Spatial organization of cellulose microfibrils and matrix polysaccharides in primary plant cell walls as imaged by multichannel atomic force microscopy

A review of polysaccharides from Ganoderma lucidum: Preparation methods, structural characteristics, bioactivities, structure-activity relationships and potential applications

Ganoderma lucidum (G. lucidum), commonly known as "fairy grass", is a saprophyte fungus belonging to the Polyporaceae family with a lengthy history of use as a food ingredient and traditional medicine in China. G. lucidum is abundant in diverse chemical compounds that encompass polysaccharides, alkaloids, steroids, terpenoids, proteins, as well as amino acids. Among these, polysaccharides as the main active ingredients prepared from G. lucidum have exhibited a multitude of biological activities, such as anti-tumor effect, antioxidant activity, antidiabetic effect, anti-inflammatory effect, immune regulation and so forth. The main methods for extracting GLPs are hot water extraction, ultrasound-assisted extraction, microwave-assisted extraction and enzyme-assisted extraction. The research on GLPs poses challenges, especially in optimizing extraction and purification processes to enhance the yield and preserve the structural characteristics. Furthermore, it remains ambiguous whether the bioactivities of GLPs are closely related to their extraction methods and structural characteristics, necessitating further exploration and elucidation of the structure-activity relationships. This review comprehensively and systematically outlines an overview of the preparation methods, structural characteristics, bioactivities, structure-activity relationships and potential applications of GLPs. The review emphasizes their therapeutic potential and health functions, providing a reference for further exploitation and application in various fields of GLPs.

Heterogeneity in Mechanical Properties of Plant Cell Walls

The acquisition and utilization of cell walls have fundamentally shaped the plant lifestyle. While the walls provide mechanical strength and enable plants to grow and occupy a three-dimensional space, successful sessile life also requires the walls to undergo dynamic modifications to accommodate size and shape changes accurately. Plant cell walls exhibit substantial mechanical heterogeneity due to the diverse polysaccharide composition and different development stages. Here, we review recent research advances, both methodological and experimental, that shed new light on the architecture of cell walls, with a focus on the mechanical heterogeneity of plant cell walls. Facilitated by advanced techniques and tools, especially atomic force microscopy (AFM), research efforts over the last decade have contributed to impressive progress in our understanding of how mechanical properties are associated with cell growth. In particular, the pivotal importance of pectin, the most complex wall polysaccharide, in wall mechanics is rapidly emerging. Pectin is regarded as an important determinant for establishing anisotropic growth patterns of elongating cells. Altogether, the diversity of plant cell walls can lead to heterogeneity in the mechanical properties, which will help to reveal how mechanical factors regulate plant cell growth and organ morphogenesis.

How Many Glucan Chains Form Plant Cellulose Microfibrils? A Mini Review

Assessing the number of glucan chains in cellulose microfibrils (CMFs) is crucial for understanding their structure-property relationships and interactions within plant cell walls. This Review examines the conclusions and limitations of the major experimental techniques that have provided insights into this question. Small-angle X-ray and neutron scattering data predominantly support an 18-chain model, although analysis is complicated by factors such as fibril coalescence and matrix polysaccharide associations. Solid-state nuclear magnetic resonance (NMR) spectroscopy allows the estimation of the CMF width from the ratio of interior to surface glucose residues. However, there is uncertainty in the assignment of NMR spectral peaks to surface or interior chains. Freeze-fracture transmission electron microscopy images show cellulose synthase complexes to be "rosettes" of six lobes each consistent with a trimer of cellulose synthase enzymes, consistent with the synthesis of 18 parallel glucan chains in the CMF. Nevertheless, the number of chains in CMFs remains to be conclusively demonstrated.

Enzymatic approaches for diversifying bioproducts from cellulosic biomass

Cellulosic biomass is the most abundantly available natural carbon-based renewable resource on Earth. Its widespread availability, combined with rising awareness, evolving policies, and changing regulations supporting sustainable practices, has propelled its role as a crucial renewable feedstock to meet the escalating demand for eco-friendly and renewable materials, chemicals, and fuels. Initially, biorefinery models using cellulosic biomass had focused on single-product platform, primarily monomeric sugars for biofuel. However, since the launch of the first pioneering cellulosic plants in 2014, these models have undergone significant revisions to adapt their biomass upgrading strategy. These changes aim to diversify the bioproduct portfolio and improve the revenue streams of cellulosic biomass biorefineries. Within this area of research and development, enzyme-based technologies can play a significant role by contributing to eco-design in producing and creating innovative bioproducts. This Feature Article highlights our strategies and recent progress in utilizing the biological diversity and inherent selectivity of enzymes to develop and continuously optimize sustainable enzyme-based technologies with distinct application approaches. We have advanced technologies for standalone platforms, which produce various forms of cellulose nanomaterials engineered with customized and enhanced properties and high yields. Additionally, we have tailored technologies for integration within a biorefinery concept. This biorefinery approach prioritizes designing tailored processes to establish bionanomaterials, such as cellulose and lignin nanoparticles, and bioactive molecules as part of a new multi-bioproduct platform for cellulosic biomass biorefineries. These innovations expand the range of bioproducts that can be produced from cellulosic biomass, transcending the conventional focus on monomeric sugars for biofuel production to include biomaterials biorefinery. This shift thereby contributes to strengthening the Bioeconomy strategy and supporting the achievement of several Sustainable Development Goals (SDGs) of the 2030 Agenda for Sustainable Development.

Dynamic Structural Change of Plant Epidermal Cell Walls under Strain

The molecular foundations of epidermal cell wall mechanics are critical for understanding structure-function relationships of primary cell walls in plants and facilitating the design of bioinspired materials. To uncover the molecular mechanisms regulating the high extensibility and strength of the cell wall, the onion epidermal wall is stretched uniaxially to various strains and cell wall structures from mesoscale to atomic scale are characterized. Upon longitudinal stretching to high strain, epidermal walls contract in the transverse direction, resulting in a reduced area. Atomic force microscopy shows that cellulose microfibrils exhibit orientation-dependent rearrangements at high strains: longitudinal microfibrils are straightened out and become highly ordered, while transverse microfibrils curve and kink. Small-angle X-ray scattering detects a 7.4 nm spacing aligned along the stretch direction at high strain, which is attributed to distances between individual cellulose microfibrils. Furthermore, wide-angle X-ray scattering reveals a widening of (004) lattice spacing and contraction of (200) lattice spacing in longitudinally aligned cellulose microfibrils at high strain, which implies longitudinal stretching of the cellulose crystal. These findings provide molecular insights into the ability of the wall to bear additional load after yielding: the aggregation of longitudinal microfibrils impedes sliding and enables further stretching of the cellulose to bear increased loads.

Single-Particle and Single-Molecule Characterization of Immobilized Enzymes: A Multiscale Path toward Optimizing Heterogeneous Biocatalysts

Heterogeneous biocatalysis is highly relevant in biotechnology as it offers several benefits and practical uses. To leverage the full potential of heterogeneous biocatalysts, the establishment of well-crafted protocols, and a deeper comprehension of enzyme immobilization on solid substrates are essential. These endeavors seek to optimize immobilized biocatalysts, ensuring maximal enzyme performance within confined spaces. For this aim, multidimensional characterization of heterogeneous biocatalysts is required. In this context, spectroscopic and microscopic methodologies conducted at different space and temporal scales can inform about the intraparticle enzyme kinetics, the enzyme spatial distribution, and the mass transport issues. In this Minireview, we identify enzyme immobilization, enzyme catalysis, and enzyme inactivation as the three main processes for which advanced characterization tools unveil fundamental information. Recent advances in operando characterization of immobilized enzymes at the single-particle (SP) and single-molecule (SM) levels inform about their functional properties, unlocking the full potential of heterogeneous biocatalysis toward biotechnological applications.

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.

Growth and tension in explosive fruit

Exploding seed pods of the common weed Cardamine hirsuta have the remarkable ability to launch seeds far from the plant. The energy for this explosion comes from tension that builds up in the fruit valves. Above a critical threshold, the fruit fractures along its dehiscence zone and the two valves coil explosively, ejecting the seeds. A common mechanism to generate tension is drying, causing tissues to shrink. However, this does not happen in C. hirsuta fruit. Instead, tension is produced by active contraction of growing exocarp cells in the outer layer of the fruit valves. Exactly how growth causes the exocarp tissue to contract and generate pulling force is unknown. Here we show that the reorientation of microtubules in the exocarp cell cortex changes the orientation of cellulose microfibrils in the cell wall and the consequent cellular growth pattern. We used mechanical modeling to show how tension emerges through growth due to the highly anisotropic orientation of load-bearing cellulose microfibrils and their effect on cell shape. By explicitly defining the cell wall as multi-layered in our model, we discovered that a cross-lamellate pattern of cellulose microfibrils further enhances the developing tension in growing cells. Therefore, the interplay of cell wall properties with turgor-driven growth enables the fruit exocarp to generate sufficient tension to power explosive seed dispersal.

Hydration and mechanical properties of arabinoxylan, (1,3;1,4)-β-glucan, and cellulose multilayer films simulating the cell wall of wheat endosperm

The cell walls of wheat endosperm, which play a pivotal role in seed germination, exhibit a laminated structure primarily composed of polysaccharides. In this study, composite multilayer films were prepared using arabinoxylan (AX), (1,3;1,4)-β-D-glucan (MLG), and cellulose nanofibers (CNFs), and the effect of polymer blend structure on cell wall hydration and mechanical properties was investigated. Atomic force microscopy and X-ray diffraction indicated that the network structure of MLG/CNF exhibits a higher degree of continuity and uniformity compared to that of AX/CNF. Mechanically, the extensive linkages between MLG and CNFs chains enhance the mechanical properties of the films. Moreover, water diffusion experiments and TD-NMR analysis revealed that water molecules diffuse faster in the network structure formed by AX. We propose a structural model of the endosperm cell wall, in which the CNFs polymer blend coated with MLG serves as the framework, and the AX network fills the gaps between them, providing diffusion channels for water molecules.

Lignin impairs Cel7A degradation of in vitro lignified cellulose by impeding enzyme movement and not by acting as a sink

BACKGROUND

Cellulose degradation by cellulases has been studied for decades due to the potential of using lignocellulosic biomass as a sustainable source of bioethanol. In plant cell walls, cellulose is bonded together and strengthened by the polyphenolic polymer, lignin. Because lignin is tightly linked to cellulose and is not digestible by cellulases, is thought to play a dominant role in limiting the efficient enzymatic degradation of plant biomass. Removal of lignin via pretreatments currently limits the cost-efficient production of ethanol from cellulose, motivating the need for a better understanding of how lignin inhibits cellulase-catalyzed degradation of lignocellulose. Work to date using bulk assays has suggested three possible inhibition mechanisms: lignin blocks access of the enzyme to cellulose, lignin impedes progress of the enzyme along cellulose, or lignin binds cellulases directly and acts as a sink.

RESULTS

We used single-molecule fluorescence microscopy to investigate the nanoscale dynamics of Cel7A from Trichoderma reesei, as it binds to and moves along purified bacterial cellulose in vitro. Lignified cellulose was generated by polymerizing coniferyl alcohol onto purified bacterial cellulose, and the degree of lignin incorporation into the cellulose meshwork was analyzed by optical and electron microscopy. We found that Cel7A preferentially bound to regions of cellulose where lignin was absent, and that in regions of high lignin density, Cel7A binding was inhibited. With increasing degrees of lignification, there was a decrease in the fraction of Cel7A that moved along cellulose rather than statically binding. Furthermore, with increasing lignification, the velocity of processive Cel7A movement decreased, as did the distance that individual Cel7A molecules moved during processive runs.

CONCLUSIONS

In an in vitro system that mimics lignified cellulose in plant cell walls, lignin did not act as a sink to sequester Cel7A and prevent it from interacting with cellulose. Instead, lignin both blocked access of Cel7A to cellulose and impeded the processive movement of Cel7A along cellulose. This work implies that strategies for improving biofuel production efficiency should target weakening interactions between lignin and cellulose surface, and further suggest that nonspecific adsorption of Cel7A to lignin is likely not a dominant mechanism of inhibition.

The nonlinear mechanics of highly extensible plant epidermal cell walls

Plant epidermal cell walls maintain the mechanical integrity of plants and restrict organ growth. Mechanical analyses can give insights into wall structure and are inputs for mechanobiology models of plant growth. To better understand the intrinsic mechanics of epidermal cell walls and how they may accommodate large deformations during growth, we analyzed a geometrically simple material, onion epidermal strips consisting of only the outer (periclinal) cell wall, ~7 μm thick. With uniaxial stretching by >40%, the wall showed complex three-phase stress-strain responses while cyclic stretching revealed reversible and irreversible deformations and elastic hysteresis. Stretching at varying strain rates and temperatures indicated the wall behaved more like a network of flexible cellulose fibers capable of sliding than a viscoelastic composite with pectin viscosity. We developed an analytic framework to quantify nonlinear wall mechanics in terms of stiffness, deformation, and energy dissipation, finding that the wall stretches by combined elastic and plastic deformation without compromising its stiffness. We also analyzed mechanical changes in slightly dehydrated walls. Their extension became stiffer and more irreversible, highlighting the influence of water on cellulose stiffness and sliding. This study offers insights into the structure and deformation modes of primary cell walls and presents a framework that is also applicable to tissues and whole organs.

A continuum mechanics model of the plant cell wall reveals interplay between enzyme action and cell wall structure

Plant cell growth is regulated through manipulation of the cell wall network, which consists of oriented cellulose microfibrils embedded within a ground matrix incorporating pectin and hemicellulose components. There remain many unknowns as to how this manipulation occurs. Experiments have shown that cellulose reorients in cell walls as the cell expands, while recent data suggest that growth is controlled by distinct collections of hemicellulose called biomechanical hotspots, which join the cellulose molecule together. The enzymes expansin and Cel12A have both been shown to induce growth of the cell wall; however, while Cel12A's wall-loosening action leads to a reduction in the cell wall strength, expansin's has been shown to increase the strength of the cell wall. In contrast, members of the XTH enzyme family hydrolyse hemicellulose but do not appear to cause wall creep. This experimentally observed behaviour still awaits a full explanation. We derive and analyse a mathematical model for the effective mechanical properties of the evolving cell wall network, incorporating cellulose microfibrils, which reorient with cell growth and are linked via biomechanical hotspots made up of regions of crosslinking hemicellulose. Assuming a visco-elastic response for the cell wall and using a continuum approach, we calculate the total stress resultant of the cell wall for a given overall growth rate. By changing appropriate parameters affecting breakage rate and viscous properties, we provide evidence for the biomechanical hotspot hypothesis and develop mechanistic understanding of the growth-inducing enzymes.

Cell Wall Pectin Content Refers to Favored Delivery of Negatively Charged Carbon Dots in Leaf Cells

In this work, we systematically investigated how cell wall and cell wall components affect the delivery of charged carbon quantum dots (CDs, from -34 to +41 mV) to leaf cells of cucumber and Arabidopsis plants. Four different types of leaf cells in cucumber and Arabidopsis were used, i.e., protoplasts (without cell wall), isolated individual cells (cell wall hydrolyzed with pectinase), regenerated individual cells (cell wall regenerated from protoplast), and intact leaf cells (intact cell wall, in planta). Leaf cells were incubated with charged CDs (0.5 mg/mL) for 2 h. Confocal imaging results showed that protoplasts, regenerated individual cells, and leaf cells showed favored uptake of the negatively charged CDs (-34 mV) compared to the PEI (polyethylenimine) coated and positively charged carbon dots [PEI600-CDs (17 mV) and PEI10K-CDs (41 mV)], while in isolated individual cells, the trend is opposite. The results of the content of the cell wall components showed that no significant changes in the total cell wall content were found between isolated individual cells and regenerated individual cells (1.28 vs 1.11 mg/106 cells), while regenerated individual cells showed significant higher pectin content [water-soluble pectin (0.13 vs 0.06 mg/106 cells, P < 0.01), chelator-soluble pectin (0.04 vs 0.01 mg/106 cells, P < 0.01), and alkaline pectin (0.02 vs 0.01 mg/106 cells, P < 0.01)] and significant lower cellulose content (0.13 vs 0.32 mg/106 cells, P < 0.01) than the isolated individual cells. No difference of the hemicellulose content was found between isolated individual cells and regenerated individual cells (0.20 vs 0.21 mg/106 cells). Our results suggest that compared with cellulose and hemicellulose in the cell wall, the pectin is a more important factor referring to the favored uptake of negatively charged carbon dots in leaf cells. Overall, this work provides a method to study the role of cell wall components in the uptake of nanoparticles in plant cells and also points out the importance of understanding the interactions between cell barriers and nanoparticles to design nanoparticles for agricultural use.

Tissue-specific directionality of cellulose synthase complex movement inferred from cellulose microfibril polarity in secondary cell walls of Arabidopsis

In plant cells, cellulose synthase complexes (CSCs) are nanoscale machines that synthesize and extrude crystalline cellulose microfibrils (CMFs) into the apoplast where CMFs are assembled with other matrix polymers into specific structures. We report the tissue-specific directionality of CSC movements of the xylem and interfascicular fiber walls of Arabidopsis stems, inferred from the polarity of CMFs determined using vibrational sum frequency generation spectroscopy. CMFs in xylems are deposited in an unidirectionally biased pattern with their alignment axes tilted about 25° off the stem axis, while interfascicular fibers are bidirectional and highly aligned along the longitudinal axis of the stem. These structures are compatible with the design of fiber-reinforced composites for tubular conduit and support pillar, respectively, suggesting that during cell development, CSC movement is regulated to produce wall structures optimized for cell-specific functions.

Regiospecific Cellulose Orientation and Anisotropic Mechanical Property in Plant Cell Walls

Cellulose microfibrils (CMFs) are a major load-bearing component in plant cell walls. Thus, their structures have been studied extensively with spectroscopic and microscopic characterization methods, but the findings from these two approaches were inconsistent, which hampers the mechanistic understanding of cell wall mechanics. Here, we report the regiospecific assembly of CMFs in the periclinal wall of plant epidermal cells. Using sum frequency generation spectroscopic imaging, we found that CMFs are highly aligned in the cell edge region where two cells form a junction, whereas they are mostly isotropic on average throughout the wall thickness in the flat face region of the epidermal cell. This subcellular-level heterogeneity in the CMF alignment provided a new perspective on tissue-level anisotropy in the tensile modulus of cell wall materials. This finding also has resolved a previous contradiction between the spectroscopic and microscopic imaging studies, which paves a foundation for better understanding of the cell wall architecture, especially structure-geometry relationships.

Cellulose assembles into helical bundles of uniform handedness in cell walls with abnormal pectin composition

Plant cells and organs grow into a remarkable diversity of shapes, as directed by cell walls composed primarily of polysaccharides such as cellulose and multiple structurally distinct pectins. The properties of the cell wall that allow for precise control of morphogenesis are distinct from those of the individual polysaccharide components. For example, cellulose, the primary determinant of cell morphology, is a chiral macromolecule that can self-assemble in vitro into larger-scale structures of consistent chirality, and yet most plant cells do not display consistent chirality in their growth. One interesting exception is the Arabidopsis thaliana rhm1 mutant, which has decreased levels of the pectin rhamnogalacturonan-I and causes conical petal epidermal cells to grow with a left-handed helical twist. Here, we show that in rhm1 the cellulose is bundled into large macrofibrils, unlike the evenly distributed microfibrils of the wild type. This cellulose bundling becomes increasingly severe over time, consistent with cellulose being synthesized normally and then self-associating into macrofibrils. We also show that in the wild type, cellulose is oriented transversely, whereas in rhm1 mutants, the cellulose forms right-handed helices that can account for the helical morphology of the petal cells. Our results indicate that when the composition of pectin is altered, cellulose can form cellular-scale chiral structures in vivo, analogous to the helicoids formed in vitro by cellulose nano-crystals. We propose that an important emergent property of the interplay between rhamnogalacturonan-I and cellulose is to permit the assembly of nonbundled cellulose structures, providing plants flexibility to orient cellulose and direct morphogenesis.

Harmonized biochemical modification of cell walls to get permission for entrance of Azospirillum sp. to rice roots

Biological nitrogen-fixation is important in increasing crop efficiency. Azospirillum is a nitrogen-fixing microorganism that naturally coexists with grasses roots. The present study was undertaken to clarify the role of rice root cell walls in the acceptance of two Azospirillum species, alone or in combination with indole-3-acetic acid (IAA) and gibberellic acid (GA3) treatments. Rice seedlings were grown in Yoshida solution for 21 days and then inoculated with A. brasilense and A. irakens in the presence of 0, 0.57, and 1.14 mM of IAA or 0, 0.29, and 0.58 mM GA3 or a combination of 1.14 mM of IAA and 0.58 mM of GA3. The results showed that the amount of hydrogen peroxide, lipid peroxidation, total nitrogen and activity of ferulic acid peroxidase, NADPH oxidase, nitrate reductase, pectin methyl esterase, cellulase, mannanase, xylanase and pectinase were significantly increased in inoculated samples treated with or without phytohormones. The highest activity of these enzymes was observed in A. brasilense- inoculated rice roots in auxin+gibberellin treatment. In the latter, the activity of phenylalanine ammonia lyase and wall ferulic acid peroxidase enzymes, the content of cell wall polysaccharide, lignin, and total phenolic compounds were the least, compared to controls and also with those samples which were inoculated with A. irakens. The results indicate an active role of the wall and its enzymes in allowing bacteria to enter the roots. Understanding this mechanism can improve the methods of inoculating bacteria into plants and increase crop efficiency, which will result in reduced use of chemical fertilizers and their destructive environmental effects.

Green nanobiopolymers for ecological applications: a step towards a sustainable environment

To minimize the usage of non-renewable resources and to maintain a sustainable environment, the exploitation of green nanobiopolymers should be enhanced. Biopolymers are generally developed from various microorganisms and plants in the specified condition. This review article discusses the current advances and trends of biopolymers, particularly in the arena of nanotechnology. In addition, discussion on various synthesis steps and structural characterization of green polymer materials like cellulose, chitin, and lignin is also encompassed. This article aims to coordinate the most recent outputs and possible future utilization of nanobiopolymers to the ecosystem with negligible effects by promoting the utilities of polymeric materials like polycaprolactones, starch, and nanocellulose. Additionally, strategic modification of cellulose into nanocellulose via rearrangement of the polymeric compound to serve various industrial and medical purposes has also been highlighted in the review. Specifically, the process of nanoencapsulation and its advancements in terms of nutritional aspects was also presented. The potential utility of green nanobiopolymers is one of the best cost-effective alternatives concerning circular economy and thereby helps to maintain sustainability.

Discretizing the cellular bases of plant morphogenesis: Emerging properties from subcellular and noisy patterning

A fundamental question in biology is how multicellular organisms robustly shape their organs. In the past decade, much progress has been made not just in identifying biochemical and biophysical factors underpinning morphogenesis, but also in analyzing their spatio-temporal dynamics. A remarkable outcome of such analyses is that morphogenesis involves high levels of heterogeneity and fluctuations at local scales. Although this could be considered as white noise to be averaged over time, there is increasing evidence that these heterogeneities and fluctuations are instructive cues for development. In this review, we highlight some of the new questions that such heterogeneities raise for plant morphogenesis. We also investigate their effects across scales, focusing on how subcellular heterogeneities contribute to organ shape robustness and evolvability.

DEFECTIVE KERNEL1 regulates cellulose synthesis and affects primary cell wall mechanics

The cell wall is one of the defining features of plants, controlling cell shape, regulating growth dynamics and hydraulic conductivity, as well as mediating plants interactions with both the external and internal environments. Here we report that a putative mechanosensitive Cys-protease DEFECTIVE KERNEL1 (DEK1) influences the mechanical properties of primary cell walls and regulation of cellulose synthesis. Our results indicate that DEK1 is an important regulator of cellulose synthesis in epidermal tissue of Arabidopsis thaliana cotyledons during early post-embryonic development. DEK1 is involved in regulation of cellulose synthase complexes (CSCs) by modifying their biosynthetic properties, possibly through interactions with various cellulose synthase regulatory proteins. Mechanical properties of the primary cell wall are altered in DEK1 modulated lines with DEK1 affecting both cell wall stiffness and the thickness of the cellulose microfibril bundles in epidermal cell walls of cotyledons.

Single-molecular insights into the breakpoint of cellulose nanofibers assembly during saccharification

Plant cellulose microfibrils are increasingly employed to produce functional nanofibers and nanocrystals for biomaterials, but their catalytic formation and conversion mechanisms remain elusive. Here, we characterize length-reduced cellulose nanofibers assembly in situ accounting for the high density of amorphous cellulose regions in the natural rice fragile culm 16 (Osfc16) mutant defective in cellulose biosynthesis using both classic and advanced atomic force microscopy (AFM) techniques equipped with a single-molecular recognition system. By employing individual types of cellulases, we observe efficient enzymatic catalysis modes in the mutant, due to amorphous and inner-broken cellulose chains elevated as breakpoints for initiating and completing cellulose hydrolyses into higher-yield fermentable sugars. Furthermore, effective chemical catalysis mode is examined in vitro for cellulose nanofibers conversion into nanocrystals with reduced dimensions. Our study addresses how plant cellulose substrates are digestible and convertible, revealing a strategy for precise engineering of cellulose substrates toward cost-effective biofuels and high-quality bioproducts.

To curve for survival: Apical hook development

Apical hook is a simple curved structure formed at the upper part of hypocotyls when dicot seeds germinate in darkness. The hook structure is transient but essential for seedlings' survival during soil emergence due to its efficient protection of the delicate shoot apex from mechanical injury. As a superb model system for studying plant differential growth, apical hook has fascinated botanists as early as the Darwin age, and significant advances have been achieved at both the morphological and molecular levels to understand how apical hook development is regulated. Here, we will mainly summarize the research progress at these two levels. We will also briefly compare the growth dynamics between apical hook and hypocotyl gravitropic bending at early seed germination phase, with the aim to deduce a certain consensus on their connections. Finally, we will outline the remaining questions and future research perspectives for apical hook development.

RGI-Type Pectic Polysaccharides Modulate Gut Microbiota in a Molecular Weight-Dependent Manner In Vitro

In this study, the fermentation characteristics of high rhamnogalacturonan I pectic polysaccharides (RGI) and free-radical degraded RGI (DRGI) were evaluated by a human fecal batch-fermentation model, and their structural properties were also investigated. As a result, the M_w of RGI decreased from 246.8 to 11.6 kDa, and the branches were broken dramatically. Fermentation showed that RGI degraded faster and produced more acetate and propionate than DRGI. Both of them reduced the Firmicutes/Bacteroidetes ratio and promoted the development of Bacteroides, Bifidobacterium, and Lactobacillus, bringing benefits to the gut ecosystem. However, the composition and metabolic pathways of the microbiota in RGI and DRGI were different. Most of the dominant bacteria of RGI (such as [Eubacterium]_eligens_group) participated in carbohydrate utilization, leading to better performance in glucolipid metabolism and energy metabolism. This work elucidated that large molecular weight matters in the gut microbiota modulatory effect of RGI-type pectic polysaccharides in vitro.

Biomechanical Weakening of Paper and Plant Cell Walls by Bacterial Expansins

Expansins are proteins that loosen plant cell walls but lack enzymatic activity. Here we describe two protocols tailored to measure the biomechanical activity of bacterial expansin. The first assay relies on the weakening of filter paper by expansin. The second assay is based on induction of creep (long-term, irreversible extension) of plant cell wall samples.

Plant Cell Wall Integrity Perturbations and Priming for Defense

A plant cell wall is a highly complex structure consisting of networks of polysaccharides, proteins, and polyphenols that dynamically change during growth and development in various tissues. The cell wall not only acts as a physical barrier but also dynamically responds to disturbances caused by biotic and abiotic stresses. Plants have well-established surveillance mechanisms to detect any cell wall perturbations. Specific immune signaling pathways are triggered to contrast biotic or abiotic forces, including cascades dedicated to reinforcing the cell wall structure. This review summarizes the recent developments in molecular mechanisms underlying maintenance of cell wall integrity in plant-pathogen and parasitic interactions. Subjects such as the effect of altered expression of endogenous plant cell-wall-related genes or apoplastic expression of microbial cell-wall-modifying enzymes on cell wall integrity are covered. Targeted genetic modifications as a tool to study the potential of cell wall elicitors, priming of signaling pathways, and the outcome of disease resistance phenotypes are also discussed. The prime importance of understanding the intricate details and complete picture of plant immunity emerges, ultimately to engineer new strategies to improve crop productivity and sustainability.

Detecting the orientation of newly-deposited crystalline cellulose with fluorescent CBM3

Cellulose microfibril patterning influences many of the mechanical attributes of plant cell walls. We developed a simple, fluorescence microscopy-based method to detect the orientation of newly-synthesized cellulose microfibrils in epidermal peels of onion and Arabidopsis. It is based on Alexa Fluor 488-tagged carbohydrate binding module 3a (CBM3a) from Clostridium thermocellum which displayed a nearly 4-fold greater binding to cell walls at pH 5.5 compared with pH 8. Binding to isolated cellulose did not display this pH dependence. At pH 7.5 fibrillar patterns at the surface of the epidermal peels were visible, corresponding to the directionality of surface cellulose microfibrils, as verified by atomic force microscopy. The fibrillar pattern was not visible as the labeling intensity increased at lower pH. The pH of greatest cell wall labeling corresponds to the isoelectric point of CBM3a, suggesting that electrostatic forces limit CBM3a penetration into the wall. Consistent with this, digestion of the wall with pectate lyase to remove homogalacturonan increased labeling intensity. We conclude that electrostatic interactions strongly influence labeling of cell walls with CBM3 and potentially other proteins, holding implications for any work that relies on penetration of protein probes such as CBMs, antibodies, or enzymes into charged polymeric substrates.

Extracting structural insights from soft X-ray scattering of biological assemblies

Resonant soft X-ray scattering (RSoXS), a technique that combines X-ray absorption spectroscopy and X-ray scattering, can probe the nano- and meso-scale structure of biological assemblies with chemical specificity. RSoXS experiments yield scattering data collected at several photon energies, for example across an elemental absorption edge of interest. Collecting a near-edge X-ray absorption fine structure (NEXAFS) spectrum complements RSoXS experiments and determines X-ray energies that are best suited for RSoXS measurements. The analysis of RSoXS data is similar in many ways to analysis of small angle X-ray scattering using hard X-rays, with an added dimension that includes an X-ray energy dependence. This chapter discusses procedures for predicting scattering contrast and thereby identifying energies suitable for RSoXS measurements using NEXAFS spectra, analyses of 2D RSoXS images through integration into 1D profiles, and strategies for elucidating the origin of RSoXS scattering features. It also discusses existing and potential methods for interpretation of RSoXS data to gain detailed structural insights into biological systems.

Numerical Simulation of Vibrational Sum Frequency Generation Intensity for Non-Centrosymmetric Domains Interspersed in an Amorphous Matrix: A Case Study for Cellulose in Plant Cell Wall

Vibrational sum frequency generation (SFG) spectroscopy can specifically probe molecular species non-centrosymmetrically arranged in a centrosymmetric or isotropic medium. This capability has been extensively utilized to detect and study molecular species present at the two-dimensional (2D) interface at which the centrosymmetry or isotropy of bulk phases is naturally broken. The same principle has been demonstrated to be very effective for the selective detection of non-centrosymmetric crystalline nanodomains interspersed in three-dimensional (3D) amorphous phases. However, the full spectral interpretation of SFG features has been difficult due to the complexity associated with the theoretical calculation of SFG responses of such 3D systems. This paper describes a numerical method to predict the relative SFG intensities of non-centrosymmetric nanodomains in 3D systems as functions of their size and concentration as well as their assembly patterns, i.e., the distributions of tilt, azimuth, and rotation angles with respect to the lab coordinate. We applied the developed method to predict changes in the CH and OH stretch modes characteristic to crystalline cellulose microfibrils distributed with various orders, which are relevant to plant cell wall structures. The same algorithm can also be applied to any SFG-active nanodomains interspersed in 3D amorphous matrices.

Molecular Insight into the Self-Assembly Process of Cellulose Iβ Microfibril

The self-assembly process of β-D-glucose oligomers on the surface of cellulose Iβ microfibril involves crystallization, and this process is analyzed herein, in terms of the length and flexibility of the oligomer chain, by means of molecular dynamics (MD) simulations. The characterization of this process involves the structural relaxation of the oligomer, the recognition of the cellulose I microfibril, and the formation of several hydrogen bonds (HBs). This process is monitored on the basis of the changes in non-bonded energies and the interaction with hydrophilic and hydrophobic crystal faces. The oligomer length is considered a parameter for capturing insight into the energy landscape and its stability in the bound form with the cellulose I microfibril. We notice that the oligomer-microfibril complexes are more stable by increasing the number of hydrogen bond interactions, which is consistent with a gain in electrostatic energy. Our studies highlight the interaction with hydrophilic crystal planes on the microfibril and the acceptor role of the flexible oligomers in HB formation. In addition, we study by MD simulation the interaction between a protofibril and the cellulose I microfibril in solution. In this case, the main interaction consists of the formation of hydrogen bonds between hydrophilic faces, and those HBs involve donor groups in the protofibril.

Expression of Exogenous GFP-CesA6 in Tobacco Enhances Cell Wall Biosynthesis and Biomass Production

Simple Summary

Cellulose is synthesized at the plasma membrane by an enzymatic complex constituted by different cellulose synthase (CesA) proteins. The overexpression of CesA genes has been assessed for increasing cellulose biosynthesis and plant biomass. In this study, we analyzed transgenic tobacco plants (F₃1 line), stably expressing the Arabidopsis CesA6 fused to GFP, for possible variations in the cellulose biosynthesis. We found that F₃1 plants were bigger than the wild-type (wt), showing significant increases of stem height, root length, and leaf area. They bloomed about 3 weeks earlier and yielded more flowers and seeds than wt. In the F₃1 leaves, the expression of the exogenous GFP-CesA6 prompted the overexpression of all CesAs involved in the synthesis of primary cell wall cellulose and of other proteins responsible for plant cell wall building and remodeling. Instead, secondary cell wall CesAs were not affected. In the F₃1 stem, showing a 3.3-fold increase of the secondary xylem thickness, both primary and secondary CesAs expression was differentially modulated. Significantly, the amounts of cellulose and matrix polysaccharides increased in the transformed seedlings. The results evidence the potentiality to overexpress primary CesAs in tobacco for biomass production increase.

Abstract

Improved cellulose biosynthesis and plant biomass represent important economic targets for several biotechnological applications including bioenergy and biofuel production. The attempts to increase the biosynthesis of cellulose by overexpressing CesAs proteins, components of the cellulose synthase complex, has not always produced consistent results. Analyses of morphological and molecular data and of the chemical composition of cell walls showed that tobacco plants (F₃1 line), stably expressing the Arabidopsis CesA6 fused to GFP, exhibits a “giant” phenotype with no apparent other morphological aberrations. In the F₃1 line, all evaluated growth parameters, such as stem and root length, leaf size, and lignified secondary xylem, were significantly higher than in wt. Furthermore, F₃1 line exhibited increased flower and seed number, and an advance of about 20 days in the anthesis. In the leaves of F₃1 seedlings, the expression of primary CesAs (NtCesA1, NtCesA3, and NtCesA6) was enhanced, as well as of proteins involved in the biosynthesis of non-cellulosic polysaccharides (xyloglucans and galacturonans, NtXyl4, NtGal10), cell wall remodeling (NtExp11 and XTHs), and cell expansion (NtPIP1.1 and NtPIP2.7). While in leaves the expression level of all secondary cell wall CesAs (NtCesA4, NtCesA7, and NtCesA8) did not change significantly, both primary and secondary CesAs were differentially expressed in the stem. The amount of cellulose and matrix polysaccharides significantly increased in the F₃1 seedlings with no differences in pectin and hemicellulose glycosyl composition. Our results highlight the potentiality to overexpress primary CesAs in tobacco plants to enhance cellulose synthesis and biomass production.

Altering arabinans increases Arabidopsis guard cell flexibility and stomatal opening

Stomata regulate plant water use and photosynthesis by controlling leaf gas exchange. They do this by reversibly opening the pore formed by two adjacent guard cells, with the limits of this movement ultimately set by the mechanical properties of the guard cell walls and surrounding epidermis.1,2 A body of evidence demonstrates that the methylation status and cellular patterning of pectin wall polymers play a core role in setting the guard cell mechanical properties, with disruption of the system leading to poorer stomatal performance.3-6 Here we present genetic and biochemical data showing that wall arabinans modulate guard cell flexibility and can be used to engineer stomata with improved performance. Specifically, we show that a short-chain linear arabinan epitope associated with the presence of rhamnogalacturonan I in the guard cell wall is required for full opening of the stomatal pore. Manipulations leading to the novel accumulation of longer-chain arabinan epitopes in guard cell walls led to an increase in the maximal pore aperture. Using computational modeling combined with atomic force microscopy, we show that this phenotype reflected a decrease in wall matrix stiffness and, consequently, increased flexing of the guard cells under turgor pressure, generating larger, rounder stomatal pores. Our results provide theoretical and experimental support for the conclusion that arabinan side chains of pectin modulate guard cell wall stiffness, setting the limits for cell flexing and, consequently, pore aperture, gas exchange, and photosynthetic assimilation.

Building an extensible cell wall

This article recounts, from my perspective of four decades in this field, evolving paradigms of primary cell wall structure and the mechanism of surface enlargement of growing cell walls. Updates of the structures, physical interactions, and roles of cellulose, xyloglucan, and pectins are presented. This leads to an example of how a conceptual depiction of wall structure can be translated into an explicit quantitative model based on molecular dynamics methods. Comparison of the model's mechanical behavior with experimental results provides insights into the molecular basis of complex mechanical behaviors of primary cell wall and uncovers the dominant role of cellulose-cellulose interactions in forming a strong yet extensible network.

Structure of In Vitro-Synthesized Cellulose Fibrils Viewed by Cryo-Electron Tomography and 13C Natural-Abundance Dynamic Nuclear Polarization Solid-State NMR

Cellulose, the most abundant biopolymer, is a central source for renewable energy and functionalized materials. In vitro synthesis of cellulose microfibrils (CMFs) has become possible using purified cellulose synthase (CESA) isoforms from Physcomitrium patens and hybrid aspen. The exact nature of these in vitro fibrils remains unknown. Here, we characterize in vitro-synthesized fibers made by CESAs present in membrane fractions of P. patens over-expressing CESA5 by cryo-electron tomography and dynamic nuclear polarization (DNP) solid-state NMR. DNP enabled measuring two-dimensional 13C-13C correlation spectra without isotope-labeling of the fibers. Results show structural similarity between in vitro fibrils and native CMF in plant cell walls. Intensity quantifications agree with the 18-chain structural model for plant CMF and indicate limited fibrillar bundling. The in vitro system thus reveals insights into cell wall synthesis and may contribute to novel cellulosic materials. The integrated DNP and cryo-electron tomography methods are also applicable to structural studies of other carbohydrate-based biomaterials.

Plant biology: Peering deeply into the structure of the onion epidermal cell wall

Cell wall ultrastructure has previously been assessed by thin-section transmission electron microscopy and by surface-based methods, such as atomic force microscopy. A new study uses electron tomography to image cellulose and pectin organization deep inside a thick epidermal cell wall.

Cryo-electron tomography of the onion cell wall shows bimodally oriented cellulose fibers and reticulated homogalacturonan networks

One hallmark of plant cells is their cell wall. They protect cells against the environment and high turgor and mediate morphogenesis through the dynamics of their mechanical and chemical properties. The walls are a complex polysaccharidic structure. Although their biochemical composition is well known, how the different components organize in the volume of the cell wall and interact with each other is not well understood and yet is key to the wall's mechanical properties. To investigate the ultrastructure of the plant cell wall, we imaged the walls of onion (Allium cepa) bulbs in a near-native state via cryo-focused ion beam milling (cryo-FIB milling) and cryo-electron tomography (cryo-ET). This allowed the high-resolution visualization of cellulose fibers in situ. We reveal the coexistence of dense fiber fields bathed in a reticulated matrix we termed "meshing," which is more abundant at the inner surface of the cell wall. The fibers adopted a regular bimodal angular distribution at all depths in the cell wall and bundled according to their orientation, creating layers within the cell wall. Concomitantly, employing homogalacturonan (HG)-specific enzymatic digestion, we observed changes in the meshing, suggesting that it is-at least in part-composed of HG pectins. We propose the following model for the construction of the abaxial epidermal primary cell wall: the cell deposits successive layers of cellulose fibers at -45° and +45° relative to the cell's long axis and secretes the surrounding HG-rich meshing proximal to the plasma membrane, which then migrates to more distal regions of the cell wall.

Molecular studies of cellulose synthase supercomplex from cotton fiber reveal its unique biochemical properties

Cotton fiber is a highly elongated and thickened single cell that produces large quantities of cellulose, which is synthesized and assembled into cell wall microfibrils by the cellulose synthase complex (CSC). In this study, we report that in cotton (Gossypium hirsutum) fibers harvested during secondary cell wall (SCW) synthesis, GhCesA 4, 7, and 8 assembled into heteromers in a previously uncharacterized 36-mer-like cellulose synthase supercomplex (CSS). This super CSC was observed in samples prepared using cotton fiber cells harvested during the SCW synthesis period but not from cotton stem tissue or any samples obtained from Arabidopsis. Knock-out of any of GhCesA 4, 7, and 8 resulted in the disappearance of the CSS and the production of fiber cells with no SCW thickening. Cotton fiber CSS showed significantly higher enzyme activity than samples prepared from knock-out cotton lines. We found that the microfibrils from the SCW of wild-type cotton fibers may contain 72 glucan chains in a bundle, unlike other plant materials studied. GhCesA4, 7, and 8 restored both the dwarf and reduced vascular bundle phenotypes of their orthologous Arabidopsis mutants, potentially by reforming the CSC hexamers. Genetic complementation was not observed when non-orthologous CesA genes were used, indicating that each of the three subunits is indispensable for CSC formation and for full cellulose synthase function. Characterization of cotton CSS will increase our understanding of the regulation of SCW biosynthesis.

The ectomycorrhizal basidiomycete Laccaria bicolor releases a GH28 polygalacturonase that plays a key role in symbiosis establishment

In ectomycorrhiza, root penetration and colonization of the intercellular space by symbiotic hyphae is thought to rely on the mechanical force that results from hyphal tip growth, enhanced by the activity of secreted cell-wall-degrading enzymes. Here, we characterize the biochemical properties of the symbiosis-induced polygalacturonase LbGH28A from the ectomycorrhizal fungus Laccaria bicolor. The transcriptional regulation of LbGH28A was measured by quantitative PCR (qPCR). The biological relevance of LbGH28A was confirmed by generating RNA interference (RNAi)-silenced LbGH28A mutants. We localized the LbGH28A protein by immunofluorescence confocal and immunogold cytochemical microscopy in poplar ectomycorrhizal roots. Quantitative PCR confirmed the induced expression of LbGH28A during ectomycorrhiza formation. Laccaria bicolor RNAi mutants have a lower ability to establish ectomycorrhiza, confirming the key role of this enzyme in symbiosis. The purified recombinant LbGH28A has its highest activity towards pectin and polygalacturonic acid. In situ localization of LbGH28A indicates that this endopolygalacturonase is located in both fungal and plant cell walls at the symbiotic hyphal front. These findings suggest that the symbiosis-induced pectinase LbGH28A is involved in the Hartig net formation and is an important determinant for successful symbiotic colonization.

Comprehensive approach of methods for microstructural analysis and analytical tools in lignocellulosic biomass assessment - A review

The trend in the modern world is to replace fossil fuels with green energy sources in order to reduce their environmental impact. The biorefinery industry, within this premise, needs to establish quantitative and qualitative analytical methods to better understand lignocellulosic biomass composition and structure. This paper presents chemical techniques (chromatography, thermal analysis, HRMS, FTIR, NIR, and NMR) and physicochemical techniques (XRD, optical and electron microscopy techniques - Confocal fluorescence, Raman, SPM, AFM, SEM, and TEM) for the microstructural characterization of lignocellulosic biomass and its derivatives. Each of these tools provides different and complementary information regarding molecular and microstructural composition of lignocellulosic biomass. Understanding these properties is essential for the design and operation of associated biomass conversion processing facilities. PAT, monitored in real-time, ensures an economical and balanced mass-energy process. This review aimed to help researchers select the most suitable analytical technique with which to investigate biomass feedstocks with recalcitrant natures.

Advances in Understanding Microbial Deterioration of Buried and Waterlogged Archaeological Woods: A Review

This review provides information on the advances made leading to an understanding of the micromorphological patterns produced during microbial degradation of lignified cell walls of buried and waterlogged archaeological woods. This knowledge not only serves as an important diagnostic signature for identifying the type(s) of microbial attacks present in such woods but also aids in the development of targeted methods for more effective preservation/restoration of wooden objects of historical and cultural importance. In this review, an outline of the chemical and ultrastructural characteristics of wood cell walls is first presented, which serves as a base for understanding the relationship of these characteristics to microbial degradation of lignocellulosic cell walls. The micromorphological patterns of the three different types of microbial attacks—soft rot, bacterial tunnelling and bacterial erosion—reported to be present in waterlogged woods are described. Then, the relevance of understanding microbial decay patterns to the preservation of waterlogged archaeological wooden artifacts is discussed, with a final section proposing research areas for future exploration.

Sodium bicarbonate reduces the cooked hardness of lotus rhizome via side chain rearrangement and pectin degradation

In this study, 0.1% (W/V) sodium bicarbonate (SB) solution was used to soften lotus rhizome, and the mechanism was characterized by monoclonal antibodies labeling (mAbs) and atomic force microscopy (AFM). The results showed that the cell wall of lotus rhizome was disintegrated under SB treatment. In addition, the mAbs results revealed that low-esterified homogalacturonan (HG) at the tricellular junction was degraded, the rearrangement of Ara and the interaction between Gal and cellulose may be related to the texture changes. Compared with distilled water treatment, SB treatment reduced the relative content of pectin from 34.1% to 19.1% while increased that of cellulose from 65.9% to 80.9%. AFM results revealed that the height of CSF skeleton decreased from about 32 nm to 1.5 nm. These results clearly demonstrate that cooking with 0.1% SB can soften lotus rhizome through degradation of pectin and arrangement of side chains of rhamnogalacturonan-Ⅰ (RG-Ⅰ).

Super-resolution imaging illuminates new dynamic behaviors of cellulose synthase

Confocal imaging has shown that CELLULOSE SYNTHASE (CESA) particles move through the plasma membrane as they synthesize cellulose. However, the resolution limit of confocal microscopy circumscribes what can be discovered about these tiny biosynthetic machines. Here, we applied Structured Illumination Microscopy (SIM), which improves resolution two-fold over confocal or widefield imaging, to explore the dynamic behaviors of CESA particles in living plant cells. SIM imaging reveals that Arabidopsis thaliana CESA particles are more than twice as dense in the plasma membrane as previously estimated, helping explain the dense arrangement of cellulose observed in new wall layers. CESA particles tracked by SIM display minimal variation in velocity, suggesting coordinated control of CESA catalytic activity within single complexes and that CESA complexes might move steadily in tandem to generate larger cellulose fibrils or bundles. SIM data also reveal that CESA particles vary in their overlaps with microtubule tracks and can complete U-turns without changing speed. CESA track patterns can vary widely between neighboring cells of similar shape, implying that cellulose patterning is not the sole determinant of cellular growth anisotropy. Together, these findings highlight SIM as a powerful tool to advance CESA imaging beyond the resolution limit of conventional light microscopy.

Characterising the mechanics of cell-cell adhesion in plants

Cell-cell adhesion is a fundamental feature of multicellular organisms. To ensure multicellular integrity, adhesion needs to be tightly controlled and maintained. In plants, cell-cell adhesion remains poorly understood. Here, we argue that to be able to understand how cell-cell adhesion works in plants, we need to understand and quantitatively measure the mechanics behind it. We first introduce cell-cell adhesion in the context of multicellularity, briefly explain the notions of adhesion strength, work and energy and present the current knowledge concerning the mechanisms of cell-cell adhesion in plants. Because still relatively little is known in plants, we then turn to animals, but also algae, bacteria, yeast and fungi, and examine how adhesion works and how it can be quantitatively measured in these systems. From this, we explore how the mechanics of cell adhesion could be quantitatively characterised in plants, opening future perspectives for understanding plant multicellularity.

Correlated mechanochemical maps of Arabidopsis thaliana primary cell walls using atomic force microscope infrared spectroscopy

Spatial heterogeneity in composition and organisation of the primary cell wall affects the mechanics of cellular morphogenesis. However, directly correlating cell wall composition, organisation and mechanics has been challenging. To overcome this barrier, we applied atomic force microscopy coupled with infrared (AFM-IR) spectroscopy to generate spatially correlated maps of chemical and mechanical properties for paraformaldehyde-fixed, intact Arabidopsis thaliana epidermal cell walls. AFM-IR spectra were deconvoluted by non-negative matrix factorisation (NMF) into a linear combination of IR spectral factors representing sets of chemical groups comprising different cell wall components. This approach enables quantification of chemical composition from IR spectral signatures and visualisation of chemical heterogeneity at nanometer resolution. Cross-correlation analysis of the spatial distribution of NMFs and mechanical properties suggests that the carbohydrate composition of cell wall junctions correlates with increased local stiffness. Together, our work establishes new methodology to use AFM-IR for the mechanochemical analysis of intact plant primary cell walls.

Cannabis Glandular Trichome Cell Walls Undergo Remodeling to Store Specialized Metabolites

The valuable cannabinoid and terpenoid metabolites of Cannabis sativa L. are produced by floral glandular trichomes. The trichomes consist of secretory disk cells, which produce the abundant lipidic metabolites, and an extracellular storage cavity. The mechanisms of apoplastic cavity formation to accumulate and store metabolites in cannabis glandular trichomes remain wholly unexplored. Here, we identify key wall components and how they change during cannabis trichome development. While glycome and monosaccharide analyses revealed that glandular trichomes have loosely bound xyloglucans and pectic polysaccharides, quantitative immunolabeling with wall-directed antibodies revealed precise spatiotemporal distributions of cell wall epitopes. An epidermal-like identity of early trichome walls matured into specialized wall domains over development. Cavity biogenesis was marked by separation of the subcuticular wall from the underlying surface wall in a homogalacturonan and α-1,5 arabinan epitope-rich zone and was associated with a reduction in fucosylated xyloglucan epitopes. As the cavity filled, a matrix with arabinogalactan and α-1,5 arabinan epitopes enclosed the metabolite droplets. At maturity, the disk cells' apical wall facing the storage cavity accumulated rhamnogalacturonan-I epitopes near the plasma membrane. Together, these data indicate that cannabis glandular trichomes undergo spatiotemporal remodeling at specific wall subdomains to facilitate storage cavity formation and metabolite storage.

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.

Plant cell adhesion and growth on artificial fibrous scaffolds as an in vitro model for plant development

Mechanistic studies of plant development would benefit from an in vitro model that mimics the endogenous physical interactions between cells and their microenvironment. Here, we present artificial scaffolds to which both solid- and liquid-cultured tobacco BY-2 cells adhere without perturbing cell morphology, division, and cortical microtubule organization. Scaffolds consisting of polyvinylidene tri-fluoroethylene (PVDF-TrFE) were prepared to mimic the cell wall’s fibrillar structure and its relative hydrophobicity and piezoelectric property. We found that cells adhered best to scaffolds consisting of nanosized aligned fibers. In addition, poling of PVDF-TrFE, which orients the fiber dipoles and renders the scaffold more piezoelectric, increased cell adhesion. Enzymatic treatments revealed that the plant cell wall polysaccharide, pectin, is largely responsible for cell adhesion to scaffolds, analogous to pectin-mediated cell adhesion in plant tissues. Together, this work establishes the first plant biomimetic scaffolds that will enable studies of how cell-cell and cell-matrix interactions affect plant developmental pathways.

Plant Cell Growth: Do Pectins Drive Lobe Formation in Arabidopsis Pavement Cells?

Pectins are conventionally thought to form a gel-like matrix between stress-bearing cellulose microfibrils in growing plant cell walls. A new study proposes a more active role in driving wall expansion. How does the proposal stack up against current evidence?

Enzymatic characterization of ancestral/group-IV clade xyloglucan endotransglycosylase/hydrolase enzymes reveals broad substrate specificities

Xyloglucan endotransglycosylase/hydrolase (XTH) enzymes play important roles in cell wall remodelling. Although previous studies have shown a pathway of evolution for XTH genes from bacterial licheninases, through plant endoglucanases (EG16), the order of development within the phylogenetic clades of true XTHs is yet to be elucidated. In addition, recent studies have revealed interesting and potentially useful patterns of transglycosylation beyond the standard xyloglucan-xyloglucan donor/acceptor substrate activities. To study evolutionary relationships and to search for enzymes with useful broad substrate specificities, genes from the 'ancestral' XTH clade of two monocots, Brachypodium distachyon and Triticum aestivum, and two eudicots, Arabidopsis thaliana and Populus tremula, were investigated. Specific activities of the heterologously produced enzymes showed remarkably broad substrate specificities. All the enzymes studied had high activity with the cellulose analogue HEC (hydroxyethyl cellulose) as well as with mixed-link β-glucan as donor substrates, when compared with the standard xyloglucan. Even more surprising was the wide range of acceptor substrates that these enzymes were able to catalyse reactions with, opening a broad range of possible roles for these enzymes, both within plants and in industrial, pharmaceutical and medical fields. Genome screening and expression analyses unexpectedly revealed that genes from this clade were found only in angiosperm genomes and were predominantly or solely expressed in reproductive tissues. We therefore posit that this phylogenetic group is significantly different and should be renamed as the group-IV clade.

Analytical implications of different methods for preparing plant cell wall material

Almost all plant cells are surrounded by a wall constructed of co-extensive networks of polysaccharides and proteoglycans. The capability to analyse cell wall components is essential for both understanding their complex biology and to fully exploit their numerous practical applications. Several biochemical and immunological techniques are used to analyse cell walls and in almost all cases the first step is the preparation of an alcohol insoluble residue (AIR). There is significant variation in the protocols used for AIR preparation, which can have a notable impact on the downstream extractability and detection of cell wall components. To explore these effects, we have formally compared ten AIR preparation methods and analysed polysaccharides subsequently extracted using high-performance anion exchange chromatography (HPAEC-PAD) and Micro Array Polymer Profiling (MAPP). Our results reveal the impact that AIR preparation has on downstream detection of cell wall components and the need for optimisation and consistency when preparing AIR.