Morphogenesis at the shoot meristem.

Shoot apical meristems are populations of stem cells which initiate the aerial parts of higher plants. Work during the last decades has revealed a complex network of molecular regulators, which control both meristem maintenance and the production of different types of organs. The behavior of this network in time and space is defined by the local interactions between regulators and also involves hormonal regulation.

Jan Traas.

Interview with Jan Traas, who studies shoot meristem function at the École normale supérieure de Lyon.

A 3D gene expression atlas of the floral meristem based on spatial reconstruction of single nucleus RNA sequencing data.

Cellular heterogeneity in growth and differentiation results in organ patterning. Single-cell transcriptomics allows characterization of gene expression heterogeneity in developing organs at unprecedented resolution. However, the original physical location of the cell is lost during this methodology.

Benchmarking of deep learning algorithms for 3D instance segmentation of confocal image datasets.

Segmenting three-dimensional (3D) microscopy images is essential for understanding phenomena like morphogenesis, cell division, cellular growth, and genetic expression patterns. Recently, deep learning (DL) pipelines have been developed, which claim to provide high accuracy segmentation of cellular images and are increasingly considered as the state of the art for image segmentation problems. However, it remains difficult to define their relative performances as the concurrent diversity and lack of uniform evaluation strategies makes it difficult to know how their results compare.

Stable establishment of organ polarity occurs several plastochrons before primordium outgrowth in Arabidopsis.

In many species, leaves are initiated at the flanks of shoot meristems. Subsequent growth usually occurs mainly in the plane of the leaf blade, which leads to the formation of a bifacial leaf with dorsoventral identities. In a classical set of surgical experiments in potato meristems, Sussex provided evidence that dorsoventrality depends on a signal emanating from the meristem center.

A multiscale analysis of early flower development in Arabidopsis provides an integrated view of molecular regulation and growth control.

We have analyzed the link between the gene regulation and growth during the early stages of flower development in Arabidopsis. Starting from time-lapse images, we generated a 4D atlas of early flower development, including cell lineage, cellular growth rates, and the expression patterns of regulatory genes. This information was introduced in MorphoNet, a web-based platform.

How Mechanical Forces Shape Plant Organs.

Plants produce organs of various shapes and sizes. While much has been learned about genetic regulation of organogenesis, the integration of mechanics in the process is also gaining attention. Here, we consider the role of forces as instructive signals in organ morphogenesis.

Visualization of cortical microtubule networks in plant cells by live imaging and immunostaining.

Cortical microtubules (CMTs) play pivotal roles during plant cell growth and division. The organization of CMTs undergoes important changes during different cellular and developmental processes. Here, we describe two methods for the visualization of CMT organization in plant cells using confocal laser scanning microscopy.

The Mechanical Feedback Theory of Leaf Lamina Formation.

The appearance of leaves with flattened laminae about 400 million years (Myr) ago had broad impacts on the Earth's ecosystem. The influential telome theory presents a model for this evolutionary transition, although it lacks plausible molecular evidence. Recently, microtubule-mediated mechanical feedback was proposed as a parsimonious alternative mechanism to explain leaf blade evolution.

Microtubule-Mediated Wall Anisotropy Contributes to Leaf Blade Flattening.

Plant organs can adopt a wide range of shapes, resulting from highly directional cell growth and divisions. We focus here on leaves and leaf-like organs in Arabidopsis and tomato, characterized by the formation of thin, flat laminae. Combining experimental approaches with 3D mechanical modeling, we provide evidence that leaf shape depends on cortical microtubule mediated cellulose deposition along the main predicted stress orientations, in particular, along the adaxial-abaxial axis in internal cell walls.

Cellular Heterogeneity in Pressure and Growth Emerges from Tissue Topology and Geometry.

Cell-to-cell heterogeneity prevails in many systems, as exemplified by cell growth, although the origin and function of such heterogeneity are often unclear. In plants, growth is physically controlled by cell wall mechanics and cell hydrostatic pressure, alias turgor pressure. Whereas cell wall heterogeneity has received extensive attention, the spatial variation of turgor pressure is often overlooked.

Xyloglucans and Microtubules Synergistically Maintain Meristem Geometry and Phyllotaxis.

The shoot apical meristem (SAM) gives rise to all aerial plant organs. Cell walls are thought to play a central role in this process, translating molecular regulation into dynamic changes in growth rate and direction, although their precise role in morphogenesis during organ formation is poorly understood. Here, we investigated the role of xyloglucans (XyGs), a major, yet functionally poorly characterized, wall component in the SAM of Arabidopsis ().

Simulating Turgor-Induced Stress Patterns in Multilayered Plant Tissues.

The intertwining between mechanics and developmental biology is extensively studied at the shoot apical meristem of land plants. Indeed, plant morphogenesis heavily relies on mechanics; tissue deformations are fueled by turgor-induced forces, and cell mechanosensitivity plays a major regulatory role in this dynamics. Since measurements of forces in growing meristems are still out of reach, our current knowledge relies mainly on theoretical and numerical models.

Organogenesis at the Shoot Apical Meristem.

Lateral organ initiation at the shoot apical meristem involves complex changes in growth rates and directions, ultimately leading to the formation of leaves, stems and flowers. Extensive molecular analysis identifies auxin and downstream transcriptional regulation as major elements in this process. This molecular regulatory network must somehow interfere with the structural elements of the cell, in particular the cell wall, to induce specific morphogenetic events.

Evidence for the Regulation of Gynoecium Morphogenesis by via Cell Wall Dynamics.

() is an atypical member of the AUXIN RESPONSE FACTOR family of transcription factors that plays a crucial role in tissue patterning in the Arabidopsis () gynoecium. Though recent insights have provided valuable information on ETT's interactions with other components of auxin signaling, the biophysical mechanisms linking ETT to its ultimate effects on gynoecium morphology were until now unknown. Here, using techniques to assess cell-wall dynamics during gynoecium growth and development, we provide a coherent body of evidence to support a model in which ETT controls the elongation of the valve tissues of the gynoecium through the positive regulation of pectin methylesterase (PME) activity in the cell wall.

Regulation of plant cell wall stiffness by mechanical stress: a mesoscale physical model.

A crucial question in developmental biology is how cell growth is coordinated in living tissue to generate complex and reproducible shapes. We address this issue here in plants, where stiff extracellular walls prevent cell migration and morphogenesis mostly results from growth driven by turgor pressure. How cells grow in response to pressure partly depends on the mechanical properties of their walls, which are generally heterogeneous, anisotropic and dynamic.

Transcriptional induction of cell wall remodelling genes is coupled to microtubule-driven growth isotropy at the shoot apex in .

The shoot apical meristem of higher plants continuously generates new tissues and organs through complex changes in growth rates and directions of its individual cells. Cell growth, which is driven by turgor pressure, largely depends on the cell walls, which allow cell expansion through synthesis and structural changes. A previous study revealed a major contribution of wall isotropy in organ emergence, through the disorganization of cortical microtubules.

Mechanical signaling in plant morphogenesis.

To control changes in shape during development, the molecular regulatory networks have to interact with the mechanical, structural components of the individual cells, in particular the cytoskeleton and the cell wall. A widely accepted hypothesis proposes that molecular regulation interferes with wall synthesis and stiffness, causing the wall polymers to yield to the internal turgor pressure. However, growth is not only the result of a rigid molecular program instructing the cells precisely what to do.

Cell-size dependent progression of the cell cycle creates homeostasis and flexibility of plant cell size.

Mean cell size at division is generally constant for specific conditions and cell types, but the mechanisms coupling cell growth and cell cycle control with cell size regulation are poorly understood in intact tissues. Here we show that the continuously dividing fields of cells within the shoot apical meristem of Arabidopsis show dynamic regulation of mean cell size dependent on developmental stage, genotype and environmental signals. We show cell size at division and cell cycle length is effectively predicted using a two-stage cell cycle model linking cell growth and two sequential cyclin dependent kinase (CDK) activities, and experimental results concur in showing that progression through both G1/S and G2/M is size dependent.

Plant Development: From Dynamics to Mechanics.

A new study analyses the complex changes in shape occurring during petal development in snapdragon. Combining simulations with quantitative analysis leads to a new model, where molecular regulators control overall organ shape through mechanical conflicts operating at the level of entire tissues.

Flower development: from morphodynamics to morphomechanics.

The shoot apical meristem (SAM) is a small population of stem cells that continuously generates organs and tissues. We will discuss here flower formation at the SAM, which involves a complex network of regulatory genes and signalling molecules. A major downstream target of this network is the extracellular matrix or cell wall, which is a local determinant for both growth rates and growth directions.

In-vivo analysis of morphogenesis in plants.

Although many molecular regulators of morphogenesis have been identified in plants, it remains largely unknown how the molecular networks influence local cell shape and how cell growth, form, and position are coordinated during tissue and organ formations. So far, analyses of gene function in morphogenesis have mainly focused on the qualitative analysis of phenotypes, often providing limited mechanistic insight into how particular factors act. For this reason, there has been a growing interest in mathematical and computational models to formalize and test hypotheses.

The heterodimeric transcription factor complex ERF115-PAT1 grants regeneration competence.

Regeneration of a tissue damaged by injury represents a physiological response for organ recovery. Although this regeneration process is conserved across multicellular taxa, plants appear to display extremely high regenerative capacities, a feature widely used in tissue culture for clonal propagation and grafting. Regenerated cells arise predominantly from pre-existing populations of division-competent cells; however, the mechanisms by which these cells are triggered to divide in response to injury remain largely elusive.

Force-Driven Polymerization and Turgor-Induced Wall Expansion.

While many molecular players involved in growth control have been identified in the past decades, it is often unknown how they mechanistically act to induce specific shape changes during development. Plant morphogenesis results from the turgor-induced yielding of the extracellular and load-bearing cell wall. Its mechanochemical equilibrium appears as a fundamental link between molecular growth regulation and the effective shape evolution of the tissue.