3-acetic acid in both hypocotyl sections and primary roots of Arabidopsis seedlings was measured. In both tissues, polar auxin transport was significantly reduced in apy2 null mutants when they were induced by estradiol to suppress the expression of APY1 by RNA interference. In the hypocotyl assays, the basal halves of APY-suppressed hypocotyls contained considerably lower free indole-3-acetic acid levels when compared with wild-type plants, and disrupted auxin transport in the APY-suppressed roots was reflected by their significant morphological abnormalities. When a green fluorescent protein fluorescence signal encoded by a DR5:green fluorescent protein construct was measured in primary roots whose apyrase expression was suppressed either genetically or chemically, the roots showed no signal asymmetry following gravistimulation, and both their growth and gravitropic curvature were inhibited. Chemicals that suppress apyrase activity also inhibit gravitropic curvature and, to a lesser extent, growth. Taken together, these results indicate that a critical step connecting apyrase suppression to growth suppression is the inhibition of polar auxin transport.In both animals and plants, cells release nucleotides into their extracellular matrix, where they function as signaling agents, inducing rapid increases in the concentration of cytosolic calcium that are transduced into downstream changes in cell physiology (Kim et al., 2006; Burnstock, 2007; Roux and Steinebrunner, 2007; Tanaka et al., 2010a, 2010b; Demidchik et al., 2011). Prominent among these downstream changes in plants are changes in the growth of cells, including the growth of pollen tubes (Steinebrunner et al., 2003), root hairs (Clark et al., 2010b), and cotton (Gossypium hirsutum) fibers (Clark et al., 2010a). These results suggest the possibility that the signaling changes induced by extracellular nucleotides intersect with signaling changes induced by one or more of the hormones that regulate plant cell growth. Consistent with this possibility, Tang et al. (2003) showed that a concentration of applied nucleotides that inhibited the gravitropic growth of roots could block the transport of the growth hormone auxin and that this effect could not be attributed to either pH changes or chelation of divalent cations. Correspondingly, Clark et al. (2010a) showed that when the application of nucleotides to cotton ovules growing in culture altered the rate of cotton fiber growth, it also induced the production of ethylene, a hormone known to regulate the growth of cotton fibers.Given the potency of extracellular nucleotides to regulate cellular activities, it would be important for cells to control the concentration of these nucleotides. In both animals and plants, the principal enzymes that limit the buildup of extracellular ATP (eATP) and extracellular ADP are ectoapyrases (apyrase; EC 3.6.1.5). These enzymes, which are nucleoside triphosphate diphosphohydrolases, are characterized by apyrase-conserved regions whose peptide sequences are highly similar throughout the plant and animal kingdoms (Clark and Roux, 2009). Based on this structural criterion, there are seven apyrases in Arabidopsis (Arabidopsis thaliana; APY1–APY7), and two of these, APY1 and APY2, share 87% protein sequence identity but are less than 30% similar to the other five apyrases. These two apyrases partially complement each other’s function and play central roles in growth control in Arabidopsis, as judged both by genetic and biochemical criteria (Wolf et al., 2007; Wu et al., 2007). Polyclonal antibodies raised to APY1 (Steinebrunner et al., 2000) inhibit the apyrase activity released into the medium of growing pollen tubes, and when these antibodies were added to the culture medium of germinated pollen, they both blocked the growth of the pollen and raised the concentration of ATP in the medium (Wu et al., 2007). Similarly, treatment of cultured cotton ovules with antibodies that recognize cotton fiber apyrase both inhibits the growth of the fibers and increases the concentration of ATP in the medium, further establishing the link between apyrase activity and regulation of the extracellular ATP concentration ([eATP]) in growing tissues (Clark et al., 2010a).Because wild-type pollen tubes expressing active APY1 or APY2 and cultured cotton fibers with wild-type apyrase activity grow at a normal rate, and because the antibodies inhibit apyrase activity (Wu et al., 2007), the growth inhibition induced by the antibodies further implicated apyrase activity as critical for the growth of these tissues. The antibodies were unlikely to enter the pollen tubes or cotton fibers, so these results also suggested that the pollen and cotton apyrases were ectoapyrases. However, these data do not rule out a possible Golgi function for APY1 and APY2 and for the cotton APY(s), as discussed by Wu et al. (2007) and Clark and Roux (2011). In fact, there is strong evidence that APY1 and APY2 are localized in the Golgi and may function there to regulate protein glycosylation and/or affect polysaccharide synthesis (Chiu et al., 2012; Schiller et al., 2012).Although the suppression of APY1/APY2 or of apyrase activity has a dramatic effect on growth, overexpression of APY1 or APY2 has much less of an effect. Constitutive expression of APY1 induces a small but statistically significant increase in the growth of etiolated hypocotyls, while overexpressing APY2 has no effect on this growth (Wu et al., 2007). This is probably because the wild-type levels of apyrase expression are near optimal for growth (Roux and Steinebrunner, 2007).The double knockout apy1apy2 is sterile, because the pollen of this mutant does not germinate (Steinebrunner et al., 2003). However, when APY1 is suppressed only approximately 60% by an inducible RNA interference (RNAi) construct in apy2 null mutants, pollen of these mutants will germinate, permitting fertilization and subsequent normal development, although the adult plants of these mutants are dwarf (Wu et al., 2007). Suppression of ectoapyrase activity would be expected to raise the equilibrium concentration of eATP (Wu et al., 2007), and since higher levels of eATP can inhibit auxin transport in roots (Tang et al., 2003), it was reasonable to hypothesize that the suppression of apyrase by RNAi could suppress auxin transport. The experiments described in this report test this hypothesis. The results indicate that suppression of APY1/APY2 expression in an inducible RNAi line, R2-4A (Wu et al., 2007), results in a significant inhibition of polar auxin transport in Arabidopsis hypocotyls and roots, with a concomitant altered distribution of endogenous auxin. Consistent with this result and with the results of Tang et al. (2003), suppression of APY1/APY2 also blocks the asymmetric distribution of a GFP reporter encoded by a DR5:GFP construct in gravistimulated primary roots of Arabidopsis seedlings and diminishes the extent of the elongation zone in these roots. These results are consistent with the novel conclusion that inhibition of auxin transport is a key step in the signaling pathway that links the inhibition of apyrase expression to growth inhibition.     

Requirement of the Auxin Polar Transport System in Early Stages of Arabidopsis Floral Bud Formation

The pin-formed mutant pin 1-1, one of the Arabidopsis flower mutants, has several structural abnormalities in inflorescence axes, flowers, and leaves. In some cases, pin1-1 forms a flower with abnormal structure (wide petals, no stamens, pistil-like structure with no ovules in the ovary) at the top of inflorescence axes. In other cases, no floral buds are formed on the axes. An independently isolated allelic mutant (pin1-2) shows similar phenotypes. These mutant phenotypes are exactly the same in wild-type plants cultured in the presence of chemical compounds known as auxin polar transport inhibitors: 9-hydroxyfluorene-9-carboxylic acid or N-(1-naphthyl)phthalamic acid. We tested the polar transport activity of indole-3-acetic acid and the endogenous amount of free indole-3-acetic acid in the tissue of inflorescence axes of the pin1 mutants and wild type. The polar transport activity in the pin 1-1 mutant and in the pin1-2 mutant was decreased to 14% and 7% of wild type, respectively. These observations strongly suggest that the normal level of polar transport activity in the inflorescence axes is required in early developmental stages of floral bud formation in Arabidopsis and that the primary function of the pin1 gene is auxin polar transport in the inflorescence axis.     

生长素在植物胚胎早期发育中的作用

有性生殖是有花植物的一个重要特征, 胚胎则是实现有性生殖和世代交替的重要载体。植物胚胎从双受精开始, 经历了合子极性建立、顶基轴形成、细胞层分化和器官形成等过程, 这些过程都受到生长素的调控。近年来的研究表明, 生长素在生物合成、极性运输和信号转导3个层面上调控胚胎的发育过程。该文以双子叶植物拟南芥(Arabidopsis thaliana)为例, 综述了生长素对胚胎早期发育过程, 包括合子极性和顶基轴建立、表皮原特化和对称模式转变、胚根原特化和根尖分生组织形成及茎尖分生组织形成等发育的调控机制。     

PIN蛋白在生长素极性运输中的作用

PIN蛋白是生长素流出栽体,它在细胞中的不对称分布决定细胞间生长素流方向.PIN蛋白网络系统决定生长素的极性运输,为植物体各部位的细胞提供了特异的位置和方向信息.从细胞水平上介绍PIN蛋白在生长素极性运输中的作用及对PIN蛋白功能调节的研究进展.     

Arabidopsis ROOT UVB SENSITIVE2/WEAK AUXIN RESPONSE1 Is Required for Polar Auxin Transport

Auxin is an essential phytohormone that regulates many aspects of plant development. To identify new genes that function in auxin signaling, we performed a genetic screen for Arabidopsis thaliana mutants with an alteration in the expression of the auxin-responsive reporter DR5rev:GFP (for green fluorescent protein). One of the mutants recovered in this screen, called weak auxin response1 (wxr1), has a defect in auxin response and exhibits a variety of auxin-related growth defects in the root. Polar auxin transport is reduced in wxr1 seedlings, resulting in auxin accumulation in the hypocotyl and cotyledons and a reduction in auxin levels in the root apex. In addition, the levels of the PIN auxin transport proteins are reduced in the wxr1 root. We also show that WXR1 is ROOT UV-B SENSITIVE2 (RUS2), a member of the broadly conserved DUF647 domain protein family found in diverse eukaryotic organisms. Our data indicate that RUS2/WXR1 is required for auxin transport and to maintain the normal levels of PIN proteins in the root.