The Arabidopsis BODENLOS gene encodes an auxin response protein inhibiting MONOPTEROS-mediated embryo patterning

Regulated proteolysis and plant development

Eukaryotes use the ubiquitin-proteasome system to control the abundance of regulatory proteins such as cell-cycle proteins and transcription factors. Over 5% of the Arabidopsis genome encodes for proteins with an apparent functional homology to components of the ubiquitin-proteasome system. This suggests that ubiquitin-mediated proteolysis has a major role in plant growth and development. Consistent with this notion, various processes, including most phytohormone responses and photomorphogenesis, have already been shown to require protein degradation in one way or another. In this review, we provide an overview of the plant ubiquitin-proteasome system and its role during Arabidopsis development. Since we consider auxin response and photomorphogenesis as particularly instructive examples, these processes are reviewed in greater detail.

Pattern formation in the vascular system of monocot and dicot plant species

Plant vascular tissues are organised in continuous strands, the longitudinal and radial patterns of which are intimately linked to the signals that direct plant architecture as a whole. Therefore, understanding the mechanisms underlying vascular tissue patterning is expected to shed light on patterning events beyond those that organise the vascular system, and thus represents a central issue in plant developmental biology. A number of recent advances, reviewed here, are leading to a more precise definition of the signals that control the formation of vascular tissues and their integration into a larger organismal context. Contents Summary 209 I. Introduction 209 II. The plant vascular system 210 III. Ontogeny of the vascular tissues 210 IV. Procambium development 210 V. The organisation of the vascular tissues 212 VI. The regulation of longitudinal vascular pattern formation 214 VII. The regulation of radial vascular pattern formation 220 VIII. Genetic screens for vascular development mutants 231 IX. Genes involved in vascular development identified through reverse genetics approaches 235 X. Conclusions and perspectives 235 Note added at the revision stage 236 Acknowledgements 236 References 236.

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.

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.

Growing up green: cellular basis of plant development

Plant development is a biphasic process. Pattern formation during embryogenesis generates a basic body organisation, including self-maintaining stem-cell systems called meristems at opposite ends of the main axis of polarity. During post-embryonic development, the meristems produce new organs with reference to the existing body, transforming the juvenile seedling into the species-specific adult plant. Studies in Arabidopsis indicate that patterning in plants involves not only cell surface interactions but also unique modes of communication such as movement of transcription factors between cells and directional transport of the signaling molecule auxin.

Stage-specific markers define early steps of procambium development inArabidopsisleaves and correlate termination of vein formation with mesophyll differentiation

During leaf development, ground meristem cells along continuous lines undergo coordinated oriented cell divisions and differentiate to form procambial cells, the precursors of all vascular cells. The molecular genetic dissection of early procambial development suffers from the lack of easily identifiable markers, especially of cell states preceding procambium formation. In this study, we have identified and characterized three reporter gene expression markers that reflect three distinct preprocambial stages, as well as one marker whose expression seems to be perfectly congruent with the appearance of procambial cells. All four markers are invariably expressed in continuous domains connected to pre-existing vasculature and their expression profiles reveal a common spatiotemporal pattern of early vein formation. We observed progressive extension of vascular strands at the preprocambial stage,suggesting that veins are initiated as freely ending preprocambial domains and that network formation occurs through subsequent fusion of these domains. Consistent with this interpretation, we demonstrate that veins are generally not programmed to become freely ending or interconnected network elements. Instead, we found that the progressive extension of preprocambial domains can be interrupted experimentally and that this leads to less complex vein patterns consisting of fewer vein orders, in which even lower-order veins become freely ending. Mesophyll differentiation turned out to be strictly correlated with the termination of preprocambial domain extension. These findings suggest that Arabidopsis vein pattern is not inherently determinate, but arises through reiterative initiation of new preprocambial branches until this process becomes terminated by the differentiation of mesophyll.

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.

Cotyledon vascular pattern2-mediated inositol (1,4,5) triphosphate signal transduction is essential for closed venation patterns of Arabidopsis foliar organs

Vein patterns in leaves and cotyledons form in a spatially regulated manner through the progressive recruitment of ground cells into vascular cell fate. To gain insight into venation patterning mechanisms, we have characterized the cotyledon vascular pattern2 (cvp2) mutants, which exhibit an increase in free vein endings and a resulting open vein network. We cloned CVP2 by a map-based cloning strategy and found that it encodes an inositol polyphosphate 5' phosphatase (5PTase). 5PTases regulate inositol (1,4,5) triphosphate (IP(3)) signal transduction by hydrolyzing IP(3) and thus terminate IP(3) signaling. CVP2 gene expression is initially broad and then gradually restricted to incipient vascular cells in several developing organs. Consistent with the inferred enzymatic activity of CVP2, IP(3) levels are elevated in cvp2 mutants. In addition, cvp2 mutants exhibit hypersensitivity to the plant hormone abscisic acid. We propose that elevated IP(3) levels in cvp2 mutants reduce ground cell recruitment into vascular cell fate, resulting in premature vein termination and, thus, in an open reticulum.

Overlapping and non-redundant functions of the Arabidopsis auxin response factors MONOPTEROS and NONPHOTOTROPIC HYPOCOTYL 4

Transcription factors of the auxin response factor (ARF) family have been implicated in auxin-dependent gene regulation, but little is known about the functions of individual ARFs in plants. Here, interaction assays, expression studies and combinations of multiple loss- and gain-of-function mutants were used to assess the roles of two ARFs, NONPHOTOTROPIC HYPOCOTYL 4 (NPH4/ARF7) and MONOPTEROS (MP/ARF5), in Arabidopsis development. Both MP and NPH4 interact strongly and selectively with themselves and with each other, and are expressed in vastly overlapping domains. We show that the regulatory properties of both genes are far more related than suggested by their single mutant phenotypes. NPH4 and MP are capable of controlling both axis formation in the embryo and auxin-dependent cell expansion. Interaction of MP and NPH4 in Arabidopsis plants is indicated by their joint requirement in a number of auxin responses and by synergistic effects associated with the co-overexpression of both genes. Finally, we demonstrate antagonistic interaction between ARF and Aux/IAA gene functions in Arabidopsis development. Overexpression of MP suppresses numerous defects associated with a gain-of-function mutation in BODENLOS (BDL)/IAA12. Together these results provide evidence for the biological relevance of ARF-ARF and ARF-Aux/IAA interaction in Arabidopsis plants and demonstrate that an individual ARF can act in both invariantly programmed pattern formation as well as in conditional responses to external signals.

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.

MASSUGU2 encodes Aux/IAA19, an auxin-regulated protein that functions together with the transcriptional activator NPH4/ARF7 to regulate differential growth responses of hypocotyl and formation of lateral roots in Arabidopsis thaliana

We have isolated a dominant, auxin-insensitive mutant of Arabidopsis thaliana, massugu2 (msg2), that displays neither hypocotyl gravitropism nor phototropism, fails to maintain an apical hook as an etiolated seedling, and is defective in lateral root formation. Yet other aspects of growth and development of msg2 plants are almost normal. These characteristics of msg2 are similar to those of another auxin-insensitive mutant, non-phototropic hypocotyl4 (nph4), which is a loss-of-function mutant of AUXIN RESPONSE FACTOR7 (ARF7) (Harper et al., 2000). Map-based cloning of the MSG2 locus reveals that all four mutant alleles result in amino acid substitutions in the conserved domain II of an Auxin/Indole-3-Acetic Acid protein, IAA19. Interestingly, auxin inducibility of MSG2/IAA19 gene expression is reduced by 65% in nph4/arf7. Moreover, MSG2/IAA19 protein binds to the C-terminal domain of NPH4/ARF7 in a Saccharomyces cerevisiae (yeast) two-hybrid assay and to the whole latter protein in vitro by pull-down assay. These results suggest that MSG2/IAA19 and NPH4/ARF7 may constitute a negative feedback loop to regulate differential growth responses of hypocotyls and lateral root formation.

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.

Roles for auxin during morphogenesis of the compound leaves of pea ( Pisum sativum)

The goal of this study was to explore the impact of the plant growth regulator auxin on the development of compound leaves in pea. Wildtype ( WT) plantlets, as well as those of two leaf mutants, acacia ( tl) and tendrilled acacia ( uni-tac) of pea ( Pisum sativum L.), were grown on media containing the auxin-transport inhibitors 2,3,5-triiodobenzoic acid (TIBA), N-(1-naphthyl)phthalamic acid (NPA), or the auxin antagonist, p-chlorophenoxyisobutyric acid (PCIB). The resulting plantlets were carefully analyzed morphologically, by scanning electron microscopy and for Uni gene expression using quantitative reverse transcription-polymerase chain reaction. Auxin transport was measured in WT leaf parts using [(14)C]indole-3-acetic acid. Relative Uni gene expression was determined in shoot tips of a range of leaf-form mutants. Morphological abnormalities were observed for all genotypes examined. The terminal tendrils on WT plants were converted to leaflets, stubs or were aborted. The number of pinna pairs produced on leaves was reduced, with the distal forms being eliminated before the proximal ones. Some leaves were converted to simple, including tri-and bilobed, forms. These treatments phenocopy the uni-tac and unifoliata ( uni) mutants of pea. In the most extreme situations, leaf blades were completely lost leaving only a pair of stipules or scale leaves. Polar auxin transport was basipetal for all leaf parts. Uni gene expression in shoot tips was significantly reduced in 60 microM NPA and TIBA. Uni mRNA was more abundant in tl, af and af tl and reduced in the uni mutants compared to WT. These results indicate that an auxin gradient plays fundamental roles in controlling morphogenesis in the compound leaves of pea and specifically it: (i). is the driving force for leaf growth and pinna determination; (ii). is necessary for pinna initiation; and (iii). controls subsequent pinna development.

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.

Diphtheria toxin-mediated cell ablation reveals interregional communication during Arabidopsis seed development

Fertilization of the female gametophyte in angiosperm plants initiates a process of coordinated development of embryo, endosperm, and seed coat that ensures the production of a viable seed. Mutant analysis has suggested that communication between the endosperm and the seed coat is an important determinant in this process. In addition, cell groups within the embryo, derived from the apical and from the basal cell, respectively, after zygote division, concertedly establish a functional root meristem, and cells in the apical region of the embryo are hypothesized to repress cell divisions in the basal cell-derived suspensor. The available evidence for these interregional communication events mostly relies on the analysis of mutant phenotypes in Arabidopsis. To provide independent and direct evidence for communication events, we used conditional domain-specific expression of the diphtheria toxin A chain (DTA) in developing Arabidopsis seeds. By using a collection of cell- or tissue-type-specific promoters, we show that the mGAL4:VP16/UAS two-component gene expression allows reliable spatiotemporal and conditional expression of the GFP:GUS reporter and the DTA gene in the developing embryo and endosperm. Expression of DTA in the protoderm of the embryo proper led to excessive proliferation of suspensor cells, sometimes resulting in the formation of secondary embryos. Endosperm-specific expression of DTA caused complete cessation of seed growth, followed by pattern defects in the embryo and embryo arrest. Taken together, the results presented here substantiate the evidence for and underline the importance of interregional communication in embryo and seed development and demonstrate the usefulness of conditional toxin expression as a method complementary to phenotypic analysis of developmental mutants.

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.

AXR3 and SHY2 interact to regulate root hair development

Signal transduction of the plant hormone auxin centres on the regulation of the abundance of members of the Aux/IAA family of transcriptional regulators, of which there are 29 in Arabidopsis. Auxin can influence Aux/IAA abundance by promoting the transcription of Aux/IAA genes and by reducing the half-life of Aux/IAA proteins. Stabilising mutations, which render Aux/IAA proteins resistant to auxin-mediated degradation, confer a wide range of phenotypes consistent with disruptions in auxin response. Interestingly, similar mutations in different family members can confer opposite phenotypic effects. To understand the molecular basis for this functional specificity in the Aux/IAA family, we have studied a pair of Aux/IAAs, which have contrasting roles in root hair development. We have found that stabilising mutations in AXR3/IAA17 blocks root hair initiation and elongation, whereas similar mutations in SHY2/IAA3 result in early initiation of root hair development and prolonged hair elongation, giving longer root hairs. The phenotypes resulting from double mutant combinations, the transient induction of expression of the proteins, and the pattern of transcription of the cognate genes suggest that root hair initiation is controlled by the relative abundance of SHY2 and AXR3 in a cell. These results suggest a general model for auxin signalling in which the modulation of the relative abundance of different Aux/IAA proteins can determine which down-stream responses are induced.

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.

Arabidopsis AXR6 encodes CUL1 implicating SCF E3 ligases in auxin regulation of embryogenesis

The AXR6 gene is required for auxin signaling in the Arabidopsis embryo and during postembryonic development. One of the effects of auxin is to stimulate degradation of the Aux/IAA auxin response proteins through the action of the ubiquitin protein ligase SCF(TIR1). Here we show that AXR6 encodes the SCF subunit CUL1. The axr6 mutations affect the ability of mutant CUL1 to assemble into stable SCF complexes resulting in reduced degradation of the SCF(TIR1) substrate AXR2/IAA7. In addition, we show that CUL1 is required for lateral organ initiation in the shoot apical meristem and the inflorescence meristem. These results indicate that the embryonic axr6 phenotype is related to a defect in SCF function and accumulation of Aux/IAA proteins such as BDL/IAA12. In addition, we show that CUL1 has a role in auxin response throughout the life cycle of the plant.

Syntaxin specificity of cytokinesis in Arabidopsis

Syntaxins interact with other SNAREs (soluble NSF-attachment protein receptors) to form structurally related complexes that mediate membrane fusion in diverse intracellular trafficking pathways. The original SNARE hypothesis postulated that each type of transport vesicle has its own distinct vesicle-SNARE that pairs up with a unique target-SNARE, or syntaxin, on the target membrane. However, recent evidence suggests that small G-proteins of the Rab family and their effectors mediate the initial contact between donor and acceptor membranes, providing complementary specificity to SNARE pairing at a later step towards membrane fusion. To assess the role of syntaxin specificity in membrane recognition requires a biological assay in which one syntaxin is replaced by other family members that do not normally function in that trafficking pathway. Here, we examine whether membrane fusion in Arabidopsis thaliana cytokinesis, which involves a plant-specific syntaxin, the cell-cycle-regulated KNOLLE (KN) protein, can be mediated by other syntaxins if expressed under the control of KN cis-regulatory sequences. Only a non-essential syntaxin was targeted to the plane of cell division and sufficiently related to KN to perform its function, thus revealing syntaxin specificity of cytokinesis.

The RUB/Nedd8 conjugation pathway is required for early development in Arabidopsis

The related-to-ubiquitin (RUB) protein is post-translationally conjugated to the cullin subunit of the SCF (SKP1, Cullin, F-box) class of ubiquitin protein ligases. Although the precise biochemical function of RUB modification is unclear, studies indicate that the modification is important for SCF function. In Arabidopsis, RUB modification of CUL1 is required for normal function of SCF(TIR1), an E3 required for response to the plant hormone auxin. In this report we show that an Arabidopsis protein called RCE1 functions as a RUB-conjugating enzyme in vivo. A mutation in the RCE1 gene results in a phenotype like that of the axr1 mutant. Most strikingly, plants deficient in both RCE1 and AXR1 have an embryonic phenotype similar to mp and bdl mutants, previously shown to be deficient in auxin signaling. Based on these results, we suggest that the RUB-conjugation pathway is required for auxin-dependent pattern formation in the developing embryo. In addition, we show that RCE1 interacts directly with the RING protein RBX1 and is present in a stable complex with SCF. We propose that RBX1 functions as an E3 for RUB modification of CUL1.

Plasma membrane H+-ATPase is involved in auxin-mediated cell elongation during wheat embryo development

Previous investigations suggested that specific auxin spatial distribution due to auxin movements to particular embryonic regions was important for normal embryonic pattern formation. To gain information on the molecular mechanism(s) by which auxin acts to direct pattern formation in specific embryonic regions, the role of a plasma membrane (PM) ATPase was evaluated as downstream target of auxin in the present study. Western-blot analysis revealed that the PM H(+)-ATPase expression level was significantly increased by auxin in wheat (Triticum aestivum) embryos (two-three times increase). In bilaterally symmetrical embryos, the spatial expression pattern of the PM H(+)-ATPase correlates with the distribution pattern of the auxin analog, tritiated 5-azidoindole-3-acetic acid. A strong immunosignal was observed in the abaxial epidermis of the scutellum and in the epidermal cells at the distal tip of this organ. Pseudoratiometric analysis using a fluorescent pH indicator showed that the pH in the apoplast of the cells expressing the PM H(+)-ATPase was in average more acidic than the apoplastic pH of nonexpressing cells. Cellulose staining of living embryos revealed that cells of the scutellum abaxial epidermis expressing the ATPase were longer than the scutellum adaxial epidermal cells, where the protein was not expressed. Our data indicate that auxin activates the proton pump resulting in apoplastic acidification, a process contributing to cell wall loosening and elongation of the scutellum. Therefore, we suggest that the PM H(+)-ATPase is a component of the auxin-signaling cascade that may direct pattern formation in embryos.

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.

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.

The RADICLELESS1 gene is required for vascular pattern formation in rice

The molecular mechanisms through which the complex patterns of plant vascular tissues are established are largely unknown. The highly ordered, yet simple, striate array of veins of rice leaves represents an attractive system to study the dynamics underlying pattern formation. Here we show that mutation in the RADICLELESS1 (RAL1) gene results in distinctive vascular pattern defects. In ral1 embryonic scutella, secondary veins are absent and in the prematurely aborted and discontinuous primary veins, cells are misaligned to each other. In ral1 leaves, longitudinal and commissural (transverse) veins display altered spacing and the commissural veins additionally show atypical branching and interruptions in their continuity. The vascular pattern alterations of ral1 occur in the context of normally shaped leaf primordia. Anatomical inspection and analysis of the expression of the procambium specification marker Oshox1-GUS and of the auxin-inducible reporter DR5-GUS demonstrates that all the vascular patterning aberrations of ral1 originate from defects in the procambium, which represents the earliest identifiable stage of vascular development. Furthermore, the ral1 mutant is unique in that procambium formation in leaf primordium development is delayed. Finally, the ral1 vascular patterning distortions are associated with a defective response to auxin and with an enhanced sensitivity to cytokinin. ral1 is the first mutant impaired in both procambium development and vascular patterning to be isolated in a monocot species.