Innovative enzymatic approach to resolve homogalacturonans based on their methylesterification pattern

TBL38 atypical homogalacturonan-acetylesterase activity and cell wall microdomain localization in Arabidopsis seed mucilage secretory cells

Plant cell walls constitute complex polysaccharidic/proteinaceous networks whose biosynthesis and dynamics implicate several cell compartments. The synthesis and remodeling of homogalacturonan pectins involve Golgi-localized methylation/acetylation and subsequent cell wall-localized demethylation/deacetylation. So far, TRICHOME BIREFRINGENCE-LIKE (TBL) family members have been described as Golgi-localized acetyltransferases targeting diverse hemicelluloses or pectins. Using seed mucilage secretory cells (MSCs) from Arabidopsis thaliana, we demonstrate the atypical localization of TBL38 restricted to a cell wall microdomain. A tbl38 mutant displays an intriguing homogalacturonan immunological phenotype in this cell wall microdomain and in an MSC surface-enriched abrasion powder. Mass spectrometry oligosaccharide profiling of this fraction reveals an increased homogalacturonan acetylation phenotype. Finally, TBL38 displays pectin acetylesterase activity in vitro. These results indicate that TBL38 is an atypical cell wall-localized TBL that displays a homogalacturonan acetylesterase activity rather than a Golgi-localized acetyltransferase activity as observed in previously studied TBLs. TBL38 function during seed development is discussed.

Characterization of Crude and Processed Pulp Cell Walls of Three Selected Mamey Accessions (Mammea americana L.)

Mamey (Mammea americana L.) is a tropical fleshy fruit native from the West Indies and northern South America. It is very appreciated for its flavor and color but has been little described. The present study investigates the composition and histochemistry of the pulp cell walls of three mamey accessions readily available in Martinique. The impact of pulp processing into puree on cell wall composition is evaluated. The histology and rheology of mamey puree are assessed considering these characterizations. Mamey pulp cell wall composition is dominated by highly methyl-esterified pectins (DM: 66.2-76.7%) of high molecular weight, and show few hemicelluloses, mainly xyloglucans. Processing reduced methyl-esterified uronic acid contents and gave purees with significantly different viscosities. Mamey puree was composed of polydisperse particles (20-2343 µm), which size distributions were different depending on the accession: Ti Jacques was dominated by smaller particles (50% had approximated diameters lower than 160 µm), Sonson's by larger particles (50% had approximated diameters higher than 900 µm), and Galion's had an intermediate profile. This new knowledge on mamey pulp is valuable for future works on mamey processing into new food products, even more so for those including cell wall polysaccharide-degrading enzymes.

ZmBELL10 interacts with other ZmBELLs and recognizes specific motifs for transcriptional activation to modulate internode patterning in maize

Plant height is an important agronomic trait that affects crop yield. Elucidating the molecular mechanism underlying plant height regulation is also an important question in developmental biology. Here, we report that a BELL transcription factor, ZmBELL10, positively regulates plant height in maize (Zea mays). Loss of ZmBELL10 function resulted in shorter internodes, fewer nodes, and smaller kernels, while ZmBELL10 overexpression increased plant height and hundred-kernel weight. Transcriptome analysis and chromatin immunoprecipitation followed by sequencing showed that ZmBELL10 recognizes specific sequences in the promoter of its target genes and activates cell division- and cell elongation-related gene expression, thereby influencing node number and internode length in maize. ZmBELL10 interacted with several other ZmBELL proteins via a spatial structure in its POX domain to form protein complexes involving ZmBELL10. All interacting proteins recognized the same DNA sequences, and their interaction with ZmBELL10 increased target gene expression. We identified the key residues in the POX domain of ZmBELL10 responsible for its protein-protein interactions, but these residues did not affect its transactivation activity. Collectively, our findings shed light on the functions of ZmBELL10 protein complexes and provide potential targets for improving plant architecture and yield in maize.

A wide diversity exists in pectin structure from thirteen apple cultivars

To emphasize that differences in pectin structure among cultivars play a crucial role in the texture and quality of fruits and vegetables, the sugar content and methyl-esterification of pectin fractions from 13 apple cultivars was studied. Cell wall polysaccharides were isolated as alcohol-insoluble solids (AIS) and subsequently extracted to yield water-soluble solids (WSS) and chelating-soluble solids (ChSS). All fractions contained significant amounts of galacturonic acid, while sugar compositions varied between cultivars. AIS and WSS pectins showed a degree of methyl-esterification (DM) > 50 %, while ChSS pectins had either a medium (~50 %) or low (<30 %) DM. Homogalacturonan as major structure was studied using enzymatic fingerprinting. Methyl-ester distribution of pectin was described by degrees of blockiness and -hydrolysis. Novel descriptive parameters were obtained by measuring the levels of methyl-esterified oligomers released by endo-PG (DB_PGme) and PL (DB_PLme). Pectin fractions differed in relative amounts of non-, moderately-, and highly methyl-esterified segments. WSS pectins were mostly lacking non-esterified GalA sequences, while ChSS pectins had medium DM and many non-methyl-esterified blocks or a low DM with many intermediate methyl-esterified GalA blocks. These findings will be of help to better understand physicochemical properties of apple and its products.

TLR 2/1 interaction of pectin depends on its chemical structure and conformation

Citrus pectins have demonstrated health benefits through direct interaction with Toll-like receptor 2. Methyl-ester distribution patterns over the homogalacturonan were found to contribute to such immunomodulatory activity, therefore molecular interactions with TLR2 were studied. Molecular-docking analysis was performed using four GalA-heptamers, GalA₇Me⁰, GalA₇Me^1,6, GalA₇Me^1,7 and GalA₇Me^2,5. The molecular relations were measured in various possible conformations. Furthermore, commercial citrus pectins were characterized by enzymatic fingerprinting using polygalacturonase and pectin-lyase to determine their methyl-ester distribution patterns. The response of 12 structurally different pectic polymers on TLR2 binding and the molecular docking with four pectic oligomers clearly demonstrated interactions with human-TLR2 in a structure-dependent way, where blocks of (non)methyl-esterified GalA were shown to inhibit TLR2/1 dimerization. Our results may be used to understand the immunomodulatory effects of certain pectins via TLR2. Knowledge of how pectins with certain methyl-ester distribution patterns bind to TLRs may lead to tailored pectins to prevent inflammation.

Determining Methyl-Esterification Patterns in Plant-Derived Homogalacturonan Pectins

Homogalacturonan (HG)-type pectins are nutrient components in plants and are widely used in the food industry. The methyl-esterification pattern is a crucial structural parameter used to assess HG pectins in terms of their nutraceutical activity. To better understand the methyl-esterification pattern of natural HG pectins from different plants, we purified twenty HG pectin-rich fractions from twelve plants and classified them by their monosaccharide composition, Fourier transform-infrared spectroscopy (FT-IR) signatures, and NMR analysis. FT-IR shows that these HG pectins are all minimally esterified, with the degree of methyl-esterification (DM) being 5 to 40%. To examine their methyl-esterification pattern by enzymatic fingerprinting, we hydrolyzed the HG pectins using endo-polygalacturonase. Hydrolyzed oligomers were derivatized with 2-aminobenzamide and subjected to liquid chromatography-fluorescence-tandem mass spectrometry (HILIC-FLR-MSn). Twenty-one types of mono-/oligo-galacturonides having DP values of 1–10 were found to contain nonesterified monomers, dimers, and trimers, as well as oligomers with 1 to 6 methyl-ester groups. In these oligo-galacturonides, MSn analysis demonstrated that the number of methyl-ester groups in the continuous sequence was 2 to 5. Mono- and di-esterified oligomers had higher percentages in total methyl-esterified groups, suggesting that these are a random methyl-esterification pattern in these HG pectins. Our study analyzes the characteristics of the methyl-esterification pattern in naturally occurring plant-derived HG pectins and findings that will be useful for further studying HG structure-function relationships.

The Class II KNOX family members KNAT3 and KNAT7 redundantly participate in Arabidopsis seed coat mucilage biosynthesis

The production of Arabidopsis seed mucilage involves complex polysaccharide biosynthetic pathways and developmental processes in seed epidermal cells. Although the polysaccharide components of Arabidopsis seed mucilage have been identified, their regulatory mechanism requires further investigation. Here, we show that Class II KNOX gene family members KNAT3 and KNAT7 play an essential role in regulating mucilage production in the early developmental stages of Arabidopsis seeds. Double mutant knat3knat7 resulted in defective seed mucilage production and columellae formation, whereas knat3 showed a normal phenotype compared with wild type, and the mucilage thickness in knat7 was slightly disturbed. Rhamnogalacturonan I (RG-I) and its biosynthetic substrates galacturonic acid and rhamnose were reduced in both the adherent and soluble mucilage of knat3knat7. Comparative transcriptome analysis on whole seeds suggested that polysaccharide, glucosinolate and anthocyanin biosynthetic pathways were specifically repressed in knat3knat7. Transient co-expression of KNAT3 and KNAT7 with promoter regions of candidate genes in Arabidopsis protoplasts revealed that both KNAT3 and KNAT7 act as positive regulators of the RG-I biosynthetic gene MUCILAGE-MODIFIED 4 (MUM4, AT1G53500). Collectively, our results demonstrate that KNAT3 and KNAT7 are multifunctional transcription factors in secondary cell wall development and redundantly modulate mucilage biosynthesis in Arabidopsis seeds.

Revealing methyl-esterification patterns of pectins by enzymatic fingerprinting: Beyond the degree of blockiness

Citrus pectins were studied by enzymatic fingerprinting using a simultaneous enzyme treatment with endo-polygalacturonase (endo-PG) from Kluyveromyces fragilis and pectin lyase (PL) from Aspergillus niger to reveal the methyl-ester distribution patterns over the pectin backbone. Using HILIC-MS combined with HPAEC enabled the separation and identification of the diagnostic oligomers released. Structural information on the pectins was provided by using novel descriptive parameters such as degree of blockiness of methyl-esterified oligomers by PG (DB_PGme) and degree of blockiness of methyl-esterified oligomers by PL (DB_PLme). This approach enabled us to clearly differentiate citrus pectins with various methyl-esterification patterns. The simultaneous use of PG and PL showed additional information, which is not revealed in digests using PG or PL alone. This approach can be valuable to differentiate pectins having the same DM and to get specific structural information on pectins and therefore to be able to better predict their physical and biochemical functionalities.

Comparison of Analytical Methods for Determining Methylesterification and Acetylation of Pectin

The esterification of galacturonic acid with methanol and/or acetic acid is important for the structural analysis of pectin. Although several methods have been reported for determining the degree of methylesterification (DM) and acetylation (DAc), the present study compares and optimizes three methods (titration, FT-IR and HPLC) using commercial citrus pectin (CP). Our results showed that the DM of CP was 47.0%, 47.6% and 48.0% as determined by titration, FT-IR and HPLC, respectively, suggesting that DM determination is nearly identical using any of these methods. However, the titration approach requires more sample than the other two. HPLC showed that the DAc of CP was 1.6%, an approach that can be used to determine the DM and DAc of pectin simultaneously. Here, we simplified and optimized sample treatment for HPLC analysis and compared it with the reported literature. Our results provide useful information for choosing appropriate methods for determining the DM and DAc of pectin based on various sample properties and experimental conditions.

Effect of solid loading on the behaviour of pectin-degrading enzymes

BACKGROUND

Pectin plays a role in the recalcitrance of plant biomass by affecting the accessibility of other cell wall components to enzymatic degradation. Elimination of pectin consequently has a positive impact on the saccharification of pectin-rich biomass. This work thus focused on the behaviour of different pectin-degrading enzymes in the presence of low (5%) to high (35%) solid loading of lemon peel.

RESULTS

High solid loading of lemon peel affected pectin solubilisation differently depending on the pectinase used. Pectin lyase was less sensitive to a reduction of water content than was a mixture of endopolygalacturonase and pectin methylesterase, regardless of whether or not the latter's mode of action is processive or not. Marked changes in water mobility were observed along with enzymatic degradation depending on the enzyme used. However, the pectin lyase resulted in less pronounced shifts in water distribution than polygalacturonase-pectin methylesterase mixtures. At similar pectin concentration, pectin solutions hindered the diffusion of hydrolases more than the solid substrate. This can be attributed to the high viscosity of the highly concentrated pectin solutions while the solid substrate may provide continuous diffusion paths through pores.

CONCLUSIONS

The increase in solid substrate loading reduced the efficiency of pectin-degrading enzymes catalysing hydrolysis more significantly than those catalysing β-elimination. LF-NMR experiments highlighted the impact of solid loading on water mobility. Compared to other enzymes and whatever the solid loading, pectin lyase led to longer relaxation times linked with the most destructuration of the solid substrate. This new information could benefit the biorefinery processing of pectin-rich plant material when enzymes are used in the treatment.

Structural Characterization of Isomeric Oligogalacturonan Mixtures Using Ultrahigh-Performance Liquid Chromatography-Charge Transfer Dissociation Mass Spectrometry

Pectins are natural polysaccharides made from galacturonic acid residues, and they are widely used as an excipient in food and pharmaceutical industries. The degree of methyl-esterification, the monomeric composition, and the linkage pattern are all important factors that influence the physical and chemical properties of pectins, such as the solubility. This work focuses on the successful online coupling of charge transfer dissociation-mass spectrometry (CTD-MS) with ultrahigh-performance liquid chromatography (UHPLC) to differentiate isomers of oligogalacturonans derived from citrus pectins. This work employed CTD fragmentation of the pectin mixtures in data-dependent acquisition mode. Compared to the UHPLC with collision-induced dissociation mass spectrometry (UHPLC-CID-MS), UHPLC-CTD-MS yielded fewer ambiguous ions and more structurally informative results. The developed UHPLC-CTD-MS method resulted in abundant cross-ring cleavages-and especially ^1,4X_n, ^1,5X_n, and ^2,4X_n ions-which helped to identify most of the isomers. The Gal A isomers differed only in the methyl group position along the galacturonic acid backbone. The combination of CTD in real time with UHPLC provides a new tool for the structural characterization of complex mixtures of oligogalacturonans and potentially other classes of oligosaccharides.

Impacts of thermal and non-thermal processing on structure and functionality of pectin in fruit- and vegetable- based products: A review

Pectin, a major polysaccharide found in the cell walls of higher plants, plays major roles in determining the physical and nutritional properties of fruit- and vegetable-based products. An in-depth understanding of the effects of processing operations on pectin structure and functionality is critical for designing better products. This review, therefore, focuses on the progress made in understanding the effects of processing on pectin structure, further on pectin functionality, consequently on product properties. The effects of processing on pectin structure are highly dependent on the processing conditions. Targeted control of pectin structure by applying various processing operations could enhance textural, rheological, nutritional properties and cloud stability of products. While it seems that optimizing product quality in terms of physical properties is counteracted by optimizing the nutritional properties. Therefore, understanding plant component biosynthesis mechanisms and processing mechanisms could be a major challenge to balance among the quality indicators of processed products.

Assembly of tomato fruit cuticles: a cross-talk between the cutin polyester and cell wall polysaccharides

The cuticle is an essential and ubiquitous biological polymer composite covering aerial plant organs, whose structural component is the cutin polyester entangled with cell wall polysaccharides. The nature of the cutin-embedded polysaccharides (CEPs) and their association with cutin polyester are still unresolved Using tomato fruit as a model, chemical and enzymatic pretreatments combined with biochemical and biophysical methods were developed to compare the fine structure of CEPs with that of the noncutinized polysaccharides (NCPs). In addition, we used tomato fruits from cutin-deficient transgenic lines cus1 (cutin synthase 1) to study the impact of cutin polymerization on the fine structure of CEPs. Cutin-embedded polysaccharides exhibit specific structural features including a high degree of esterification (i.e. methylation and acetylation), a low ramification of rhamnogalacturonan (RGI), and a high crystallinity of cellulose. In addition to decreasing cutin deposition and polymerization, cus1 silencing induced a specific modification of CEPs, especially on pectin content, while NCPs were not affected. This new evidence of the structural specificities of CEPs and of the cross-talk between cutin polymerization and polysaccharides provides new hypotheses concerning the formation of these complex lipopolysaccharide edifices.

Influence of Pectin Structural Properties on Interactions with Divalent Cations and Its Associated Functionalities

Pectin is an anionic cell wall polysaccharide which is known to interact with divalent cations via its nonmethylesterified galacturonic acid units. Due to its cation-binding capacity, extracted pectin is frequently used for several purposes, such as a gelling agent in food products or as a biosorbent to remove toxic metals from waste water. Pectin can, however, possess a large variability in molecular structure, which influences its cation-binding capacity. Besides the pectin structure, several extrinsic factors, such as cation type or pH, have been shown to define the cation binding of pectin. This review paper focuses on the research progress in the field of pectin-divalent cation interactions and associated functional properties. In addition, it addresses the main research gaps and challenges in order to clearly understand the influence of pectin structural properties on its divalent cation-binding capacity and associated functionalities. This review reveals that many factors, including pectin molecular structure and extrinsic factors, influence pectin-cation interactions and its associated functionalities, which makes it difficult to predict the pectin-cation-binding capacity. Despite the limited information available, determination of the cation-binding capacity of pectins with distinct structural properties using equilibrium adsorption experiments or isothermal titration calorimetry is a promising tool to gain fundamental insights into pectin-cation interactions. These insights can then be used in targeted pectin structural modification, in order to optimize the cation-binding capacity and to promote pectin-cation interactions, for instance for a structure build-up in food products without compromising the mineral nutrition value.

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.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Charge Transfer Dissociation of Complex Oligosaccharides: Comparison with Collision-Induced Dissociation and Extreme Ultraviolet Dissociative Photoionization

The structural characterization of oligosaccharides still challenges the field of analytical chemistry. Tandem mass spectrometry offers many advantages toward this aim, although the generic fragmentation method (low-energy collision-induced dissociation) shows clear limitations and is often insufficient to retrieve some essential structural information on these molecules. In this work, we present the first application of helium charge transfer dissociation (He-CTD) to characterize the structure of complex oligosaccharides. We compare this method with low-energy collision-induced dissociation and extreme-ultraviolet dissociative photoionization (XUV-DPI), which was shown previously to ensure the successful characterization of complex glycans. Similarly to what could be obtained by XUV-DPI, He-CTD provides a complete description of the investigated structures by producing many informative cross-ring fragments and no ambiguous fragmentation. Unlike XUV-DPI, which is performed at a synchrotron source, He-CTD has the undeniable advantage of being implementable in a conventional benchtop ion trap in a conventional laboratory setting. Graphical Abstract ᅟ.

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.

PME58 plays a role in pectin distribution during seed coat mucilage extrusion through homogalacturonan modification

Pectins are major components of plant primary cell walls. They include homogalacturonans (HGs), which are the most abundant pectin and can be the target of apoplastic enzymes like pectin methylesterases (PMEs) that control their methylesterification level. Several PMEs are expressed in the seed coat of Arabidopsis thaliana, particularly in mucilage secretory cells (MSCs). On the basis of public transcriptomic data, seven PME genes were selected and checked for their seed-specific expression by quantitative reverse transcription PCR. Of these, PME58 presented the highest level of expression and was specifically expressed in MSCs at the early stages of seed development. pme58 mutants presented two discrete phenotypes: (i) their adherent mucilage was less stained by ruthenium red when compared to wild-type seeds, but only in the presence of EDTA, a Ca(2+)chelator; and (ii) the MSC surface area was decreased. These phenotypes are the consequence of an increase in the degree of HG methylesterification connected to a decrease in PME activity. Analysis of the sugar composition of soluble and adherent mucilage showed that, in the presence of EDTA, sugars of adherent mucilage were more readily extracted in pme58 mutants. Immunolabelling with LM19, an antibody that preferentially recognizes unesterified HGs, also showed that molecular interactions with HGs were modified in the adherent mucilage of pme58 mutants, suggesting a role of PME58 in mucilage structure and organization. In conclusion, PME58 is the first PME identified to play a direct role in seed mucilage structure.

Structure and Properties of a Non-processive, Salt-requiring, and Acidophilic Pectin Methylesterase from Aspergillus niger Provide Insights into the Key Determinants of Processivity Control

Many pectin methylesterases (PMEs) are expressed in plants to modify plant cell-wall pectins for various physiological roles. These pectins are also attacked by PMEs from phytopathogens and phytophagous insects. The de-methylesterification by PMEs of the O6-methyl ester groups of the homogalacturonan component of pectin, exposing galacturonic acids, can occur processively or non-processively, respectively, describing sequential versus single de-methylesterification events occurring before enzyme-substrate dissociation. The high resolution x-ray structures of a PME from Aspergillus niger in deglycosylated and Asn-linked N-acetylglucosamine-stub forms reveal a 10⅔-turn parallel β-helix (similar to but with less extensive loops than bacterial, plant, and insect PMEs). Capillary electrophoresis shows that this PME is non-processive, halophilic, and acidophilic. Molecular dynamics simulations and electrostatic potential calculations reveal very different behavior and properties compared with processive PMEs. Specifically, uncorrelated rotations are observed about the glycosidic bonds of a partially de-methyl-esterified decasaccharide model substrate, in sharp contrast to the correlated rotations of processive PMEs, and the substrate-binding groove is negatively not positively charged.

The Deconstruction of Pectic Rhamnogalacturonan I Unmasks the Occurrence of a Novel Arabinogalactan Oligosaccharide Epitope

Rhamnogalacturonan I (RGI) is a pectic polysaccharide composed of a backbone of alternating rhamnose and galacturonic acid residues with side chains containing galactose and/or arabinose residues. The structure of these side chains and the degree of substitution of rhamnose residues are extremely variable and depend on species, organs, cell types and developmental stages. Deciphering RGI function requires extending the current set of monoclonal antibodies (mAbs) directed to this polymer. Here, we describe the generation of a new mAb that recognizes a heterogeneous subdomain of RGI. The mAb, INRA-AGI-1, was produced by immunization of mice with RGI oligosaccharides isolated from potato tubers. These oligomers consisted of highly branched RGI backbones substituted with short side chains. INRA-AGI-1 bound specifically to RGI isolated from galactan-rich cell walls and displayed no binding to other pectic domains. In order to identify its RGI-related epitope, potato RGI oligosaccharides were fractionated by anion-exchange chromatography. Antibody recognition was assessed for each chromatographic fraction. INRA-AGI-1 recognizes a linear chain of (1→4)-linked galactose and (1→5)-linked arabinose residues. By combining the use of INRA-AGI-1 with LM5, LM6 and INRA-RU1 mAbs and enzymatic pre-treatments, evidence is presented of spatial differences in RGI motif distribution within individual cell walls of potato tubers and carrot roots. These observations raise questions about the biosynthesis and assembly of pectin structural domains and their integration and remodeling in cell walls.

Tuning of Pectin Methylesterification: PECTIN METHYLESTERASE INHIBITOR 7 MODULATES THE PROCESSIVE ACTIVITY OF CO-EXPRESSED PECTIN METHYLESTERASE 3 IN A pH-DEPENDENT MANNER

Pectin methylesterases (PMEs) catalyze the demethylesterification of homogalacturonan domains of pectin in plant cell walls and are regulated by endogenous pectin methylesterase inhibitors (PMEIs). In Arabidopsis dark-grown hypocotyls, one PME (AtPME3) and one PMEI (AtPMEI7) were identified as potential interacting proteins. Using RT-quantitative PCR analysis and gene promoter::GUS fusions, we first showed that AtPME3 and AtPMEI7 genes had overlapping patterns of expression in etiolated hypocotyls. The two proteins were identified in hypocotyl cell wall extracts by proteomics. To investigate the potential interaction between AtPME3 and AtPMEI7, both proteins were expressed in a heterologous system and purified by affinity chromatography. The activity of recombinant AtPME3 was characterized on homogalacturonans (HGs) with distinct degrees/patterns of methylesterification. AtPME3 showed the highest activity at pH 7.5 on HG substrates with a degree of methylesterification between 60 and 80% and a random distribution of methyl esters. On the best HG substrate, AtPME3 generates long non-methylesterified stretches and leaves short highly methylesterified zones, indicating that it acts as a processive enzyme. The recombinant AtPMEI7 and AtPME3 interaction reduces the level of demethylesterification of the HG substrate but does not inhibit the processivity of the enzyme. These data suggest that the AtPME3·AtPMEI7 complex is not covalently linked and could, depending on the pH, be alternately formed and dissociated. Docking analysis indicated that the inhibition of AtPME3 could occur via the interaction of AtPMEI7 with a PME ligand-binding cleft structure. All of these data indicate that AtPME3 and AtPMEI7 could be partners involved in the fine tuning of HG methylesterification during plant development.

Methylesterase behaviour is related to polysaccharide organisation in model systems mimicking cell walls

Pectin gels and pectin-cellulose binary gels were used as cell wall-mimicking systems to investigate the diffusion ability of a fungal pectin methylesterase. Increasing content of cellulose in the gel appears to result: (i) in longer demethylated blocks thus favouring AaPME processivity, and (ii) in accelerated enzyme kinetics. To better understand this unexpected behaviour, a method was set up to investigate the gel porosity as a function of the cellulose content by following the passive diffusion of three pullulans having different hydrodynamic volumes. Like the enzyme, the pullulans diffused more efficiently in the gels containing the highest proportions of cellulose. Altogether, these results suggest that the gel settled differently during formation according to the respective proportions of the two polysaccharides. With cellulose present, a fraction of pectin would form close interactions with the microfibrils resulting in a larger volume accessible to diffusing molecules. This volume would be related to the cellulose concentration.

Multiscale investigation of mealiness in apple: an atypical role for a pectin methylesterase during fruit maturation

BACKGROUND

Apple fruit mealiness is one of the most important textural problems that results from an undesirable ripening process during storage. This phenotype is characterized by textural deterioration described as soft, grainy and dry fruit. Despite several studies, little is known about mealiness development and the associated molecular events. In this study, we integrated phenotypic, microscopic, transcriptomic and biochemical analyses to gain insights into the molecular basis of mealiness development.

RESULTS

Instrumental texture characterization allowed the refinement of the definition of apple mealiness. In parallel, a new and simple quantitative test to assess this phenotype was developed.

CONCLUSIONS

These data support the role of PME in cell wall remodelling during apple fruit development and ripening and suggest a local action of these enzymes. Mealiness may partially result from qualitative and spatial variations of pectin microarchitecture rather than quantitative pectin differences, and these changes may occur early in fruit development. The specific MdPME2 gene highlighted in this study could be a good early marker of texture unfavourable trait in apple.

The wheat grain contains pectic domains exhibiting specific spatial and development-associated distribution

Cell walls are complex structures surrounding plant cells with a composition that varies among species and even within a species between organs, cell types and development stages. For years, cell walls in wheat grains were described as simple walls consisting mostly of arabinoxylans and mixed-linked beta glucans. Proteomic and transcriptomic studies identified enzyme families involved in the synthesis of many more cell wall polysaccharides in the wheat grains. Here we describe the discovery of pectic domains in wheat grain using monoclonal antibodies and enzymatic treatment to degrade the major cell wall polymers. Distinct spatial distributions were observed for rhamnogalacturonan I present in the endosperm and mostly in the aleurone layer and homogalacturonan especially found in the outer layers, and tight developmental regulations were unveiled. We also uncovered a massive deposition of homogalacturonan via large vesicular bodies in the seed coat (testa) beneath a thick cuticle during development. Our findings raise questions about the function of pectin in wheat grain.

PECTIN METHYLESTERASE INHIBITOR6 promotes Arabidopsis mucilage release by limiting methylesterification of homogalacturonan in seed coat epidermal cells

Imbibed seeds of the Arabidopsis thaliana accession Djarly are affected in mucilage release from seed coat epidermal cells. The impaired locus was identified as a pectin methylesterase inhibitor gene, PECTIN METHYLESTERASE INHIBITOR6 (PMEI6), specifically expressed in seed coat epidermal cells at the time when mucilage polysaccharides are accumulated. This spatio-temporal regulation appears to be modulated by GLABRA2 and LEUNIG HOMOLOG/MUCILAGE MODIFIED1, as expression of PMEI6 is reduced in mutants of these transcription regulators. In pmei6, mucilage release was delayed and outer cell walls of epidermal cells did not fragment. Pectin methylesterases (PMEs) demethylate homogalacturonan (HG), and the majority of HG found in wild-type mucilage was in fact derived from outer cell wall fragments. This correlated with the absence of methylesterified HG labeling in pmei6, whereas transgenic plants expressing the PMEI6 coding sequence under the control of the 35S promoter had increased labeling of cell wall fragments. Activity tests on seeds from pmei6 and 35S:PMEI6 transgenic plants showed that PMEI6 inhibits endogenous PME activities, in agreement with reduced overall methylesterification of mucilage fractions and demucilaged seeds. Another regulator of PME activity in seed coat epidermal cells, the subtilisin-like Ser protease SBT1.7, acts on different PMEs, as a pmei6 sbt1.7 mutant showed an additive phenotype.

Cell wall polysaccharide chemistry of peach genotypes with contrasted textures and other fruit traits

Cell wall composition, pectin, and hemicellulose fine structure variation were assessed in peach and related genotypes with contrasted texture and fruit shape. Cell walls were prepared from four commercial peaches, eight genotypes from the Jalousia × Fantasia peach cross, and six genotypes from the Earlygold peach × Texas almond cross. Sugar composition was determined chemically while fine structure of homogalacturonan pectin and xyloglucan hemicellulose were assessed by coupling pectin lyase and glucanase degradation, respectively, with MALDI-TOF MS analysis of the degradation products. The results indicate clear compositional and structural differences between the parents and their related genotypes on the basis of pectin versus cellulose/hemicellulose content and on the fine structure of homogalacturonan and xyloglucan. A relation between methyl- and acetyl-esterification of pectin with fruit shape is revealed in the Fantasia × Jalousia peach genotypes.