P. V. Mikshina

Specific type of secondary cell wall formed by plant fibers

The review sums data indicating that, in many plant fibers, the secondary cell wall contains so-called gelatinous layers of peculiar structure along with those of common (xylan) structure. Sometimes these gelatinous layers comprise the main bulk of the cell wall. Key characteristics of gelatinous cell wall are presented and compared with those of classic xylan-type cell wall. The process of gelatinous cell wall formation is considered in detail for flax phloem fibers; several characteristic features of this process were revealed: intense rearrangement of already deposited cell-wall layers, unusual dynamics of Golgi vesicles, the occurrence of the stage-specific polysaccharide with specific properties, high activity of β-galactosidase, and the presence of substantial amount of free galactose. Similarity and differences in the gelatinous cell wall formation in the fibers of various plant species are discussed.

Aspen Tension Wood Fibers Contain β-(1---> 4)-Galactans and Acidic Arabinogalactans Retained by Cellulose Microfibrils in Gelatinous Walls.

Entrapment of pectic galactan and acidic arabinogalactan II within cellulose fibrils is a distinctive feature of aspen tension wood gelatinous fibers with contractile properties. Contractile cell walls are found in various plant organs and tissues such as tendrils, contractile roots, and tension wood. The tension-generating mechanism is not known but is thought to involve special cell wall architecture. We previously postulated that tension could result from the entrapment of certain matrix polymers within cellulose microfibrils. As reported here, this hypothesis was corroborated by sequential extraction and analysis of cell wall polymers that are retained by cellulose microfibrils in tension wood and normal wood of hybrid aspen (Populus tremula × Populus tremuloides). β-(1→4)-Galactan and type II arabinogalactan were the main large matrix polymers retained by cellulose microfibrils that were specifically found in tension wood. Xyloglucan was detected mostly in oligomeric form in the alkali-labile fraction and was enriched in tension wood. β-(1→4)-Galactan and rhamnogalacturonan I backbone epitopes were localized in the gelatinous cell wall layer. Type II arabinogalactans retained by cellulose microfibrils had a higher content of (methyl)glucuronic acid and galactose in tension wood than in normal wood. Thus, β-(1→4)-galactan and a specialized form of type II arabinogalactan are trapped by cellulose microfibrils specifically in tension wood and, thus, are the main candidate polymers for the generation of tensional stresses by the entrapment mechanism. We also found high β-galactosidase activity accompanying tension wood differentiation and propose a testable hypothesis that such activity might regulate galactan entrapment and, thus, mechanical properties of cell walls in tension wood.

Cellulosic Fibers: Role of Matrix Polysaccharides in Structure and Function

Cellulose, being the major cell wall component and the most abundant organic matter, produced by living organisms, is not uniformly distributed within plant tissues. There are numerous cells, like the parenchyma ones, which even at maturity have thin cell wall. Thick cell walls are characteristic for the tissues with mechanical function. Among those, there are cell walls, which contain several major components, and those, the predominant component of which is cellulose. The most pure natural cellulose is considered to be present in cotton seed hairs (sometimes erroneously called “cotton fibers”) – over 90% of cell wall [1]. Very close to this value is a special group of plant fibers – cellulosic or gelatinous fibers, the proportion of cellulose in which amounts for 85-90% [2,3]. The cell wall thickness in such fibers may reach 15 m, as compared to 0.2 m in cells with thin cell wall. So, the very significant portion of total plant cellulose may be concentrated within the gelatinous fibers, making them the important source for production of biofuels and bio-based products. An additional attractiveness of cellulosic fibers for such applications comes from the fact that gelatinous cell wall layers are devoid of lignin – the major hurdle in using plant biomass [1].

Formation of plant cell wall supramolecular structure

Plant cell wall is an example of a widespread natural supramolecular structure: its components are considered to be the most abundant organic compounds renewable by living organisms. Plant cell wall includes numerous components, mainly polysaccharidic; its formation is largely based on carbohydrate-carbohydrate interactions. In contrast to the extracellular matrix of most other organisms, the plant cell compartment located outside the plasma membrane is so structured that has been named “wall”. The present review summarizes data on the mechanisms of formation of this supramolecular structure and considers major difficulties and results of research. Existing approaches to the study of interactions between polysaccharides during plant cell wall formation have been analyzed, including: (i) characterization of the structure of natural polysaccharide complexes obtained during cell wall fractionation; (ii) analysis of the interactions between polysaccharides “at mixing in a tube”; (iii) study of the interactions between isolated individual plant cell wall matrix polysaccharides and microfibrils formed by cellulose-synthesizing microorganisms; and (iv) investigation of cell wall formation and modification directly in plant objects. The key stages in formation of plant cell wall supramolecular structure are defined and characterized as follows: (i) formation of cellulose microfibrils; (ii) interactions between matrix polysaccharides within Golgi apparatus substructures; (iii) interaction between matrix polysaccharides, newly secreted outside the plasma membrane, and cellulose microfibrils during formation of the latter; (iv) packaging of the formed complexes and individual polysaccharides in cell wall layers; and (v) modification of deposited cell wall layers.

Physicochemical properties of complex rhamnogalacturonan I from gelatinous cell walls of flax fibers.

The physicochemical properties of flax fiber cell wall rhamnogalacturonan I (RG-I) and its fragments, obtained after galactanase treatment (fraction G1), were characterized. RG-I retains its hydrodynamic volume after its molecular weight decreases by approximately half, as revealed by SEC. Two techniques, DLS and NMR, with different principles of diffusion experiment were used to establish the reasons for this property of RG-I. Three possible types of particles were revealed by DLS depending on the concentration of the RG-I and G1 solutions (2-2.5, 15-20, and 150-200 nm). It was determined by BPP-LED experiments that the backbone of the RG-I was 1.3-1.9-fold more mobile than the side chains. The obtained data suggest a novel type of pectin spatial organization-the formation of RG-I associates with the backbone at the periphery and the interaction between the side chains to form a core zone.

Spatial structure of plant cell wall polysaccharides and its functional significance

Plant polysaccharides comprise the major portion of organic matter in the biosphere. The cell wall built on the basis of polysaccharides is the key feature of a plant organism largely determining its biology. All together, around 10 types of polysaccharide backbones, which can be decorated by different substituents giving rise to endless diversity of carbohydrate structures, are present in cell walls of higher plants. Each of the numerous cell types present in plants has cell wall with specific parameters, the features of which mostly arise from the structure of polymeric components. The structure of polysaccharides is not directly encoded by the genome and has variability in many parameters (molecular weight, length, and location of side chains, presence of modifying groups, etc.). The extent of such variability is limited by the “functional fitting” of the polymer, which is largely based on spatial organization of the polysaccharide and its ability to form supramolecular complexes of an appropriate type. Consequently, the carrier of the functional specificity is not the certain molecular structure but the certain type of the molecules having a certain degree of heterogeneity. This review summarizes the data on structural features of plant cell wall polysaccharides, considers formation of supramolecular complexes, gives examples of tissue- and stage-specific polysaccharides and functionally significant carbohydrate-carbohydrate interactions in plant cell wall, and presents approaches to analyze the spatial structure of polysaccharides and their complexes.

Free galactose and galactosidase activity in the course of flax fiber development

Composition and content of free monosaccharides and β-galactosidase activity were determined in the course of development of the flax (Linum usitatissimum L.) tissues containing bast fibers. In the stem regions where the fibers were at the stage of formation of the secondary cell wall of gelatinous type, the content of free galactose was high (14 mM) and 13–20 times greater than in the upper part of the stem where the fibers were at the stage of intrusive growth. Pulse-chase experiments demonstrated the differences in the metabolism of individual low-molecular sugars. In respect to glucose and sucrose, all the examined characteristics (content, absolute and specific radioactivity, and the temporal changes of these indices) were identical in the stem regions wherein the fibers were at different stages of development. Labeled galactose was detected only in the stem regions where the fibers were at the stage of secondary cell wall formation. The specific radioactivity of glucose and sucrose reached the maximum immediately after photosynthesis in the presence of 14CO2 and changed in the same way as in the primary products of photosynthesis. The time-course of label incorporation into galactose indicated that this monosaccharide arose as a result of hydrolytic processes. At the stage of secondary cell wall formation, high activity of β-galactosidase was observed, with tissue- and stage-specific fiber β-1,4-galactan as its substrate.

Tissue-specific rhamnogalacturonan I forms the gel with hyperelastic properties.

Rhamnogalacturonans I are complex pectin polysaccharides extremely variable in structure and properties and widely represented in various sources. The complexity and diversity of the structure of rhamnogalacturonans I are the reasons for the limited information about the properties and supramolecular organization of these polysaccharides, including the relationship between these parameters and the functions of rhamnogalacturonans I in plant cells. In the present work, on the example of rhamnogalacturonan I from flax gelatinous fibers, the ability of this type of pectic polysaccharides to form at physiological concentrations hydrogels with hyperelastic properties was revealed for the first time. According to IR spectroscopy, water molecules are more tightly retained in the gelling rhamnogalacturonan I from flax fiber cell wall in comparison with the non-gelling rhamnogalacturonan I from primary cell wall of potato. With increase in strength of water binding by rhamnogalacturonan I, there is an increase in elastic modulus and decrease in Poisson’s ratio of gel formed by this polysaccharide. The model of hyperelastic rhamnogalacturonan I capture by laterally interacting cellulose microfibrils, constructed using the finite element method, confirmed the suitability of rhamnogalacturonan I gel with the established properties for the function in the gelatinous cell wall, allowing consideration of this tissue- and stage-specific pectic polysaccharide as an important factor in creation of gelatinous fiber contractility.

Glucuronoarabinoxylan extracted by treatment with endoxylanase from different zones of growing maize root

Glucuronoarabinoxylan is a key tethering glucan in the primary cell wall of cereals. Glucuronoarabinoxylan was extracted from different zones of maize (Zea mays L.) roots using endoxylanase that specifically cleaves β-(1,4)-glycoside bond between two consequent unsubstituted xylose residues. Changes in polysaccharide structure during elongation growth were characterized. Glucuronoarabinoxylan extractable after the endoxylanase treatment consisted of high molecular weight (30–400 kDa) and low molecular weight (<10 kDa) fractions. The presence of high molecular weight derivatives indicated that part of the natural glucuronoarabinoxylan is not digestible by the endoxylanase. This could be due to the revealed peculiar structural features, such as high level of substitution of xylose, absence of unsubstituted xylose residues existing in sequence, and significant degree of acetylation. In maize root meristem the indigestible fraction was 98% of the total extracted glucuronoarabinoxylan. This portion decreases to 47% during elongation. Also, the average molecular weight of indigestible glucuronoarabinoxylan reduced twofold. These changes in the ratio of glucuronoarabinoxylan fragments with different structure during root cell growth could reflect a transition of polysaccharide from its separating (highly substituted indigestible glucuronoarabinoxylan) form to that binding to cellulose microfibrils or other glucuronoarabinoxylan molecules and, hence, retarding growth.

Metrics of rhamnogalacturonan I with β-(1→4)-linked galactan side chains and structural basis for its self-aggregation

Within the family of plant cell wall polysaccharides rhamnogalacturonans I are the most diverse and structurally complex members. In present study we characterize the 3-dimensional structures and dynamic features of the constituents of RG-I along MD trajectories. It is demonstrated that extended threefold helical structure of the rhamnogalacturonan linear backbone is the most energetically favorable motif. Branching helps to stabilize a conformer of the backbone twisted along 1→2 glycosidic linkage triggering the orientation of long side chains without altering the extended overall backbone chain conformation. Formation of anti-parallel pairing of the β-galactan side chains allows us to suggest a novel mode of non-covalent cross-linking in pectins. Studied structural elements are organized to report the first attempt to characterize 3D structure of RG-I focusing on the special case of flax tertiary cell wall and elucidate the structural basis underlying the formation of RG-I self-associates and functional role of RG-I in planta.