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

Abstract

Bast fiber peels of developing flax ( Linum usitatissimum L.) stem contain tissue-specific soluble (1→4)-β-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)-β-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)-β-galactans, the stem sections were treated with buffer, which elutes buffer-soluble galactan. The soluble high molecular mass (1→4)-β-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.