Pectins in secondary cell walls: modifications during cell wall assembly and maturation

Pectins are the major components of middle lamellae and primary cell wall. Though in relatively small proportion, they are also present in secondary cell walls and can play there an important role, which received little attention. This is especially true for secondary cell wall of gelatinous type, which is characteristic for many plant fibers. Pectic molecules can be so tightly bound to the secondary cell wall cellulose, that they cannot be extracted even after treatment with concentrated alkali, which confirms their role as an important player in cell wall assembly. Developing flax fibres, while making their secondary cell wall, contain a particular tissue- and stage-specific high-molecular mass pectic galactan, which can be collected from homogenization buffer, and is not fixed yet within the cell wall. The galactan comes out from the specific Golgi-derived vesicles and can be separated from the other polymers by gel filtration due to its considerable molecular weight (900-2000 kDa), providing a rare opportunity to characterize individual polysaccharides before they are incorporated into the cell wall. Moreover, the fiber-specificity of the galactan gives possibility to trace the metabolism of certain polymers in certain cell type, while analyzing whole stem portions. The set of approaches permitted to analyze the composition of several pools: the galactan from Golgi vesicles, the galactan from various fractions of the cell wall and the galactose mono- and oligomers. Label redistribution in pulse-chase experiments indicated that disassembly of the nascent galactan into smaller blocks occurred before or during the interaction with cellulose. A significant amount of free galactose is present specifically in fibers and only at the stage of secondary cell wall formation. The activity of beta-galactosidase, which corresponded in time and place with the content of free galactose was measured. The changes in the galactan structure are discussed from the perspective of its role in cell wall assembly and maturation.