Auxin Flow Research Articles

Transfusion tracheids in the conifer leaves of Thuja plicata (Cupressaceae) are derived from parenchyma and their differentiation is induced by auxin

Conifer leaves are characterized by the differentiation of transfusion tracheids either adjacent to the vascular bundle or away from bundles. Toward uncovering the mechanism regulating this differentiation, we tested the hypotheses that transfusion tracheids differentiate from parenchyma rather than from procambium and that auxin acts as an inducer of this process. • Transfusion tracheids were studied at different developmental stages in both dissected and cleared juvenile and mature leaves. Auxin accumulation was induced by application of either auxin to juvenile leaves or of auxin transport inhibitors in lanolin to stems. • Transfusion tracheids originate from parenchyma cells during late stages of leaf development, after the activity of the procambium has ceased. Transfusion tracheids differentiate also in the leaf tip, a region in which there are no procambial cells. Application of either auxin or auxin transport inhibitors resulted in a significant increase in transfusion tracheids in leaves. Disruption of the leaf vascular bundle combined with auxin application resulted in direct differentiation of transfusion tracheids from parenchyma cells; the regeneration of a vascular bundle around the disruption was polar and supports both hypotheses. • The results provide experimental support for a parenchymatic origin of the transfusion tracheids in a conifer leaf and for auxin acting as an inducer of these cells. Our results suggest a new model in which auxin production in the leaf apex continues after primary tracheids and parenchyma cells have differentiated, and this late auxin flow induces transfusion tracheids from parenchyma cells.

Polarity, Continuity, and Alignment in Plant Vascular Strands

Plant vascular cells are joined end to end along uninterrupted lines to connect shoot organs with roots; vascular strands are thus polar, continuous, and internally aligned. What controls the formation of vascular strands with these properties? The "auxin canalization hypothesis"-based on positive feedback between auxin flow through a cell and the cell's capacity for auxin transport-predicts the selection of continuous files of cells that transport auxin polarly, thus accounting for the polarity and continuity of vascular strands. By contrast, polar, continuous auxin transport-though required-is insufficient to promote internal alignment of vascular strands, implicating additional factors. The auxin canalization hypothesis was derived from the response of mature tissue to auxin application but is consistent with molecular and cellular events in embryo axis formation and shoot organ development. Objections to the hypothesis have been raised based on vascular organizations in callus tissue and shoot organs but seem unsupported by available evidence. Other objections call instead for further research; yet the inductive and orienting influence of auxin on continuous vascular differentiation remains unique.

Juicy Stories on Female Reproductive Tissue Development: Coordinating the Hormone Flows

In the past 20-30 years, developmental biologists have made tremendous progress in identifying genes required for the specification of individual cell types of an organ and in describing how they interact in genetic networks. In comparison, very little is known about the mechanisms that regulate tissue polarity and overall organ patterning. Gynoecia and fruits from members of the Brassicaceae family of flowering plants provide excellent model systems to study organ patterning and tissue specification because they become partitioned into distinct domains whose formation is determined by polarity establishment both at a cellular and whole tissue level. Interactions among key regulators of Arabidopsis gynoecium and fruit development have revealed a network of upstream transcription factor activities required for such tissue differentiation. Regulation of the plant hormone auxin is emerging as both an immediate downstream output and input of these activities, and here we aim to provide an overview of the current knowledge regarding the link between auxin and female reproductive development in plants. In this review, we will also demonstrate how available data can be exploited in a mathematical modeling approach to reveal and understand the feedback regulatory circuits that underpin the polarity establishment, necessary to guide auxin flows.

Transcriptomic analysis highlights epigenetic and transcriptional regulation during zygotic embryo development of Pinus pinaster

BackgroundIt is during embryogenesis that the plant body plan is established and the meristems responsible for all post-embryonic growth are specified. The molecular mechanisms governing conifer embryogenesis are still largely unknown. Their elucidation may contribute valuable information to clarify if the distinct features of embryo development in angiosperms and gymnosperms result from differential gene regulation. To address this issue, we have performed the first transcriptomic analysis of zygotic embryo development in a conifer species (Pinus pinaster) focusing our study in particular on regulatory genes playing important roles during plant embryo development, namely epigenetic regulators and transcription factors.ResultsMicroarray analysis of P. pinaster zygotic embryogenesis was performed at five periods of embryo development from early developing to mature embryos. Our results show that most changes in transcript levels occurred in the first and the last embryo stage-to-stage transitions, namely early to pre-cotyledonary embryo and cotyledonary to mature embryo. An analysis of functional categories for genes that were differentially expressed through embryogenesis highlighted several epigenetic regulation mechanisms. While putative orthologs of transcripts associated with mechanisms that target transposable elements and repetitive sequences were strongly expressed in early embryogenesis, PRC2-mediated repression of genes seemed more relevant during late embryogenesis. On the other hand, functions related to sRNA pathways appeared differentially regulated across all stages of embryo development with a prevalence of miRNA functions in mid to late embryogenesis. Identification of putative transcription factor genes differentially regulated between consecutive embryo stages was strongly suggestive of the relevance of auxin responses and regulation of auxin carriers during early embryogenesis. Such responses could be involved in establishing embryo patterning. Later in development, transcripts with homology to genes acting on modulation of auxin flow and determination of adaxial-abaxial polarity were up-regulated, as were putative orthologs of genes required for meristem formation and function as well as establishment of organ boundaries. Comparative analysis with A. thaliana embryogenesis also highlighted genes involved in auxin-mediated responses, as well as epigenetic regulation, indicating highly correlated transcript profiles between the two species.ConclusionsThis is the first report of a time-course transcriptomic analysis of zygotic embryogenesis in a conifer. Taken together our results show that epigenetic regulation and transcriptional control related to auxin transport and response are critical during early to mid stages of pine embryogenesis and that important events during embryogenesis seem to be coordinated by putative orthologs of major developmental regulators in angiosperms.

ERECTA Family Genes Regulate Auxin Transport in the Shoot Apical Meristem and Forming Leaf Primordia

Leaves are produced postembryonically at the flanks of the shoot apical meristem. Their initiation is induced by a positive feedback loop between auxin and its transporter PIN-FORMED1 (PIN1). The expression and polarity of PIN1 in the shoot apical meristem is thought to be regulated primarily by auxin concentration and flow. The formation of an auxin maximum in the L1 layer of the meristem is the first sign of leaf initiation and is promptly followed by auxin flow into the inner tissues, formation of the midvein, and appearance of the primordium bulge. The ERECTA family genes (ERfs) encode leucine-rich repeat receptor-like kinases, and in Arabidopsis (Arabidopsis thaliana), this gene family consists of ERECTA (ER), ERECTA-LIKE1 (ERL1), and ERL2. Here, we show that ERfs regulate auxin transport during leaf initiation. The shoot apical meristem of the er erl1 erl2 triple mutant produces leaf primordia at a significantly reduced rate and with altered phyllotaxy. This phenotype is likely due to deficiencies in auxin transport in the shoot apex, as judged by altered expression of PIN1, the auxin reporter DR5rev::GFP, and the auxin-inducible genes MONOPTEROS, INDOLE-3-ACETIC ACID INDUCIBLE1 (IAA1), and IAA19. In er erl1 erl2, auxin presumably accumulates in the L1 layer of the meristem, unable to flow into the vasculature of a hypocotyl. Our data demonstrate that ERfs are essential for PIN1 expression in the forming midvein of future leaf primordia and in the vasculature of emerging leaves.

Nicotiana tabacum actin-depolymerizing factor 2 is involved in actin-driven, auxin-dependent patterning

Polar transport of auxin has been identified as a central element of pattern formation. To address the underlying cellular mechanisms, we use the tobacco cell line (Nicotiana tabacum L. cv. Bright Yellow 2; BY-2) as model. We showed previously that cell divisions within a cell file are synchronized by polar auxin flow, linked to the organization of actin filaments (AF) which, in turn, is modified via actin-binding proteins (ABPs). From a preparatory study for disturbed division synchrony in cell lines overexpressing different ABPs, we identified the actin depolymerizing factor 2 (ADF2). A cell line overexpressing GFP-NtADF2 was specifically affected in division synchrony. The cell division pattern could be rescued by addition of Phosphatidylinositol 4,5-bisphosphate (PIP2) or by phalloidin. These observations allow to draw first conclusions on the pathway linking auxin signalling via actin reorganization to synchronized cell division placing the regulation of cortical actin turnover by ADF2 into the focus.

TOR tour to auxin

Auxin is a principal growth hormone in plants. A study by Schepetilnikov et al (2013) now reports that TOR is activated by auxin and regulates expression of auxin‐responsive genes by facilitating translation reinitiation of mRNAs with uORFs at their 5′ UTRs. In this way, TOR acts at the crossroad of the plasma membrane and nuclear auxin signalling pathways to link auxin with general nutrient signalling. Auxin signalling acts through concentration gradients to regulate developmental processes in plants and the flow of Auxin can be rapidly altered by environmental stimuli resulting in the redirection of plant growth. Auxin concentrations are read and translated into gradients of regulatory proteins, as seen for the PLETHORA (PLT) transcription factors, which determine cellular states, such as stem cell maintenance, cell proliferation, and cell elongation. The currently dominating view is that auxin is perceived by the nuclear TIR/AFB receptors, F‐box proteins, which regulate turnover of the AUX/IAA transcriptional repressors to control gene transcription through a family of AUXIN RESPONSE FACTORs (ARFs; Figure 1; Hayashi, 2012). Schepetilnikov et al (2013) now show that auxin also controls the abundance of key regulators in a post‐transcriptional manner involving selective protein translation. They report that auxin activates TARGET OF RAPAMICIN (TOR), the evolutionary conserved central growth regulator in yeast, plants, and animals. TOR is known to control protein translation through translation initiation in yeast and animals, but its role is less well understood in plants. It was previously …

MATHEMATICAL MODELING OF AUXIN TRANSPORT IN PROTOXYLEM AND PROTOPHLOEM OF ARABIDOPSIS THALIANA ROOT TIPS

Phytohormone auxin is the main regulator of plant growth and development. Nonuniform auxin distribution in plant tissue sets positional information, which determines morphogenesis. Auxin is transported in tissue by means of diffusion and active transport through the cell membrane. There is a number of auxin carriers performing its influx into a cell (AUX\LAX family) or efflux from a cell (PIN, PGP families). The paper presents mathematical models for auxin transport in vascular tissues of Arabidopsis thaliana L.root tip, namely protophloem and protoxylem. Tissue specificity of auxin active transport was considered in these models. There is PIN-mediated auxin efflux in both protoxylem and protophloem, but AUX1-mediated influx exists only in protophloem. Optimal values of parameters were adjusted for model solutions to fit the experimentally observed auxin distributions in the root tip. Based on simulation results we predicted that shoot-derived auxin flow to protophloem is lower than one to protoxylem, and the efficiency of PIN-mediated auxin transport in protophloem is higher than in protoxylem. In summary, our simulation showed that despite the same auxin distribution pattern, provascular tissues in the root tip differ in dynamics of auxin transport.

Transforming a Stem into a Bush

Without careful pruning, a rose bush would grow into a single stem with leaves. This is why gardeners invest in a good pair of clippers—cutting the leading shoot helps activate axillary buds which typically form at the base of each leaf near the stem and give rise to new shoots. Yet, axillary buds sometimes remain dormant despite pruning, and many plant species activate branching without a snip from the gardener. As to how a plant regulates this unpredictable branch growth, there are two competing theories that point to strigolactone, a recently discovered plant hormone. One theory suggests strigolactone primarily acts to regulate gene expression in axillary buds. New findings published this week in PLOS Biology support an alternative theory, which suggests strigolactone regulates bud growth by modulating the cellular flow of auxin, a plant growth hormone that influences development. The findings explain how plants coordinate growth and development. According to a debated theory proposed by Ottoline Leyser and her colleagues at the University of Cambridge, the auxin transport network helps distribute branch growth across the plant in response to changing environmental conditions and resource availability. This auxin transport canalization model suggests that cells in the axillary bud serve as a source of auxin while the stem and roots provide a sink. As auxin flows across the cell membrane, more exporter proteins are inserted into the plasma membrane. This increases the flow of auxin out of the cell, and leads to the insertion of even more exporter proteins. As this stream of auxin flows to the stem and roots, the axillary buds can potentially produce even more hormone. Leyser believes this positive feedback loop triggers buds to grow into new shoots. After an axillary bud is activated, other buds must compete for access to sinks in the stem. The first few shoots dominate the local flow of auxin, making it harder for neighboring buds to establish the positive feedback of auxin export needed for activity. As a result, an axillary bud might not be able to activate after shoots start to grow elsewhere on the stem. In previous work, Leyser and her colleagues showed that strigolactone reduces cellular levels of PIN1, a protein that exports auxin out of plant cells. As PIN1 accumulation requires a gene expressed in vascular associated cells, it’s possible that strigolactone systemically inhibits canalization throughout the plant. But this theory conflicts with other research showing that strigolactone upregulates a transcription factor that inhibits bud growth, suggesting that strigolactone acts locally within the bud. To test these alternative hypotheses, Leyser and her team grew mutant Arabidopsis plants incapable of synthesizing strigolactone or responding to its signal. By genetically modifying plants to produce fluorescently labeled PIN1 proteins, the researchers could track this auxin exporter in plant tissue. In keeping with prior findings, these strigolactone mutants grew more shoots and accumulated more PIN1. Similarly, treating mutants with synthetic strigolactone (GR24) reduced branching. Yet, the story is a little more complex— it turns out strigolactone can both activate and repress shoot branching. Mutants weakly capable of transporting auxin showed elevated branching after a low dose of strigalactone, yet after a high dose, branching was reduced. In double mutants that lacked the ability to produce strigolactone and also suffered weak auxin transport, low doses of strigalactone reduced branching while high doses restored branching to levels of untreated plants. If strigalactone acts as a direct inhibitor of bud growth, these findings would be difficult to explain. It appears strigalactone can either promote or inhibit bud growth depending on the auxin transport status of the treated plant, although the underlying mechanisms remain unclear. Leyser and her team further investigated the hormone’s systemic function with a computational model, which suggested that branching can occur in one of two ways. First, if PIN1 proteins remain in the membrane after insertion, auxin transport can be rapidly established and new buds activate. In contrast, if the rate of PIN1 insertion is low, relatively less auxin flows out of the cell. This reduces competition for auxin transport down the stem, and an active bud is less likely to inhibit its neighbors. If strigolactone acts to promote removal of PIN1 proteins from the membrane, then the modeling results and experimental data together explain how strigolactone can either activate or repress shoot branching depending on the auxin transport status of the plants. It’s a new way of thinking about shoot growth, and it’s not without controversy. Auxin canalization theory is contentious for at least two reasons—it assumes that

Strigolactone Can Promote or Inhibit Shoot Branching by Triggering Rapid Depletion of the Auxin Efflux Protein PIN1 from the Plasma Membrane

Author SummaryPlants can adapt their form to suit the environment in which they are growing. For example, genetically identical plants can develop as a single unbranched stem or as a highly ramified bush. This broad developmental potential is possible because the shoot system is produced continuously by growing tips, known as shoot meristems. Meristems produce the stem and leaves of a shoot, and at the base of each leaf, a new meristem is formed. This meristem can remain dormant as a small bud or activate to produce a branch. Thus, the shoot system is a community of shoot meristems, the combined activity and inactivity of which shape shoot form. Here we provide evidence that growth is balanced across the Arabidopsis shoot system by competition between the shoot meristems. This competition is likely mediated by the requirement of meristems to export the plant hormone auxin in order to activate bud outgrowth. In our model, auxin in the main stem, exported from active branches, can prevent auxin export by dormant buds, thus preventing their activation. Our findings show that a second hormone, strigolactone, increases the level of competition between branches by making auxin export harder to establish. Together, these hormones balance growth across the shoot system, adjusting it according to the environmental conditions in which a plant is growing.

Auxin-associated initiation of vascular cell differentiation by LONESOME HIGHWAY

Plant vascular tissues are essential for the existence of land plants. Many studies of transcriptional regulation and cell-cell communication have revealed the process underlying the development of vascular tissues from vascular initial cells. However, the initiation of vascular cell differentiation is still a mystery. Here, we report that LONESOME HIGHWAY (LHW), which encodes a bHLH transcription factor, is expressed in pericycle-vascular mother cells at the globular embryo stage and is required for proper asymmetric cell division to generate vascular initial cells. In addition, ectopic expression of LHW elicits an ectopic auxin response. Moreover, LHW is required for the correct expression patterns of components related to auxin flow, such as PIN-FORMED 1 (PIN1), MONOPTEROS (MP) and ATHB-8, and ATHB-8 partially rescues the vascular defects of lhw. These results suggest that LHW functions as a key regulator to initiate vascular cell differentiation in association with auxin regulation.

Auxin regulates first leaf development and promotes the formation of protocorm trichomes and rhizome-like structures in developing seedlings of Spathoglottis plicata (Orchidaceae)

Auxin flows in a polar manner to target tissues and exert its morphogenic effect. Preventing auxin movement, with polar auxin transport (PAT) inhibitors, or increasing auxin levels in tissues through exogenous application can provide a means for assessing the importance of appropriate tissue distribution and concentration of this hormone during development. The formulation of culture media for micropropagation has been the primary focus of most orchid tissue culture research, a goal that unveils seedling hormone responses at a single point in development. This study was unique because it evaluated the auxin response of orchids during three stages of seedling development. Seedlings were grown on standard culture media for 10, 35 and 85 days. Each group was sub-cultured onto auxinand/or PAT inhibitor-containing media for an additional 10, 30 and 60 days, respectively. Data were collected on first leaf initiation, trichome formation and the appearance of propagative structures. In the 20-day seedlings, auxins and PAT inhibitors promoted precocious formation and random placement of protocorm hairs rather than in tufts, as seen in older, control seedlings. The 65-day seedlings formed protocorm-like bodies, rhizome-like growths from the stem, and fleshy leaves with trichomes. Seedlings cultured for 145 days developed microshoots or callus growth in the axils of older leaves and exhibited necrosis of original seedling roots and leaves. In general, exogenously applied auxin promoted the reversion of differentiated Spathoglottis plicata seedling tissue to a morphology that had propagative properties. Additionally, auxins commonly induced hair formation, which suggests that protocorm hairs may be root hair-like in nature. This work characterized three auxin growth responses in S. plicata seedlings that have not been reported in orchids: (i) the inhibition of first leaf initiation and abnormal first leaf morphology; (ii) the promotion of trichome formation; and (iii) the induction of rhizome-like structures and microshoots at a specific stage in seedling development.

Gravity Sensing and Signal Transduction in Vascular Plant Primary Roots

During gravitropism, the potential energy of gravity is converted into a biochemical signal. How this transfer occurs remains one of the most exciting mysteries in plant cell biology. New experiments are filling in pieces of the puzzle. In this review, we introduce gravitropism and give an overview of what we know about gravity sensing in roots of vascular plants, with special highlight on recent papers. When plant roots are reoriented sideways, amyloplast resedimentation in the columella cells is a key initial step in gravity sensing. This process somehow leads to cytoplasmic alkalinization of these cells followed by relocalization of auxin efflux carriers (PINs). This changes auxin flow throughout the root, generating a lateral gradient of auxin across the cap that upon transmission to the elongation zone leads to differential cell elongation and gravibending. We will present the evidence for and against the following players having a role in transferring the signal from the amyloplast sedimentation into the auxin signaling cascade: mechanosensitive ion channels, actin, calcium ions, inositol trisphosphate, receptors/ligands, ARG1/ARL2, spermine, and the TOC complex. We also outline auxin transport and signaling during gravitropism.

Diverting the downhill flow of auxin to steer growth during tropisms

Polar auxin transport can be likened to water following the path of least resistance as it flows downhill. In the case of auxin, the hill is the difference in electrochemical potential of the auxin anion (IAA(-)). H(+)-ATPases and H(+)-IAA symporters at the plasma membrane create the electrical and IAA(-) concentration gradients that constitute this thermodynamic hill. PIN and ABCB transporters also at the plasma membrane bias the direction and limit the rate of downhill flow out of the cell. This article will present the thermodynamic basis for this view and critically examine how well the molecular biological descriptions of the polar auxin transport process fit the framework. An auxin concentration gradient across an organ has long been recognized as the cause of bending growth during tropisms. Its generation can be viewed as a result of redirected polar auxin transport. This article will examine how molecular regulation of the paths of least resistance to auxin efflux diverts the downhill flow of auxin to steer growth during tropisms.

Auxin biology in roots

Auxin regulates almost every aspect of plant growth and development. Its intracellular concentration is controlled by biosynthesis and degradation. In addition, there is an pool that consists of the conjugates with sugars, amino acids, and peptides. Some of the conjugates reversely release auxin, enabling alternative methods to regulate auxin concentrations. Auxin concentrations are also affected by transport. Besides the long distance delivery through the phloem, auxin is transported across the cell by influx and efflux carriers, from the shoot to root with maximum concentration at the root tip. At the root tip, the auxin flow reverses, and shootward auxin transport occurs. An auxin gradient formed this way is indispensable for proper development, maintenance of the meristem, and cell identity. The formation of root hairs is auxin-dependent. Auxin controls not only the initiation of root hairs but also regulates their elongation. In Arabidopsis thaliana, auxin accumulates in atrichoblasts and it is supplied to trichoblasts. Before hair initiation, randomization of cortical microtubule arrays is observed in lettuce seedlings. This action is promoted by auxin and is indispensable to hair formation. Furthermore, light promotes CMT randomization and root hair initiation via auxin signaling. Ethylene is another promoter of root hair formation. Ethylene affects auxin signaling and vice versa. Interactions between these hormones are synergistic for root growth inhibition but antagonistic for lateral root formation. Reactive oxygen species also regulate various responses in plants. They play an important role during root hair elongation, although their precise relationship with auxin is yet not clear.

Root gravitropism and root hair development constitute coupled developmental responses regulated by auxin homeostasis in the Arabidopsis root apex

Active polar transport establishes directional auxin flow and the generation of local auxin gradients implicated in plant responses and development. Auxin modulates gravitropism at the root tip and root hair morphogenesis at the differentiation zone. Genetic and biochemical analyses provide evidence for defective basipetal auxin transport in trh1 roots. The trh1, pin2, axr2 and aux1 mutants, and transgenic plants overexpressing PIN1, all showing impaired gravity response and root hair development, revealed ectopic PIN1 localization. The auxin antagonist hypaphorine blocked root hair elongation and caused moderate agravitropic root growth, also leading to PIN1 mislocalization. These results suggest that auxin imbalance leads to proximal and distal developmental defects in Arabidopsis root apex, associated with agravitropic root growth and root hair phenotype, respectively, providing evidence that these two auxin-regulated processes are coupled. Cell-specific subcellular localization of TRH1-YFP in stele and epidermis supports TRH1 engagement in auxin transport, and hence impaired function in trh1 causes dual defects of auxin imbalance. The interplay between intrinsic cues determining root epidermal cell fate through the TTG/GL2 pathway and environmental cues including abiotic stresses modulates root hair morphogenesis. As a consequence of auxin imbalance in Arabidopsis root apex, ectopic PIN1 mislocalization could be a risk aversion mechanism to trigger root developmental responses ensuring root growth plasticity.

Intracellular auxin transport in pollen

Cellular auxin homeostasis is controlled at many levels that include auxin biosynthesis, auxin metabolism, and auxin transport. In addition to intercellular auxin transport, auxin homeostasis is modulated by auxin flow through the endoplasmic reticulum (ER). PIN5, a member of the auxin efflux facilitators PIN protein family, was the first protein to be characterized as an intracellular auxin transporter. We demonstrated that PIN8, the closest member of the PIN family to PIN5, represents another ER-residing auxin transporter. PIN8 is specifically expressed in the male gametophyte and is located in the ER. By combining genetic, physiological, cellular and biochemical data we demonstrated a role for PIN8 in intracellular auxin homeostasis. Although our investigation shed light on intracellular auxin transport in pollen, the physiological function of PIN8 still remains to be elucidated. Here we discuss our data taking in consideration other recent findings.

Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism

Gravity profoundly influences plant growth and development. Plants respond to changes in orientation by using gravitropic responses to modify their growth. Cholodny and Went hypothesized over 80 years ago that plants bend in response to a gravity stimulus by generating a lateral gradient of a growth regulator at an organ's apex, later found to be auxin. Auxin regulates root growth by targeting Aux/IAA repressor proteins for degradation. We used an Aux/IAA-based reporter, domain II (DII)-VENUS, in conjunction with a mathematical model to quantify auxin redistribution following a gravity stimulus. Our multidisciplinary approach revealed that auxin is rapidly redistributed to the lower side of the root within minutes of a 90° gravity stimulus. Unexpectedly, auxin asymmetry was rapidly lost as bending root tips reached an angle of 40° to the horizontal. We hypothesize roots use a "tipping point" mechanism that operates to reverse the asymmetric auxin flow at the midpoint of root bending. These mechanistic insights illustrate the scientific value of developing quantitative reporters such as DII-VENUS in conjunction with parameterized mathematical models to provide high-resolution kinetics of hormone redistribution.

Transcription factor WRKY23 assists auxin distribution patterns during Arabidopsis root development through local control on flavonol biosynthesis

Gradients of the plant hormone auxin, which depend on its active intercellular transport, are crucial for the maintenance of root meristematic activity. This directional transport is largely orchestrated by a complex interaction of specific influx and efflux carriers that mediate the auxin flow into and out of cells, respectively. Besides these transport proteins, plant-specific polyphenolic compounds known as flavonols have been shown to act as endogenous regulators of auxin transport. However, only limited information is available on how flavonol synthesis is developmentally regulated. Using reduction-of-function and overexpression approaches in parallel, we demonstrate that the WRKY23 transcription factor is needed for proper root growth and development by stimulating the local biosynthesis of flavonols. The expression of WRKY23 itself is controlled by auxin through the Auxin Response Factor 7 (ARF7) and ARF19 transcriptional response pathway. Our results suggest a model in which WRKY23 is part of a transcriptional feedback loop of auxin on its own transport through local regulation of flavonol biosynthesis.

Early Embryogenesis in Flowering Plants: Setting Up the Basic Body Pattern

Early embryogenesis is the critical developmental phase during which the basic features of the plant body are established: the apical-basal axis of polarity, different tissue layers, and both the root pole and the shoot pole. Polarization of the zygote correlates with the generation of apical and basal (embryonic and extraembryonic) cell fates. Whereas mechanisms of zygote polarization are still largely unknown, distinct expression domains of WOX family transcription factors as well as directional auxin transport and local auxin response are known to be involved in early apical-basal patterning. Radial patterning of tissue layers appears to be mediated by cell-cell communication involving both peptide signaling and transcription factor movement. Although the initiation of the shoot pole is still unclear, the apical organization of the embryo depends on both the proper establishment of transcription factor expression domains and, for cotyledon initiation, upward auxin flow in the protoderm. Here we focus on the essential patterning processes, drawing mainly on data from Arabidopsis thaliana and also including relevant data from other species if available.