Binding of arabinan or galactan during cellulose synthesis is extensive and reversible

Influence of pectin and hemicelluloses on physical properties of bacterial cellulose

The properties of bacterial cellulose (BC)-based films produced by in situ biosynthesis with various polysaccharides (water-soluble pectin, arabinan, rhamnogalacturonan I, arabinoxylan, xyloglucan, glucomannan) were investigated. The addition of the polysaccharides to the bacterial growth environment changed the composition of the films by incorporating characteristic monosaccharides. BC-based films contained up to 26.7 % of non-cellulosic polysaccharides. The applied modification had a clear impact on water sorption and caused a decrease in the thermal stability of most BC films, which was connected with the depletion of geometrical dimensions of cellulose nanofibers observed with AFM. The FT-IR and Raman spectra demonstrated a decrease in % Iα of cellulose films, most notably for xyloglucan and glucomannan, as well as a change in their degree of crystallinity and the length of cellulose chains. The addition of xyloglucan had the most pronounced effect on film hardening; the other additives had a similar but lesser effect.

Bacterial cellulose nanofibers used as nanofillers improved the fresh-keeping performance of gelatin-based edible coating for fresh-cut apples

In this study, bacterial cellulose nanofibers (BCNs) (0%, 1%, 2%, and 3%) were used as nanofillers to prepare gelatin-based edible films, and their physical properties and fresh-keeping performance were investigated. The microstructure observation showed that the BCNs were well dispersed in the gelatin-based edible films and the surface roughness of the films increased with the increase of BCNs content. X-ray diffraction and thermogravimetric analysis showed that the crystallinity and thermal stability of the film were significantly increased with the increase of BCNs. Fourier-transform infrared spectroscopy analysis suggested that hydrogen bond interactions occurred between BCNs and gelatin polymers, leading to improved mechanical properties with the increase of BCNs content. Furthermore, the barrier performance was also improved with the increase of BCNs content, where gelatin-based edible films with 2% BCNs showed the best mechanical property. Meanwhile, the gelatin-based film-forming solutions (FFSs) containing different BCNs were coated on the fresh-cut apples and the corresponding fresh-keeping performance was investigated. The results showed that the fresh-keeping parameters of fresh-cut apples coated with FFSs containing BCNs were better as compared with those of pure gelatin FFSs. Moreover, the fresh-keeping parameters were improved with the increase of BCNs, especially the FFSs containing 2% BCNs that showed the best fresh-keeping parameters. Therefore, BCNs, used as nanofillers, are an excellent enhancer to improve the fresh-keeping performance of the gelatin-based edible coating, showing a promising potential application in the food preservation field.

Analysis of pectin-cellulose interaction in cell wall of lotus rhizome with assistance of ball-milling and galactosidase

The current study sought to depict the structural feature of polysaccharides extracted from Na2CO3 unextractable fraction (LUN) of lotus rhizome using galactosidase with assistance of ball milling. The extracted polysaccharides were a complex of cellulose microfibrils and the RG-I structural domain of pectin, and the top three monosaccharides were glucose, galactose and galactose uronic acid, which allowed to tune the properties of the enzyme-hydrolyzed polysaccharide from LUN after 15 and 45 min of ball milling. The data of XRD revealed that pectin has a masking effect on the diffraction peaks of cellulose components. The removing of the polysaccharides could increase the degree of crystallinity and the pectin-cellulose interaction mainly occured through the galactan side chain was speculated. Textural characterization by SEM exhibited a cross-linked rod-like structure, which is similar to the structure of cellulose microfibrils. The morphological analysis of AFM revealed that L15-P (enzyme-hydrolyzed polysaccharide from LUN after 15 min of ball milling) contained relatively ordered and uniform network structures. Overall, the present study provides an important insight into cell wall of lotus rhizome matrix polysaccharide.

The salt-activated CBF1/CBF2/CBF3-GALS1 module fine-tunes galactan-induced salt hypersensitivity in Arabidopsis

Plant growth and development are significantly hampered in saline environments, limiting agricultural productivity. Thus, it is crucial to unravel the mechanism underlying plant responses to salt stress. β-1,4-Galactan (galactan), which forms the side chains of pectic rhamnogalacturonan I, enhances plant sensitivity to high-salt stress. Galactan is synthesized by GALACTAN SYNTHASE1 (GALS1). We previously showed that NaCl relieves the direct suppression of GALS1 transcription by the transcription factors BPC1 and BPC2 to induce the excess accumulation of galactan in Arabidopsis (Arabidopsis thaliana). However, how plants adapt to this unfavorable environment remains unclear. Here, we determined that the transcription factors CBF1, CBF2, and CBF3 directly interact with the GALS1 promoter and repress its expression, leading to reduced galactan accumulation and enhanced salt tolerance. Salt stress enhances the binding of CBF1/CBF2/CBF3 to the GALS1 promoter by inducing CBF1/CBF2/CBF3 transcription and accumulation. Genetic analysis suggested that CBF1/CBF2/CBF3 function upstream of GALS1 to modulate salt-induced galactan biosynthesis and the salt response. CBF1/CBF2/CBF3 and BPC1/BPC2 function in parallel to regulate GALS1 expression, thereby modulating the salt response. Our results reveal a mechanism in which salt-activated CBF1/CBF2/CBF3 inhibit BPC1/BPC2-regulated GALS1 expression to alleviate galactan-induced salt hypersensitivity, providing an activation/deactivation fine-tune mechanism for dynamic regulation of GALS1 expression under salt stress in Arabidopsis.

Elongated galactan side chains mediate cellulose-pectin interactions in engineered Arabidopsis secondary cell walls

The plant secondary cell wall is a thickened matrix of polysaccharides and lignin deposited at the cessation of growth in some cells. It forms the majority of carbon in lignocellulosic biomass, and it is an abundant and renewable source for forage, fiber, materials, fuels, and bioproducts. The complex structure and arrangement of the cell wall polymers mean that the carbon is difficult to access in an economical and sustainable way. One solution is to alter the cell wall polymer structure so that it is more suited to downstream processing. However, it remains difficult to predict what the effects of this engineering will be on the assembly, architecture, and properties of the cell wall. Here, we make use of Arabidopsis plants expressing a suite of genes to increase pectic galactan chain length in the secondary cell wall. Using multi-dimensional solid-state nuclear magnetic resonance, we show that increasing galactan chain length enhances pectin-cellulose spatial contacts and increases cellulose crystallinity. We also found that the increased galactan content leads to fewer spatial contacts of cellulose with xyloglucan and the backbone of pectin. Hence, we propose that the elongated galactan side chains compete with xyloglucan and the pectic backbone for cellulose interactions. Due to the galactan topology, this may result in comparatively weak interactions and disrupt the cell wall architecture. Therefore, introduction of this strategy into trees or other bioenergy crops would benefit from cell-specific expression strategies to avoid negative effects on plant growth.

Cell corner middle lamella in hydroids of dendroid moss Hypnodendron menziesii gametophyte is prominently thickened: a proposed role in the mechanical support function

Significantly thickened corner middle lamella of the hydroid cell wall in the stipe of dendroid moss Hypnodendron menziesii has a mechanical support function. The hydroid cell walls of the erect stipe of Hypnodendron menziesii were investigated using light microscopy (LM), transmission electron microscopy (TEM), and TEM-immunogold labeling in support of the proposed biomechanical function for the highly thickened cell corner middle lamellae. The statistical analyses of dimensions of hydroid cell and wall parameters revealed a strong positive correlation between the area of hydroid cell and (i) the hydroid cell walls adhering to thick corner middle lamella, (ii) the area of the thick cell wall at hydroid corners, and (iii) the maximum thickness of cell wall at hydroid corners. The total area of the thick cell wall at the hydroid corners concomitantly increased with the area of the hydroid cell wall adhering to the middle lamella, and with the increased number of hydroids surrounding a reference hydroid. The results suggest that markedly thickened middle lamellae of the hydroid cell wall in Hypnodendron likely function by preventing hydroid cells from collapsing under the tensile forces generated from the transpirational pull on the water column. The specific localization of (1→4)- β-D-galactan and (1,5)-α-L-arabinan in the interface region of the hydroid cell wall and the thick middle lamella is consistent with these cell wall components being involved in the mechanical strengthening of the interface through firm adhesion as well as elasticity, ensuring the structural stability of this cell wall region, which may be prone to delamination/fracturing from the various internal and external pressures imposed. The copious presence of homogalacturonan in the thick middle lamella may further enhance the strength and flexibility of hydroid cell walls.

Structural and biochemical insight into a modular β-1,4-galactan synthase in plants

Rhamnogalacturonan I (RGI) is a structurally complex pectic polysaccharide with a backbone of alternating rhamnose and galacturonic acid residues substituted with arabinan and galactan side chains. Galactan synthase 1 (GalS1) transfers galactose and arabinose to either extend or cap the β-1,4-galactan side chains of RGI, respectively. Here we report the structure of GalS1 from Populus trichocarpa, showing a modular protein consisting of an N-terminal domain that represents the founding member of a new family of carbohydrate-binding module, CBM95, and a C-terminal glycosyltransferase family 92 (GT92) catalytic domain that adopts a GT-A fold. GalS1 exists as a dimer in vitro, with stem domains interacting across the chains in a 'handshake' orientation that is essential for maintaining stability and activity. In addition to understanding the enzymatic mechanism of GalS1, we gained insight into the donor and acceptor substrate binding sites using deep evolutionary analysis, molecular simulations and biochemical studies. Combining all the results, a mechanism for GalS1 catalysis and a new model for pectic galactan side-chain addition are proposed.

Temporal cell wall changes during cold acclimation and deacclimation and their potential involvement in freezing tolerance and growth

Plants adapt to freezing stress through cold acclimation, which is induced by nonfreezing low temperatures and accompanied by growth arrest. A later increase in temperature after cold acclimation leads to rapid loss of freezing tolerance and growth resumption, a process called deacclimation. Appropriate regulation of the trade-off between freezing tolerance and growth is necessary for efficient plant development in a changing environment. The cell wall, which mainly consists of polysaccharide polymers, is involved in both freezing tolerance and growth. Still, it is unclear how the balance between freezing tolerance and growth is affected during cold acclimation and deacclimation by the changes in cell wall structure and what role is played by its monosaccharide composition. Therefore, to elucidate the regulatory mechanisms controlling freezing tolerance and growth during cold acclimation and deacclimation, we investigated cell wall changes in detail by sequential fractionation and monosaccharide composition analysis in the model plant Arabidopsis thaliana, for which a plethora of information and mutant lines are available. We found that arabinogalactan proteins and pectic galactan changed in close coordination with changes in freezing tolerance and growth during cold acclimation and deacclimation. On the other hand, arabinan and xyloglucan did not return to nonacclimation levels after deacclimation but stabilized at cold acclimation levels. This indicates that deacclimation does not completely restore cell wall composition to the nonacclimated state but rather changes it to a specific novel composition that is probably a consequence of the loss of freezing tolerance and provides conditions for growth resumption.

Auxin triggers pectin modification during rootlet emergence in white lupin

Emergence of secondary roots through parental tissue is a highly controlled developmental process. Although the model plant Arabidopsis has been useful to uncover the predominant role of auxin in this process, its simple root structure is not representative of how emergence takes place in most plants, which display more complex root anatomy. White lupin is a legume crop producing structures called cluster roots, where closely spaced rootlets emerge synchronously. Rootlet primordia push their way through several cortical cell layers while maintaining the parent root integrity, reflecting more generally the lateral root emergence process in most multilayered species. In this study, we showed that lupin rootlet emergence is associated with an upregulation of cell wall pectin modifying and degrading genes under the active control of auxin. Among them, we identified LaPG3, a polygalacturonase gene typically expressed in cells surrounding the rootlet primordium and we showed that its downregulation delays emergence. Immunolabeling of pectin epitopes and their quantification uncovered a gradual pectin demethylesterification in the emergence zone, which was further enhanced by auxin treatment, revealing a direct hormonal control of cell wall properties. We also report rhamnogalacturonan-I modifications affecting cortical cells that undergo separation as a consequence of primordium outgrowth. In conclusion, we describe a model of how external tissues in front of rootlet primordia display cell wall modifications to allow for the passage of newly formed rootlets.

Arabinogalactan Proteins: Focus on the Role in Cellulose Synthesis and Deposition during Plant Cell Wall Biogenesis

Arabinogalactan proteins (AGPs) belong to a family of glycoproteins that are widely present in plants. AGPs are mostly composed of a protein backbone decorated with complex carbohydrate side chains and are usually anchored to the plasma membrane or secreted extracellularly. A trickle of compelling biochemical and genetic evidence has demonstrated that AGPs make exciting candidates for a multitude of vital activities related to plant growth and development. However, because of the diversity of AGPs, functional redundancy of AGP family members, and blunt-force research tools, the precise functions of AGPs and their mechanisms of action remain elusive. In this review, we put together the current knowledge about the characteristics, classification, and identification of AGPs and make a summary of the biological functions of AGPs in multiple phases of plant reproduction and developmental processes. In addition, we especially discuss deeply the potential mechanisms for AGP action in different biological processes via their impacts on cellulose synthesis and deposition based on previous studies. Particularly, five hypothetical models that may explain the AGP involvement in cellulose synthesis and deposition during plant cell wall biogenesis are proposed. AGPs open a new avenue for understanding cellulose synthesis and deposition in plants.

Cell wall β-1,4-galactan regulated by the BPC1/BPC2-GALS1 module aggravates salt sensitivity in Arabidopsis thaliana

Salinity severely reduces plant growth and limits agricultural productivity. Dynamic changes and rearrangement of the plant cell wall is an important response to salt stress, but relatively little is known about the biological importance of specific cell wall components in the response. Here, we demonstrate a specific function of β-1,4-galactan in salt hypersensitivity. We found that salt stress induces the accumulation of β-1,4-galactan in root cell walls by up regulating the expression of GALACTAN SYNTHASE 1 (GALS1), which encodes a β-1,4-galactan synthase. The accumulation of β-1,4-galactan negatively affects salt tolerance. Exogenous application of D-galactose (D-Gal) causes an increase in β-1,4-galactan levels in the wild type and GALS1 mutants, especially in GALS1 overexpressors, which correlated with the aggravated salt hypersensitivity. Furthermore, we discovered that the BARLEY B RECOMBINANT/BASIC PENTACYSTEINE transcription factors BPC1/BPC2 positively regulate plant salt tolerance by repressing GALS1 expression and β-1,4-galactan accumulation. Genetic analysis suggested that GALS1 is genetically epistatic to BPC1/BPC2 with respect to the control of salt sensitivity as well as accumulation of β-1,4-galactan. Taken together, our results reveal a new regulatory mechanism by which β-1,4-galactan regulated by the BPC1/BPC2-GALS1 module aggravates salt sensitivity in Arabidopsis thaliana.

Production of Microbial Cellulose Films from Green Tea (Camellia Sinensis) Kombucha with Various Carbon Sources

The aim of this study was to evaluate the production of microbial cellulose films (MCFs) in culture media based on green tea and different carbon sources, using two microbial consortia (COr and CFr). During the fermentation process, there was a reduction in the total soluble solids (TSS) content and pH, as well as an increase in the acidity in all treatments. Furthermore, fluctuations in the total sugar content and proteins during the fermentation process were associated with the consumption of carbon and nitrogen sources, as well as the production of MCFs. In the color analysis, a decrease in the L* value was observed while the rest of the parameters remained stable. Production of films was observed between days 6 and 9 of fermentation; the preferred substrate for COr was glucose (wet base yields = 603.61% and dry base yields = 22.37%), whereas for CFr was dextrose (wet base yields = 601.49% and dry base yields = 28.14%). Finally, the MCFs produced by COr and CFr showed a homogeneous, thick appearance, slight flexibility, and the characteristic brown color of the fermentation medium.

Wood hemicelluloses exert distinct biomechanical contributions to cellulose fibrillar networks

Hemicelluloses, a family of heterogeneous polysaccharides with complex molecular structures, constitute a fundamental component of lignocellulosic biomass. However, the contribution of each hemicellulose type to the mechanical properties of secondary plant cell walls remains elusive. Here we homogeneously incorporate different combinations of extracted and purified hemicelluloses (xylans and glucomannans) from softwood and hardwood species into self-assembled networks during cellulose biosynthesis in a bacterial model, without altering the morphology and the crystallinity of the cellulose bundles. These composite hydrogels can be therefore envisioned as models of secondary plant cell walls prior to lignification. The incorporated hemicelluloses exhibit both a rigid phase having close interactions with cellulose, together with a flexible phase contributing to the multiscale architecture of the bacterial cellulose hydrogels. The wood hemicelluloses exhibit distinct biomechanical contributions, with glucomannans increasing the elastic modulus in compression, and xylans contributing to a dramatic increase of the elongation at break under tension. These diverging effects cannot be explained solely from the nature of their direct interactions with cellulose, but can be related to the distinct molecular structure of wood xylans and mannans, the multiphase architecture of the hydrogels and the aggregative effects amongst hemicellulose-coated fibrils. Our study contributes to understanding the specific roles of wood xylans and glucomannans in the biomechanical integrity of secondary cell walls in tension and compression and has significance for the development of lignocellulosic materials with controlled assembly and tailored mechanical properties.

Hemicelluloses are an essential constituent of plant cell walls, but the individual biomechanical roles remain elusive. Here the authors report on the interaction of wood hemicellulose with bacterial cellulose during deposition and explore the resultant fibrillar architecture and mechanical properties.

Cellulose, pectin and water in cell walls determine apple flesh viscoelastic mechanical properties

The viscoelastic mechanical properties are important quality traits for fleshy fruit uses. The contribution of cell wall polysaccharides chemistry and organization on their variability was studied in six varieties of apple. Correlation between damping and storage modulus of plasmolyzed tissue distinguished better apple varieties on their viscoelasticity than fresh samples. Galactose, arabinose and uronic acids correlated positively with the storage modulus of fresh apple samples (E'_f). These corresponded to 4-linked galactan but no specific arabinose linkage. Galacturonic acid branched on O-3 and terminal rhamnose correlated negatively with E'_f. These correlations formed two groups of fruit except for branched methyl-esterified galacturonic. Solid-state ¹³C NMR spectroscopy analyses showed that E'_f correlated negatively with cellulose C4 T_1ρ^H relaxation and positively with pectin methyl esters T_HH proton diffusion. The results point to the key roles of pectin structure and hydration and cellulose microfibrils distribution on apple mechanical properties.

β-(1,4)-Galactan remodelling in Arabidopsis cell walls affects the xyloglucan structure during elongation

Galactan turnover occurs during cell elongation and affects the cell wall xyloglucan structure which is involved in the interaction between cellulose and xyloglucan. β-(1,4)-Galactan is one of the main side chains of rhamnogalacturonan I. Although the specific function of this polymer has not been completely established, it has been related to different developmental processes. To study β-(1,4)-galactan function, we have generated transgenic Arabidopsis plants overproducing chickpea βI-Gal β-galactosidase under the 35S CaMV promoter (35S::βI-Gal) to reduce galactan side chains in muro. Likewise, an Arabidopsis double loss-of-function mutant for BGAL1 and BGAL3 Arabidopsis β-galactosidases (bgal1/bgal3) has been obtained to increase galactan levels. The characterization of these plants has confirmed the role of β-(1,4)-galactan in cell growth, and demonstrated that the turnover of this pectic side chain occurs during cell elongation, at least in Arabidopsis etiolated hypocotyls and floral stem internodes. The results indicate that BGAL1 and BGAL3 β-galactosidases act in a coordinate way during cell elongation. In addition, this work indicates that galactan plays a role in the maintenance of the cell wall architecture during this process. Our results point to an involvement of the β-(1,4)-galactan in the xyloglucan structure and the interaction between cellulose and xyloglucan.

Expanding roles for pectins in plant development

Pectins are complex cell wall polysaccharides important for many aspects of plant development. Recent studies have discovered extensive physical interactions between pectins and other cell wall components, implicating pectins in new molecular functions. Pectins are often localized in spatially-restricted patterns, and some of these non-uniform pectin distributions contribute to multiple aspects of plant development, including the morphogenesis of cells and organs. Furthermore, a growing number of mutants affecting cell wall composition have begun to reveal the distinct contributions of different pectins to plant development. This review discusses the interactions of pectins with other cell wall components, the functions of pectins in controlling cellular morphology, and how non-uniform pectin composition can be an important determinant of developmental processes.

Fruit Softening: Revisiting the Role of Pectin

Fruit softening, which is a major determinant of shelf life and commercial value, is the consequence of multiple cellular processes, including extensive remodeling of cell wall structure. Recently, it has been shown that pectate lyase (PL), an enzyme that degrades de-esterified pectin in the primary wall, is a major contributing factor to tomato fruit softening. Studies of pectin structure, distribution, and dynamics have indicated that pectins are more tightly integrated with cellulose microfibrils than previously thought and have novel structural features, including branches of the main polymer backbone. Moreover, recent studies of the significance of pectinases, such as PL and polygalacturonase, are consistent with a causal relationship between pectin degradation and a major effect on fruit softening.

Composition and structure of tuber cell walls affect in vitro digestibility of potato (Solanum tuberosum L.)

The digestibility of starchy foods, such as potatoes, can be characterized by the proportion of starch that is rapidly digestible by in vitro hydrolysis (rapidly digestible starch, RDS). This study evaluated the RDS content in a potato germplasm collection consisting of 98 genotypes and identified three advanced lines, Crop39, Crop71 and Crop85, where cooked potato RDS content was significantly lower than that of their respective isolated starches (P < 0.05). In Crop39, Crop71 and Crop85, the properties of their isolated starch did not differ significantly from that of five control lines with higher RDS contents. Cell wall analyses revealed that, compared with other lines tested, Crop39, Crop71 and Crop85 had at least four times the amount of rhamnogalacturonan-I (RG-I) galactan side-chains that were very firmly attached to the wall and requiring 4 M KOH for extraction. Pectin solubilization during cooking was also remarkably low (2-4%) in these three lines compared with other lines tested (7-19%). The findings suggest that possession of higher amounts of RG-I galactan that interact strongly with cellulose may provide a sturdier wall that better resists solubilization during cooking, and effectively impedes access of digestive enzymes for starch hydrolysis in an in vitro model.

Emerging models on the regulation of intercellular transport by plasmodesmata-associated callose

The intercellular transport of molecules through membranous channels that traverse the cell walls-so-called plasmodesmata-is of fundamental importance for plant development. Regulation of plasmodesmata aperture (and transport capacity) is mediated by changes in the flanking cell walls, mainly via the synthesis/degradation (turnover) of the (1,3)-β-glucan polymer callose. The role of callose in organ development and in plant environmental responses is well recognized, but detailed understanding of the mechanisms regulating its accumulation and its effects on the structure and permeability of the channels is still missing. We compiled information on the molecular components and signalling pathways involved in callose turnover at plasmodesmata and, more generally, on the structural and mechanical properties of (1,3)-β-glucan polymers in cell walls. Based on this revision, we propose models integrating callose, cell walls, and the regulation of plasmodesmata structure and intercellular communication. We also highlight new tools and interdisciplinary approaches that can be applied to gain further insight into the effects of modifying callose in cell walls and its consequences for intercellular signalling.

Examining the contribution of cell wall polysaccharides to the mechanical properties of apple parenchyma tissue using exogenous enzymes

The viscoelastic mechanical properties of water-rich plant tissues are fundamental for many aspects of organ physiology and plant functioning. These properties are determined partly by the water in cellular vacuole and partly by the mechanical properties of the cell wall, the latter varying according to the composition and organization of its polysaccharides. In this study, relationships between the viscoelastic properties of apple cortex parenchyma tissue and cell wall pectin, hemicelluloses, and cellulose structures were studied by infusing the tissue with selected sets of purified enzymes in a controlled osmoticum. The results showed that tissue elasticity and viscosity were related, and controlled to variable extents by all the targeted polysaccharides. Among them, pectic homogalacturonan domains, crystalline cellulose, and fucosylated xyloglucan were revealed as being of prime importance in determining the viscoelastic mechanical properties of apple cortex tissue.

The influences of added polysaccharides on the properties of bacterial crystalline nanocellulose

Acid hydrolyzed bacterial crystalline nanocellulose (BCNC) with different nanofiber morphologies, geometrical dimensions, crystalline structure and mechanical properties were obtained by adding different polysaccharides into the growing culture medium. Arabinogalactan had little effect on the characteristics of BCNC due to its negligible binding affinity to bacterial cellulose (BC). Bacterial exopolysaccharides were capable of modulating the bundling of cellulose microfibrils during BC formation, resulting in BCNC with bundled nanocrystals, high crystallinity, a less sulfated surface, and improved thermal stability and tensile properties. Xylan/BCNC and xyloglucan/BCNC exhibited the most significant improvements, including an increased length and aspect ratio, a significantly less sulfated surface and superior thermal stability and tensile properties. It is hypothesized that the improvement in CNC characteristics results from a change in amorphous cellulose formation in the native BC. This study also suggests that improved feedstocks for producing CNCs may be obtained by modulating hemicellulose production in plants.

Remodeling of pectin and hemicelluloses in tomato pericarp during fruit growth

Tomato fruit texture depends on histology and cell wall architecture, both under genetic and developmental controls. If ripening related cell wall modifications have been well documented with regard to softening, little is known about cell wall construction during early fruit development. Identification of key events and their kinetics with regard to tissue architecture and cell wall development can provide new insights on early phases of texture elaboration. In this study, changes in pectin and hemicellulose chemical characteristics and location were investigated in the pericarp tissue of tomato (Solanum lycopersicon var Levovil) at four stages of development (7, 14 and 21day after anthesis (DPA) and mature green stages). Analysis of cell wall composition and polysaccharide structure revealed that both are continuously modified during fruit development. At early stages, the relative high rhamnose content in cell walls indicates a high synthesis of rhamnogalacturonan I next to homogalacturonan. Fine tuning of rhamnogalacturonan I side chains appears to occur from the cell expansion phase until prior to the mature green stage. Cell wall polysaccharide remodelling also concerns xyloglucans and (galacto)glucomannans, the major hemicelluloses in tomato cell walls. In situ localization of cell wall polysaccharides in pericarp tissue revealed non-ramified RG-I rich pectin and XyG at cellular junctions and in the middle lamella of young fruit. Blocks of non-methyl esterified homogalacturonan are detected as soon as 14 DPA in the mesocarp and remained restricted to cell corner and middle lamella whatever the stages. These results point to new questions about the role of pectin RGI and XyG in cell adhesion and its maintenance during cell expansion.

Cell wall dynamics during apple development and storage involves hemicellulose modifications and related expressed genes

BACKGROUND

Fruit quality depends on a series of biochemical events that modify appearance, flavour and texture throughout fruit development and ripening. Cell wall polysaccharide remodelling largely contributes to the elaboration of fleshy fruit texture. Although several genes and enzymes involved in cell wall polysaccharide biosynthesis and modifications are known, their coordinated activity in these processes is yet to be discovered.

RESULTS

Combined transcriptomic and biochemical analyses allowed the identification of putative enzymes and related annotated members of gene families involved in cell wall polysaccharide composition and structural changes during apple fruit growth and ripening. The early development genes were mainly related to cell wall biosynthesis and degradation with a particular target on hemicelluloses. Fine structural evolutions of galactoglucomannan were strongly correlated with mannan synthase, glucanase (GH9) and β-galactosidase gene expression. In contrast, fewer genes related to pectin metabolism and cell expansion (expansin genes) were observed in ripening fruit combined with expected changes in cell wall polysaccharide composition.

CONCLUSIONS

Hemicelluloses undergo major structural changes particularly during early fruit development. The high number of early expressed β-galactosidase genes questions their function on galactosylated structures during fruit development and storage. Their activity and cell wall substrate remains to be identified. Moreover, new insights into the potential role of peroxidases and transporters, along with cell wall metabolism open the way to further studies on concomitant mechanisms involved in cell wall assembly/disassembly during fruit development and storage.

Plant cell wall extensibility: connecting plant cell growth with cell wall structure, mechanics, and the action of wall-modifying enzymes

The advent of user-friendly instruments for measuring force/deflection curves of plant surfaces at high spatial resolution has resulted in a recent outpouring of reports of the 'Young's modulus' of plant cell walls. The stimulus for these mechanical measurements comes from biomechanical models of morphogenesis of meristems and other tissues, as well as single cells, in which cell wall stress feeds back to regulate microtubule organization, auxin transport, cellulose deposition, and future growth directionality. In this article I review the differences between elastic modulus and wall extensibility in the context of cell growth. Some of the inherent complexities, assumptions, and potential pitfalls in the interpretation of indentation force/deflection curves are discussed. Reported values of elastic moduli from surface indentation measurements appear to be 10- to >1000-fold smaller than realistic tensile elastic moduli in the plane of plant cell walls. Potential reasons for this disparity are discussed, but further work is needed to make sense of the huge range in reported values. The significance of wall stress relaxation for growth is reviewed and connected to recent advances and remaining enigmas in our concepts of how cellulose, hemicellulose, and pectins are assembled to make an extensible cell wall. A comparison of the loosening action of α-expansin and Cel12A endoglucanase is used to illustrate two different ways in which cell walls may be made more extensible and the divergent effects on wall mechanics.

We be jammin': an update on pectin biosynthesis, trafficking and dynamics

Pectins are complex polysaccharides that contain acidic sugars and are major determinants of the cohesion, adhesion, extensibility, porosity and electrostatic potential of plant cell walls. Recent evidence has solidified their positions as key regulators of cellular growth and tissue morphogenesis, although important details of how they achieve this regulation are still missing. Pectins are also hypothesized to function as ligands for wall integrity sensors that enable plant cells to respond to intrinsic defects in wall biomechanics and to wall degradation by attacking pathogens. This update highlights recent advances in our understanding of the biosynthesis of pectins, how they are delivered to the cell surface and become incorporated into the cell wall matrix and how pectins are modified over time in the apoplast. It also poses unanswered questions for further research into this enigmatic but essential class of carbohydrate polymers.

Diffusion of macromolecules in self-assembled cellulose/hemicellulose hydrogels

Cellulose hydrogels are extensively applied in many biotechnological fields and are also used as models for plant cell walls. We synthesised model cellulosic hydrogels containing hemicelluloses, as a biomimetic of plant cell walls, in order to study the role of hemicelluloses on their mass transport properties. Microbial cellulose is able to self-assemble into composites when hemicelluloses, such as xyloglucan and arabinoxylan, are present in the incubation media, leading to hydrogels with different nano and microstructures. We investigated the diffusivities of a series of fluorescently labelled dextrans, of different molecular weight, and proteins, including a plant pectin methyl esterase (PME), using fluorescence recovery after photobleaching (FRAP). The presence of xyloglucan, known to be able to crosslink cellulose fibres, confirmed by scanning electron microscopy (SEM) and (13)C NMR, reduced mobility of macromolecules of molecular weight higher than 10 kDa, reflected in lower diffusion coefficients. Furthermore PME diffusion was reduced in composites containing xyloglucan, despite the lack of a particular binding motif in PME for this polysaccharide, suggesting possible non-specific interactions between PME and this hemicellulose. In contrast, hydrogels containing arabinoxylan coating cellulose fibres showed enhanced diffusivity of the molecules studied. The different diffusivities were related to the architectural features found in the composites as a function of polysaccharide composition. Our results show the effect of model hemicelluloses in the mass transport properties of cellulose networks in highly hydrated environments relevant to understanding the role of hemicelluloses in the permeability of plant cell walls and aiding design of plant based materials with tailored properties.