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 crop

Occurrence of cell-specific galactan is coinciding with bast fiber developmental transition in flax

Bast fiber peels of developing flax (Linum usitatissimum L.) stem contain tissue-specific soluble (1 --> 4)-beta-galactan. The occurrence and localization of this polymer have been investigated at various stages of bast fiber development. Tissue-specific galactan (molecular mass >2000 kDa) emerged right below the snap point-the manually identified spot on the stem, above which the fiber cells elongate, while below no more elongation occurs, but intensive secondary cell wall thickening takes place. In the course of plant development the amount of tissue-specific galactan gradually declined and in mature plants the polymer disappeared from the fraction, being either degraded or fixed within the cell wall. The immunolocalization of (1 --> 4)-beta-galactans with LM5 antibody revealed that in flax stem these polymers were present only in bast fiber cells. Two layers were well pronounced within the secondary cell wall, the density of gold particles in the inner layer was 5.2 +/- 1.5 times higher than in the outer layer. To distinguish soluble galactan from other (1 --> 4)-beta-galactans, the stem sections were treated with buffer, which elutes buffer-soluble galactan. The soluble high molecular mass (1 --> 4)-beta-galactan is a bast fiber cell-specific polymer, emerging at the transition from cell elongation to cell wall thickening. It is localized in the inner portion of the secondary cell wall and is modified or degraded in the course of further plant development. This polymer belongs to the rarely observed plant cell wall polysaccharides, which only occurs at one specific stage of cell development, suggesting that it could play a prominent role in cellulose deposition and secondary cell wall formation in flax fibers.. (C) 2003 Elsevier B.V. All rights reserved