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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.

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We investigated the synthesis and turnover of cell wall polysaccharides of the flax (Linum usifafissimum L.) plant during development of the phloem fibers. One-month-old flax plants were exposed to a 40-min pulse with 1 4 C 0 , followed by 8-h, 24-h, and 1-month periods of chase with ambient CO,, 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.Flax (Linum usifafissimum L.) is an important source of natural fibers in many parts of the world. The fibers originate from procambial cells in the protophloem (Esau, 1977), where bundles of several dozen cells each encircle the vascular cylinder and produce, at maturity, thick walls containing about 70% cellulose (McDougall, 1993). We investigated the noncellulosic polysaccharides of developing flax plants, of fiber-rich peels that can be stripped from the plant during growth, and of the developing xylem (Gorshkova ,et al., 1996b). These investigations revealed several features specific to changes in the flax cell walls during development. As with other species with type I primary cell walls (Carpita and Gibeaut, 1993), the growing tip of the flax plant is rich in pectic substances and a (fucogalacto)XyG. A 4-linked xylan branched with nonreducing terminal 4-O-methylglucuronosyl units accumulates to more than 60% of the mass of the developing xylem. The fiber- 723rich peels are also enriched in pectic substances, XyG, and the 4-linked xylan but also contain 5-linked arabinans, (galacto)glucomannans, and substantial amounts of a buffer-soluble, 4-linked galactan branched at the 0 -2 and 0-3 positions with terminal-galactosyl units (Gorshkova et al., 199613). Our efforts are now focused on the biosynthesis of the polysaccharides in specific tissue and cell types, in particular, the phloem fibers.Plant cell wall metabolism has been studied primarily by labeling excised plant organs or tissues, suspension cultures, and isolated protoplasts with exogenous sugars or their derivatives in pulse-chase experiments. Excision of plant parts results in an alteration of cell metabolism, including growth regulator status, membrane potential, etc., that may affect cell wall processes, particula...