Plant oligosaccharides - outsiders among elicitors?

This review substantiates the need to study the plant oligoglycome. The available information on oligosaccharins - physiologically active fragments of plant cell wall polysaccharides - is summarized. The diversity of such compounds in chemical composition, origin, and proved biological activity is highlighted. At the same time, plant oligosaccharides can be considered as outsiders among elicitors of various natures in research intensity of recent decades. This review discusses the reasons for such attitude towards these regulators, which are largely connected with difficulties in isolation and identification. Together with that, approaches are suggested whose potentials can be used to study oligosaccharins. The topics of oligosaccharide metabolism in plants, including the ways of formation, transport, and inactivation are presented, together with data on biological activity and interaction with plant hormones. The current viewpoints on the mode of oligosaccharin action - perception, signal transduction, and possible "targets" - are considered. The potential uses of such compounds in medicine, food industry, agriculture, and biotechnology are discussed.

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.

Arrangement of mixed-linkage glucan and glucuronoarabinoxylan in the cell walls of growing maize roots.

BACKGROUND AND AIMS: Plant cell enlargement is unambiguously coupled to changes in cell wall architecture, and as such various studies have examined the modification of the proportions and structures of glucuronoarabinoxylan and mixed-linkage glucan in the course of cell elongation in grasses. However, there is still no clear understanding of the mutual arrangement of these matrix polymers with cellulose microfibrils and of the modification of this architecture during cell growth. This study aimed to determine the correspondence between the fine structure of grass cell walls and the course of the elongation process in roots of maize (Zea mays). METHODS: Enzymatic hydrolysis followed by biochemical analysis of derivatives was coupled with immunohistochemical detection of cell wall epitopes at different stages of cell development in a series of maize root zones. KEY RESULTS: Two xylan-directed antibodies (LM11 and ABX) have distinct patterns of primary cell wall labelling in cross-sections of growing maize roots. The LM11 epitopes were masked by mixed-linkage glucan and were revealed only after lichenase treatment. They could be removed from the section by xylanase treatment. Accessibility of ABX epitopes was not affected by the lichenase treatment. Xylanase treatment released only part of the cell wall glucuronoarabinoxylan and produced two types of products: high-substituted (released in polymeric form) and low-substituted (released as low-molecular-mass fragments). The amount of the latter was highly correlated with the amount of mixed-linkage glucan. CONCLUSIONS: Three domains of glucuronoarabinoxylan were determined: one separating cellulose microfibrils, one interacting with them and a middle domain between the two, which links them. The middle domain is masked by the mixed-linkage glucan. A model is proposed in which the mixed-linkage glucan serves as a gel-like filler of the space between the separating domain of the glucuronoarabinoxylan and the cellulose microfibrils. Space for glucan is provided along the middle domain, the proportion of which increases during cell elongation.

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.

Reconstruction and analysis of the gene regulatory network for cell wall function in Arabidopsis thaliana L. leaves in response to water deficit.

The plant cell wall represents the outer compartment of the plant cell, which provides a physical barrier and triggers signaling cascades under the influence of biotic and abiotic stressors. Drought is a factor that negatively affects both plant growth and development. Cell wall proteins (CWP) play an important role in the plant response to water deficit. The adaptation mechanisms of the cell wall to water loss are of interest for identifying important genetic factors determining plant drought resistance and provide valuable information on biomarkers for further selection aimed at increasing the yield of crop plants. Using ANDSystem, a gene network describing the regulation of CWPs under water restriction conditions was reconstructed. The analysis of the gene network and the transcriptome data analysis allowed prioritizing transcription factors (TF) based on their enrichment of differentially expressed genes regulated by them. As a result, scores were calculated, acting as indicators of the association of TFs with water deficit. On the basis of the score values, eight most significant TFs were selected. The highest priority was given to the TF GBF3. CWPs were prioritized according to the criterion of summing up the scores of transcription factors regulating these genes. Among the most prioritized CWPs were the AT5G03350 gene encoding a lectin-like protein, AT4G20860 encoding BBE-like 22 required for the oxidation of cellulose degradation products, and AT4G37800 encoding xyloglucan endotransglucosylase/ hydrolase 7. Overall, the implemented algorithm could be used for prediction of regulatory interactions between transcription factors and target genes encoding cell wall proteins in plants.

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 ß-(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.

[Cell wall proteins of plant fibers].

The cell wall of the same type - phloem fibers (Linum usitatissimum L.), active forming the thick secondary cell wall, - was obtained. Weakly bound cell wall proteins of phloem fibers were extracted and it subsequent separation and obtaining mass spectra was carried out. For identification and attachment of identified proteins to a specific cell compartments a variety ofbioinformatics methods was used. Were identified 93 proteins, many of which were defined as predicted, putative or hypothetical. At the same 21 proteins were identified as cell-wall protein. The absence of such marker proteins of the primary cell wall as xyloglucan-endotransglycosilase, expansins indirectly confirms that in the sample for extraction of proteins dominated the secondary cell wall.

[Senescence and apoptosis of protoplasts from flax fibers: an ultrastructural analysis].

Plant fibers represent specialized cells that perform a mechanical function. Their development includes the following phases, typical for the most plant cells: anlage, extension growth, specialization, senescence, and apoptosis. Ultrastructural analysis of these cells has been carried out at the late phases of their development (senescence and apoptosis) using flax phloem fibers, a classical object for the analysis of sclerenchyma fiber formation. The results of the performed analysis show that flax fiber protoplasts remain viable until the end ofa vegetation season. The ultrastructural analysis of flax phloem fibers has not revealed any typical apoptosis manifestations. Gradual degradation of the cytoplasm starts during the active thickening of a secondary cell wall and manifests via the intensification of autolytic processes, causing a partial loss of cell content. The final stage represents the breaking of tonoplast integrity. The obtained data allow us to suppose that the apoptosis of flax fibers occurs during their senescence, and its program is similar to the cell death program realized in the xylem fibers of woody plants.

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.

[Biogenesis of plant fibers].

The fiber (in terms of plant biology) is an individual cells characterized by spindle shape, length of up to several centimeters, well developed cell wall, and mechanical function. The review summarizes different, sometimes contradictory view points about duration, segregation and mechanisms of realization of individual stages of fiber biogenesis. Initiation and coordinated and intrusive growth are considered, as well as formation of secondary cell wall, including its gelatinous layers, and senescence. Biogenesis of fibers ontogenetically related to various tissues has been analyzed and the data about marker stage-specific characters of these cells. The data summarized in this review willow not only deeper understanding the development of cells with such unique characters, but also interpret the growth mechanisms for much more cell types, in which it is more difficult to identify individual stages of biogenesis than in the sclerenchyme fibers.

Pathogen-induced conditioning of the primary xylem vessels - a prerequisite for the formation of bacterial emboli by Pectobacterium atrosepticum.

Representatives of Pectobacterium genus are some of the most harmful phytopathogens in the world. In the present study, we have elucidated novel aspects of plant-Pectobacterium atrosepticum interactions. This bacterium was recently demonstrated to form specific 'multicellular' structures - bacterial emboli in the xylem vessels of infected plants. In our work, we showed that the process of formation of these structures includes the pathogen-induced reactions of the plant. The colonisation of the plant by P. atrosepticum is coupled with the release of a pectic polysaccharide, rhamnogalacturonan I, into the vessel lumen from the plant cell wall. This polysaccharide gives rise to a gel that serves as a matrix for bacterial emboli. P. atrosepticum-caused infection involves an increase of reactive oxygen species (ROS) levels in the vessels, creating the conditions for the scission of polysaccharides and modification of plant cell wall composition. Both the release of rhamnogalacturonan I and the increase in ROS precede colonisation of the vessels by bacteria and occur only in the primary xylem vessels, the same as the subsequent formation of bacterial emboli. Since the appearance of rhamnogalacturonan I and increase in ROS levels do not hamper the bacterial cells and form a basis for the assembly of bacterial emboli, these reactions may be regarded as part of the susceptible response of the plant. Bacterial emboli thus represent the products of host-pathogen integration, since the formation of these structures requires the action of both partners.

Turnover of Galactans and Other Cell Wall Polysaccharides during Development of Flax Plants.

We investigated the synthesis and turnover of cell wall polysaccharides of the flax (Linum usitatissimum L.) plant during development of the phloem fibers. One-month-old flax plants were exposed to a 40-min pulse with 14CO2 followed by 8-h, 24-h, and 1-month periods of chase with ambient CO2, and radioactivity in cell wall sugars was determined in various plant parts. The relative radioactivity of glucose in noncellulosic polysaccharides was the highest compared with all other cell wall sugars immediately after the pulse and decreased substantially during the subsequent chase. The relative radioactivities of the other cell wall sugars changed with differing rates, indicating turnover of specific polysaccharides. Notably, after 1 month of chase there was a marked decrease in the proportional mass and total radioactivity in cell wall galactose, indicating a long-term turnover of the galactans enriched in the fiber-containing tissues. The ratio of radiolabeled xylose to arabinose also increased during the chase, indicating a turnover of arabinose-containing polymers and interconversion to xylose. The pattern of label redistribution differed between organs, indicating that the cell wall turnover processes are tissue- and cell-specific.

Cell-Wall Polysaccharides of Developing Flax Plants.

Flax (Linum usitatissimum L.) fibers originate from procambial cells of the protophloem and develop in cortical bundles that encircle the vascular cylinder. We determined the polysaccharide composition of the cell walls from various organs of the developing flax plant, from fiber-rich strips peeled from the stem, and from the xylem. Ammonium oxalate-soluble polysaccharides from all tissues contained 5-linked arabinans with low degrees of branching, rhamnogalacturonans, and polygalacturonic acid. The fiber-rich peels contained, in addition, substantial amounts of a buffer-soluble, 4-linked galactan branched at the 0-2 and 0-3 positions with nonreducing terminal-galactosyl units. The cross-linking glycans from all tissues were (fucogalacto)xyloglucan, typical of type-I cell walls, xylans containing (1->)-[beta]-D-xylosyl units branched exclusively at the xylosyl O-2 with t-(4-O-methyl)-glucosyluronic acid units, and (galacto)glucomannans. Tissues containing predominantly primary cell wall contained a larger proportion of xyloglucan. The xylem cells were composed of about 60% 4-xylans, 32% cellulose, and small amounts of pectin and the other cross-linking polysaccharides. The noncellulosic polysaccharides of flax exhibit an uncommonly low degree of branching compared to similar polysaccharides from other flowering plants. Although the relative abundance of the various noncellulosic polysaccharides varies widely among the different cell types, the linkage structure and degree of branching of several of the noncellulosic polysaccharides are invariant.

[Some effects of radiation hormesis for bacterial and yeast cells]. / Nekotorye éffekty radiatsionnogo gormezisa bakterial'nykh i drozhzhevykh kletok.

A comparative study of chronic and acute action of ionizing radiation on the processes of aging and dying off of bacterial and yeast cells was carried out. It was ascertained that chronic action of ionizing radiation, 2-10,000 times exceeded the natural background, resulted in slowing down of aging and dying off of both pro- and eukaryotic cells. A single acute irradiation of yeast also resulted in the retardation of dying off of the yeast cells surviving after irradiation. The data is presented demonstrating a great increase in the survival of yeast cells under their repeated irradiation after recovery from potentially lethal radiation.

Intrusive growth of flax phloem fibers is of intercalary type.

Flax (Linum usitatissimum L.) phloem fibers elongate considerably during their development and intrude between existing cells. We questioned whether fiber elongation is caused by cell tip growth or intercalary growth. Cells with tip growth are characterized by having two specific zones of cytoplasm in the cell tip, one with vesicles and no large organelles at the very tip and one with various organelles amongst others longitudinally arranged cortical microtubules in the subapex. Such zones were not observed in elongating flax fibers. Instead, organelles moved into the very tip region, and cortical microtubules showed transversal and helical configurations as known for cells growing in intercalary way. In addition, pulse-chase experiments with Calcofluor White resulted in a spotted fluorescence in the cell wall all over the length of the fiber. Therefore, it is concluded that fiber elongation is not achieved by tip growth but by intercalary growth. The intrusively growing fiber is a coenocytic cell that has no plasmodesmata, making the fibers a symplastically isolated domain within the stem.