S. B. Chemikosova

The snap point : a transition point in Linum usitatissimum bast fiber development

The developing stem of fibre flax (Linum usitatissimum L.) contains a specific region called the ‘snap point’, where the fiber-enriched bast tissues considerably change their mechanical properties. The snap point was found to be present during a restricted period of plant development */the fast growth phase, and to disappear when stem growth was completed. To relate this snap point to bast fiber formation stages, the number of bast fiber cells and the thickness of their cell walls were followed on the stained cross-sections of the flax stem throughout plant development, using the progressing snap point as the reference. The snap point was shown to be the spot, above which the elongation of bast fiber cells is fully completed. This fast growth stage is the period when the maximum length of all bast fibers in the mature plant (a major characteristic of flax fiber quality) is fixed and would not be changed later. Autoradiography was used to visualize the mode of flax bast fiber elongation above the snap point. The even distribution of label was indicative for surface (diffusive) growth type. Elongation of individual fiber cells was estimated to take only 2 � /4 days with a rate of 1 � /2 cm per day, while cell wall thickening occurs mainly below snap point and lasts around 2 months. The special cell wall structural order, characteristic for mature bast fibers, first appeared at the snap point in the outer layer of the secondary cell wall. Schemes are included, illustrating the course of cell wall thickening and the localization of various stages of fiber formation on the stem of growing flax plant. The established exact localization and duration of flax bast fiber formation stages, and the existence of snap point as the manually identified morphological reference for the transition, permit to separate the bast fibers at different stages of development and make flax an attractive model system to study the functional genomics of fiber formation in technical crops. # 2003 Elsevier B.V. All rights reserved.

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.

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.

The effect of soil drought on the phloem fiber development in long-fiber flax

The effects of soil drought on various stages of phloem fiber development during the period of flax (Linum usitatissimum L.) rapid growth were assessed. The formation of the secondary cell wall was shown to be most retarded. The content of a tissue-specific galactan was reduced especially sharply, and its side chains were changed. Under conditions of pronounced stress-induced plant growth retardation, fiber intrusive growth was suppressed relatively softly: their number on the stem transverse sections was reduced only by 16%. However, this determined irreversible diversity in the fiber length in various stem regions. Such insignificant suppression of intrusive growth under osmotic stress (simultaneously with substantial retardation of plant growth and metabolism inhibition) indicates the functioning of special mechanisms of its regulation.

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.

Tissue-specific processes during cell wall formation in flax fiber

Flax (Linum usitatissimum L.) phloem fiber elongation is separate from secondary cell wall formation. The indicator for the developmental transition is the manually determinable “snap point”. Sharp increase in the mechanical strength at certain level of flax stem. It helped to characterize fiber-specific and stage-specific processes: soluble galactan turnover (revealed in pulse-chase experiments), specialized Golgi vesicle accumulation, and cell wall postsynthetic modification.

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.

Transcriptome portrait of cellulose-enriched flax fibres at advanced stage of specialization

Functional specialization of cells is among the most fundamental processes of higher organism ontogenesis. The major obstacle to studying this phenomenon in plants is the difficulty of isolating certain types of cells at defined stages of in planta development for in-depth analysis. A rare opportunity is given by the developed model system of flax (Linum usitatissimum L.) phloem fibres that can be purified from the surrounding tissues at the stage of the tertiary cell wall deposition. The performed comparison of the whole transcriptome profile in isolated fibres and other portions of the flax stem, together with fibre metabolism characterization, helped to elucidate the general picture of the advanced stage of plant cell specialization and to reveal novel participants potentially involved in fibre metabolism regulation and cell wall formation. Down-regulation of all genes encoding proteins involved in xylan and lignin synthesis and up-regulation of genes for the specific set of transcription factors transcribed during tertiary cell wall formation were revealed. The increased abundance of transcripts for several glycosyltransferases indicated the enzymes that may be involved in synthesis of fibre-specific version of rhamnogalacturonan I.

Variability in the composition of tissue-specific galactan from flax fibers

Tissue-specific galactan of sclerenchyma fibers, with cell walls of the gelatinous type, was examined in flax plants (Linum usitatissimum L.) of 23 various genotypes. The content and average degree of polymerization of side chains of galactan were estimated before its deposition into the cell wall. The variability of the analyzed parameters of tissue-specific galactan from flax fibers was high; within the same genotype, the scope of paratypic variability between replicates and years of research was comparable to variability between different genotypes. The average length of side chains in the studied samples ranged from 5 to 41 galactose residues. The average degrees of polymerization of galactan side chains in flax fibers was found to be discrete, which could be explained by block assemblage of the polymer in the Golgi apparatus.