Experimental Manipulation of Pectin Architecture in the Cell Wall of the Unicellular Charophyte, Penium Margaritaceum

Chemically induced phenotype plasticity in the unicellular zygnematophyte, Penium margaritaceum

Phenotypic plasticity allows a plant cell to alter its structure and function in response to external pressure. This adaptive phenomenon has also been important in the evolution of plants including the emergence of land plants from a streptophyte alga. Penium margaritaceum is a unicellular zygnematophyte (i.e., the group of streptophyte algae that is sister to land plants) that was employed in order to study phenotypic plasticity with a focus on the role of subcellular expansion centers and the cell wall in this process. Live cell fluorescence labeling, immunofluorescence labeling, transmission electron microscopy, and scanning electron microscopy showed significant subcellular changes and alterations to the cell wall. When treated with the actin-perturbing agent, cytochalasin E, cytokinesis is arrested and cells are transformed into pseudo-filaments made of up to eight or more cellular units. When treated with the cyclin-dependent kinase (CDK) inhibitor, roscovitine, cells converted to a unique phenotype with a narrow isthmus zone.

Structure and growth of plant cell walls

Plant cells build nanofibrillar walls that are central to plant growth, morphogenesis and mechanics. Starting from simple sugars, three groups of polysaccharides, namely, cellulose, hemicelluloses and pectins, with very different physical properties are assembled by the cell to make a strong yet extensible wall. This Review describes the physics of wall growth and its regulation by cellular processes such as cellulose production by cellulose synthase, modulation of wall pH by plasma membrane H+-ATPase, wall loosening by expansin and signalling by plant hormones such as auxin and brassinosteroid. In addition, this Review discusses the nuanced roles, properties and interactions of cellulose, matrix polysaccharides and cell wall proteins and describes how wall stress and wall loosening cooperatively result in cell wall growth.

The extracellular matrix of green algae

Green algae display a wide range of extracellular matrix (ECM) components that include various types of cell walls (CW), scales, crystalline glycoprotein coverings, hydrophobic compounds, and complex gels or mucilage. Recently, new information derived from genomic/transcriptomic screening, advanced biochemical analyses, immunocytochemical studies, and ecophysiology has significantly enhanced and refined our understanding of the green algal ECM. In the later diverging charophyte group of green algae, the CW and other ECM components provide insight into the evolution of plants and the ways the ECM modulates during environmental stress. Chlorophytes produce diverse ECM components, many of which have been exploited for various uses in medicine, food, and biofuel production. This review highlights major advances in ECM studies of green algae.

Structural insights of cell wall integrity signaling during development and immunity

A communication system between plant cells and their surrounding cell wall is required to coordinate development, immunity, and the integration of environmental cues. This communication network is facilitated by a large pool of membrane- and cell-wall-anchored proteins that can potentially interact with the matrix or its fragments, promoting cell wall patterning or eliciting cellular responses that may lead to changes in the architecture and chemistry of the wall. A mechanistic understanding of how these receptors and cell wall proteins recognize and interact with cell wall epitopes would be key to a better understanding of all plant processes that require cell wall remodeling such as expansion, morphogenesis, and defense responses. This review focuses on the latest developments in structurally and biochemically characterized receptors and protein complexes implicated in reading and regulating cell wall integrity and immunity.

Plant cell wall patterning and expansion mediated by protein-peptide-polysaccharide interaction

Assembly of cell wall polysaccharides into specific patterns is required for plant growth. A complex of RAPID ALKALINIZATION FACTOR 4 (RALF4) and its cell wall-anchored LEUCINE-RICH REPEAT EXTENSIN 8 (LRX8)-interacting protein is crucial for cell wall integrity during pollen tube growth, but its molecular connection with the cell wall is unknown. Here, we show that LRX8-RALF4 complexes adopt a heterotetrametric configuration in vivo, displaying a dendritic distribution. The LRX8-RALF4 complex specifically interacts with demethylesterified pectins in a charge-dependent manner through RALF4's polycationic surface. The LRX8-RALF4-pectin interaction exerts a condensing effect, patterning the cell wall's polymers into a reticulated network essential for wall integrity and expansion. Our work uncovers a dual structural and signaling role for RALF4 in pollen tube growth and in the assembly of complex extracellular polymers.

Zygnematophycean algae: Possible models for cellular and evolutionary biology

Plant terrestrialization was a critical event for our planet. For the study of plant evolution, charophytes have received a great deal of attention because of their phylogenetic position. Among charophytes, the class Zygnematophyceae is the closest lineage to land plants. During sexual reproduction, they show isogamous conjugation by immotile gametes, which is characteristic of zygnematophycean algae. Here, we introduce the genera Mougeotia, Penium, and Closterium, which are representative model organisms of Zygnematophyceae in terms of chloroplast photorelocation movement, the cell wall, and sexual reproduction, respectively.

Bricks out of the wall: polysaccharide extramural functions

Plant polysaccharides are components of plant cell walls and/or store energy. However, this oversimplified classification neglects the fact that some cell wall polysaccharides and glycoproteins can localize outside the relatively sharp boundaries of the apoplastic moiety, where they adopt functions not directly related to the cell wall. Such polysaccharide multifunctionality (or 'moonlighting') is overlooked in current research, and in most cases the underlying mechanisms that give rise to unconventional ex muro trafficking, targeting, and functions of polysaccharides and glycoproteins remain elusive. This review highlights major examples of the extramural occurrence of various glycan cell wall components, discusses the possible significance and implications of these phenomena for plant physiology, and lists exciting open questions to be addressed by future research.

The cell biology of charophytes: Exploring the past and models for the future

Charophytes (Streptophyta) represent a diverse assemblage of extant green algae that are the sister lineage to land plants. About 500-600+ million years ago, a charophyte progenitor successfully colonized land and subsequently gave rise to land plants. Charophytes have diverse but relatively simple body plans that make them highly attractive organisms for many areas of biological research. At the cellular level, many charophytes have been used for deciphering cytoskeletal networks and their dynamics, membrane trafficking, extracellular matrix secretion, and cell division mechanisms. Some charophytes live in challenging habitats and have become excellent models for elucidating the cellular and molecular effects of various abiotic stressors on plant cells. Recent sequencing of several charophyte genomes has also opened doors for the dissection of biosynthetic and signaling pathways. While we are only in an infancy stage of elucidating the cell biology of charophytes, the future application of novel analytical methodologies in charophyte studies that include a broader survey of inclusive taxa will enhance our understanding of plant evolution and cell dynamics.

Cell Wall Signaling in Plant Development and Defense

Plant architecture fundamentally differs from that of other multicellular organisms in that individual cells serve as osmotic bricks, defined by the equilibrium between the internal turgor pressure and the mechanical resistance of the surrounding cell wall, which constitutes the interface between plant cells and their environment. The state and integrity of the cell wall are constantly monitored by cell wall surveillance pathways, which relay information to the cell interior. A recent surge of discoveries has led to significant advances in both mechanistic and conceptual insights into a multitude of cell wall response pathways that play diverse roles in the development, defense, stress response, and maintenance of structural integrity of the cell. However, these advances have also revealed the complexity of cell wall sensing, and many more questions remain to be answered, for example, regarding the mechanisms of cell wall perception, the molecular players in this process, and how cell wall-related signals are transduced and integrated into cellular behavior. This review provides an overview of the mechanistic and conceptual insights obtained so far and highlights areas for future discoveries in this exciting area of plant biology.

The role of pectin phase separation in plant cell wall assembly and growth

A rapidly increasing body of literature suggests that many biological processes are driven by phase separation within polymer mixtures. Liquid-liquid phase separation can lead to the formation of membrane-less organelles, which are thought to play a wide variety of roles in cell metabolism, gene regulation or signaling. One of the characteristics of these systems is that they are poised at phase transition boundaries, which makes them perfectly suited to elicit robust cellular responses to often very small changes in the cell's "environment". Recent observations suggest that, also in the semi-solid environment of plant cell walls, phase separation not only plays a role in wall patterning, hydration and stress relaxation during growth, but also may provide a driving force for cell wall expansion. In this context, pectins, the major polyanionic polysaccharides in the walls of growing cells, appear to play a critical role. Here, we will discuss (i) our current understanding of the structure-function relationship of pectins, (ii) in vivo evidence that pectin modification can drive critical phase transitions in the cell wall, (iii) how such phase transitions may drive cell wall expansion in addition to turgor pressure and (iv) the periodic cellular processes that may control phase transitions underlying cell wall assembly and expansion.

The effects of osmotic stress on the cell wall-plasma membrane domains of the unicellular streptophyte, Penium margaritaceum

Penium margaritaceum is a unicellular zygnematophyte (basal Streptophyteor Charophyte) that has been used as a model organism for the study of cell walls of Streptophytes and for elucidating organismal adaptations that were key in the evolution of land plants.. When Penium is incubated in sorbitol-enhance medium, i.e., hyperosmotic medium, 1000-1500 Hechtian strands form within minutes and connect the plasma membrane to the cell wall. As cells acclimate to this osmotic stress over time, further significant changes occur at the cell wall and plasma membrane domains. The homogalacturonan lattice of the outer cell wall layer is significantly reduced and is accompanied by the formation of a highly elongate, "filamentous" phenotype. Distinct peripheral thickenings appear between the CW and plasma membrane and contain membranous components and a branched granular matrix. Monoclonal antibody labeling of these thickenings indicates the presence of rhamnogalacturonan-I epitopes. Acclimatization also results in the proliferation of the cell's vacuolar networks and macroautophagy. Penium's ability to acclimatize to osmotic stress offers insight into the transition of ancient zygnematophytes from an aquatic to terrestrial existence.

An aptamer highly specific to cellulose enables the analysis of the association of cellulose with matrix cell wall polymers in vitro and in muro

The current toolbox of cell wall-directed molecular probes has been pivotal for advancing basic and application-oriented plant carbohydrate research; however, it still exhibits limitations regarding target diversity and specificity. Scarcity of probes targeting intramolecular associations between cell wall polymers particularly hinders our understanding of the cell wall microstructure and affects the development of effective means for its efficient deconstruction for bioconversion. Here we report a detailed characterization of a cellulose-binding DNA aptamer CELAPT MINI using a combination of various in vitro biochemical, biophysical, and molecular biology techniques. Our results show evidence for its high specificity towards long non-substituted β-(1-4)-glucan chains in both crystalline and amorphous forms. Fluorescent conjugates of CELAPT MINI are applicable as in situ cellulose probes and are well suited for various microscopy techniques, including super-resolution imaging. Compatibility of fluorescent CELAPT MINI variants with immunodetection of cell wall matrix polymers enabled them simultaneously to resolve the fibrillar organization of complex cellulose-enriched pulp material and to quantify the level of cellulose masking by xyloglucan and xylan. Using enzymatically, chemically, or genetically modulated Brachypodium internode sections we showed the diversity in cell wall packing among various cell types and even cell wall microdomains. We showed that xylan is the most prominent, but not the only, cellulose-masking agent in Brachypodium internode tissues. These results collectively highlight the hitherto unexplored potential to expand the cell wall probing toolbox with highly specific and versatile in vitro generated polynucleotide probes.

Arabinogalactan in the side chain of pectin from persimmon is involved in the interaction with small intestinal epithelial cells

Pectin in Diospyros kaki (persimmon) is a complex polysaccharide and is classified as a dietary fiber. Pectin is characterized by the presence of side chains of neutral sugars, such as galactose residues; however, the structure and properties of these sugars vary greatly depending on the plant species from which it is derived. Here, we report the structural features of pectin extracted from persimmon. The polysaccharide was low-methoxy pectin with a degree of methyl esterification <50% and ratio of side chain galactan to arabinan in the rhamnogalacturonan-I region of pectin of 3-20. To investigate the physiological function of pectin from persimmon, we performed a coculture assay using Caco-2 cells. As a result, it was shown that the proliferation of undifferentiated Caco-2 cells was promoted, and further, the importance of arabinogalactan among the pectin structures was shown.

Current and future advances in fluorescence-based visualization of plant cell wall components and cell wall biosynthetic machineries

Plant cell wall-derived biomass serves as a renewable source of energy and materials with increasing importance. The cell walls are biomacromolecular assemblies defined by a fine arrangement of different classes of polysaccharides, proteoglycans, and aromatic polymers and are one of the most complex structures in Nature. One of the most challenging tasks of cell biology and biomass biotechnology research is to image the structure and organization of this complex matrix, as well as to visualize the compartmentalized, multiplayer biosynthetic machineries that build the elaborate cell wall architecture. Better knowledge of the plant cells, cell walls, and whole tissue is essential for bioengineering efforts and for designing efficient strategies of industrial deconstruction of the cell wall-derived biomass and its saccharification. Cell wall-directed molecular probes and analysis by light microscopy, which is capable of imaging with a high level of specificity, little sample processing, and often in real time, are important tools to understand cell wall assemblies. This review provides a comprehensive overview about the possibilities for fluorescence label-based imaging techniques and a variety of probing methods, discussing both well-established and emerging tools. Examples of applications of these tools are provided. We also list and discuss the advantages and limitations of the methods. Specifically, we elaborate on what are the most important considerations when applying a particular technique for plants, the potential for future development, and how the plant cell wall field might be inspired by advances in the biomedical and general cell biology fields.

Dampening the DAMPs: How Plants Maintain the Homeostasis of Cell Wall Molecular Patterns and Avoid Hyper-Immunity

Several oligosaccharide fragments derived from plant cell walls activate plant immunity and behave as typical damage-associated molecular patterns (DAMPs). Some of them also behave as negative regulators of growth and development, and due to their antithetic effect on immunity and growth, their concentrations, activity, time of formation, and localization is critical for the so-called "growth-defense trade-off." Moreover, like in animals, over accumulation of DAMPs in plants provokes deleterious physiological effects and may cause hyper-immunity if the cellular mechanisms controlling their homeostasis fail. Recently, a mechanism has been discovered that controls the activity of two well-known plant DAMPs, oligogalacturonides (OGs), released upon hydrolysis of homogalacturonan (HG), and cellodextrins (CDs), products of cellulose breakdown. The potential homeostatic mechanism involves specific oxidases belonging to the family of berberine bridge enzyme-like (BBE-like) proteins. Oxidation of OGs and CDs not only inactivates their DAMP activity, but also makes them a significantly less desirable food source for microbial pathogens. The evidence that oxidation and inactivation of OGs and CDs may be a general strategy of plants for controlling the homeostasis of DAMPs is discussed. The possibility exists of discovering additional oxidative and/or inactivating enzymes targeting other DAMP molecules both in the plant and in animal kingdoms.