AtPIN4 mediates sink-driven auxin gradients and root patterning in Arabidopsis

Complex regulation of Arabidopsis AGR1/PIN2-mediated root gravitropic response and basipetal auxin transport by cantharidin-sensitive protein phosphatases

Polar auxin transport, mediated by two distinct plasma membrane-localized auxin influx and efflux carrier proteins/complexes, plays an important role in many plant growth and developmental processes including tropic responses to gravity and light, development of lateral roots and patterning in embryogenesis. We have previously shown that the Arabidopsis AGRAVITROPIC 1/PIN2 gene encodes an auxin efflux component regulating root gravitropism and basipetal auxin transport. However, the regulatory mechanism underlying the function of AGR1/PIN2 is largely unknown. Recently, protein phosphorylation and dephosphorylation mediated by protein kinases and phosphatases, respectively, have been implicated in regulating polar auxin transport and root gravitropism. Here, we examined the effects of chemical inhibitors of protein phosphatases on root gravitropism and basipetal auxin transport, as well as the expression pattern of AGR1/PIN2 gene and the localization of AGR1/PIN2 protein. We also examined the effects of inhibitors of vesicle trafficking and protein kinases. Our data suggest that protein phosphatases, sensitive to cantharidin and okadaic acid, are likely involved in regulating AGR1/PIN2-mediated root basipetal auxin transport and gravitropism, as well as auxin response in the root central elongation zone (CEZ). BFA-sensitive vesicle trafficking may be required for the cycling of AGR1/PIN2 between plasma membrane and the BFA compartment, but not for the AGR1/PIN2-mediated root basipetal auxin transport and auxin response in CEZ cells.

Laser capture microdissection for the analysis of gene expression during embryogenesis of Arabidopsis

It is during embryogenesis that the body plan of the developing plant is established. Analysis of gene expression during embryogenesis has been limited due to the technical difficulty of accessing the developing embryo. Here we demonstrate that laser capture microdissection can be applied to the analysis of embryogenesis. We show how this technique can be used in concert with DNA microarray for the large-scale analysis of gene expression in apical and basal domains of the globular-stage and heart-stage embryo, respectively, when critical events of polarity, symmetry and biochemical differentiation are established. This high resolution spatial analysis shows that up to approximately 65% of the genome is expressed in the developing embryo, and that differential expression of a number of gene classes can be detected. We discuss the validity of this approach for the functional analysis of both published and previously uncharacterized essential genes.

Stepwise understanding of root development

Recent studies using Arabidopsis propose a framework of root development and pattern formation that can be divided to three processes. First, a positional signal that is delivered from neighboring cells controls the fate of undifferentiated cells. Then, cell fate is fixed through a protein-network that includes various transcription factors. Finally, the expression of a particular gene-set leads to fate-dependent cell differentiation, resulting in oriented cell division, cell specification and cell elongation. In addition, these processes could be modified by chromatin stabilization and protein degradation. We focus on three fundamental patterns of root development, circumferential pattern, radial pattern and proximo-distal pattern, and on novel approaches to identify genes that are responsible for the spatiotemporal regulation of root development.

Auxin and embryo axis formation: the ends in sight?

The major axis of polarity of the plant embryo serves as a reference for the formation of meristems and, thus, for all subsequent development. Mechanisms underlying the establishment of the embryo axis itself have remained elusive. This is now changing with recent reports documenting a role for auxin in embryo axis formation. Auxin accumulates dynamically at specific positions that correlate with developmental decisions in early embryogenesis, and this ties developmental decisions to both transport regulators and components of the response machinery. A major challenge for the future is to determine how auxin-dependent processes interact with other as yet unknown factors to mediate differential gene expression patterns in early embryogenesis.

The PIN auxin efflux facilitator network controls growth and patterning in Arabidopsis roots

Local accumulation of the plant growth regulator auxin mediates pattern formation in Arabidopsis roots and influences outgrowth and development of lateral root- and shoot-derived primordia. However, it has remained unclear how auxin can simultaneously regulate patterning and organ outgrowth and how its distribution is stabilized in a primordium-specific manner. Here we show that five PIN genes collectively control auxin distribution to regulate cell division and cell expansion in the primary root. Furthermore, the joint action of these genes has an important role in pattern formation by focusing the auxin maximum and restricting the expression domain of PLETHORA (PLT) genes, major determinants for root stem cell specification. In turn, PLT genes are required for PIN gene transcription to stabilize the auxin maximum at the distal root tip. Our data reveal an interaction network of auxin transport facilitators and root fate determinants that control patterning and growth of the root primordium.

Auxins

Auxin is a multifactorial phytohormone that is required for cell division. Fine gradients determine points of developmental change in time and space. It is associated intimately with the axiality of plant growth, and increasing doses lead to cell expansion or inhibition of cell expansion in different tissues. From embryonic patterning to fruit dehiscence every plant process has some involvement with auxin as a hormonal signal, including responses to wounding. Moreover, synthetic auxins have widespread uses as agrochemicals, particularly as selective herbicides. Despite the importance of auxin as a plant signal the pathways of its biosynthesis are still not clear. Much more is known about auxin perception and the mechanisms through which gene transcription is regulated. One receptor has been identified, and protein crystallography data has explained its auxin-binding capacity, but this is likely to control only a subset of auxin-mediated responses. Little is known of the signal transduction intermediates. A second receptor has been nominated and may be involved in controlling auxin-mediated gene transcription. A complex set of proteins comprising signalosome and proteasome contribute to the regulation of sets of transcription factors to confer regulation by derepression. A set of auxin transport proteins has been described with associated regulatory interactors, and these account for polar auxin flow and the control of auxin movements across cells, tissues, and around the plant. The gradients these transport systems build regulate the responses of growth and differentiation, including the plant's response to gravity. These areas are described and discussed by relating the physiology of the whole plant to the details of genetic and protein activities.

Gravitropic bending and plant hormones

Gravitropism is a complex multistep process that redirects the growth of roots and various above-ground organs in response to changes in the direction of the gravity vector. The anatomy and morphology of these graviresponding organs indicates a certain spatial separation between the sensing region and the responding one, a situation that strongly suggests the requirement of phytohormones as mediators to coordinate the process. The Cholodny-Went hypothesis suggested auxin as the main mediator of gravitropism. So far, ample evidence has been gathered with regard to auxin asymmetrical detection, polar and lateral transport involving influx and efflux carriers, response signaling pathway, and possible modes of action in differential cell elongation, supports its major role in gravitropism at least in roots. However, it is becoming clear that the participation of other hormones, acting in concert with auxin, is necessary as well. Of particular importance is the role of ethylene in shoot gravitropism, possibly associated with the modulation of auxin transport or sensitivity, and the key role implicated for cytokinin as the putative root cap inhibitor that controls early root gravitropism. Therefore, the major advances in the understanding of transport and signaling of auxin, ethylene, and cytokinin may shed light on the possibly tight and complicated interactions between them in gravitropism. Not much convincing evidence has been accumulated regarding the participation of other phytohormones, such as gibberellins, abscisic acid, brassinosteroids, jasmonates, and salicylic acid, in gravitropism. However, the emerging concept of cooperative hormone action opens new possibilities for a better understanding of the complex interactions of all phytohormones and their possible synergistic effects and involvement in the gravitropic bending process.

The PLETHORA genes mediate patterning of the Arabidopsis root stem cell niche

A small organizing center, the quiescent center (QC), maintains stem cells in the Arabidopsis root and defines the stem cell niche. The phytohormone auxin influences the position of this niche by an unknown mechanism. Here, we identify the PLETHORA1 (PLT1) and PLT2 genes encoding AP2 class putative transcription factors, which are essential for QC specification and stem cell activity. The PLT genes are transcribed in response to auxin accumulation and are dependent on auxin response transcription factors. Distal PLT transcript accumulation creates an overlap with the radial expression domains of SHORT-ROOT and SCARECROW, providing positional information for the stem cell niche. Furthermore, the PLT genes are activated in the basal embryo region that gives rise to hypocotyl, root, and root stem cells and, when ectopically expressed, transform apical regions to these identities. Thus, the PLT genes are key effectors for establishment of the stem cell niche during embryonic pattern formation.

PIN and AUX/LAX proteins: their role in auxin accumulation

There is mounting evidence that plant tissues specialized for auxin transport coincide with local maxima in the auxin concentration. This result is difficult to reconcile with the traditional model of auxin transport, which relies on high levels of auxin efflux carrier expression. Because transporting cells maintain high levels of auxin efflux, one naively expects a depletion of auxin relative to surrounding tissues. Here I use a computer model of auxin transport in a background of parenchyma cells to evaluate the possible roles of the PIN and AUX/LAX families of putative auxin carriers in auxin accumulation. I describe two effective accumulation strategies and review the evidence that these strategies are used by plants.

Mechanisms of Pattern Formation in Plant Embryogenesis

Many of the patterning mechanisms in plants were discovered while studying postembryonic processes and resemble mechanisms operating during animal development. The emergent role of the plant hormone auxin, however, seems to represent a plant-specific solution to multicellular patterning. This review summarizes our knowledge on how diverse mechanisms that were first dissected at the postembryonic level are now beginning to provide an understanding of plant embryogenesis.

A PINOID-dependent binary switch in apical-basal PIN polar targeting directs auxin efflux

Polar transport-dependent local accumulation of auxin provides positional cues for multiple plant patterning processes. This directional auxin flow depends on the polar subcellular localization of the PIN auxin efflux regulators. Overexpression of the PINOID protein kinase induces a basal-to-apical shift in PIN localization, resulting in the loss of auxin gradients and strong defects in embryo and seedling roots. Conversely, pid loss of function induces an apical-to-basal shift in PIN1 polar targeting at the inflorescence apex, accompanied by defective organogenesis. Our results show that a PINOID-dependent binary switch controls PIN polarity and mediates changes in auxin flow to create local gradients for patterning processes.

Potassium carrier TRH1 is required for auxin transport in Arabidopsis roots

Disruption of the TRH1 potassium transporter impairs root hair development in Arabidopsis, and also affects root gravitropic behaviour. Rescue of these morphological defects by exogenous auxin indicates a link between TRH1 activity and auxin transport. In agreement with this hypothesis, the rate of auxin translocation from shoots to roots and efflux of [3H]IAA in isolated root segments were reduced in the trh1 mutant, but efflux of radiolabelled auxin was accelerated in yeast cells transformed with the TRH1 gene. In roots, Pro(TRH1):GUS expression was localized to the root cap cells which are known to be the sites of gravity perception and are central for the redistribution of auxin fluxes. Consistent with these findings, auxin-dependent DR5:GUS promoter-reporter construct was misexpressed in the trh1 mutant indicating that partial block of auxin transport through the root cap is associated with upstream accumulation of the phytohormone in protoxylem cells. When [K+] in the medium was reduced from 20 to 0.1 mm, wild type roots showed mild agravitropic phenotype and DR5:GUS misexpression in stelar cells. This pattern of response to low external [K+] was also affected by trh1 mutation. We conclude that the TRH1 carrier is an important part of auxin transport system in Arabidopsis roots.

Spatial and Temporal Tracing of Vessel Differentiation in Young Arabidopsis Seedlings by the Expression of an Immature Tracheary Element-specific Promoter

The vascular system is a complex tissue composed of several vascular cell types. However, little is known about the differentiation process of each vascular cell in situ. In this study, we found that the expression of the Zinnia cysteine protease 4 (ZCP4) promoter is restricted to only immature tracheary elements (TEs) in situ. Therefore, we monitored the early TE differentiation process in young Arabidopsis seedlings using a fusion gene of the ZCP4 promoter and the beta-glucuronidase gene as a molecular marker. This approach revealed unique processes of vessel differentiation during early seedling development, in which discontinuous initiation of vessel element differentiation occurs at distinct regions, followed by the simultaneous differentiation of protoxylem vessels and bidirectional differentiation of metaxylem vessels to form a vessel in the plant body.

Auxin signals--turning genes on and turning cells around

The extremely wide spectrum of the plant processes that are influenced by auxin raises the question of how signals conveyed by a single molecule can trigger such a variety of responses. Although many aspects of auxin function remain elusive, others have become genetically tractable. The identification of crucial genes in auxin signal transduction and auxin transport in the past few years has led to molecularly testable concepts of how auxin signals regulate gene activities in individual cells, and how the polar transport of auxin could impact on patterning processes throughout the plant.

Network building: transcriptional circuits in the root

Genes that control cell fate in all major root tissues have been described in Arabidopsis and new candidates that pattern the root apical meristem have been identified. Several phytohormones have long been known to affect root growth; it now appears that they have at least one common entry point into the transcriptional networks that regulate root growth. A map of gene expression in the Arabidopsis root has identified many new candidate genes that feed into and out of these transcriptional controls.

Between the sheets: inter-cell-layer communication in plant development

The cells of plant meristems and embryos are arranged in an organized, and sometimes extremely beautiful, layered pattern. This pattern is maintained by the controlled orientation of cell divisions within layers. However, despite this layered structure, cell behaviour during plant development is not lineage dependent, and does not occur in a mosaic fashion. Many studies, both classical and recent, have shown that plant cell identity can be re-specified according to position, allowing plants to show remarkable developmental plasticity. However, the layered structure of meristems and the implications of this during plant development, remain subjects of some speculation. Of particular interest is the question of how cell layers communicate, and how communication between cell layers could allow coordinated developmental processes to take place. Recent research has uncovered several examples both of the molecular mechanisms by which cell layers can communicate, and of how this communication can infringe on developmental processes. A range of examples is used to illustrate the diversity of mechanisms potentially implicated in cell-layer communication during plant development.

The PIN and LAX families of auxin transport genes in Medicago truncatula

Auxin transport proteins may be involved in nodule development. As a prelude to investigating the roles of these proteins in nodule development, we took advantage of the genetic and molecular resources available in the legume Medicago truncatula to characterize the gene families encoding auxin efflux and influx carriers. We identified ten auxin efflux carrier sequences (MtPINs) and five auxin influx/permease sequences (MtLAXs). The genomic sequence of each of these fifteen genes was determined, the genes were mapped on the publicly available map of M. truncatula, and their expression was examined in shoot and root tissue of nodulating plants. With one exception, transcripts of all MtPIN genes were detected. The expression of MtPIN2 was limited to nodulating roots, while transcripts of all other expressed genes were detected in both shoots and roots. Both the PIN and LAX gene families contain more members in M. truncatula than in Arabidopsis, but the gene families are not significantly expanded. Sequence comparison of the M. truncatula PIN and LAX genes with PIN and LAX genes from other dicots and monocots indicates that both gene families share a common overall structure, with areas of high homology both within M. truncatula and across species boundaries. Molecular phylogenies of both the PIN and LAX gene families were constructed. Combined with intron position and expression data, the phylogenies were used to assign relationships between MtPIN and MtLAX genes and the orthologous Arabidopsis PIN and LAX genes. MtPIN2 and MtPIN7 appear to be the result of a recent gene duplication with subsequent divergence of expression patterns. These results set the stage for the use of these genes in research on the role of auxin in nodulation.

PIN-FORMED1 and PINOID regulate boundary formation and cotyledon development in Arabidopsis embryogenesis

In dicotyledonous plants, two cotyledons are formed at bilaterally symmetric positions in the apical region of the embryo. Single mutations in the PIN-FORMED1 (PIN1) and PINOID (PID) genes, which mediate auxin-dependent organ formation, moderately disrupt the symmetric patterning of cotyledons. We report that the pin1 pid double mutant displays a striking phenotype that completely lacks cotyledons and bilateral symmetry. In the double mutant embryo, the expression domains of CUP-SHAPED COTYLEDON1 (CUC1), CUC2 and SHOOT MERISTEMLESS (STM), the functions of which are normally required to repress growth at cotyledon boundaries, expand to the periphery and overlap with a cotyledon-specific marker, FILAMENTOUS FLOWER. Elimination of CUC1, CUC2 or STM activity leads to recovery of cotyledon growth in the double mutant, suggesting that the negative regulation of these boundary genes by PIN1 and PID is sufficient for primordium growth. We also show that PID mRNA is localized mainly to the boundaries of cotyledon primordia and early expression of PID mRNA is dependent on PIN1. Our results demonstrate the redundant roles of PIN1 and PID in the establishment of bilateral symmetry, as well as in the promotion of cotyledon outgrowth, the latter of which involves the negative regulation of CUC1, CUC2 and STM genes, which are boundary-specific downstream effectors.

Arabidopsis Yellow Stripe-Like2 (YSL2): a metal-regulated gene encoding a plasma membrane transporter of nicotianamine-metal complexes

The Yellow Stripe-Like (YSL) family of proteins has been identified based on sequence similarity to maize Yellow Stripe1 (YS1), the transporter responsible for the primary uptake of iron from the soil. YS1 transports iron that is complexed by specific plant-derived Fe(III) chelators called phytosiderophores (PS). Non-grass species of plants neither make nor use PS, yet YSL family members are found in non-grass species (monocot, dicot, gymnosperm, and moss species) including Arabidopsis thaliana. YSLs in non-grasses have been hypothesized to transport metals complexed by nicotianamine (NA), an iron chelator that is structurally similar to PS and which is found in all higher plants. Here we show that Arabidopsis YSL2 (At5g24380) transports iron and copper when these metals are chelated by NA. YSL2 is expressed in many cell types in both roots and shoots, suggesting that diverse cell types obtain metals as metal-NA complexes. YSL2 transcription is regulated by the levels of iron and copper in the growth medium. Based on its expression pattern, a major function of the YSL2 appears to be in the lateral movement of metals in the vasculature.

Variation in expression and protein localization of the PIN family of auxin efflux facilitator proteins in flavonoid mutants with altered auxin transport in Arabidopsis thaliana

Aglycone flavonols are thought to modulate auxin transport in Arabidopsis thaliana via an as yet undefined mechanism. Biochemical studies suggest that flavonoids interact with regulatory proteins rather than directly with the PIN auxin efflux facilitator proteins. Auxin transport is enhanced in the absence of flavonoids (transparent testa4 [tt4]) and reduced in the presence of excess flavonols (tt7 and tt3). Steady state PIN mRNA levels in roots inversely correlate with auxin movement in tt mutants. PIN gene transcription and protein localization in flavonoid-deficient mutants appear to be modulated by developmental cues and are auxin responsive. Modulation of PIN gene expression and protein distribution by localized auxin accumulations occurs in the wild type as well. Flavonoids inhibit auxin transport primarily at the shoot apex and root tip and appear to modulate vesicular cycling of PIN1 at the root tip. In some auxin-accumulating tissues, flavonoid increases and changes in flavonoid speciation are subsequent to auxin accumulation.

Plant stem cells: divergent pathways and common themes in shoots and roots

Stem cells in plant shoot and root meristems are maintained throughout the life of the plant and produce somatic daughter cells that make up the body of the plant. Plant stem cells can also be derived from somatic cells in vivo and in vitro. Recent findings are refining our knowledge of signaling pathways that define stem cell fate and specify either shoot or root stem cell function. New evidence also highlights a role for epigenetic mechanisms in controlling stem cell fate.

An auxin-responsive SCARECROW-like transcriptional activator interacts with histone deacetylase

Members of the plant-specific GRAS family of putative transcription factors are involved in various aspects of plant development. SCARECROW (SCR) is a member of this protein family and plays a significant role in the radial patterning of both roots and shoots. However, little is known about the regulation of SCR expression and its mode of action in plants. Here, we report on the isolation and characterization of a Brassica napus SCARECROW-like protein, BnSCL1, isolated by selecting for proteins that interact with the Arabidopsis histone deacetylase AtHDA19 in a yeast two-hybrid screen. BnSCL1 contains domains conserved in the GRAS family of proteins, interacts with AtHDA19 through a VHIID domain, and exerts transcription activation of reporter genes . BnSCL1 is expressed predominantly in the roots, where its expression is regulated by auxin, as it also is in shoots and mature leaves. These results indicate that BnSCL1 is a member of the GRAS family, and suggest that its mode of action in plant auxin response may involve interaction with HDA19.

Plant hormones and homeoboxes: bridging the gap?

Plant hormones are signalling molecules that control growth and development. Growth of the aerial parts of higher plants requires the continuous activity of the shoot apical meristem, a small mound of cells at the apex of a plant. KNOTTED1-like HOMEOBOX (KNOX) genes are involved in regulating meristem activity, however, little is known about how this regulation is mediated. Recent evidence suggests that KNOX transcription factors may control meristem development by regulating the balance of activities of multiple hormones.

The HALTED ROOT gene encoding the 26S proteasome subunit RPT2a is essential for the maintenance of Arabidopsis meristems

In higher plants, post-embryonic development is dependent on the activity of the root and shoot apical meristem (RAM and SAM). The quiescent center (QC) in the RAM and the organizing center (OC) in the SAM are known to be essential for the maintenance of meristematic activity. To understand the mechanism that maintains post-embryonic meristems, we isolated an Arabidopsis mutant, halted root (hlr). In this mutant, the cellular organization was disrupted in post-embryonic meristems both in the root and in the shoot, and their meristematic activity was reduced or became abnormal. We showed that the mutant RAM lost its QC identity after germination, which was specified during embryogenesis, whereas the identity of differentiated tissues was maintained. In the post-embryonic SAM, the expression pattern of a typical OC marker gene, WUSCHEL, was disturbed in the mutant. These observations indicate that the HLR gene is essential to maintain the cellular organization and normal nature of the RAM and SAM. The HLR gene encodes RPT2a, which is a subunit of the 26S proteasome that degrades key proteins in diverse cellular processes. We showed that the HLR gene was expressed both in the RAM and in the SAM, including in the QC and the OC, respectively, and that the activity of proteasomes were reduced in the mutant. We propose that proteasome-dependent programmed proteolysis is required to maintain the meristem integrity both in the shoot and in the root.

Traffic jams affect plant development and signal transduction

Analysis of the Arabidopsis thaliana endomembrane system has shown that plant cell viability depends on a properly functioning vacuole and intact vesicular trafficking. The endomembrane system is also essential for various aspects of plant development and signal transduction. In this review, we discuss examples of these newly discovered roles for the endomembrane system in plants, and new experimental approaches and technologies that are based on high-throughput screens, which combine chemical genetics and automated confocal microscopy.

Determination of cell fate in apical meristems

Although roots and shoots exhibit profound differences in their pattern of organogenesis, both apices share the capacity for indeterminate growth. Ongoing molecular and genetic analyses have revealed relatively little overlap between the genes that regulate organogenesis in the root and shoot apices. In the shoot, an ensemble of transcription factors lays the foundations for the leaf, in which indeterminacy is exchanged for more limited and polarized growth. Class-I KNOX genes are downregulated in the anlagen of the leaf early in its establishment, but are maintained in other regions of the shoot apex. This persistent expression of KNOX genes may serve to prevent the precocious determination of apical initial derivatives, and thus may allow the production of a large number of pluripotent cells from a relatively small number of stem cells. Greater commonality between roots and shoots is seen in mechanisms that underlie histogenesis and radial-patterning processes. Recent work suggests that undetermined stem cells in both the root and the shoot may be maintained by related mechanisms, which feature regulation of WUSCHEL-like organizer activities by feedback mechanisms that involve receptor-like kinases.

Expression dynamics of WOX genes mark cell fate decisions during early embryonic patterning in Arabidopsis thaliana

During embryonic pattern formation, the main body axes are established and cells of different developmental fates are specified from a single-cell zygote. Despite the fundamental importance of this process, in plants, the underlying mechanisms are largely unknown. We show that expression dynamics of novel WOX (WUSCHEL related homeobox) gene family members reveal early embryonic patterning events in Arabidopsis. WOX2 and WOX8 are co-expressed in the egg cell and zygote and become confined to the apical and basal daughter cells of the zygote, respectively, by its asymmetric division. WOX2 not only marks apical descendants of the zygote, but is also functionally required for their correct development, suggesting that the asymmetric division of the plant zygote separates determinants of apical and basal cell fates. WOX9 expression is initiated in the basal daughter cell of the zygote and subsequently shifts into the descendants of the apical daughter apparently in response to signaling from the embryo proper. Expression of WOX5 shows that identity of the quiescent center is initiated very early in the hypophyseal cell, and highlights molecular and developmental similarities between the stem cell niches of root and shoot meristems. Together, our data suggest that during plant embryogenesis region-specific transcription programs are initiated very early in single precursor cells and that WOX genes play an important role in this process.

A MAPKK Kinase Gene Regulates Extra-Embryonic Cell Fate in Arabidopsis

The Arabidopsis zygote divides asymmetrically into an embryonic apical cell and a basal cell with mostly extra-embryonic fate. This fundamental asymmetry sets the stage for further embryonic development, but the events mediating it are poorly understood. We have identified a MAPKK kinase gene, named YODA, that promotes extra-embryonic cell fates in the basal lineage. In loss-of-function mutants, the zygote does not elongate properly, and the cells of the basal lineage are eventually incorporated into the embryo instead of differentiating the extra-embryonic suspensor. Gain-of-function alleles cause exaggerated growth of the suspensor and can suppress embryonic development to a degree where no recognizable proembryo is formed. Our results imply that a MAP kinase cascade acts as a molecular switch promoting extra-embryonic fate.

Ups and downs of tissue and planar polarity in plants

The polar orientation of cells within a tissue is an intensively studied research area in animal cells. The term planar polarity refers to the common polar arrangement of cells within the plane of an epithelium. In plants, the subcellular analysis of tissue polarity has been limited by the lack of appropriate markers. Recently, research on plant tissue polarity has come of age. Advances are based on studies of Arabidopsis patterning, cell polarity and auxin transport mutants employing the coordinated, polar localization of auxin transporters and the planar polarity of root epidermal hairs as markers. These approaches have revealed auxin transport and response, vesicular trafficking, membrane sterol and cytoskeletal requirements of tissue polarity. This review summarizes recent progress in research on vascular tissue and planar epidermal polarity in the Arabidopsis root and compares it to findings on planar polarity in animals and cell polarity in yeast.

Partial loss-of-function alleles reveal a role for GNOM in auxin transport-related, post-embryonic development of Arabidopsis

The Arabidopsis GNOM gene encodes an ARF GDP/GTP exchange factor involved in embryonic axis formation and polar localisation of the auxin efflux regulator PIN1. To examine whether GNOM also plays a role in post-embryonic development and to clarify its involvement in auxin transport, we have characterised newly isolated weak gnom alleles as well as trans-heterozygotes of complementing strong alleles. These genotypes form a phenotypic series of GNOM activity in post-embryonic development, with auxin-related defects, especially in the maintenance of primary root meristem activity and in the initiation and organisation of lateral root primordia. Our results suggest a model for GNOM action mediating auxin transport in both embryogenesis and post-embryonic organ development.

Auxin transport synchronizes the pattern of cell division in a tobacco cell line

The open morphogenesis of plants requires coordination of patterning by intercellular signals. The tobacco (Nicotiana tabacum cv Virginia Bright Italia) cell line VBI-0 provides a simple model system to study the role of intercellular communication in patterning. In this cell line, singular cells divide axially to produce linear cell files of distinct polarity. The trigger for this axial division is exogenous auxin. When frequency distributions of files are constructed over the number of cells per file during the exponential phase of the culture, even numbers are found to be frequent, whereas files consisting of uneven numbers of cells are rare. We can simulate these distributions with a mathematical model derived from nonlinear dynamics, which describes a chain of cell-division oscillators where elementary oscillators are coupled unidirectionally and where the number of oscillators is not conserved. The model predicts several nonintuitive properties of our experimental system. For instance, files consisting of six cells are more frequent than expected from a strictly binary division system. More centrally, the model predicts a polar transport of the coordinating signal. We therefore tested the patterns obtained after treatment with 1-N-naphthylphthalamic acid, an inhibitor of auxin efflux carriers. Using low concentrations of 1-N-naphthylphthalamic acid that leave cell division and axiality of division unaltered, we observe that the frequencies of files with even and uneven cell numbers are equalized. Our findings are discussed in the context of auxin transport as synchronizing signal in cell patterning.

Regulation of phyllotaxis by polar auxin transport

The regular arrangement of leaves around a plant's stem, called phyllotaxis, has for centuries attracted the attention of philosophers, mathematicians and natural scientists; however, to date, studies of phyllotaxis have been largely theoretical. Leaves and flowers are formed from the shoot apical meristem, triggered by the plant hormone auxin. Auxin is transported through plant tissues by specific cellular influx and efflux carrier proteins. Here we show that proteins involved in auxin transport regulate phyllotaxis. Our data indicate that auxin is transported upwards into the meristem through the epidermis and the outermost meristem cell layer. Existing leaf primordia act as sinks, redistributing auxin and creating its heterogeneous distribution in the meristem. Auxin accumulation occurs only at certain minimal distances from existing primordia, defining the position of future primordia. This model for phyllotaxis accounts for its reiterative nature, as well as its regularity and stability.

The case for morphogens in plants

Plants and animals have evolved as multicellular organisms independently of one another. This raises the intriguing question of whether plants and animals have developed similar or distinct patterning strategies to establish their body plans. Animals use concentration gradients of signals termed morphogens for tissue patterning, but whether they are also used by plants is unclear. Here we compare and contrast the plant hormone auxin with animal morphogens, and speculate as to whether plants have independently evolved similar mechanisms to regulate pattern formation.

Efflux-dependent auxin gradients establish the apical–basal axis of Arabidopsis

Axis formation occurs in plants, as in animals, during early embryogenesis. However, the underlying mechanism is not known. Here we show that the first manifestation of the apical-basal axis in plants, the asymmetric division of the zygote, produces a basal cell that transports and an apical cell that responds to the signalling molecule auxin. This apical-basal auxin activity gradient triggers the specification of apical embryo structures and is actively maintained by a novel component of auxin efflux, PIN7, which is located apically in the basal cell. Later, the developmentally regulated reversal of PIN7 and onset of PIN1 polar localization reorganize the auxin gradient for specification of the basal root pole. An analysis of pin quadruple mutants identifies PIN-dependent transport as an essential part of the mechanism for embryo axis formation. Our results indicate how the establishment of cell polarity, polar auxin efflux and local auxin response result in apical-basal axis formation of the embryo, and thus determine the axiality of the adult plant.

Local, Efflux-Dependent Auxin Gradients as a Common Module for Plant Organ Formation

Plants, compared to animals, exhibit an amazing adaptability and plasticity in their development. This is largely dependent on the ability of plants to form new organs, such as lateral roots, leaves, and flowers during postembryonic development. Organ primordia develop from founder cell populations into organs by coordinated cell division and differentiation. Here, we show that organ formation in Arabidopsis involves dynamic gradients of the signaling molecule auxin with maxima at the primordia tips. These gradients are mediated by cellular efflux requiring asymmetrically localized PIN proteins, which represent a functionally redundant network for auxin distribution in both aerial and underground organs. PIN1 polar localization undergoes a dynamic rearrangement, which correlates with establishment of auxin gradients and primordium development. Our results suggest that PIN-dependent, local auxin gradients represent a common module for formation of all plant organs, regardless of their mature morphology or developmental origin.

The FORKED genes are essential for distal vein meeting in Arabidopsis

As in most dicotyledonous plants, the leaves and cotyledons of Arabidopsis have a closed, reticulate venation pattern. This pattern is proposed to be generated through canalization of the hormone auxin. We have identified two genes, FORKED 1 (FKD1) and FORKED 2 (FKD2), that are necessary for the closed venation pattern: mutations in either gene result in an open venation pattern that lacks distal meeting. In fkd1 leaves and cotyledons, the defect is first evident in the provascular tissue, such that the distal end of the newly forming vein does not connect to the previously formed, more distal vein. Plants doubly mutant for both genes have widespread defects in leaf venation, suggesting that the genes function in an overlapping manner at the distal junctions, but act redundantly throughout leaf veins. Expression of an auxin responsive reporter gene is reduced in fkd1 leaves, suggesting that FKD1 is necessary for the auxin response that directs vascular tissue development. The reduction in reporter gene expression and the fkd1 phenotype are relieved in the presence of auxin transport inhibition. The restoration of vein junctions in situations where auxin concentrations are increased indicates that distal vein junctions are sites of low auxin concentration and are particularly sensitive to reduced FKD1 and FKD2 activity.

The polycotyledon mutant of tomato shows enhanced polar auxin transport

The polycotyledon mutant of tomato (Lycopersicon esculentum L. cv Ailsa Craig) showed altered development during embryogenesis and during vegetative and reproductive phases. The phenotype was pleiotropic and included the formation of extra cotyledons, changes in leaf shape, increased number of flowers (indeterminacy) with abnormal floral organs, the formation of epiphyllous structures, and altered gravitropism. The earliest defects were observed at the transition from the globular to the heart stage of embryogenesis with the formation of multiple cotyledons. Epidermal cells in the mutant embryo were smaller and less expanded compared with wild type. Examination of polar auxin transport (PAT) showed a striking enhancement in the case of the mutant. Increase in PAT did not appear to be caused by a decrease in flavonoids because the mutant had normal flavonoid levels. Application of 2,3,5-triiodobenzoic acid, an inhibitor of polar transport of auxin, rescued postgermination phenotypes of young seedlings. Our analysis reveals a level of control that negatively regulates PAT in tomato and its contribution to plant development and organogenesis.

Polar auxin transport in the wood-forming tissues of hybrid aspen is under simultaneous control of developmental and environmental signals

Recent research has highlighted the importance of auxin concentration gradients during plant development. Establishment of these gradients is believed to involve polar auxin transport through specialized carrier proteins. We have used an experimental system, the wood-forming tissue of hybrid aspen, which allows tissue-specific expression analysis of auxin carrier genes and quantification of endogenous concentrations of the hormone. As part of this study, we isolated the putative polar auxin transport genes, PttLAX1-PttLAX3 and PttPIN1-PttPIN3, belonging to the AUX1-like family of influx and PIN1-like efflux carriers, respectively. Analysis of PttLAX and PttPIN expression suggests that specific positions in a concentration gradient of the hormone are associated with different stages of vascular cambium development and expression of specific members of the auxin transport gene families. We were also able demonstrate positive feedback of auxin on polar auxin transport genes. Entry into dormancy at the end of a growing season leads to a loss of auxin transport capacity, paralleled by reduced expression of PttLAX and PttPIN genes. Furthermore, data from field experiments show that production of the molecular components of the auxin transport machinery is governed by environmental controls. Our findings collectively demonstrate that trees have developed mechanisms to modulate auxin transport in the vascular meristem in response to developmental and environmental cues.

[Graviresponse in higher plants and its regulation in molecular bases: relevance to growth and development, and auxin polar transport in etiolated pea seedlings]

We review the graviresponse under true and simulated microgravity conditions on a clinostat in higher plants, and its regulation in molecular bases, especially on the aspect of auxin polar transport in etiolated pea (Pisum sativum L. cv. Alaska) seedlings which were the plant materials subjected to STS-95 space experiments. True and simulated microgravity conditions substantially affected growth and development in etiolated pea seedlings, especially the direction of growth of stems and roots, resulting in automorphosis. In etiolated pea seedlings grown in space, epicotyls were the most oriented toward the direction far from the cotyledons, and roots grew toward the aerial space of Plant Growth Chamber. Automorphosis observed in space were well simulated by a clinorotation on a 3-dimensional clinostat and also phenocopied by the application of auxin polar transport inhibitors of 2,3,5-triiodobenzoic acid, N-(1-naphtyl)phthalamic acid and 9-hydroxyfluorene-9-carboxylic acid. Judging from the results described above together with the fact that activities of auxin polar transport in epicotyls of etiolated pea seedlings grown in space substantially were reduced, auxin polar transport seems to be closely related to automorphosis. Strenuous efforts to learn in molecular levels how gravity contributes to the auxin polar transport in etiolated pea epicotyls resulted in successful identification of PsPIN2 and PsAUX1 genes located in plasma membrane which products are considered to be putative efflux and influx carriers of auxin, respectively. Based on the results of expression of PsPIN2 and PsAUX1 genes under various gravistimulations, a possible role of PsPIN2 and PsAUX1 genes for auxin polar transport in etiolated pea seedlings will be discussed.

Automated whole mount localisation techniques for plant seedlings

Plant biology is currently experiencing a growing demand for easy and reliable mRNA and protein localisation techniques. Here, we present novel whole mount in situ hybridisation and immunolocalisation protocols, suitable to localise mRNAs and proteins in Arabidopsis seedlings. We demonstrate that these methods can be used in different organs of Arabidopsis seedlings as well as in other plant species. In order to achieve better reproducibility and higher throughput, we modified these protocols for automation to be performed by a liquid handling robot. In addition, we show that other procedures such as reporter enzyme assays and tissue clearing can be similarly automated. We present examples of application of our protocols including mRNA localisation and proteins and epitope tag (co)localisations which demonstrate that these methods provide reliable and versatile tools for expression, localisation and anatomical studies in plants.

Quiescent center formation in maize roots is associated with an auxin-regulated oxidizing environment

Embedded within the meristem of all Angiosperm roots is a population of slowly dividing cells designated the quiescent center (QC). In maize roots the QC can constitute upwards of 800-1200 cells, most of which spend an extended period of time (180-200 hours) in the G(1) phase of the cell cycle. How the QC forms and is maintained is not known. Here we report that cells of the QC are characterized by their highly oxidized status. Glutathione and ascorbic acid occur predominately in the oxidized forms in the QC. This is contrasted with the status of these redox intermediates in adjacent, rapidly dividing cells in the root meristem, in which the reduced forms of these two species are favored. Using a redox sensitive fluorescent dye we were able to visualize an overall oxidizing environment in the QC, and we also made comparisons with the adjacent, rapidly dividing cells in the root meristem. Altering the distribution of auxin and the location of the auxin maximum in the root tip activates the QC, and cells leave G(1) and enter mitosis. Commencement of relatively more rapid cell division in the QC is preceded by changes in the overall redox status of the QC, which becomes less oxidizing. We discuss how the position of the auxin maximum may influence the redox status of the QC and thereby modulate the cell cycle.

Gravity-regulated differential auxin transport from columella to lateral root cap cells

Gravity-induced root curvature has long been considered to be regulated by differential distribution of the plant hormone auxin. However, the cells establishing these gradients, and the transport mechanisms involved, remain to be identified. Here, we describe a GFP-based auxin biosensor to monitor auxin during Arabidopsis root gravitropism at cellular resolution. We identify elevated auxin levels at the root apex in columella cells, the site of gravity perception, and an asymmetric auxin flux from these cells to the lateral root cap (LRC) and toward the elongation zone after gravistimulation. We differentiate between an efflux-dependent lateral auxin transport from columella to LRC cells, and an efflux- and influx-dependent basipetal transport from the LRC to the elongation zone. We further demonstrate that endogenous gravitropic auxin gradients develop even in the presence of an exogenous source of auxin. Live-cell auxin imaging provides unprecedented insights into gravity-regulated auxin flux at cellular resolution, and strongly suggests that this flux is a prerequisite for root gravitropism.

The Arabidopsis mutant alh1 illustrates a cross talk between ethylene and auxin

Ethylene or its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) can stimulate hypocotyl elongation in light-grown Arabidopsis seedlings. A mutant, designated ACC-related long hypocotyl 1 (alh1), that displayed a long hypocotyl in the light in the absence of the hormone was characterized. Etiolated alh1 seedlings overproduced ethylene and had an exaggerated apical hook and a thicker hypocotyl, although no difference in hypocotyl length was observed when compared with wild type. Alh1 plants were less sensitive to ethylene, as reflected by reduction of ACC-mediated inhibition of hypocotyl growth in the dark and delay in flowering and leaf senescence. Alh1 also had an altered response to auxin, whereas auxin levels in whole alh1 seedlings remained unaffected. In contrast to wild type, alh1 seedlings showed a limited hypocotyl elongation when treated with indole-3-acetic acid. Alh1 roots had a faster response to gravity. Furthermore, the hypocotyl elongation of alh1 and of ACC-treated wild type was reverted by auxin transport inhibitors. In addition, auxin up-regulated genes were ectopically expressed in hypocotyls upon ACC treatment, suggesting that the ethylene response is mediated by auxins. Together, these data indicate that alh1 is altered in the cross talk between ethylene and auxins, probably at the level of auxin transport.

Auxin signaling in Arabidopsis leaf vascular development

A number of observations have implicated auxin in the formation of vascular tissues in plant organs. These include vascular strand formation in response to local auxin application, the effects of impaired auxin transport on vascular patterns and suggestive phenotypes of Arabidopsis auxin response mutants. In this study, we have used molecular markers to visualize auxin response patterns in developing Arabidopsis leaves as well as Arabidopsis mutants and transgenic plants to trace pathways of auxin signal transduction controlling the expression of early procambial genes. We show that in young Arabidopsis leaf primordia, molecular auxin response patterns presage sites of procambial differentiation. This is the case not only in normal development but also upon experimental manipulation of auxin transport suggesting that local auxin signals are instrumental in patterning Arabidopsis leaf vasculature. We further found that the activity of the Arabidopsis gene MONOPTEROS, which is required for proper vascular differentiation, is also essential in a spectrum of auxin responses, which include the regulation of rapidly auxin-inducible AUX/IAA genes, and discovered the tissue-specific vascular expression profile of the class I homeodomain-leucine zipper gene, AtHB20. Interestingly, MONOPTEROS activity is a limiting factor in the expression of AtHB8 and AtHB20, two genes encoding transcriptional regulators expressed early in procambial development. Our observations connect general auxin signaling with early controls of vascular differentiation and suggest molecular mechanisms for auxin signaling in patterned cell differentiation.

Simple hormones but complex signalling

It has not been easy to make sense of the pleiotropic effects of plant hormones, especially of auxins; but now, it has become possible to study these effects within the framework of what we know about signal transduction in general. Changes in local auxin concentrations, perhaps even actively maintained auxin gradients, signal to networks of transcription factors, which in turn signal to downstream effectors. Transcription factors can also signal back to hormone biosynthetic pathways.

The beta-subunit of the Arabidopsis G protein negatively regulates auxin-induced cell division and affects multiple developmental processes

Plant cells respond to low concentrations of auxin by cell expansion, and at a slightly higher concentration, these cells divide. Previous work revealed that null mutants of the alpha-subunit of a putative heterotrimeric G protein (GPA1) have reduced cell division. Here, we show that this prototypical G protein complex acts mechanistically by controlling auxin sensitivity toward cell division. Loss-of-function G protein mutants have altered auxin-mediated cell division throughout development, especially during the auxin-induced formation of lateral and adventitious root primordia. Ectopic expression of the wild-type Galpha-subunit phenocopies the Gbeta mutants (auxin hypersensitivity), probably by sequestering the Gbetagamma-subunits, whereas overexpression of Gbeta reduces auxin sensitivity and a constitutively active (Q222L) mutant Galpha behaves like the wild type. These data are consistent with a model in which Gbetagamma acts as a negative regulator of auxin-induced cell division. Accordingly, basal repression of approximately one-third of the identified auxin-regulated genes (47 of 150 upregulated genes among 8300 quantitated) is lost in the Gbeta transcript-null mutant. Included among these are genes that encode proteins proposed to control cell division in root primordia formation as well as several novel genes. These results suggest that although auxin-regulated cell division is not coupled directly by a G protein, the Gbeta-subunit attenuates this auxin pathway upstream of the control of mRNA steady state levels.

A no hydrotropic response root mutant that responds positively to gravitropism in Arabidopsis

For most plants survival depends upon the capacity of root tips to sense and move towards water and other nutrients in the soil. Because land plants cannot escape environmental stress they use developmental solutions to remodel themselves in order to better adapt to the new conditions. The primary site for perception of underground signals is the root cap (RC). Plant roots have positive hydrotropic response and modify their growth direction in search of water. Using a screening system with a water potential gradient, we isolated a no hydrotropic response (nhr) semi-dominant mutant of Arabidopsis that continued to grow downwardly into the medium with the lowest water potential contrary to the positive hydrotropic and negative gravitropic response seen in wild type-roots. The lack of hydrotropic response of nhr1 roots was confirmed in a system with a gradient in air moisture. The root gravitropic response of nhr1 seedlings was significantly faster in comparison with those of wild type. The frequency of the waving pattern in nhr1 roots was increased compared to those of wild type. nhr1 seedlings had abnormal root cap morphogenesis and reduced root growth sensitivity to abscisic acid (ABA) and the polar auxin transport inhibitor N-(1-naphtyl)phtalamic acid (NPA). These results showed that hydrotropism is amenable to genetic analysis and that an ABA signaling pathway participates in sensing water potential gradients through the root cap.

Mutations in a gene for plastid ribosomal protein S6-like protein reveal a novel developmental process required for the correct organization of lateral root meristem in Arabidopsis

Arabidopsis rfc3 mutants have previously been isolated with an altered fatty acid composition of membrane lipids. In this study, rfc3 was found to have a sucrose-conditional defect in the patterning of distal elements in the lateral root meristem. By utilizing this feature, a sucrose-sensitive process important for lateral root development was localized to the growing portion of rfc3 primary root. Because lateral root formation occurs at a later stage, this finding suggests the existence of an RFC3-dependent, non-primordium autonomous signal playing a role in the organization of lateral root meristem. Map-based cloning of RFC3 gene revealed that it encodes a plastid-localized ribosomal protein S6-like protein and provides a potential link between control of plastid gene expression and LR development.