Biology subjects

Charrier, B.

Publications and source records attributed to Charrier, B..

10 recordsLinked to original sources

bioRxiv2026

Radical cell identity bifurcation in Saccharina embryos coincides with the expression of newly acquired genes.

Brown algae evolved independently from animals, land plants and other algae, and we know very little about the spatio-temporal dynamics of their embryogenesis. Here, we used time-lapse, bright-field microscopy to study cell division and lineage development during early embryogenesis in the kelp Saccharina latissima, a large brown alga. We discovered a radical change of cell identity as early as the 4-cell stage: after fertilization, the zygote underwent two or three unequal cell divisions before the basal cell - that closest to the maternal tissue - stopped dividing and radically differentiated into a hyperpolarized cell, the rhizoid, which anchors the embryo to the substrate. RNA-seq analysis showed that differentiation of rhizoid cells was preceded by expression of 130 basal cell-specific genes. Phylostratigraphic analysis further revealed that more than 40% of these basal cell-specific genes appeared after the emergence of the brown algae group, and their functions are largely unknown. By contrast, the apical cell predominantly expressed more ancestral, metabolism-related genes, and it continued to divide to produce the long, blade-shaped thallus of the alga. The early and radical nature of cell differentiation in Saccharina embryos, combined with differential gene expression from various evolutionary periods, highlights the unique mechanisms of embryogenesis of this alga.

developmental biology↗

bioRxiv2026

LipoTag: A minimal motif for live and functional imaging of plant cell membranes.

The plant plasma membrane is a highly dynamic structure that is crucial for cell compartmentalization, the maintenance of (bio)chemical gradients, signaling and cell growth and responses to stress. In plants, plasma membranes are tightly connected to the cell walls that encase them. These cell walls can act as diffusion barriers and prevent the use of a wide range of synthetic fluorescent probes that have been developed to study animal cell membranes, which lack a cell wall, with live functional imaging. Here, we introduce LipoTag, a minimal chemical motif that, upon chemical conjugation, transforms hydrophobic fluorophores into water-soluble, membrane-targeted probes that can permeate plant cell walls to reach their intended location. LipoTag uses a localized positive charge in combination with a short aliphatic spacer to direct cargo to the plasma membrane. We used LipoTag to develop a suite of membrane-specific fluorescent probes that work in walled organisms beyond the plant kingdom. In addition, we used LipoTag to develop functional reporters for the quantitative imaging of membrane density, lipid order and membrane oxidation in living plant tissues. LipoTag forms a modular platform for exploring the plant plasma membrane with a suite of contemporary imaging modalities.

plant biology↗

bioRxiv2026

Cell position is more important than cell shape or age for the acquisition of cell identity in the brown alga Ectocarpus.

The brown alga Ectocarpus is a morphologically simple organism, with cells in the growing filament changing shape and position over time. In this study, we investigated the roles of cell shape, position, and age in establishing cell identity in Ectocarpus. Using laser capture microdissection, we isolated cells with different relative ages, shapes and positions from young sporophytes, and analysed their transcriptomes using differential RNA-seq to attribute a molecular signature to each cell. Five distinct cell transcriptomic signatures were found along the filament of WT Ectocarpus; the apical cell signature differed most from the others. To dissociate cell shape, age, and position, we analysed the transcriptomes of two mutants in which the relationships between these three parameters were disrupted. We found that the transcriptomic profile observed in the WT was significantly altered in these mutants. Overall, our data revealed that molecular cell signatures depend primarily on cell position along the filament and secondarily on cell shape.

developmental biology↗

bioRxiv2025

SBIS, a new orange fluorescent vital probe for the 4D imaging of brown algal cells

Living cells of brown algae are difficult to observe in 3D because pigments such as fucoxanthin and chlorophyll diffract light. Furthermore, at the beginning of their life, brown algae develop slowly in seawater. To gain insight into the 3D shape and size of brown algal cells during embryogenesis, we designed a fluorescence probe that efficiently and selectively labels the plasma membrane. Styryl benzoindoleninium sulfonate (SBIS) is a bright orange fluorogenic probe that is soluble and virtually non-emissive in seawater and is activated upon binding to the plasma membrane. Unlike Calcofluor White, SBIS enables observation of cells at thicknesses of up to 25 {micro}m. More importantly, SBIS allows three-dimensional observation of the cells in the growing uniseriate filaments of Ectocarpus sp., the polystichous filaments of Sphacelaria rigidula and the cellular monolayered lamina of Saccharina latissima over periods of up to seven days. Altogether, these properties allow visualization of the entire cell contours in living brown algae, making the study of early development at the cellular level in 4D now possible in these marine organisms.

cell biology↗

bioRxiv2024

The longitudinal growth of the embryo of the kelp Saccharina depends on actin filaments that control the formation of an alginate corset in the cell wall

The initiation of embryogenesis in the kelp Saccharina latissima is accompanied by significant anisotropy in cell shape. Using monoclonal antibodies, we show that this anisotropy coincides with a spatio-temporal pattern of accumulation of alginates in the cell wall of the zygote and embryo. Alginates rich in guluronates as well as sulphated fucans show a homogeneous distribution in the embryo throughout Phase I of embryogenesis, but mannuronate alginates accumulate mainly on the sides of the zygote and embryo, disappearing as the embryo enlarges at the start of Phase II. This pattern depends on the presence of cortical actin filaments. In contrast, within the embryo lamina, the alginate composition of the walls newly formed by cytokinesis is not affected by the depolymerisation of actin filaments. Thus, in addition to revealing the existence of a mannuronate-rich alginate corset that may restrict the enlargement of the zygote and the embryo, thereby promoting the formation of the apico-basal growth axis, we demonstrate stage- and cytoskeleton-dependent differences in cell wall deposition in Saccharina embryos.

developmental biology↗

bioRxiv2024

CarboTag: a modular approach for live and functional imaging of plant cell walls

Plant cells are contained inside a rigid network of cell walls. Cell walls are highly dynamic structures that act both as a structural material and as a hub for a wide range of signaling processes. Despite its crucial role in all aspects of the plant life cycle, live dynamical imaging of the cell wall and its functional properties has remained challenging. Here, we introduce CarboTag, a modular toolbox for live functional imaging of plant walls. CarboTag relies on a small molecular motif, a pyridine boronic acid, that targets its cargo to the cell wall, is non-toxic and ensures rapid tissue permeation. We designed a suite of cell wall imaging probes based on CarboTag in any desired color for multiplexing. Moreover, we created new functional reporters for live quantitative imaging of key cell wall features: network porosity, cell wall pH and the presence of reactive oxygen species. CarboTag opens the way to dynamical and quantitative mapping of cell wall responses at subcellular resolution.

plant biology↗

bioRxiv2024

MUM, a maternal unknown message, inhibits early establishment of the mediolateral axis in the embryo of the kelp Saccharina latissima

Brown algae are multicellular photosynthetic organisms that have evolved independently of plants and other algae. Apart from studies on the polarisation of the Fucus zygote in the 1990s, little is known about the mechanisms controlling the embryogenesis of these organisms. Here, we studied the determinism of embryogenesis in the kelp Saccharina latissima, focusing on the formation of its body axes. This alga initially develops an oblong embryo formed of a cell monolayer, which subsequently thickens; growth axes are then established in temporally distinct stages, starting with the formation of a dominant apico-basal axis. Our experiments focused on the role of the stalk, an empty cell that retains the embryo attached to the maternal tissue, in the development of the growth axes in mature embryos. In response to the removal of the stalk, the embryos developed as a monolayered disc rather than an elongated blade, demonstrating that attachment to the stalk inhibits the formation of the medio-lateral axis at the onset of embryogenesis. In addition, compared with embryos attached to the stalk, the cells of detached embryos were smaller and displayed an altered shape. The topology of the tissue was also disrupted, as cells had a higher number of cell neighbours. Observation of cell division patterns just after removal of the stalk showed that the stalk represses longitudinal cell divisions, thereby reinforcing the establishment of the main apico-basal axis. This unique quantitative study of brown algal embryogenesis revealed that, in kelps, a signal from maternal tissue (MUM for maternal unknown message) is necessary for the establishment of growth axes at the onset of embryogenesis and of the organisation of growing embryonic tissues. In addition, we discovered that, although the stalk persists for several weeks until the embryo reaches at least 500 cells, MUM is emitted in the first 4 days after fertilisation only, before the embryo reaches the 8-cell stage. Finally, transplantation experiments indicated that MUM does not diffuse in seawater, but requires contact between the embryo and the stalk. The potential chemical or mechanical nature of MUM is discussed.

developmental biology↗

bioRxiv2021

Tip growth in the brown alga Ectocarpus is controlled by a RHO-GAP-BAR domain protein independently from F-actin organisation

The brown alga Ectocarpus is a filamentous seaweed that grows by tip growth and branching. In the morphometric mutant etoile, tip growth is slower than in the WT and eventually stops. In this paper, we show that the causal etoile mutation is a null mutation in a bi-domain BAR-RhoGAP gene. By quantitative RT-PCR, we showed that ETOILE is ubiquitously expressed in prostrate filaments of the Ectocarpus sporophyte, and is downregulated in the etoile mutant. We immunolocalised both domains of the protein in WT and etoile, as well as RAC1, the known target of Rho-GAP enzymes. Thus, ETOILE would be localised at the apical cell dome where it would control the localisation of EsRAC1 to the plasma membrane. Actin staining showed that the mutant is not affected in F-actin structures. Overall, these results suggest that in Ectocarpus, BAR-RhoGAP controls tip growth by controlling RAC1 localization and through an actin-independent mechanism.

developmental biology↗

bioRxiv2021

Acquisition of cell identity in the brown alga Ectocarpus: which of time, cell shape or position matters most?

During development, cells undergo simultaneous changes of different types that together depict cell "identity". In the multicellular brown alga Ectocarpus sp., while ageing, cells change shape and relative position within the filament. Understanding how these factors act and interact to specify cell identity requires markers of cell identity and the ability to genetically separate age, shape and position. Here we used laser capture microdissection (LCM) to isolate specific cell types from young sporophytes of Ectocarpus, and performed differential RNA-seq analysis. Transcriptome profiles of cell types in the wild-type strain provided signatures of the five cell types that can be identified by shape and position. In two mutants, where the relationship between cell shape, position and age are altered, transcriptome signatures revealed that little differential expression could be identified when only shape was perturbed. More generally, although the two mutants are characterised by opposite morphological phenotypes, their transcriptomes were remarkably similar. We concluded that despite the robustness of cell differentiation during WT development, neither the shape nor the position of the cell could serve as a faithful gauge for tracking differentiation.

developmental biology↗

bioRxiv2021

Growth and immunolocalisation of the brown alga Ectocarpus in a microfluidic environment

PDMS chips have proven to be suitable environments for the growth of several filamentous organisms. However, depending on the specimen, the pattern of growth and cell differentiation has been rarely investigated. We monitored the developmental pattern of the brown alga Ectocarpus inside a PDMS lab-on-chip. Two main methods of inoculation of the lab-on-chip were tested, i.e. by injection of spores or by insertion of sporophyte filaments into the chamber. Growth rate, growth trajectory, cell differentiation, and branching were the main development steps that were monitored for 20 days inside 25 m or 40 m parallel channels under standard light and temperature conditions. They were shown to be similar to those observed in non-constrained in-vitro conditions. Labelling of Ectocarpus cell wall polysaccharides - both with calcofluor for cellulose, and by immunolocalisation for alginates with monoclonal antibodies-showed expected patterns when compared to open space growth using either epifluorescence or confocal microscopy. Overall this article describes the experimental conditions for observing and studying the basic unaltered processes of brown algal growth using microfluidic technology, which provides the basis for future biochemical and biological research.

cell biology↗