Homofusion of Golgi secretory vesicles in flax phloem fibers during formation of the gelatinous secondary cell wall

The gelatinous type of secondary cell wall is present in tension wood and in phloem fibers of many plants. It is characterized by the absence of xylan and lignin, a high cellulose content and axially orientated microfibrils in the huge S2 layer. In flax phloem fiber, the major non-cellulosic component of such cell walls is tissue-specific galactan, which is tightly bound to cellulose. Ultrastructural analysis of flax fiber revealed that initiation of gelatinous secondary cell wall formation was accompanied by the accumulation of specific Golgi vesicles, which had a characteristic bicolor (dark-light) appearance and were easily distinguishable from vesicles made in different tissues and during the other stages of fiber development. Many of the bicolor vesicles appeared to fuse with each other, forming large vacuoles. The largest observed was 4 mu m in diameter. Bicolor vesicles and vacuoles fused with the plasma membrane and spread their content in a characteristic syringe-like manner, covering a significant area of periplasm and forming dark stripes on the inner wall surface. Both Golgi derivatives and cell wall layers were labeled by LM5 antibody, indicating the presence of tissue- and stage-specific (1 -> 4)-beta-galactan. We suggest that this specific type of galactan secretion, which allows coverage of a large area of periplasm, is designed to increase the chance of the galactan meeting the cellulose microfibrils while they are still in the process of construction. The membrane fusion machinery of flax fiber must possess special components, which may be crucial for the formation of the gelatinous type cell wall.