Disrupting two Arabidopsis thaliana xylosyltransferase genes results in plants deficient in xyloglucan, a major primary cell wall component

Probing homogalacturonan in situ: A comprehensive review of available molecular recognition tools

Among the major plant cell wall components, pectic-type homogalacturonan emerges as a chemically and spatiotemporally dynamic matrix-forming agent embedded within the cell wall through various inter- and intramolecular interactions. Its abundance, localization, and chemistry profoundly influence cell wall biomechanics and all facets of plant physiology. Precise tracking of homogalacturonan in a native context is crucial for understanding cell wall organization, particularly the relation between molecular structure and function. It also enables the detailed characterization of plant-based resources for industrial, food, and biomedical applications. This review offers a comprehensive and focused survey of the state-of-the-art molecular recognition tools being employed to visualize homogalacturonan in diverse plant samples. We particularly highlight homogalacturonan-specific monoclonal antibodies, which represent the most used and well-established probes. However, we also discuss less common reagents, such as fluorophores, oligosaccharide-based probes, carbohydratebinding modules, and whole enzymes, as well as emerging chemical biology approaches exemplified by click chemistry. We critically evaluate their strengths, limitations, and suitability for given research objectives and provide the most notable examples of their usage. Lastly, we outline the anticipated future expansion of an advanced, improved range of new molecular tools, which holds the potential to overcome some of the current experimental hurdles.

Camelot: a computer-automated micro-extensometer with low-cost optical tracking

BACKGROUND

Plant growth and morphogenesis is a mechanical process controlled by genetic and molecular networks. Measuring mechanical properties at various scales is necessary to understand how these processes interact. However, obtaining a device to perform the measurements on plant samples of choice poses technical challenges and is often limited by high cost and availability of specialized components, the adequacy of which needs to be verified. Developing software to control and integrate the different pieces of equipment can be a complex task.

RESULTS

To overcome these challenges, we have developed a computer automated micro-extensometer combined with low-cost optical tracking (Camelot) that facilitates measurements of elasticity, creep, and yield stress. It consists of three primary components: a force sensor with a sample attachment point, an actuator with a second attachment point, and a camera. To monitor force, we use a parallel beam sensor, commonly used in digital weighing scales. To stretch the sample, we use a stepper motor with a screw mechanism moving a stage along linear rail. To monitor sample deformation, a compact digital microscope or a microscope camera is used. The system is controlled by MorphoRobotX, an integrated open-source software environment for mechanical experimentation. We first tested the basic Camelot setup, equipped with a digital microscope to track landmarks on the sample surface. We demonstrate that the system has sufficient accuracy to measure the stiffness in delicate plant samples, the etiolated hypocotyls of Arabidopsis, and were able to measure stiffness differences between wild type and a xyloglucan-deficient mutant. Next, we placed Camelot on an inverted microscope and used a C-mount microscope camera to track displacement of cell junctions. We stretched onion epidermal peels in longitudinal and transverse directions and obtained results similar to those previously published. Finally, we used the setup coupled with an upright confocal microscope and measured anisotropic deformation of individual epidermal cells during stretching of an Arabidopsis leaf.

CONCLUSIONS

The portability and suitability of Camelot for high-resolution optical tracking under a microscope make it an ideal tool for researchers in resource-limited settings or those pursuing exploratory biomechanics work.

Analysis of xyloglucan metabolism mutants highlights the prominent role of xylose cleavage in seed dormancy

Seed dormancy is an adaptive trait that delays germination until environmental conditions become favorable for seedling survival and growth. Germination has been shown to depend on the mechanical resistance strength of the covering layers (testa and endosperm) that counteracts the growth force of the embryo. Cell wall remodeling is essential in the regulation of germination processes. In Arabidopsis thaliana, the side chain trimming of xyloglucans (XyG), the major hemicellulose in cell walls, by the apoplastic XYLOSIDASE1 (XYL1), has been previously shown to regulate XyG side chain length and seed dormancy. To investigate to what extent side chain complexity impacts on cell wall mechanical properties and regulates seed germination, xyl1 mutations were combined here with mutations in the two other glycosidases, the fucosidase AXY8 and the beta-galactosidase BGAL10. Analysis of germination phenotypes in axy8 bgal10 xyl1 and in several XyG biosynthesis mutants did not show any link between dormancy depth and side chain length. The very specific effect of xyl1 on seed dormancy in single and multiple mutants was clearly correlated with alterations in XyG intracellular localization, together with release and oxidation of free oligosaccharides (XGO). Accumulation of oxidized XGO could negatively impact cell wall remodeling by impairing remobilization and polarized secretion in cell walls, thus reducing growth anisotropy in elongating organs and modifying mechanical characteristics in seed tissues.

Time-resolved tracking of cellulose biosynthesis and assembly during cell wall regeneration in live Arabidopsis protoplasts

Cellulose, the most abundant polysaccharide on earth composing plant cell walls, is synthesized by coordinated action of multiple enzymes in cellulose synthase complexes embedded within the plasma membrane. Multiple chains of cellulose fibrils form intertwined extracellular matrix networks. It remains largely unknown how newly synthesized cellulose is assembled into an intricate fibril network on cell surfaces. Here, we have established an in vivo time-resolved imaging platform to continuously visualize cellulose biosynthesis and fibril network assembly on Arabidopsis thaliana protoplast surfaces as the primary cell wall regenerates. Our observations provide the basis for a model of cellulose fibril network development in protoplasts driven by an interplay of multiscale dynamics that includes rapid diffusion and coalescence of nascent cellulose fibrils, processive elongation of single fibrils, and cellulose fibrillar network rearrangement during maturation. This study provides fresh insights into the dynamic and mechanistic aspects of cell wall synthesis at the single-cell level.

Global transcriptional modulation and nutritional status of soybean plants following foliar application of zinc borate as a suspension concentrate fertilizer

The management of micronutrients, such as boron (B) and zinc (Zn), is critical for plant growth and crop yields. One method of rapid intervention crop management to mitigate nutritional deficiency is the foliar supply of B and Zn. Our study investigates the effect of foliar-supplied B and Zn availability on the global transcriptional modulation in soybean (Glycine max). The transcriptional response to B was more widespread compared to Zn. RNA-Seq of leaves under different B levels revealed modulated genes with potential roles in nutritional homeostasis and stress response that may be key to controlling B status in the plant. We also identified putative B transporters whose expression levels were significantly affected by B supplementation with foliar fertilization of plants growing under low B conditions. Furthermore, a gene lacking functional annotation (Glyma.03G180300) emerged as a novel potential marker of B status. Two genes (Glyma.16G118000, Glyma.16G199000) were consistently induced by Zn availability, highlighting their potential as biomarkers for assessing its status in soybean plants. This work advances our understanding of nutritional homeostasis in soybean plants and identifies target genes and potential molecular mechanisms involved in nutritional response. Our study informs fertilizer design targeting specific nutrient transporters, thereby enhancing nutrient efficiency in crops.

Chitosan stimulates root hair callose deposition, endomembrane dynamics, and inhibits root hair growth

Although angiosperm plants generally react to immunity elicitors like chitin or chitosan by the cell wall callose deposition, this response in particular cell types, especially upon chitosan treatment, is not fully understood. Here we show that the growing root hairs (RHs) of Arabidopsis can respond to a mild (0.001%) chitosan treatment by the callose deposition and by a deceleration of the RH growth. We demonstrate that the glucan synthase-like 5/PMR4 is vital for chitosan-induced callose deposition but not for RH growth inhibition. Upon the higher chitosan concentration (0.01%) treatment, RHs do not deposit callose, while growth inhibition is prominent. To understand the molecular and cellular mechanisms underpinning the responses to two chitosan treatments, we analysed early Ca2+ and defence-related signalling, gene expression, cell wall and RH cellular endomembrane modifications. Chitosan-induced callose deposition is also present in the several other plant species, including functionally analogous and evolutionarily only distantly related RH-like structures such as rhizoids of bryophytes. Our results point to the RH callose deposition as a conserved strategy of soil-anchoring plant cells to cope with mild biotic stress. However, high chitosan concentration prominently disturbs RH intracellular dynamics, tip-localised endomembrane compartments, growth and viability, precluding callose deposition.

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.

Xyloglucan side chains enable polysaccharide secretion to the plant cell wall

Plant cell walls are essential for growth. The cell wall hemicellulose xyloglucan (XyG) is produced in the Golgi apparatus before secretion. Loss of the Arabidopsis galactosyltransferase MURUS3 (MUR3) decreases XyG d-galactose side chains and causes intracellular aggregations and dwarfism. It is unknown how changing XyG synthesis can broadly impact organelle organization and growth. We show that intracellular aggregations are not unique to mur3 and are found in multiple mutant lines with reduced XyG D-galactose side chains. mur3 aggregations disrupt subcellular trafficking and induce formation of intracellular cell-wall-like fragments. Addition of d-galacturonic acid onto XyG can restore growth and prevent mur3 aggregations. These results indicate that the presence, but not the composition, of XyG side chains is essential, likely by ensuring XyG solubility. Our results suggest that XyG polysaccharides are synthesized in a highly substituted form for efficient secretion and then later modified by cell-wall-localized enzymes to fine-tune cell wall properties.

New insights on β-glycan synthases using in vitro GT-array (i-GT-ray) platform

Cellulose and hemicellulose are the major structural β-glycan polysaccharides in cell walls of land plants. They are characterized by a backbone of β-(1,3)- and/or β-(1,4)-linked sugars such as glucose, mannose, or xylose. The backbones of these polymers are produced by processive glycosyltransferases (GTs) called synthases having multiple transmembrane domains anchoring them to the membrane. Thus, they are among the most difficult membrane proteins to test in vitro and to purify. Recently, we developed an in vitro GT-array (i-GTray) platform and showed that non-processive type II membrane GTs could be produced via cell-free system in a soluble and active form and tested in this platform. To determine whether i-GT-ray platform is adequate for the production and testing of β-glycan synthases, we tested five synthases involved in cellulose, xyloglucan, (gluco)mannan, and β-(1,3)(1,4)-mixed-linkage glucan synthesis. Our results revealed unsuspected features of these enzymes. For example, all these synthases could be produced in a soluble and active form and are active in the absence of detergent or membrane lipids, and none of them required a primer for initiation of synthesis. All synthases produced ethanol-insoluble products that were susceptible to the appropriate hydrolases (i.e., cellulase, lichenase, mannanase). Using this platform, we showed that AtCslC4 and AtXXT1 interact directly to form an active xyloglucan synthase that produced xylosylated cello-oligosaccharides (up to three xylosyl residues) when supplied with UDP-Glc and UDP-Xyl. i-GTray platform represents a simple and powerful functional genomics tool for discovery of new insights of synthase activities and can be adapted to other enzymes.

CELLULOSE SYNTHASE-LIKE C proteins modulate cell wall establishment during ethylene-mediated root growth inhibition in rice

The cell wall shapes plant cell morphogenesis and affects the plasticity of organ growth. However, the way in which cell wall establishment is regulated by ethylene remains largely elusive. Here, by analyzing cell wall patterns, cell wall composition and gene expression in rice (Oryza sativa, L.) roots, we found that ethylene induces cell wall thickening and the expression of cell wall synthesis-related genes, including CELLULOSE SYNTHASE-LIKE C1, 2, 7, 9, 10 (OsCSLC1, 2, 7, 9, 10) and CELLULOSE SYNTHASE A3, 4, 7, 9 (OsCESA3, 4, 7, 9). Overexpression and mutant analyses revealed that OsCSLC2 and its homologs function in ethylene-mediated induction of xyloglucan biosynthesis mainly in the cell wall of root epidermal cells. Moreover, OsCESA-catalyzed cellulose deposition in the cell wall was enhanced by ethylene. OsCSLC-mediated xyloglucan biosynthesis likely plays an important role in restricting cell wall extension and cell elongation during the ethylene response in rice roots. Genetically, OsCSLC2 acts downstream of ETHYLENE-INSENSITIVE3-LIKE1 (OsEIL1)-mediated ethylene signaling, and OsCSLC1, 2, 7, 9 are directly activated by OsEIL1. Furthermore, the auxin signaling pathway is synergistically involved in these regulatory processes. These findings link plant hormone signaling with cell wall establishment, broadening our understanding of root growth plasticity in rice and other crops.

Architecture and functions of stomatal cell walls in eudicots and grasses

BACKGROUND

Like all plant cells, the guard cells of stomatal complexes are encased in cell walls that are composed of diverse, interacting networks of polysaccharide polymers. The properties of these cell walls underpin the dynamic deformations that occur in guard cells as they expand and contract to drive the opening and closing of the stomatal pore, the regulation of which is crucial for photosynthesis and water transport in plants.

SCOPE

Our understanding of how cell wall mechanics are influenced by the nanoscale assembly of cell wall polymers in guard cell walls, how this architecture changes over stomatal development, maturation and ageing and how the cell walls of stomatal guard cells might be tuned to optimize stomatal responses to dynamic environmental stimuli is still in its infancy.

CONCLUSION

In this review, we discuss advances in our ability to probe experimentally and to model the structure and dynamics of guard cell walls quantitatively across a range of plant species, highlighting new ideas and exciting opportunities for further research into these actively moving plant cells.

Xyloglucan deficiency leads to a reduction in turgor pressure and changes in cell wall properties, affecting early seedling establishment

Xyloglucan is believed to play a significant role in cell wall mechanics of dicot plants. Surprisingly, Arabidopsis plants defective in xyloglucan biosynthesis exhibit nearly normal growth and development. We investigated a mutant line, cslc-Δ5, lacking activity in all five Arabidopsis cellulose synthase like-C (CSLC) genes responsible for xyloglucan backbone biosynthesis. We observed that this xyloglucan-deficient line exhibited reduced cellulose crystallinity and increased pectin levels, suggesting the existence of feedback mechanisms that regulate wall composition to compensate for the absence of xyloglucan. These alterations in cell wall composition in the xyloglucan-absent plants were further linked to a decrease in cell wall elastic modulus and rupture stress, as observed through atomic force microscopy (AFM) and extensometer-based techniques. This raised questions about how plants with such modified cell wall properties can maintain normal growth. Our investigation revealed two key factors contributing to this phenomenon. First, measurements of turgor pressure, a primary driver of plant growth, revealed that cslc-Δ5 plants have reduced turgor, preventing the compromised walls from bursting while still allowing growth to occur. Second, we discovered the conservation of elastic asymmetry (ratio of axial to transverse wall elasticity) in the mutant, suggesting an additional mechanism contributing to the maintenance of normal growth. This novel feedback mechanism between cell wall composition and mechanical properties, coupled with turgor pressure regulation, plays a central role in the control of plant growth and is critical for seedling establishment in a mechanically challenging environment by affecting shoot emergence and root penetration.

The plant cell wall-dynamic, strong, and adaptable-is a natural shapeshifter

Mythology is replete with good and evil shapeshifters, who, by definition, display great adaptability and assume many different forms-with several even turning themselves into trees. Cell walls certainly fit this definition as they can undergo subtle or dramatic changes in structure, assume many shapes, and perform many functions. In this review, we cover the evolution of knowledge of the structures, biosynthesis, and functions of the 5 major cell wall polymer types that range from deceptively simple to fiendishly complex. Along the way, we recognize some of the colorful historical figures who shaped cell wall research over the past 100 years. The shapeshifter analogy emerges more clearly as we examine the evolving proposals for how cell walls are constructed to allow growth while remaining strong, the complex signaling involved in maintaining cell wall integrity and defense against disease, and the ways cell walls adapt as they progress from birth, through growth to maturation, and in the end, often function long after cell death. We predict the next century of progress will include deciphering cell type-specific wall polymers; regulation at all levels of polymer production, crosslinks, and architecture; and how walls respond to developmental and environmental signals to drive plant success in diverse environments.

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.

Spaceflight impacts xyloglucan oligosaccharide abundance in Arabidopsis thaliana root cell walls

Over the course of more than a decade, space biology investigations have consistently indicated that cell wall remodeling occurs in a variety of spaceflight-grown plants. Here, we describe a mass spectrometric method to study the fundamental composition of xyloglucan, the most abundant hemicellulose in dicot cell walls, in space-grown plants. Four representative Arabidopsis root samples, from a previously conducted spaceflight experiment - Advanced Plant EXperiment - 04 (APEX-04), were used to investigate changes in xyloglucan oligosaccharides abundances in spaceflight-grown plants compared to ground controls. In situ localized enzymatic digestions and surface sampling mass spectrometry analysis provided spatial resolution of the changes in xyloglucan oligosaccharides abundances. Overall, the results showed that oligosaccharide XXLG/XLXG and XXFG branching patterns were more abundant in the lateral roots of spaceflight-grown plants, while XXXG, XLFG, and XLFG/XLFG were more abundant in the lateral roots of ground control plants. In the primary roots, XXFG had a higher abundance in ground controls than in spaceflight plants. This methodology of analyzing the basic components of the cell wall in this paper highlights two important findings. First, that are differences in the composition of xyloglucan oligosaccharides in spaceflight root cell walls compared to ground controls and, second, most of these differences are observed in the lateral roots. Thus, the methodology described in this paper provides insights into spaceflight cell wall modifications for future investigations.

Rapid alkalinization factor 22 has a structural and signalling role in root hair cell wall assembly

Pressurized cells with strong walls make up the hydrostatic skeleton of plants. Assembly and expansion of such stressed walls depend on a family of secreted RAPID ALKALINIZATION FACTOR (RALF) peptides, which bind both a membrane receptor complex and wall-localized LEUCINE-RICH REPEAT EXTENSIN (LRXs) in a mutually exclusive way. Here we show that, in root hairs, the RALF22 peptide has a dual structural and signalling role in cell expansion. Together with LRX1, it directs the compaction of charged pectin polymers at the root hair tip into periodic circumferential rings. Free RALF22 induces the formation of a complex with LORELEI-LIKE-GPI-ANCHORED PROTEIN 1 and FERONIA, triggering adaptive cellular responses. These findings show how a peptide simultaneously functions as a structural component organizing cell wall architecture and as a feedback signalling molecule that regulates this process depending on its interaction partners. This mechanism may also underlie wall assembly and expansion in other plant cell types.

Auxin acts upstream of nitric oxide to regulate cell wall xyloglucan and its aluminium-binding capacity in Arabidopsis thaliana

MAIN CONCLUSION

Auxin acts upstream of NO through NOA and XXT5 pathways to regulate the binding capacity of the root cell wall to Al. In our previous study, we identified an unknown mechanism by which 1-naphthaleneacetic acid (NAA) decreased the fixation of aluminum (Al) in the cell wall. Here, we observed that external application of the nitric oxide (NO) donor S-nitrosoglutathion (GSNO) increased the inhibition of Al on root elongation. Further analysis indicated that GSNO could induce Al accumulation in the roots and root cell walls, which is consistent with lower xyloglucan content. In comparison to the Columbia-0 (Col-0) wild type (WT), endogenous NO-reduced mutants noa1 (NOA pathway) and nia1nia2 (NR pathway) were more resistant to Al, with lower root Al content, higher xyloglucan content, and more Al accumulation in the root cell walls. By contrast, the xxt5 mutant with reduced xyloglucan content exhibited an Al-sensitive phenotype. Interestingly, Al treatment increased the endogenous auxin and NO levels, and the auxin levels induced under Al stress further stimulated NO production. Auxin application reduced Al retention in hemicellulose and decreased the xyloglucan content, similar to the effects observed with GSNO. In yucca and aux1-7 mutants, exogenous application of NO resulted in responses similar to those of the WT, whereas exogenous auxin had little effect on the noa1 mutant under Al stress. In addition, as auxin had similar effects on the nia1nia2 mutant and the WT, exogenous auxin and NO had little effect on the xxt5 mutant under Al stress, further confirming that auxin acts upstream of NO through NOA and XXT5 pathways to regulate the binding capacity of the root cell wall to Al.

Flexible Pectin Nanopatterning Drives Cell Wall Organization in Plants

Plant cell walls are abundant sources of materials and energy. Nevertheless, cell wall nanostructure, specifically how pectins interact with cellulose and hemicelluloses to construct a robust and flexible biomaterial, is poorly understood. X-ray scattering measurements are minimally invasive and can reveal ultrastructural, compositional, and physical properties of materials. Resonant X-ray scattering takes advantage of compositional differences by tuning the energy of the incident X-ray to absorption edges of specific elements in a material. Using Tender Resonant X-ray Scattering (TReXS) at the calcium K-edge to study hypocotyls of the model plant, Arabidopsis thaliana, we detected distinctive Ca features that we hypothesize correspond to previously unreported Ca-Homogalacturonan (Ca-HG) nanostructures. When Ca-HG structures were perturbed by chemical and enzymatic treatments, cellulose microfibrils were also rearranged. Moreover, Ca-HG nanostructure was altered in mutants with abnormal cellulose, pectin, or hemicellulose content. Our results indicate direct structural interlinks between components of the plant cell wall at the nanoscale and reveal mechanisms that underpin both the structural integrity of these components and the molecular architecture of the plant cell wall.

Growth-inhibiting effects of the unconventional plant APYRASE 7 of Arabidopsis thaliana influences the LRX/RALF/FER growth regulatory module

Plant cell growth involves coordination of numerous processes and signaling cascades among the different cellular compartments to concomitantly enlarge the protoplast and the surrounding cell wall. The cell wall integrity-sensing process involves the extracellular LRX (LRR-Extensin) proteins that bind RALF (Rapid ALkalinization Factor) peptide hormones and, in vegetative tissues, interact with the transmembrane receptor kinase FERONIA (FER). This LRX/RALF/FER signaling module influences cell wall composition and regulates cell growth. The numerous proteins involved in or influenced by this module are beginning to be characterized. In a genetic screen, mutations in Apyrase 7 (APY7) were identified to suppress growth defects observed in lrx1 and fer mutants. APY7 encodes a Golgi-localized NTP-diphosphohydrolase, but opposed to other apyrases of Arabidopsis, APY7 revealed to be a negative regulator of cell growth. APY7 modulates the growth-inhibiting effect of RALF1, influences the cell wall architecture and -composition, and alters the pH of the extracellular matrix, all of which affect cell growth. Together, this study reveals a function of APY7 in cell wall formation and cell growth that is connected to growth processes influenced by the LRX/RALF/FER signaling module.

Woody plant cell walls: Fundamentals and utilization

Cell walls in plants, particularly forest trees, are the major carbon sink of the terrestrial ecosystem. Chemical and biosynthetic features of plant cell walls were revealed early on, focusing mostly on herbaceous model species. Recent developments in genomics, transcriptomics, epigenomics, transgenesis, and associated analytical techniques are enabling novel insights into formation of woody cell walls. Here, we review multilevel regulation of cell wall biosynthesis in forest tree species. We highlight current approaches to engineering cell walls as potential feedstock for materials and energy and survey reported field tests of such engineered transgenic trees. We outline opportunities and challenges in future research to better understand cell type biogenesis for more efficient wood cell wall modification and utilization for biomaterials or for enhanced carbon capture and storage.

Engineering of substrate specificity in a plant cell-wall modifying enzyme through alterations of carboxyl-terminal amino acid residues

Structural determinants of substrate recognition remain inadequately defined in broad specific cell-wall modifying enzymes, termed xyloglucan xyloglucosyl transferases (XETs). Here, we investigate the Tropaeolum majus seed TmXET6.3 isoform, a member of the GH16_20 subfamily of the GH16 network. This enzyme recognises xyloglucan (XG)-derived donors and acceptors, and a wide spectrum of other chiefly saccharide substrates, although it lacks the activity with homogalacturonan (pectin) fragments. We focus on defining the functionality of carboxyl-terminal residues in TmXET6.3, which extend acceptor binding regions in the GH16_20 subfamily but are absent in the related GH16_21 subfamily. Site-directed mutagenesis using double to quintuple mutants in the carboxyl-terminal region - substitutions emulated on barley XETs recognising the XG/penta-galacturonide acceptor substrate pair - demonstrated that this activity could be gained in TmXET6.3. We demonstrate the roles of semi-conserved Arg238 and Lys237 residues, introducing a net positive charge in the carboxyl-terminal region (which complements a negative charge of the acidic penta-galacturonide) for the transfer of xyloglucan fragments. Experimental data, supported by molecular modelling of TmXET6.3 with the XG oligosaccharide donor and penta-galacturonide acceptor substrates, indicated that they could be accommodated in the active site. Our findings support the conclusion on the significance of positively charged residues at the carboxyl terminus of TmXET6.3 and suggest that a broad specificity could be engineered via modifications of an acceptor binding site. The definition of substrate specificity in XETs should prove invaluable for defining the structure, dynamics, and function of plant cell walls, and their metabolism; these data could be applicable in various biotechnologies.

Galactosylation of xyloglucan is essential for the stabilization of the actin cytoskeleton and endomembrane system through the proper assembly of cell walls

Xyloglucan, a major hemicellulose, interacts with cellulose and pectin to assemble primary cell walls in plants. Loss of the xyloglucan galactosyltransferase MURUS3 (MUR3) leads to the deficiency of galactosylated xyloglucan and perturbs plant growth. However, it is unclear whether defects in xyloglucan galactosylation influence the synthesis of other wall polysaccharides, cell wall integrity, cytoskeleton behaviour, and endomembrane homeostasis. Here, we found that in mur3-7 etiolated seedlings cellulose was reduced, CELLULOSE SYNTHASE (CESA) genes were down-regulated, the density and mobility of cellulose synthase complexes (CSCs) were decreased, and cellulose microfibrils become discontinuous. Pectin, rhamnogalacturonan II (RGII), and boron contents were reduced in mur3-7 plants, and B-RGII cross-linking was abnormal. Wall porosity and thickness were significantly increased in mur3-7 seedlings. Endomembrane aggregation was also apparent in the mur3-7 mutant. Furthermore, mutant seedlings and their actin filaments were more sensitive to Latrunculin A (LatA) treatment. However, all defects in mur3-7 mutants were substantially restored by exogenous boric acid application. Our study reveals the importance of MUR3-mediated xyloglucan galactosylation for cell wall structural assembly and homeostasis, which is required for the stabilization of the actin cytoskeleton and the endomembrane system.

Glycome profiling and immunohistochemistry uncover changes in cell walls of Arabidopsis thaliana roots during spaceflight

A large and diverse library of glycan-directed monoclonal antibodies (mAbs) was used to determine if plant cell walls are modified by low-gravity conditions encountered during spaceflight. This method called glycome profiling (glycomics) revealed global differences in non-cellulosic cell wall epitopes in Arabidopsis thaliana root extracts recovered from RNA purification columns between seedlings grown on the International Space Station-based Vegetable Production System and paired ground (1-g) controls. Immunohistochemistry on 11-day-old seedling primary root sections showed that ten of twenty-two mAbs that exhibited spaceflight-induced increases in binding through glycomics, labeled space-grown roots more intensely than those from the ground. The ten mAbs recognized xyloglucan, xylan, and arabinogalactan epitopes. Notably, three xylem-enriched unsubstituted xylan backbone epitopes were more intensely labeled in space-grown roots than in ground-grown roots, suggesting that the spaceflight environment accelerated root secondary cell wall formation. This study highlights the feasibility of glycomics for high-throughput evaluation of cell wall glycans using only root high alkaline extracts from RNA purification columns, and subsequent validation of these results by immunohistochemistry. This approach will benefit plant space biological studies because it extends the analyses possible from the limited amounts of samples returned from spaceflight and help uncover microgravity-induced tissue-specific changes in plant cell walls.

Increased gene expression variability hinders the formation of regional mechanical conflicts leading to reduced organ shape robustness

To relate gene networks and organ shape, one needs to address two wicked problems: i) Gene expression is often variable locally, and shape is reproducible globally; ii) gene expression can have cascading effects on tissue mechanics, with possibly counterintuitive consequences for the final organ shape. Here, we address such wicked problems, taking advantage of simpler plant organ development where shape only emerges from cell division and elongation. We confirm that mutation in VERNALIZATION INDEPENDENCE 3 (VIP3), a subunit of the conserved polymerase-associated factor 1 complex (Paf1C), increases gene expression variability in Arabidopsis. Then, we focused on the Arabidopsis sepal, which exhibits a reproducible shape and stereotypical regional growth patterns. In vip3 sepals, we measured higher growth heterogeneity between adjacent cells. This even culminated in the presence of negatively growing cells in specific growth conditions. Interestingly, such increased local noise interfered with the stereotypical regional pattern of growth. We previously showed that regional differential growth at the wild-type sepal tip triggers a mechanical conflict, to which cells resist by reinforcing their walls, leading to growth arrest. In vip3, the disturbed regional growth pattern delayed organ growth arrest and increased final organ shape variability. Altogether, we propose that gene expression variability is managed by Paf1C to ensure organ robustness by building up mechanical conflicts at the regional scale, instead of the local scale.

Identification of a xyloglucan beta-xylopyranosyltransferase from Vaccinium corymbosum

Plant cell walls contain the hemicellulose xyloglucan, whose fine structure may vary depending on cell type, tissue, and/or plant species. Most but not all of the glycosyltransferases involved in the biosynthesis of xyloglucan sidechains have been identified. Here, we report the identification of several functional glycosyltransferases from blueberry (Vaccinium corymbosum bluecrop). Among those transferases is a hitherto elusive Xyloglucan:Beta-xylosylTransferase (XBT). Heterologous expression of VcXBT in the Arabidopsis thaliana double mutant mur3 xlt2, where xyloglucan consists only of an unsubstituted xylosylated glucan core structure, results in the production of the xylopyranose-containing "U" sidechain as characterized by mass spectrometry, glycosidic linkage, and NMR analysis. The introduction of the additional xylopyranosyl residue rescues the dwarfed phenotype of the untransformed Arabidopsis mur3 xlt2 mutant to wild-type height. Structural protein analysis using Alphafold of this and other related xyloglucan glycosyltransferase family 47 proteins not only identifies potential domains that might influence the regioselectivity of these enzymes but also gives hints to specific amino acids that might determine the donor-substrate specificity of these glycosyltransferases.

The Arabidopsis xylosyltransferases, XXT3, XXT4, and XXT5, are essential to complete the fully xylosylated glucan backbone XXXG-type structure of xyloglucans

Although most xyloglucans (XyGs) biosynthesis enzymes have been identified, the molecular mechanism that defines XyG branching patterns is unclear. Four out of five XyG xylosyltransferases (XXT1, XXT2, XXT4, and XXT5) are known to add the xylosyl residue from UDP-xylose onto a glucan backbone chain; however, the function of XXT3 has yet to be demonstrated. Single xxt3 and triple xxt3xxt4xxt5 mutant Arabidopsis (Arabidopsis thaliana) plants were generated using CRISPR-Cas9 technology to determine the specific function of XXT3. Combined biochemical, bioinformatic, and morphological data conclusively established for the first time that XXT3, together with XXT4 and XXT5, adds xylosyl residue specifically at the third glucose in the glucan chain to synthesize XXXG-type XyGs. We propose that the specificity of XXT3, XXT4, and XXT5 is directed toward the prior synthesis of the acceptor substrate by the other two enzymes, XXT1 and XXT2. We also conclude that XXT5 plays a dominant role in the synthesis of XXXG-type XyGs, while XXT3 and XXT4 complementarily contribute their activities in a tissue-specific manner. The newly generated xxt3xxt4xxt5 mutant produces only XXGG-type XyGs, which further helps to understand the impact of structurally deficient polysaccharides on plant cell wall organization, growth, and development.

Disruption of a DUF247 Containing Protein Alters Cell Wall Polysaccharides and Reduces Growth in Arabidopsis

Plant cell wall biosynthesis is a complex process that requires proteins and enzymes from glycan synthesis to wall assembly. We show that disruption of At3g50120 (DUF247-1), a member of the DUF247 multigene family containing 28 genes in Arabidopsis, results in alterations to the structure and composition of cell wall polysaccharides and reduced growth and plant size. An ELISA using cell wall antibodies shows that the mutants also exhibit ~50% reductions in xyloglucan (XyG), glucuronoxylan (GX) and heteromannan (HM) epitopes in the NaOH fraction and ~50% increases in homogalacturonan (HG) epitopes in the CDTA fraction. Furthermore, the polymer sizes of XyGs and GXs are reduced with concomitant increases in short-chain polymers, while those of HGs and mHGs are slightly increased. Complementation using 35S:DUF247-1 partially recovers the XyG and HG content, but not those of GX and HM, suggesting that DUF247-1 is more closely associated with XyGs and HGs. DUF247-1 is expressed throughout Arabidopsis, particularly in vascular and developing tissues, and its disruption affects the expression of other gene members, indicating a regulatory control role within the gene family. Our results demonstrate that DUF247-1 is required for normal cell wall composition and structure and Arabidopsis growth.

Open questions in plant cell wall synthesis

Plant cells are surrounded by strong yet flexible polysaccharide-based cell walls that support the cell while also allowing growth by cell expansion. Plant cell wall research has advanced tremendously in recent years. Sequenced genomes of many model and crop plants have facilitated cataloging and characterization of many enzymes involved in cell wall synthesis. Structural information has been generated for several important cell wall synthesizing enzymes. Important tools have been developed including antibodies raised against a variety of cell wall polysaccharides and glycoproteins, collections of enzyme clones and synthetic glycan arrays for characterizing enzymes, herbicides that specifically affect cell wall synthesis, live-cell imaging probes to track cell wall synthesis, and an inducible secondary cell wall synthesis system. Despite these advances, and often because of the new information they provide, many open questions about plant cell wall polysaccharide synthesis persist. This article highlights some of the key questions that remain open, reviews the data supporting different hypotheses that address these questions, and discusses technological developments that may answer these questions in the future.

In silico studies of plant primary cell walls - structure and mechanics

Primary plant cell wall (PCW) is a highly organized network, its performance is dependent on cellulose, hemicellulose and pectic polysaccharides, their properties, interactions and assemblies. Their mutual relationships and functions in the cell wall can be better understood by means of conceptual models of their higher-order structures. Knowledge unified in the form of a conceptual model allows predictions to be made about the properties and behaviour of the system under study. Ongoing research in this field has resulted in a number of conceptual models of the cell wall. However, due to the currently limited research methods, the community of cell wall researchers have not reached a consensus favouring one model over another. Herein we present yet another research technique - numerical modelling - which is capable of resolving this issue. Even at the current stage of development of numerical techniques, due to their complexity, the in silico reconstruction of PCW remains a challenge for computational simulations. However, some difficulties have been overcome, thereby making it possible to produce advanced approximations of PCW structure and mechanics. This review summarizes the results concerning the simulation of polysaccharide interactions in PCW with regard to network fine structure, supramolecular properties and polysaccharide binding affinity. The in silico mechanical models presented herein incorporate certain physical and biomechanical aspects of cell wall architecture for the purposes of undertaking critical testing to bring about advances in our understanding of the mechanisms controlling cells and limiting cell wall expansion.

Structures of the xyloglucans in the monocotyledon family Araceae (aroids)

The xyloglucans of all aquatic Araceae species examined had unusual structures compared with those of other non-commelinid monocotyledon families previously examined. The aquatic Araceae species Lemna minor was earlier shown to have xyloglucans with a different structure from the fucogalactoxyloglucans of other non-commelinid monocotyledons. We investigated 26 Araceae species (including L. minor), from five of the seven subfamilies. All seven aquatic species examined had xyloglucans that were unusual in having one or two of three features: < 77% XXXG core motif [L. minor (Lemnoideae) and Orontium aquaticum (Orontioideae)]; no fucosylation [L. minor (Lemnoideae), Cryptocoryne aponogetonifolia, and Lagenandra ovata (Aroideae, Rheophytes clade)]; and > 14% oligosaccharide units with S or D side chains [Spirodela polyrhiza and Landoltia punctata (Lemnoideae) and Pistia stratiotes (Aroideae, Dracunculus clade)]. Orontioideae and Lemnoideae are the two most basal subfamilies, with all species being aquatic, and Aroideae is the most derived. Two terrestrial species [Dieffenbachia seguine and Spathicarpa hastifolia (Aroideae, Zantedeschia clade)] also had xyloglucans without fucose indicating this feature was not unique to aquatic species.

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.

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.

Morphogenesis and cell wall composition of trichomes and their function in response to salt in halophyte Salsola ferganica

BACKGROUND

To survive harsh environmental conditions, desert plants show various adaptions, such as the evolution of trichomes, which are protective epidermal protrusions. Currently, the morphogenesis and function of trichomes in desert plants are not well understood. Salsola ferganica is an annual halophyte distributed in cold deserts; at the seedling stage, its rod-shaped true leaves are covered with long and thick trichomes and are affected by habitat conditions. Therefore, we evaluated the trichomes on morphogenesis and cell wall composition of S. ferganica compared to Arabidopsis thaliana and cotton, related gene expression, and preliminary function in salt accumulation of the leaves.

RESULTS

The trichomes of S. ferganica were initiated from the epidermal primordium, followed by two to three rounds of cell division to form a multicellular trichome, while some genes associated with them were positively involved. Cell wall composition analysis showed that different polysaccharides including heavily methyl-esterified and fully de-esterified pectins (before maturation, probably in the primary wall), xyloglucans (in the mid-early and middle stages, probably in the secondary wall), and extensin (during the whole developmental period) were detected, which were different from those found in trichomes of Arabidopsis and cotton. Moreover, trichome development was affected by abiotic stress, and might accumulate salt from the mesophyll cells and secrete outside.

CONCLUSIONS

S. ferganica has multicellular, non-branched trichomes that undergo two to three rounds of cell division and are affected by abiotic stress. They have a unique cell wall composition which is different from that of Arabidopsis and cotton. Furthermore, several genes positively or negatively regulate trichome development. Our findings should contribute to our further understanding of the biogenesis and adaptation of plant accessory structures in desert plant species.

Shank-localized cell wall growth contributes to Arabidopsis root hair elongation

Root hairs are highly elongated tubular extensions of root epidermal cells with a plethora of physiological functions, particularly in establishing the root-rhizosphere interface. Anisotropic expansion of root hairs is generally thought to be exclusively mediated by tip growth-a highly controlled apically localized secretion of cell wall material-enriched vesicles that drives the extension of the apical dome. Here we show that tip growth is not the only mode of root hair elongation. We identified events of substantial shank-localized cell wall expansion along the polar growth axis of Arabidopsis root hairs using morphometric analysis with quantum dots. These regions expanded after in vivo immunolocalization using cell wall-directed antibodies and appeared as distinct bands that were devoid of cell wall labelling. Application of a novel click chemistry-enabled galactose analogue for pulse chase and real-time imaging allowed us to label xyloglucan, a major root hair glycan, and demonstrate its de novo deposition and enzymatic remodelling in these shank regions. Our data reveal a previously unknown aspect of root hair growth in which both tip- and shank-localized dynamic cell wall deposition and remodelling contribute to root hair elongation.

Eudicot primary cell wall glucomannan is related in synthesis, structure, and function to xyloglucan

Hemicellulose polysaccharides influence assembly and properties of the plant primary cell wall (PCW), perhaps by interacting with cellulose to affect the deposition and bundling of cellulose fibrils. However, the functional differences between plant cell wall hemicelluloses such as glucomannan, xylan, and xyloglucan (XyG) remain unclear. As the most abundant hemicellulose, XyG is considered important in eudicot PCWs, but plants devoid of XyG show relatively mild phenotypes. We report here that a patterned β-galactoglucomannan (β-GGM) is widespread in eudicot PCWs and shows remarkable similarities to XyG. The sugar linkages forming the backbone and side chains of β-GGM are analogous to those that make up XyG, and moreover, these linkages are formed by glycosyltransferases from the same CAZy families. Solid-state nuclear magnetic resonance indicated that β-GGM shows low mobility in the cell wall, consistent with interaction with cellulose. Although Arabidopsis β-GGM synthesis mutants show no obvious growth defects, genetic crosses between β-GGM and XyG mutants produce exacerbated phenotypes compared with XyG mutants. These findings demonstrate a related role of these two similar but distinct classes of hemicelluloses in PCWs. This work opens avenues to study the roles of β-GGM and XyG in PCWs.

Enzymatic fingerprinting reveals specific xyloglucan and pectin signatures in the cell wall purified with primary plasmodesmata

Plasmodesmata (PD) pores connect neighbouring plant cells and enable direct transport across the cell wall. Understanding the molecular composition of these structures is essential to address their formation and later dynamic regulation. Here we provide a biochemical characterisation of the cell wall co-purified with primary PD of Arabidopsis thaliana cell cultures. To achieve this result we combined subcellular fractionation, polysaccharide analyses and enzymatic fingerprinting approaches. Relative to the rest of the cell wall, specific patterns were observed in the PD fraction. Most xyloglucans, although possibly not abundant as a group, were fucosylated. Homogalacturonans displayed short methylated stretches while rhamnogalacturonan I species were remarkably abundant. Full rhamnogalacturonan II forms, highly methyl-acetylated, were also present. We additionally showed that these domains, compared to the broad wall, are less affected by wall modifying activities during a time interval of days. Overall, the protocol and the data presented here open new opportunities for the study of wall polysaccharides associated with PD.

Noncoding RNAs responsive to nitric oxide and their protein-coding gene targets shed light on root hair formation in Arabidopsis thaliana

An overview of the total Arabidopsis thaliana transcriptome, described previously by our research group, pointed some noncoding RNA (ncRNA) as participants in the restoration of hair-root phenotype in A. thaliana rhd6 mutants, leading us to a deeper investigation. A transcriptional gene expression profiling of seedling roots was performed aiming to identify ncRNA responsive to nitric oxide (GSNO) and auxin (IAA), and their involvement in root hair formation in the rhd6 null mutant. We identified 3,631 ncRNAs, including new ones, in A. thaliana and differential expression (DE) analysis between the following: 1) GSNO-treated rhd6 vs. untreated rhd6, 2) IAA-treated rhd6 vs. untreated rhd6, 3) GSNO-treated rhd6 vs. IAA-treated rhd6, and 4) WS-2 vs. untreated rhd6 detected the greatest number of DE genes in GSNO-treated rhd6. We detected hundreds of in silico interactions among ncRNA and protein-coding genes (PCGs), highlighting MIR5658 and MIR171 precursors highly upregulated in GSNO-treated rhd6 and wild type, respectively. Those ncRNA interact with many DE PCGs involved in hormone signaling, cell wall development, transcription factors, and root hair formation, becoming candidate genes in cell wall modulation and restoration of root hair phenotype by GSNO treatment. Our data shed light on how GSNO modulates ncRNA and their PCG targets in A. thaliana root hair formation.

The cell wall of hornworts and liverworts: innovations in early land plant evolution?

An important step for plant diversification was the transition from freshwater to terrestrial habitats. The bryophytes and all vascular plants share a common ancestor that was probably the first to adapt to life on land. A polysaccharide-rich cell wall was necessary to cope with newly faced environmental conditions. Therefore, some pre-requisites for terrestrial life have to be shared in the lineages of modern bryophytes and vascular plants. This review focuses on hornwort and liverwort cell walls and aims to provide an overview on shared and divergent polysaccharide features between these two groups of bryophytes and vascular plants. Analytical, immunocytochemical, and bioinformatic data were analysed. The major classes of polysaccharides-cellulose, hemicelluloses, and pectins-seem to be present but have diversified structurally during evolution. Some polysaccharide groups show structural characteristics which separate hornworts from the other bryophytes or are too poorly studied in detail to be able to draw absolute conclusions. Hydroxyproline-rich glycoprotein backbones are found in hornworts and liverworts, and show differences in, for example, the occurrence of glycosylphosphatidylinositol (GPI)-anchored arabinogalactan-proteins, while glycosylation is practically unstudied. Overall, the data are an appeal to researchers in the field to gain more knowledge on cell wall structures in order to understand the changes with regard to bryophyte evolution.

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.

Expansin-mediated developmental and adaptive responses: A matter of cell wall biomechanics?

Biomechanical properties of the cell wall (CW) are important for many developmental and adaptive responses in plants. Expansins were shown to mediate pH-dependent CW enlargement via a process called CW loosening. Here, we provide a brief overview of expansin occurrence in plant and non-plant species, their structure and mode of action including the role of hormone-regulated CW acidification in the control of expansin activity. We depict the historical as well as recent CW models, discuss the role of expansins in the CW biomechanics and address the developmental importance of expansin-regulated CW loosening in cell elongation and new primordia formation. We summarise the data published so far on the role of expansins in the abiotic stress response as well as the rather scarce evidence and hypotheses on the possible mechanisms underlying expansin-mediated abiotic stress resistance. Finally, we wrap it up by highlighting possible future directions in expansin research.

How Cell Geometry and Cellular Patterning Influence Tissue Stiffness

Cell growth in plants occurs due to relaxation of the cell wall in response to mechanical forces generated by turgor pressure. Growth can be anisotropic, with the principal direction of growth often correlating with the direction of lower stiffness of the cell wall. However, extensometer experiments on onion epidermal peels have shown that the tissue is stiffer in the principal direction of growth. Here, we used a combination of microextensometer experiments on epidermal onion peels and finite element method (FEM) modeling to investigate how cell geometry and cellular patterning affects mechanical measurements made at the tissue level. Simulations with isotropic cell-wall material parameters showed that the orientation of elongated cells influences tissue apparent stiffness, with the tissue appearing much softer in the transverse versus the longitudinal directions. Our simulations suggest that although extensometer experiments show that the onion tissue is stiffer when stretched in the longitudinal direction, the effect of cellular geometry means that the wall is in fact softer in this direction, matching the primary growth direction of the cells.

Xyloglucan remodelling enzymes and the mechanics of plant seed and fruit biology

This article comments on: Di Marzo M, Ebeling Viana V, Banfi C, Cassina V, Corti R, Herrera-Ubaldo H, Babolin N, Guazzotti A, Kiegle E, Gregis V, de Folter S, Sampedro J, Mantegazza F, Colombo L, Ezquer I. 2022. Cell wall modifications by α-XYLOSIDASE1 are required for the control of seed and fruit size. Journal of Experimental Botany 73, 1499–1515.

Reconsidering the function of the xyloglucan endotransglucosylase/hydrolase family

Plants possess an outer cell layer called the cell wall. This matrix comprises various molecules, such as polysaccharides and proteins, and serves a wide array of physiologically important functions. This structure is not static but rather flexible in response to the environment. One of the factors responsible for this plasticity is the xyloglucan endotransglucosylase/hydrolase (XTH) family, which cleaves and reconnects xyloglucan molecules. Since xyloglucan molecules have been hypothesised to tether cellulose microfibrils forming the main load-bearing network in the primary cell wall, XTHs have been thought to play a central role in cell wall loosening for plant cell expansion. However, multiple lines of recent evidence have questioned this classic model. Nevertheless, reverse genetic analyses have proven the biological importance of XTHs; therefore, a major challenge at present is to reconsider the role of XTHs in planta. Recent advances in analytical techniques have allowed for gathering rich information on the structure of the primary cell wall. Thus, the integration of accumulated knowledge in current XTH studies may offer a turning point for unveiling the precise functions of XTHs. In the present review, we redefine the biological function of the XTH family based on the recent architectural model of the cell wall. We highlight three key findings regarding this enzyme family: (1) XTHs are not strictly required for cell wall loosening during plant cell expansion but play vital roles in response to specific biotic or abiotic stresses; (2) in addition to their transglycosylase activity, the hydrolase activity of XTHs is involved in physiological benefits; and (3) XTHs can recognise a wide range of polysaccharides other than xyloglucans.

Elucidating connections between the strigolactone biosynthesis pathway, flavonoid production and root system architecture in Arabidopsis thaliana

Strigolactones (SLs) are the most recently discovered phytohormones, and their roles in root architecture and metabolism are not fully understood. Here, we investigated four MORE AXILLARY GROWTH (MAX) SL mutants in Arabidopsis thaliana, max3-9, max4-1, max1-1 and max2-1, as well as the SL receptor mutant d14-1 and karrikin receptor mutant kai2-2. By characterising max2-1 and max4-1, we found that variation in SL biosynthesis modified multiple metabolic pathways in root tissue, including that of xyloglucan, triterpenoids, fatty acids and flavonoids. The transcription of key flavonoid biosynthetic genes, including TRANSPARENT TESTA4 (TT4) and TRANSPARENT TESTA5 (TT5) was downregulated in max2 roots and seedlings, indicating that the proposed MAX2 regulation of flavonoid biosynthesis has a widespread effect. We found an enrichment of BRI1-EMS-SUPPRESSOR 1 (BES1) targets amongst genes specifically altered in the max2 mutant, reflecting that the regulation of flavonoid biosynthesis likely occurs through the MAX2 degradation of BES1, a key brassinosteroid-related transcription factor. Finally, flavonoid accumulation decreased in max2-1 roots, supporting a role for MAX2 in regulating both SL and flavonoid biosynthesis.

Using CRISPR-Cas9 Technology to Eliminate Xyloglucan in Tobacco Cell Walls and Change the Uptake and Translocation of Inorganic Arsenic

Xyloglucan is a quantitatively major polysaccharide in the primary cell walls of flowering plants and has been reported to affect plants' ability to tolerate toxic elements. However, it is not known if altering the amounts of xyloglucan in the wall influences the uptake and translocation of inorganic arsenic (As). Here, we identified two Nicotiana tabacum genes that encode xyloglucan-specific xylosyltransferases (XXT), which we named NtXXT1 and NtXXT2. We used CRISPR-Cas9 technology to generate ntxxt1, ntxxt2, and ntxxt1/2 mutant tobacco plants to determine if preventing xyloglucan synthesis affects plant growth and their ability to accumulate As. We show that NtXXT1 and NtXXT2 are required for xyloglucan biosynthesis because no discernible amounts of xyloglucan were present in the cell walls of the ntxxt1/2 double mutant. The tobacco double mutant (ntxxt1/2) and the corresponding Arabidopsis mutant (atxxt1/2) do not have severe growth defects but do have a short root hair phenotype and a slow growth rate. This phenotype is rescued by overexpressing NtXXT1 or NtXXT2 in atxxt1/2. Growing ntxxt mutants in the presence of AsIII or AsV showed that the absence of cell wall xyloglucan affects the accumulation and translocation of As. Most notably, root retention of As increased substantially and the amounts of As translocated to the shoots decreased in ntxxt1/2. Our results suggest that xyloglucan-deficient plants provide a strategy for the phytoremediation of As contaminated soils.

Characterization and Interaction Analysis of the Secondary Cell Wall Synthesis-Related Transcription Factor PmMYB7 in Pinus massoniana Lamb

In vascular plants, the importance of R2R3-myeloblastosis (R2R3-MYB) transcription factors (TFs) in the formation of secondary cell walls (SCWs) has long been a controversial topic due to the lack of empirical evidence of an association between TFs and downstream target genes. Here, we found that the transcription factor PmMYB7, which belongs to the R2R3-MYB subfamily, is involved in lignin biosynthesis in Pinus massoniana. PmMYB7 was highly expressed in lignified tissues and upon abiotic stress. As a bait carrier, the PmMYB7 protein had no toxicity or autoactivation in the nucleus. Forty-seven proteins were screened from the P. massoniana yeast library. These proteins were predicted to be mainly involved in resistance, abiotic stress, cell wall biosynthesis, and cell development. We found that the PmMYB7 protein interacted with caffeoyl CoA 3-O-methyltransferase-2 (PmCCoAOMT2)—which is involved in lignin biosynthesis—but not with beta-1, 2-xylosyltransferase (PmXYXT1) yeast two-hybrid (Y2H) studies. Our in vivo coimmunoprecipitation (Co-IP) assay further showed that the PmMYB7 and PmCCoAOMT2 proteins could interact. Therefore, we concluded that PmMYB7 is an upstream TF that can interact with PmCCoAOMT2 in plant cells. These findings lay a foundation for further research on the function of PmMYB7, lignin biosynthesis and molecular breeding in P. massoniana.

Tissue-specific transcriptomics reveal functional differences in floral development

Flowers are produced by floral meristems, groups of stem cells that give rise to floral organs. In grasses, including the major cereal crops, flowers (florets) are contained in spikelets, which contain one to many florets, depending on the species. Importantly, not all grass florets are developmentally equivalent, and one or more florets are often sterile or abort in each spikelet. Members of the Andropogoneae tribe, including maize (Zea mays), produce spikelets with two florets; the upper and lower florets are usually dimorphic, and the lower floret is greatly reduced compared to the upper floret. In maize ears, early development appears identical in both florets but the lower floret ultimately aborts. To gain insight into the functional differences between florets with different fates, we used laser capture microdissection coupled with RNA-sequencing to globally examine gene expression in upper and lower floral meristems in maize. Differentially expressed genes were involved in hormone regulation, cell wall, sugar, and energy homeostasis. Furthermore, cell wall modifications and sugar accumulation differed between the upper and lower florets. Finally, we identified a boundary domain between upper and lower florets, which we hypothesize is important for floral meristem activity. We propose a model in which growth is suppressed in the lower floret by limiting sugar availability and upregulating genes involved in growth repression. This growth repression module may also regulate floret fertility in other grasses and potentially be modulated to engineer more productive cereal crops.

The Xyloglucan Endotransglucosylase/Hydrolase Gene XTH22/TCH4 Regulates Plant Growth by Disrupting the Cell Wall Homeostasis in Arabidopsis under Boron Deficiency

TCH4 is a xyloglucan endotransglucosylase/hydrolase (XTH) family member. Extensive studies have shown that XTHs are very important in cell wall homeostasis for plant growth and development. Boron (B), as an essential micronutrient for plants, plays an essential role in the cross-linking of cell wall pectin. However, the effect of B on cell wall organization is unclear. This study aimed to explore the mechanism of plant adaption to B stress by investigating the role of TCH4 in cell wall homeostasis. We conducted both plate and hydroponic cultures of wild-type Col-0 and overexpression and gene knockout lines of XTH22/TCH4 to analyze the phenotype, components, and characteristics of the cell wall using immunofluorescence, atomic force microscopy (AFM), and transmission electron microscopy (TEM). B deficiency induces the expression of TCH4. The overexpression lines of TCH4 presented more sensitivity to B deficiency than the wild-type Col-0, while the knockout lines of TCH4 were more resistant to low B stress. Up-regulation of TCH4 influenced the ratio of chelator-soluble pectin to alkali-soluble pectin and decreased the degree of methylesterification of pectin under B-deficient conditions. Moreover, we found that B deficiency disturbed the arrangement of cellulose, enlarged the gap between cellulose microfibrils, and decreased the mechanical strength of the cell wall, leading to the formation of a thickened and deformed triangular region of the cell wall. These symptoms were more profound in the TCH4 overexpression lines. Consistently, compared with Col-0, the O₂^- and MDA contents in the TCH4 overexpression lines increased under B-deficient conditions. This study identified the B-deficiency-induced TCH4 gene, which regulates cell wall homeostasis to influence plant growth under B-deficient conditions.

Plant cell mechanobiology: Greater than the sum of its parts

The ability to sense and respond to physical forces is critical for the proper function of cells, tissues, and organisms across the evolutionary tree. Plants sense gravity, osmotic conditions, pathogen invasion, wind, and the presence of barriers in the soil, and dynamically integrate internal and external stimuli during every stage of growth and development. While the field of plant mechanobiology is growing, much is still poorly understood-including the interplay between mechanical and biochemical information at the single-cell level. In this review, we provide an overview of the mechanical properties of three main components of the plant cell and the mechanoperceptive pathways that link them, with an emphasis on areas of complexity and interaction. We discuss the concept of mechanical homeostasis, or "mechanostasis," and examine the ways in which cellular structures and pathways serve to maintain it. We argue that viewing mechanics and mechanotransduction as emergent properties of the plant cell can be a useful conceptual framework for synthesizing current knowledge and driving future research.