The shift to 3D growth during embryogenesis of kelp species, atlas of cell division and differentiation of Saccharina latissima

In most organisms, 3D growth takes place at the onset of embryogenesis. In some brown algae, 3D growth occurs later in development, when the organism consists of several hundred cells. We studied the cellular events that take place when 3D growth is established in the embryo of the brown alga Saccharina, a kelp species. Semi-thin sections, taken from where growth shifts from 2D to 3D, show that 3D growth first initiates from symmetrical cell division in the monolayered lamina, and then is enhanced through a series of asymmetrical cell divisions in a peripheral monolayer of cells called the meristoderm. Then, daughter cells rapidly differentiate into cortical and medullary cells, characterised by their position, size and shape. In essence, 3D growth in kelps is based on a series of differentiation steps that occur rapidly after the initiation of a bilayered lamina, followed by further growth of the established differentiated tissues. Our study depicts the cellular landscape necessary to study cell-fate programming in the context of a novel mode of 3D growth in an organism phylogenetically distant from plants and animals.

Cultivation and Imaging of S. latissima Embryo Monolayered Cell Sheets Inside Microfluidic Devices

The culturing and investigation of individual marine specimens in lab environments is crucial to further our understanding of this highly complex ecosystem. However, the obtained results and their relevance are often limited by a lack of suitable experimental setups enabling controlled specimen growth in a natural environment while allowing for precise monitoring and in-depth observations. In this work, we explore the viability of a microfluidic device for the investigation of the growth of the alga Saccharina latissima to enable high-resolution imaging by confining the samples, which usually grow in 3D, to a single 2D plane. We evaluate the specimen’s health based on various factors such as its growth rate, cell shape, and major developmental steps with regard to the device’s operating parameters and flow conditions before demonstrating its compatibility with state-of-the-art microscopy imaging technologies such as the skeletonisation of the specimen through calcofluor white-based vital staining of its cell contours as well as the immunolocalisation of the specimen’s cell wall. Furthermore, by making use of the on-chip characterisation capabilities, we investigate the influence of altered environmental illuminations on the embryonic development using blue and red light. Finally, live tracking of fluorescent microspheres deposited on the surface of the embryo permits the quantitative characterisation of growth at various locations of the organism.

Targeted Laser Ablation in the Embryo of Saccharina latissima

In Saccharina latissima, the embryo develops as a monolayered cell sheet called the lamina or the blade. Each embryo cell is easy to observe, readily distinguishable from its neighbors, and can be individually targeted. For decades, laser ablation has been used to study embryo development. Here, a protocol for cell-specific laser ablation was developed for early embryos of the brown alga S. latissima. The presented work includes: (1) the preparation of Saccharina embryos, with a description of the critical parameters, including culture conditions, (2) the laser ablation settings, and (3) the monitoring of the subsequent growth of the irradiated embryo using time-lapse microscopy. In addition, details are provided on the optimal conditions for transporting the embryos from the imaging platform back to the lab, which can profoundly affect subsequent embryo development. Algae belonging to the order Laminariales display embryogenesis patterns similar to Saccharina; this protocol can thus be easily transferred to other species in this taxon.

Design Principles of Branching Morphogenesis in Filamentous Organisms

The radiation of life on Earth was accompanied by the diversification of multicellular body plans in the eukaryotic kingdoms Animalia, Plantae, Fungi and Chromista. Branching forms are ubiquitous in nature and evolved repeatedly in the above lineages. The developmental and genetic basis of branch formation is well studied in the three-dimensional shoot and root systems of land plants, and in animal organs such as the lung, kidney, mammary gland, vasculature, etc. Notably, recent thought-provoking studies combining experimental analysis and computational modeling of branching patterns in whole animal organs have identified global patterning rules and proposed unifying principles of branching morphogenesis. Filamentous branching forms represent one of the simplest expressions of the multicellular body plan and constitute a key step in the evolution of morphological complexity. Similarities between simple and complex branching forms distantly related in evolution are compelling, raising the question whether shared mechanisms underlie their development. Here, we focus on filamentous branching organisms that represent major study models from three distinct eukaryotic kingdoms, including the moss Physcomitrella patens (Plantae), the brown alga Ectocarpus sp. (Chromista), and the ascomycetes Neurospora crassa and Aspergillus nidulans (Fungi), and bring to light developmental regulatory mechanisms and design principles common to these lineages. Throughout the review we explore how the regulatory mechanisms of branching morphogenesis identified in other models, and in particular animal organs, may inform our thinking on filamentous systems and thereby advance our understanding of the diverse strategies deployed across the eukaryotic tree of life to evolve similar forms.

Alginates along the filament of the brown alga Ectocarpus help cells cope with stress

Ectocarpus is a filamentous brown alga, which cell wall is composed mainly of alginates and fucans (80%), two non-crystalline polysaccharide classes. Alginates are linear chains of epimers of 1,4-linked uronic acids, β-D-mannuronic acid (M) and α-L-guluronic acid (G). Previous physico-chemical studies showed that G-rich alginate gels are stiffer than M-rich alginate gels when prepared in vitro with calcium. In order to assess the possible role of alginates in Ectocarpus, we first immunolocalised M-rich or G-rich alginates using specific monoclonal antibodies along the filament. As a second step, we calculated the tensile stress experienced by the cell wall along the filament, and varied it with hypertonic or hypotonic solutions. As a third step, we measured the stiffness of the cell along the filament, using cell deformation measurements and atomic force microscopy. Overlapping of the three sets of data allowed to show that alginates co-localise with the stiffest and most stressed areas of the filament, namely the dome of the apical cell and the shanks of the central round cells. In addition, no major distinction between M-rich and G-rich alginate spatial patterns could be observed. Altogether, these results support that both M-rich and G-rich alginates play similar roles in stiffening the cell wall where the tensile stress is high and exposes cells to bursting, and that these roles are independent from cell growth and differentiation.

Gazing at Cell Wall Expansion under a Golden Light

In plants, cell growth is constrained by a stiff cell wall, at least this is the way textbooks usually present it. Accordingly, many studies have focused on the elasticity and plasticity of the cell wall as prerequisites for expansion during growth. With their specific evolutionary history, cell wall composition, and environment, brown algae present a unique configuration offering a new perspective on the involvement of the cell wall, viewed as an inert material yet with intrinsic mechanical properties, in growth. In light of recent findings, we explore here how much of the functional relationship between cell wall chemistry and intrinsic mechanics on the one hand, and growth on the other hand, has been uncovered in brown algae.

The brown algal mode of tip growth: Keeping stress under control

Tip growth has been studied in pollen tubes, root hairs, and fungal and oomycete hyphae and is the most widely distributed unidirectional growth process on the planet. It ensures spatial colonization, nutrient predation, fertilization, and symbiosis with growth speeds of up to 800 μm h-1. Although turgor-driven growth is intuitively conceivable, a closer examination of the physical processes at work in tip growth raises a paradox: growth occurs where biophysical forces are low, because of the increase in curvature in the tip. All tip-growing cells studied so far rely on the modulation of cell wall extensibility via the polarized excretion of cell wall-loosening compounds at the tip. Here, we used a series of quantitative measurements at the cellular level and a biophysical simulation approach to show that the brown alga Ectocarpus has an original tip-growth mechanism. In this alga, the establishment of a steep gradient in cell wall thickness can compensate for the variation in tip curvature, thereby modulating wall stress within the tip cell. Bootstrap analyses support the robustness of the process, and experiments with fluorescence recovery after photobleaching (FRAP) confirmed the active vesicle trafficking in the shanks of the apical cell, as inferred from the model. In response to auxin, biophysical measurements change in agreement with the model. Although we cannot strictly exclude the involvement of a gradient in mechanical properties in Ectocarpus morphogenesis, the viscoplastic model of cell wall mechanics strongly suggests that brown algae have evolved an alternative strategy of tip growth. This strategy is largely based on the control of cell wall thickness rather than fluctuations in cell wall mechanical properties.

Furthering knowledge of seaweed growth and development to facilitate sustainable aquaculture

Macroalgae (seaweeds) are the subject of increasing interest for their potential as a source of valuable, sustainable biomass in the food, feed, chemical and pharmaceutical industries. Compared with microalgae, the pace of knowledge acquisition in seaweeds is slower despite the availability of whole-genome sequences and model organisms for the major seaweed groups. This is partly a consequence of specific hurdles related to the large size of these organisms and their slow growth. As a result, this basic scientific field is falling behind, despite the societal and economic importance of these organisms. Here, we argue that sustainable management of seaweed aquaculture requires fundamental understanding of the underlying biological mechanisms controlling macroalgal life cycles - from the production of germ cells to the growth and fertility of the adult organisms - using diverse approaches requiring a broad range of technological tools. This Viewpoint highlights several examples of basic research on macroalgal developmental biology that could enable the step-changes which are required to adequately meet the demands of the aquaculture sector.

Morphoelasticity in the development of brown alga Ectocarpus siliculosus: from cell rounding to branching

A biomechanical model is proposed for the growth of the brown alga Ectocarpus siliculosus Featuring ramified uniseriate filaments, this alga has two modes of growth: apical growth and intercalary growth with branching. Apical growth occurs upon the mitosis of a young cell at one extremity and leads to a new tip cell followed by a cylindrical cell, whereas branching mainly occurs when a cylindrical cell becomes rounded and swells, forming a spherical cell. Given the continuous interplay between cell growth and swelling, a poroelastic model combining osmotic pressure and volumetric growth is considered for the whole cell, cytoplasm and cell wall. The model recovers the morphogenetic transformations of mature cells: transformation of a cylindrical shape into spherical shape with a volumetric increase, and then lateral branching. Our simulations show that the poro-elastic model, including the Mooney-Rivlin approach for hyper-elastic materials, can correctly reproduce the observations. In particular, branching appears to be a plasticity effect due to the high level of tension created after the increase in volume of mature cells.

Surfing amongst Oil-Tankers: Connecting Emerging Research Fields to the Current International Landscape

The COST Action Phycomorph (FA1406) was initiated in 2015 from a handful of academic researchers, and now joins together 19 European countries and nine international partners. Phycomorph's goal is to coordinate and develop research on developmental biology in macroalgae. This is an ambitious project, as the related scientific community is small, the concepts are complex, and there is currently limited knowledge of these organisms and there are few technologies to study them. Here we report the first step in achieving this enterprise, the creation of the Phycomorph network. We share the associated strengths, pitfalls, and prospects for setting up the network in the hope that this might guide similar efforts in other fields.

Male-specific colon motility dysfunction in the TashT mouse line

BACKGROUND

In Hirschsprung disease (HSCR), the absence of myenteric neural ganglia in the distal bowel prevents motility and thereby causes functional intestinal obstruction. Although surgical resection of the aganglionic segment allows HSCR children to survive this condition, a number of patients still suffer from impaired motility despite having myenteric ganglia in their postoperative distal bowel. Such phenomenon is also observed in patients suffering from other enteric neuropathies and, in both cases, colonic dysmotility is believed to result from abnormalities of myenteric ganglia and/or associated interstitial cells of Cajal (ICC). To better understand this, we used a recently described HSCR mouse model called TashT.

METHODS

Intestinal motility parameters were assessed and correlated with extent of aganglionosis and with neuronal density in ganglionated regions. The neural composition of the myenteric plexus and the status of ICC networks was also evaluated using immunofluorescence.

KEY RESULTS

TashT(Tg/Tg) mice display a strong male bias in the severity of both colonic aganglionosis and hypoganglionosis, which are associated with male-specific reduced colonic motility. TashT(Tg/Tg) male mice also exhibit a specific increase in nNos(+) neurons that is restricted to the most distal ganglionated regions. In contrast, Calretinin(+) myenteric neurons, Sox10(+) myenteric glial cells, and cKit(+) ICC are not affected in TashT(Tg/Tg) mice.

CONCLUSIONS AND INFERENCES

Male-specific impairment of colonic motility in TashT(Tg/Tg) mice is associated with both severe hypoganglionosis and myenteric neuronal imbalance. Considering these parameters in the clinic might be important for the management of postoperative HSCR patients.

Production of genetically and developmentally modified seaweeds: exploiting the potential of artificial selection techniques

Plant feedstock with specific, modified developmental features has been a quest for centuries. Since the development and spread of agriculture, there has been a desire for plants producing disproportionate-or more abundant and more nutritional-biomass that meet human needs better than their native counterparts. Seaweed aquaculture, targeted for human consumption and the production of various raw materials, is a rapidly expanding field and its stakeholders have increasing vested interest for cost-effective and lucrative seaweed cultivation processes. Thus, scientific research on seaweed development is particularly timely: the potential for expansion of seaweed cultivation depends on the sector's capacity to produce seaweeds with modified morphological features (e.g., thicker blades), higher growth rates or delayed (or even no) fertility. Here, we review the various technical approaches used to modify development in macroalgae, which have attracted little attention from developmental biologists to date. Because seaweed (or marine macroalgae) anatomy is much less complex than that of land plants and because seaweeds belong to three different eukaryotic phyla, the mechanisms controlling their morphogenesis are key to understanding their development. Here, we present efficient sources of developmentally and genetically modified seaweeds-somatic variants, artificial hybrids and mutants-as well as the future potential of these techniques.

The green seaweed Ulva: a model system to study morphogenesis

Green macroalgae, mostly represented by the Ulvophyceae, the main multicellular branch of the Chlorophyceae, constitute important primary producers of marine and brackish coastal ecosystems. Ulva or sea lettuce species are some of the most abundant representatives, being ubiquitous in coastal benthic communities around the world. Nonetheless the genus also remains largely understudied. This review highlights Ulva as an exciting novel model organism for studies of algal growth, development and morphogenesis as well as mutualistic interactions. The key reasons that Ulva is potentially such a good model system are: (i) patterns of Ulva development can drive ecologically important events, such as the increasing number of green tides observed worldwide as a result of eutrophication of coastal waters, (ii) Ulva growth is symbiotic, with proper development requiring close association with bacterial epiphytes, (iii) Ulva is extremely developmentally plastic, which can shed light on the transition from simple to complex multicellularity and (iv) Ulva will provide additional information about the evolution of the green lineage.

Localization of causal locus in the genome of the brown macroalga Ectocarpus: NGS-based mapping and positional cloning approaches

Mutagenesis is the only process by which unpredicted biological gene function can be identified. Despite that several macroalgal developmental mutants have been generated, their causal mutation was never identified, because experimental conditions were not gathered at that time. Today, progresses in macroalgal genomics and judicious choices of suitable genetic models make mutated gene identification possible. This article presents a comparative study of two methods aiming at identifying a genetic locus in the brown alga Ectocarpus siliculosus: positional cloning and Next-Generation Sequencing (NGS)-based mapping. Once necessary preliminary experimental tools were gathered, we tested both analyses on an Ectocarpus morphogenetic mutant. We show how a narrower localization results from the combination of the two methods. Advantages and drawbacks of these two approaches as well as potential transfer to other macroalgae are discussed.

Laser capture microdissection in Ectocarpus siliculosus: the pathway to cell-specific transcriptomics in brown algae

Laser capture microdissection (LCM) facilitates the isolation of individual cells from tissue sections, and when combined with RNA amplification techniques, it is an extremely powerful tool for examining genome-wide expression profiles in specific cell-types. LCM has been widely used to address various biological questions in both animal and plant systems, however, no attempt has been made so far to transfer LCM technology to macroalgae. Macroalgae are a collection of widespread eukaryotes living in fresh and marine water. In line with the collective effort to promote molecular investigations of macroalgal biology, here we demonstrate the feasibility of using LCM and cell-specific transcriptomics to study development of the brown alga Ectocarpus siliculosus. We describe a workflow comprising cultivation and fixation of algae on glass slides, laser microdissection, and RNA amplification. To illustrate the effectiveness of the procedure, we show qPCR data and metrics obtained from cell-specific transcriptomes generated from both upright and prostrate filaments of Ectocarpus.

Computational prediction and experimental validation of microRNAs in the brown alga Ectocarpus siliculosus

We used an in silico approach to predict microRNAs (miRNAs) genome-wide in the brown alga Ectocarpus siliculosus. As brown algae are phylogenetically distant from both animals and land plants, our approach relied on features shared by all known organisms, excluding sequence conservation, genome localization and pattern of base-pairing with the target. We predicted between 500 and 1500 miRNAs candidates, depending on the values of the energetic parameters used to filter the potential precursors. Using quantitative polymerase chain reaction assays, we confirmed the existence of 22 miRNAs among 72 candidates tested, and of 8 predicted precursors. In addition, we compared the expression of miRNAs and their precursors in two life cycle states (sporophyte, gametophyte) and under salt stress. Several miRNA precursors, Argonaute and DICER messenger RNAs were differentially expressed in these conditions. Finally, we analyzed the gene organization and the target functions of the predicted candidates. This showed that E. siliculosus miRNA genes are, like plant miRNA genes, rarely clustered and, like animal miRNA genes, often located in introns. Among the predicted targets, several widely conserved functional domains are significantly overrepresented, like kinesin, nucleotide-binding/APAF-1, R proteins and CED-4 (NB-ARC) and tetratricopeptide repeats. The combination of computational and experimental approaches thus emphasizes the originality of molecular and cellular processes in brown algae.

Brown algal morphogenesis: atomic force microscopy as a tool to study the role of mechanical forces

Over the last few years, a growing interest has been directed toward the use of macroalgae as a source of energy, food and molecules for the cosmetic and pharmaceutical industries. Besides this, macroalgal development remains poorly understood compared to other multicellular organisms. Brown algae (Phaeophyceae) form a monophyletic lineage of usually large multicellular algae which evolved independently from land plants. In their environment, they are subjected to strong mechanical forces (current, waves, and tide), in response to which they modify rapidly and reversibly their morphology. Because of their specific cellular features (cell wall composition, cytoskeleton organization), deciphering how they cope with these forces might help discover new control mechanisms of cell wall softening and cellulose synthesis. Despite the current scarcity in knowledge on brown algal cell wall dynamics and protein composition, we will illustrate, in the light of methods adapted to Ectocarpus siliculosus, to what extent atomic force microscopy can contribute to advance this field of investigation.

Culture methods and mutant generation in the filamentous brown algae Ectocarpus siliculosus

Ectocarpus siliculosus is a small filamentous alga that has recently emerged as the new model for fundamental research on brown algae. Here, we describe the basic culture protocols for propagating and collecting E. siliculosus material that can then be used in all types of molecular biology, biochemistry and cell biology techniques. In addition, procedures for carrying out genetic experiments (generation of mutants and genetic segregation analyses) on E. siliculosus are described.

Plant Proteus: brown algal morphological plasticity and underlying developmental mechanisms

Brown algae are multicellular photosynthetic marine organisms, ubiquitous on rocky intertidal shores at cold and temperate latitudes. Nevertheless, little is known about many aspects of their biology, particularly their development. Given their phylogenetic distance (1.6 billion years) from other plant organisms (land plants, and green and red algae), brown algae harbor a high, as-yet undiscovered diversity of biological mechanisms governing their development. They also show great morphological plasticity, responding to specific environmental constraints, such as sea currents, reduced light availability, grazer attacks, desiccation and UV exposure. Here, we show that brown algal morphogenesis is rather simple and flexible, and review recent genomic data on the cellular and molecular mechanisms known to date that can possibly account for this developmental strategy.

Space-time decoupling in the branching process in the mutant étoile of the filamentous brown alga Ectocarpus siliculosus

Ectocarpus siliculosus is being developed as a model organism for brown algal genetics and genomics. Brown algae are phylogenetically distant from the other multicellular phyla (green lineage, red algae, fungi and metazoan) and therefore might offer the opportunity to study novel and alternative developmental processes that lead to the establishment of multicellularity. E. siliculosus develops as uniseriate filaments, thereby displaying one of the simplest architectures among multicellular organisms. The young sporophyte grows as a primary filament and then branching occurs, preferentially at the center of the filament. We recently described the first morphogenetic mutant étoile (etl) in a brown alga, produced by UVB mutagenesis in E. siliculosus. We showed that a single recessive mutation was responsible for a defect in both cell differentiation and the very early branching pattern (first and second branch emergences). Here, we supplement this study by reporting the branching defects observed subsequently, i.e. for the later stages corresponding to the emergence of up to the first six secondary filaments, and we show that the branching process is composed of at least two distinct components: time and position.

ETOILE regulates developmental patterning in the filamentous brown alga Ectocarpus siliculosus

Brown algae are multicellular marine organisms evolutionarily distant from both metazoans and land plants. The molecular or cellular mechanisms that govern the developmental patterning in brown algae are poorly characterized. Here, we report the first morphogenetic mutant, étoile (etl), produced in the brown algal model Ectocarpus siliculosus. Genetic, cellular, and morphometric analyses showed that a single recessive locus, ETL, regulates cell differentiation: etl cells display thickening of the extracellular matrix (ECM), and the elongated, apical, and actively dividing E cells are underrepresented. As a result of this defect, the overrepresentation of round, branch-initiating R cells in the etl mutant leads to the rapid induction of the branching process at the expense of the uniaxial growth in the primary filament. Computational modeling allowed the simulation of the etl mutant phenotype by including a modified response to the neighborhood information in the division rules used to specify wild-type development. Microarray experiments supported the hypothesis of a defect in cell-cell communication, as primarily Lin-Notch-domain transmembrane proteins, which share similarities with metazoan Notch proteins involved in binary cell differentiation were repressed in etl. Thus, our study highlights the role of the ECM and of novel transmembrane proteins in cell-cell communication during the establishment of the developmental pattern in this brown alga.

The Ectocarpus genome and the independent evolution of multicellularity in brown algae

Brown algae (Phaeophyceae) are complex photosynthetic organisms with a very different evolutionary history to green plants, to which they are only distantly related. These seaweeds are the dominant species in rocky coastal ecosystems and they exhibit many interesting adaptations to these, often harsh, environments. Brown algae are also one of only a small number of eukaryotic lineages that have evolved complex multicellularity (Fig. 1). We report the 214 million base pair (Mbp) genome sequence of the filamentous seaweed Ectocarpus siliculosus (Dillwyn) Lyngbye, a model organism for brown algae, closely related to the kelps (Fig. 1). Genome features such as the presence of an extended set of light-harvesting and pigment biosynthesis genes and new metabolic processes such as halide metabolism help explain the ability of this organism to cope with the highly variable tidal environment. The evolution of multicellularity in this lineage is correlated with the presence of a rich array of signal transduction genes. Of particular interest is the presence of a family of receptor kinases, as the independent evolution of related molecules has been linked with the emergence of multicellularity in both the animal and green plant lineages. The Ectocarpus genome sequence represents an important step towards developing this organism as a model species, providing the possibility to combine genomic and genetic approaches to explore these and other aspects of brown algal biology further.

Auxin metabolism and function in the multicellular brown alga Ectocarpus siliculosus

Ectocarpus siliculosus is a small brown alga that has recently been developed as a genetic model. Its thallus is filamentous, initially organized as a main primary filament composed of elongated cells and round cells, from which branches differentiate. Modeling of its early development suggests the involvement of very local positional information mediated by cell-cell recognition. However, this model also indicates that an additional mechanism is required to ensure proper organization of the branching pattern. In this paper, we show that auxin indole-3-acetic acid (IAA) is detectable in mature E. siliculosus organisms and that it is present mainly at the apices of the filaments in the early stages of development. An in silico survey of auxin biosynthesis, conjugation, response, and transport genes showed that mainly IAA biosynthesis genes from land plants have homologs in the E. siliculosus genome. In addition, application of exogenous auxins and 2,3,5-triiodobenzoic acid had different effects depending on the developmental stage of the organism, and we propose a model in which auxin is involved in the negative control of progression in the developmental program. Furthermore, we identified an auxin-inducible gene called EsGRP1 from a small-scale microarray experiment and showed that its expression in a series of morphogenetic mutants was positively correlated with both their elongated-to-round cell ratio and their progression in the developmental program. Altogether, these data suggest that IAA is used by the brown alga Ectocarpus to relay cell-cell positional information and induces a signaling pathway different from that known in land plants.

A stochastic 1D nearest-neighbour automaton models early development of the brown alga Ectocarpus siliculosus

Early development of the filamentous brown alga Ectocarpus siliculosus (Dillwyn) Lyngbye involves two cell types that are arranged in a polymorphic, but constrained, pattern. The present study aimed to decipher the cellular processes responsible for the establishment of this pattern. Thorough observations characterised five different events of division and differentiation that occurred during the early development. The hypothesis that a local control is responsible for these processes was tested. To do so, Ectomat, a stochastic automaton in which each cell only interacts with its closest neighbour(s), was created. The probabilities for the five events were adjusted to fit to the observations. Simulations with Ectomat reconstructed most of the essential properties of the sporophyte development, in terms of cell-type proportion, relative position and growth dynamics. The whole organism properties emerged by applying local transition rules. In conclusion, no global position information system was required at this development stage. Randomly occurring cell events, driven by simple contact interactions, are sufficient to account for the early filament development and establishment of the cell-type pattern of E. siliculosus.

EARLY DEVELOPMENT PATTERN OF THE BROWN ALGA ECTOCARPUS SILICULOSUS (ECTOCARPALES, PHAEOPHYCEAE) SPOROPHYTE(1)

The distant phylogenetic position of brown macroalgae from the other multicellular phyla offers the opportunity to study novel and alternative developmental processes involved in the establishment of multicellularity. At present, however, very little information is available about developmental patterning in this group. Ectocarpus siliculosus (Dillwyn) Lyngb. has uniseriate filaments and displays one of the simplest architectures in the Phaeophyceae. The aim of this study was to decipher the morphogenetic steps that lead to the development of the Ectocarpus sporophyte. We carried out a detailed morphometric study of the events that occurred between gamete germination and the 100-cell stage. This analysis was performed on two ecologically distant isolates to assess plasticity in developmental patterning within this species. Cell sizes were measured in both isolates, allowing the definition of two main cell types based on their shape (round and elongated). On average, the filament is composed of about 40% round cells, which are present in the central region of the filament, but different combinations of the two cell types within filaments were observed and quantified. Young sporophytes grew apically, with elongated cells progressively differentiating into round cells. Secondary filaments emerged preferentially on round cells, primarily from the older central cells. Statistical analyses showed that the pattern of branching was regulated to ensure a stereotyped architecture. This description of the developmental patterning during the growth of the E. siliculosus sporophyte will serve as a base for more detailed studies of development, in this species and in brown algae in general.

Normalisation genes for expression analyses in the brown alga model Ectocarpus siliculosus

BACKGROUND

Brown algae are plant multi-cellular organisms occupying most of the world coasts and are essential actors in the constitution of ecological niches at the shoreline. Ectocarpus siliculosus is an emerging model for brown algal research. Its genome has been sequenced, and several tools are being developed to perform analyses at different levels of cell organization, including transcriptomic expression analyses. Several topics, including physiological responses to osmotic stress and to exposure to contaminants and solvents are being studied in order to better understand the adaptive capacity of brown algae to pollution and environmental changes. A series of genes that can be used to normalise expression analyses is required for these studies.

RESULTS

We monitored the expression of 13 genes under 21 different culture conditions. These included genes encoding proteins and factors involved in protein translation (ribosomal protein 26S, EF1alpha, IF2A, IF4E) and protein degradation (ubiquitin, ubiquitin conjugating enzyme) or folding (cyclophilin), and proteins involved in both the structure of the cytoskeleton (tubulin alpha, actin, actin-related proteins) and its trafficking function (dynein), as well as a protein implicated in carbon metabolism (glucose 6-phosphate dehydrogenase). The stability of their expression level was assessed using the Ct range, and by applying both the geNorm and the Normfinder principles of calculation.

CONCLUSION

Comparisons of the data obtained with the three methods of calculation indicated that EF1alpha (EF1a) was the best reference gene for normalisation. The normalisation factor should be calculated with at least two genes, alpha tubulin, ubiquitin-conjugating enzyme or actin-related proteins being good partners of EF1a. Our results exclude actin as a good normalisation gene, and, in this, are in agreement with previous studies in other organisms.

Life-cycle-generation-specific developmental processes are modified in the immediate upright mutant of the brown alga Ectocarpus siliculosus

Development of the sporophyte and gametophyte generations of the brown alga E. siliculosus involves two different patterns of early development, which begin with either a symmetric or an asymmetric division of the initial cell, respectively. A mutant, immediate upright (imm), was isolated that exhibited several characteristics typical of the gametophyte during the early development of the sporophyte generation. Genetic analyses showed that imm is a recessive, single-locus Mendelian factor and analysis of gene expression in this mutant indicated that the regulation of a number of life-cycle-regulated genes is specifically modified in imm mutant sporophytes. Thus, IMM appears to be a regulatory locus that controls part of the sporophyte-specific developmental programme, the mutant exhibiting partial homeotic conversion of the sporophyte into the gametophyte, a phenomenon that has not been described previously.

Development and physiology of the brown alga Ectocarpus siliculosus: two centuries of research

Brown algae share several important features with land plants, such as their photoautotrophic nature and their cellulose-containing wall, but the two groups are distantly related from an evolutionary point of view. The heterokont phylum, to which the brown algae belong, is a eukaryotic crown group that is phylogenetically distinct not only from the green lineage, but also from the red algae and the opisthokont phylum (fungi and animals). As a result of this independent evolutionary history, the brown algae exhibit many novel features and, moreover, have evolved complex multicellular development independently of the other major groups already mentioned. In 2004, a consortium of laboratories, including the Station Biologique in Roscoff and Genoscope, initiated a project to sequence the genome of Ectocarpus siliculosus, a small filamentous brown alga that is found in temperate, coastal environments throughout the globe. The E. siliculosus genome, which is currently being annotated, is expected to be the first completely characterized genome of a multicellular alga. In this review we look back over two centuries of work on this brown alga and highlight the advances that have led to the choice of E. siliculosus as a genomic and genetic model organism for the brown algae.

Complex life cycles of multicellular eukaryotes: new approaches based on the use of model organisms

A wide variety of life cycles can be found in the different groups of multicellular eukaryotes. Here we provide an overview of this variety, and review some of the theoretical arguments that have been put forward to explain the evolutionary stability of different life cycle strategies. We also describe recent progress in the analysis of the haploid-diploid life cycle of the model angiosperm Arabidopsis thaliana and show how new molecular data are providing a means to test some of the theoretical predictions. Finally, we describe an emerging model organism from the brown algae, Ectocarpus siliculosus, and highlight the potential of this system for the investigation of the mechanisms that regulate complex life cycles.

The Arabidopsis thaliana GSK3/Shaggy like kinase AtSK3-2 modulates floral cell expansion

The GSK3/Shaggy family of serine/threonine protein kinases is involved in a series of biological processes in animals, plants and yeast [Charrier et al. (2002) Plant Physiol 130:577-590; Jope and Johnson (2004) Trends Biochem Sci 29:95-102; Li and Nam (2002) Science 295:1299-1301; Piao et al. (2001) Plant J 27:305-314]. In Arabidopsis thaliana, out of the 10 members of the GSK3/Shaggy-like gene family (AtSKs), a biological function has been assigned to only 1 member (AtSK2-1) by mutation. In the present work, a study was undertaken to elucidate the function of AtSK3-2. We have generated mutated versions of the A. thaliana Shaggy-like kinase 3-2 (AtSK3-2), in which Lys(167) and Arg(178), respectively homologues to Lys(85) and Arg(96) of the mammal GSK3beta, were modified into Ala by site-directed mutagenesis. In vitro kinase activity assays of the mutated recombinant protein AtSK3-2-R178A showed that the "primed activity" of the mutated kinase was reduced by 90% while the "non-primed" activity was only 20% reduced compared to the wild-type protein kinase. However, the mutant protein AtSK3-2-K167A showed no activity. Arabidopsis transgenic lines over-expressing AtSK3-2-R178A displayed smaller floral organs, namely pedicels, sepals and petals. Conversely, over-expression of both the wild-type AtSK3-2 protein and the AtSK3-2-K167A mutated version, displayed no altered morphogenesis. Scanning electron microscopic analyses of the AtSK3-2-R178A transgenic plants clearly showed a reduced cell size in flower organs, in which quantitative RT-PCR expression analyses of cell wall expansion enzymes showed reduced transcript levels of three xyloglucan endotransglycosylases (XET), namely XTH22 (TCH4), XTH23 (XTR6) and XTH30 (XTR4). Our data show that AtSK3-2 plays an important role in the control of cell elongation in flower development.

[A Picasso among brown algae: the arduous conquest of symmetry by Ectocarpus]

In response to environmental constraints, living organisms organise their body according to axes, rotation and translation plans, or asymmetries. Cellular and molecular processes are involved in the establishment of this architecture. Hence, this review aims at presenting the molecular mechanisms controlling the main symmetries and axes in plants. Several genes, coding for transcription factors, have been identified in land plants (mainly Arabidopsis thaliana), as controlling the establishment of apico-basal and adaxial-abaxial axes mainly. The establishment of these axes allows the development in other spatial directions of radial or bilateral symmetries. These processes seem in most cases to be under the control of the phytohormone auxin. In brown algae, which are all multicellular marine plants, polarity plans are less obvious than in land plants. The development of the model brown alga Ectocarpus siliculosus is currently being studied. E. siliculosus develops a filamentous architecture, and primary observations show that branching along the main axis occurs in a non-stereotyped and regular way, even though it is mainly centred. However, more detailed morphometrical studies, accompanied by probabilistic analyses, have shown that, among the overall population of individuals, organisms obey yet unidentified biological constraints, that aim at refining the radial symmetry as the organism grows. The role of this symmetry in the adaptation of E. siliculosus to its environment, as well as the molecular actors involved in this process, are currently under study in our laboratory.

Organization and Expression of the GSK3/Shaggy Kinase Gene Family in the Moss Physcomitrella patens Suggest Early Gene Multiplication in Land Plants and an Ancestral Response to Osmotic Stress

GSK3/Shaggy kinases are involved in a wide range of fundamental processes in animal development and metabolism. In angiosperm plants, these kinases are encoded by moderate-sized gene families, which appear to have a complex set of functions. Here, we present the characterization of five members of the GSK3/Shaggy gene family in the bryophyte Physcomitrella patens. The P. patens GSK3/Shaggy kinases (PpSK) are organized in a group of closely related paralogues with respect to their gene sequence and structure. Indeed, a phylogenetic analysis of the GSK3/Shaggy kinase sequences from plants and animals showed that the five PpSK proteins are monophyletic, and closer to subgroups I and IV described in angiosperms. Expression analyses performed by quantitative real-time RT-PCR on a wide range of growing conditions showed that PpSK genes responded only to either desiccation, PEG or sorbitol. As demonstrated by both inductions of marker genes and protonemal cell plasmolyses, these treatments resulted in a hyperosmotic stress. Altogether, these data suggest that (1) GSK3/Shaggy kinase gene multiplication occurred early in plant evolution, before the separation between bryophytes and vascular plants, and (2) both gene loss and duplication occurred in the ancestor of P. patens along with functional gene diversification in angiosperms. However, conservation of the transcriptional responses between Physcomitrella and Arabidopsis suggests the identification of an ancestral response of the GSK3/Shaggy kinases genes to osmotic stress.

Real-time PCR: what relevance to plant studies?

The appearance of genetically modified organisms on the food market a few years ago, and the demand for more precise and reliable techniques to detect foreign (transgenic or pathogenic) DNA in edible plants, have been the driving force for the introduction of real-time PCR techniques in plant research. This was followed by numerous fundamental research applications aiming to study the expression profiles of endogenous genes and multigene families. Since then, the interest in this technique in the plant scientist community has increased exponentially. This review describes the technical features of quantitative real-time PCR that are especially relevant to plant research, and summarizes its present and future applications.

Expression profiling of the whole Arabidopsis shaggy-like kinase multigene family by real-time reverse transcriptase-polymerase chain reaction

The recent publication of the complete sequence of the Arabidopsis genome allowed us to identify and characterize the last two members of the SHAGGY-like kinase (AtSK) gene family. As a result, the study of the overall spatio-temporal organization of the whole AtSK family in Arabidopsis has become an achievable and necessary aim to understand the role of each SHAGGY-like kinase during plant development. An analysis of the transcript level of the 10 members of the family has been performed using the technique of real-time quantitative reverse transcriptase-polymerase chain reaction. Transcript levels in several organs, under different growth conditions, were analyzed. To calibrate the results obtained, a number of other genes, such as those coding for the two MAP3Kepsilons and the two MAP4Kalphas, as well as the stress response marker RD29A; the small subunit of the Rubisco photosynthetic enzyme Ats1A; the MEDEA chromatin remodeling factor; and the SCARECROW, ASYMMETRIC LEAVES 1, and SUPERMAN transcription factors all involved in key steps of plant development were used. The analysis of our data revealed that eight of the 10 genes of the AtSK family displayed a pseudo-constitutive expression pattern at the organ level. Conversely, AtSK13 responded to osmotic changes and saline treatment, whereas AtSK31 was flower specific and responded to osmotic changes and darkness.

Transgenic tobacco plants expressing the Drosophila Polycomb (Pc) chromodomain show developmental alterations: possible role of Pc chromodomain proteins in chromatin-mediated gene regulation in plants

The chromodomain of the Drosophila Polycomb (Pc) protein has been introduced into tobacco nuclei to determine its location in the nucleus and its effect on plant development. Pc is a repressor of homeotic Drosophila genes that shares a well-conserved, although not identical, chromodomain with a structural heterochromatin component, Heterochromatin Protein 1. The chromodomains might therefore play a common role in chromatin repression. An analysis of transgenic plants expressing the Pc chromodomain, which was linked to the green fluorescent protein, suggested that the Pc chromodomain has distinct target regions in the plant genome. Transgenic plants expressing the Pc chromodomain had phenotypic abnormalities in their leaves and flowers, indicating a disruption in development. In axillary shoot buds of plants displaying altered leaf phenotypes, enhanced expression of a homeodomain gene, which is downregulated in wild-type leaves, was found. In Drosophila, Pc has been shown to possess distinct chromosome binding activity and to be involved in the regulation of development-specific genes. Our results support the assumptions that the heterologous chromodomain affects related functions in Drosophila and in plants, and that chromatin modification mechanisms are involved in the regulation of certain plant genes, in a manner similar to chromatin-mediated gene regulation in Drosophila.

Bigfoot. a new family of MITE elements characterized from the Medicago genus

We have characterized from the legume plant Medicago a new family of miniature inverted-repeat transposable elements (MITE), called the Bigfoot transposable elements. Two of these insertion elements are present only in a single allele of two different M. sativa genes. Using a PCR strategy we have isolated 19 other Bigfoot elements from the M. sativa and M. truncatula genomes. They differ from the previously characterized MITEs by their sequence, a target site of 9 bp and a partially clustered genomic distribution. In addition, we show that they exhibit a significantly stable secondary structure. These elements may represent up to 0.1% of the genome of the outcrossing Medicago sativa but are present at a reduced copy number in the genome of the autogamous M. truncatula plant, revealing major differences in the genome organization of these two plants.

The expression pattern of alfalfa flavanone 3-hydroxylase promoter-gus fusion in Nicotiana benthamiana correlates with the presence of flavonoids detected in situ

Flavanone 3-hydroxylase is an enzyme acting in the central part of the flavonoid biosynthesis pathway. It is generally encoded by a single gene and seems to have a key position for the regulation in this pathway. These two features make a single f3h promoter-gus fusion a suitable tool to study both the f3h expression and the regulation of this pathway. We present here the spatial and temporal analysis of the expression of an alfalfa flavanone 3-hydroxylase (f3h) promoter-gus fusion introduced into Nicotiana benthamiana. The Medicago sativa (alfalfa) f3h promoter directed gus expression in flowers, stems, leaves and roots. In flowers, GUS activity was observed in pollen grains, in ovules, in ovary placenta and in the epidermis, medullary parenchyma, trichomes and second cortical cellular layer surrounding the vascular bundles of the peduncle. In stems, GUS activity was detected at the same places as in the peduncle except for the medullary parenchyma. In roots, we found GUS staining in root hairs, epidermis and in the vascular bundles of the elongated zone. Finally, in leaves, the f3h promoter expressed essentially in the stalk cells of the multicellular trichomes. The expression pattern of the f3h-gus fusion was correlated to the presence of flavonoids in situ. These data indicate that this construct can be very useful to study factors controlling the production of flavonoids.

Molecular characterization and expression of alfalfa (Medicago sativa L.) flavanone-3-hydroxylase and dihydroflavonol-4-reductase encoding genes

Flavonoids are plant phenolic compounds involved in leguminous plant-microbe interactions. Genes implied in the central branch (chalcone synthase (CHS), chalcone isomerase (CHI)) or in the isoflavonoid branch of the flavonoid biosynthesis pathway have been characterized in Medicago sativa. No information is available to date, however, on genes whose products are involved in the synthesis of other types of flavonoids. In this paper we present the genomic organization as well as the nucleotide sequence of one flavanone-3-hydroxylase (F3H) encoding gene of M. sativa, containing two introns and exhibiting 82-89% similarity at the amino acid level to other F3H proteins. This is the first report on the genomic organization of a f3h gene so far. We present also the sequence of a partial dihydroflavonol-4-reductase (DFR) M. sativa cDNA clone. Southern blot experiments indicated that f3h and dfr genes are each represented by a single gene within the tetraploid genome of M. sativa. By a combination of Northern blot and RT-PCR analysis, we showed that both f3h and dfr genes are expressed in flowers, nodules and roots, with a pattern distinct from chs expression. Finally, we show that dfr is expressed in M. sativa leaves whereas f3h is not. The role played by these two genes in organs other than flowers remains to be determined.

Modulation by catecholamines and thyrotrophin of cyclic AMP response to beta-adrenergic stimulation by cultured porcine thyroid cells

Thyroid cell cyclic AMP synthesis is stimulated by beta-adrenergic agonists. We have characterized this sensitivity on cultured porcine thyroid cells and have studied its modulation by chronic treatment with thyrotrophin. The synthesis of cyclic AMP in intact porcine thyroid cells in primary culture was stimulated by the beta-adrenergic agonist, isoproterenol. This stimulation was dose-dependent and was inhibited by the beta-adrenergic antagonists propranolol and alprenolol. The cell responsiveness (i.e. the response elicited by 5 microM-isoproterenol after 5-min stimulation) was increased when the cells were cultured in the absence of thyrotrophin. Thyrotrophin, when present in the culture medium at the onset of culturing, inhibited this increase. A concentration of 100 microunits. thyrotrophin/ml was sufficient to reduce the cyclic AMP response to 15% of its control value. Prostaglandin E2 or dibutyryl cyclic AMP did not mimic the effect of thyrotrophin. The low sensitivity of thyrotrophin-treated cells to beta-adrenergic agonists could be explained by a decreased number of beta-adrenergic receptors. [125I]Iodohydroxybenzyl pindolol specific binding was ten times greater in membrane preparations of control cells than in membranes derived from thyrotrophin-treated cells. The beta-adrenergic sensitivity of cultured thyroid cells was also decreased after long-term treatment by terbutaline. A time- and dose-dependent desensitization was observed.

Polarity of three-dimensional structures derived from isolated hog thyroid cells in primary culture

When cultured in polystyrene dishes subjected to previous treatment and supplied with a serum-containing medium, hog thyroid cells form monolayers displaying dome-like arrangements after three to four days. Cells involved in formation of "domes" are morphologically polarized; the apical microvilli of these cells point toward the culture medium. When the tissue is cultured in untreated polystyrene dishes, thyroid cells remain in suspension; their aggregates swell progressively and form hollow spheres encompassed by a single layer of cells. The polarity of the cells forming such spheres is inverse in comparison to the condition characteristic of the intact thyroid gland. When culture medium is supplemented with TSH, PGE1, PGE2 or dBC, structures resembling true follicles are formed in both types of cultures. Gelatin, added to suspension cultures, is also capable of promoting follicle formation. Cultured thyroid cells regularly form an epithelial layer as a result of the interaction of cellular processes. However, the polarization of this layer depends on culture conditions. Thus, structures with either a normal follicle-like polarization of their cells or showing an inverted type of polarization can be obtained.

Effect of gelatin on the cyclic AMP response of primocultured hog thyroid cells to acute thyrotropin stimulation

The cyclic AMP response of cultured hog thyroid cells to acute thyrotropin stimulation was shown to be under a dual regulatory control by thyrotropin: both positive and negative regulation have been described. When added to the culture medium, gelatin (0.25%) promoted the reorganization of the cells into folicle-like structures, as does thyrotropin. Unlike thyrotropin, gelatin did not induce an increase in intracellular cyclic AMP but enhanced the acute cyclic AMP response to thyrotropin in cells cultured in gelatin-containing medium. When both gelatin and thyrotropin were present, the positive effect of low concentrations of hormone (less than 50 microU/ml) was increased whereas the refractory process observed in the presence of higher concentrations of hormone (greater than 50 microU/ml) was unchanged. These effects of gelatin might be mediated by interaction of the denatured collagen molecules with external proteins of the plasma membrane of thyroid cells.

Modulation of adenylate cyclase/cyclic AMP response by thyrotropin and prostaglandin E2 in cultured thyroid cells. 1. Negative regulation

Isolated porcine thyroid cells, cultured in the presence of thyrotropin (greater than or equal to 0.25 mU/ml) or prostaglandin E2 (greater than or equal to 0.1 micron), showed decreased adenosine 3':5'-monophosphate (cyclic AMP) response to further thyrotropin or prostaglandin E2 stimulation, respectively. Kinetics of the refractory process to thyrotropin and prostaglandin E2 are different: (a) maximal refractoriness to prostaglandin E2 was attained after 2--6 h exposure to prostaglandin E2 while refractoriness to thyrotropin was maximal only after 12--24 h; (b) the degree of refractoriness to prostaglandin E2 was much greater than that to thyrotropin. Refractoriness to thyrotropin or prostaglandin E2 is characterized: by specificity for each thyroid stimulator; by dependence upon the dose of thyrotropin or prostaglandin E2 in culture, e.g. induction of high degree of refractoriness with 0.5 mU/ml thyrotropin (or 1 micron prostaglandin E2), which elicits only a small cyclic AMP increase; by time requirement for induction; by partial effect; by changes of maximum activation of cyclic AMP response; by reversibility. This refractoriness of the cyclic AMP response was not induced by dibutyryl adenosine 3':5'-monophosphate. It was not attributed to increased cyclic AMP-phosphodiesterase activity, but to alterations in the receptor-adenylate cyclase system. Prevention of refractoriness to thyrotropin or prostaglandin E2 by incubation of cells in the presence of actinomycin D, puromycin and cycloheximide suggests that new RNA and protein syntheses are required for the development of the refractory state.