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Auxin is a key plant morphogenetic signal (1) but tools to analyse dynamically its distribution and signalling during development are still limited. Auxin perception directly triggers the degradation of Aux/IAA repressor proteins (2-6). Here we describe a novel Aux/IAA-based auxin signalling sensor termed DII-VENUS that was engineered in the model plant Arabidopsis thaliana. The VENUS fast maturing form of yellow fluorescent protein (7) was fused in-frame to the Aux/IAA auxin-interaction domain (termed domain II; DII)5 and expressed under a constitutive promoter. We initially show that DII-VENUS abundance is dependent on auxin, its TIRl/AFBs co-receptors (4-6,8) and proteasome activities. Next, we demonstrate that DII-VENUS provides a map of relative auxin distribution at cellular resolution in different tissues. DII-VENUS is also rapidly degraded in response to auxin and we used it to visualize dynamic changes in cellular auxin distribution successfully during two developmental responses, the root gravitropic response and lateral organ production at the shoot apex. Our results illustrate the value of developing response input sensors such as DII-VENUS to provide high-resolution spatio-temporal information about hormone distribution and response during plant growth and development., Central to auxin signalling is the ubiquitin- and proteasome-dependent degradation of Aux/IAA catalysed by the SCF-type E3 ubiquitin-ligase complexes [SCF.sup.TIR1/AFB1-5] (refs 2-6, 8). Aux/IAA repressors form heterodimers with transcription factors [...]

Polarized epithelia are fundamental to multicellular life. In animal epithelia, conserved junctional complexes establish membrane diffusion barriers, cellular adherence and sealing of the extracellular space (1). Plant cellular barriers are of independent evolutionary origin. The root endodermis strongly resembles a polarized epithelium and functions in nutrient uptake and stress resistance (2). Its defining features are the Casparian strips, belts of specialized cell wall material that generate an extracellular diffusion barrier (2). The mechanisms localizing Casparian strips are unknown. Here we identify and characterize a family of transmembrane proteins of previously unknown function. These 'CASPs' (Casparian strip membrane domain proteins) specifically mark a membrane domain that predicts the formation of Casparian strips. CASP1 displays numerous features required for a constituent of a plant junctional complex: it forms complexes with other CASPs; it becomes immobile upon localization; and it sediments like a large polymer. CASP double mutants display disorganized Casparian strips, demonstrating a role for CASPs in structuring and localizing this cell wall modification. To our knowledge, CASPs are the first molecular factors that are shown to establish a plasma membrane and extracellular diffusion barrier in plants, and represent a novel way of epithelial barrier formation in eukaryotes., In 1865 Robert Caspary described a novel cell type surrounding the vascular cylinder of roots (3). Caspary named this layer 'Schutzscheide', protective sheath, but during the twentieth century it was [...]

Like animals, the mature plant body develops via successive sets of instructions that determine cell fate, patterning, and organogenesis. In the coordination of various developmental programs, several plant hormones play decisive roles, among which auxin is the best-documented hormonal signal. Despite the broad range of processes influenced by auxin, how such a single signaling molecule can be translated into a multitude of distinct responses remains unclear. In Arabidopsis thaliana, lateral root development is a classic example of a developmental process that is controlled by auxin at multiple stages. Therefore, we used lateral root formation as a model system to gain insight into the multifunctionality of auxin. We were able to demonstrate the complementary and sequential action of two discrete auxin response modules, the previously described SOLITARY ROOT/INDOLE-3-ACETIC ACID (IAA)14-AUXIN REPONSE FACTOR (ARF)7-ARF19-dependent lateral root initiation module and the successive BODENLOS/IAA12-MONOPTEROS/ARF5-dependent module, both of which are required for proper organogenesis. The genetic framework in which two successive auxin response modules control early steps of a developmental process adds an extra dimension to the complexity of auxin's action. AUXIN/INDOLE-3-ACETIC ACID | AUXIN RESPONSE FACTOR | cell cycle | lateral root www.pnas.org/cgi/doi/10.1073/pnas.0915001107

At the onset of lateral root initiation in Arabidopsis thaliana, the phytohormone auxin activates xylem pole pericycle cells for asymmetric cell division. However, the molecular events leading from auxin to lateral root initiation are poorly understood, in part because the few responsive cells in the process are embedded in the root and are thus difficult to access. A lateral root induction system, in which most xylem pole pericycle cells were synchronously activated by auxin transport inhibition followed by auxin application, was used for microarray transcript profiling. Of 4,600 genes analyzed, 906 significantly differentially regulated genes were identified that could he grouped into six major clusters. Basically, three major patterns were discerned representing induced, repressed, and transiently expressed genes. Analysis of the coregulated genes, which were specific for each time point, provided new insight into the molecular regulation and signal transduction preceding lateral root initiation in Arabidopsis. The reproducible expression profiles during a time course allowed us to define four stages that precede the cell division in the pericycle. These early stages were characterized by [G.sub.1] cell cycle block, auxin perception, and signal transduction, followed by progression over [G.sub.1]/S transition and [G.sub.2]/M transition. All these processes took place within 6 h after transfer from N--1--naphthylphthalamic acid to 1--naphthalene acetic acid. These results indicate that this lateral root induction system represents a unique synchronized system that allows the systematic study of the developmental program upstream of the cell cycle activation during lateral root initiation. auxin | cell cycle | microarray

Byline: Carmem-Lara Manes (1,2), Tom Beeckman (1), Tita Ritsema (1,3), Marc Van Montagu (1), Koen Goethals (1), Marcelle Holsters (1) Keywords: Endophyte; Epiphyte; Rhodococcus fascians; Shoot induction; Tissue colonization Abstract: Arabidopsis thaliana (L.) Heynh. plants were challenged with Rhodococcus fascians at several developmental stages and using different inoculation procedures. A variety of morphological alterations was scored on the infected plants some of them resembled phenotypes of A. thaliana mutants in their shoot apical meristem (SAM) organization. Infection with R. fascians did not affect SAM organization in wild type nor in SAM mutants. Anatomical studies on the new organs formed after infection with R. fascians demonstrated extensive bacterial colonization. Colonization and concomitant production of specific signals are the likely cause of malformations. Author Affiliation: (1) Department of Plant Systems Biology, Flanders Interuniversity Institute for Biotechnology, Ghent University, Technologiepark 927, 9052, Gent, Belgium (2) Laboratoire Arago, Observatoire Oceanique, Banyuls-sur-Mer, France (3) Molecular Plant Physiology, University Utrecht, Utrecht, The Netherlands Article History: Registration Date: 07/01/2003 Received Date: 11/04/2003 Accepted Date: 12/12/2003 Online Date: 13/02/2004

Byline: Tom Beeckman (1), Riet De Rycke (1), Ronald Viane (2), Dirk Inze (1) Keywords: Keywords: Arabidopsis thaliana, Embryogenesis, Pigmentation, Seed coat Abstract: seed coat development using light and transmission electron microscopy revealed major morphological changes associated with the transition of the integuments into the mature seed coat. By the use of a metachromatic staining procedure, cytological events such as the production of phenolic compounds and acidic polysaccharides were followed. Immediately after fertilization, the cells of the inner epidermis of the inner integument became vacuolated and subsequently accumulated pigment within them. This pigment started to disappear from the cytoplasm at the torpedo stage of the embryo, as it became green. During the torpedo stage, mucilage began to accumulate in the cells of the external epidermis of the outer integument. Furthermore, starch grains accumulated against the central part of the inner periclinal wall of these cells, resulting in the formation of small pyramidal domes that persisted until seed maturity. At the maturation stage, when the embryo became dormant and colourless, a new pigment accumulation was observed in an amorphous layer derived from remnants of crushed integument layers. This second pigment layer was responsible for the brown seed colour. These results show that seed coat formation may proceed in a coordinated way with the developmental phases of embryogenesis. Author Affiliation: (1) Department of Plant Genetics, Flanders Interuniversity Institute for Biotechnology, Ghent University, K.L. Ledeganckstraat 35, B--9000 Gent, Belgium, BE (2) Department of Biology, Ghent University, B--9000 Gent, Belgium, BE Article note: Received 25 May 1999/ Accepted in revised form 10 February 2000