Elongating maize root: zone-specific combinations of polysaccharides from type I and type II primary cell walls.

The dynamics of cell wall polysaccharides may modulate the cell wall mechanics and thus control the expansion growth of plant cells. The unique composition of type II primary cell wall characteristic of grasses suggests that they employ specific mechanisms for cell enlargement. We characterized the transcriptomes in five zones along maize root, clustered the expression of genes for numerous glycosyltransferases and performed extensive immunohistochemical analysis to relate the changes in cell wall polysaccharides to critical stages of cell development in Poaceae. Specific patterns of cell wall formation differentiate the initiation, realization and cessation of elongation growth. Cell walls of meristem and early elongation zone represent a mixture of type I and type II specific polysaccharides. Xyloglucans and homogalacturonans are synthesized there actively together with mixed-linkage glucans and glucuronoarabinoxylans. Rhamnogalacturonans-I with the side-chains of branched 1,4-galactan and arabinan persisted in cell walls throughout the development. Thus, the machinery to generate the type I primary cell wall constituents is completely established and operates. The expression of glycosyltransferases responsible for mixed-linkage glucan and glucuronoarabinoxylan synthesis peaks at active or late elongation. These findings widen the number of jigsaw pieces which should be put together to solve the puzzle of grass cell growth.

AFM analysis reveals polymorphism of purified flax rhamnogalacturonans I of distinct functional types.

Functionally distinct polymers organized on the basis of rhamnogalacturonan I (RG-I) backbone with more than a half of rhamnose residues substituted by the side chains containing mostly galactose were purified from flaxseed mucilage, the primary cell wall of young hypocotyls and tertiary cell walls of bast fibers and characterized by atomic force microscopy. Seed mucilage RG-I with short side chains and unusual O3 substitution showed loose coils or star-like conformations. Primary cell wall RG-I, which included polygalacturonan (PGA) fragments, represented micellar objects and rare long chains. Pure RG-I with long galactan side chains, which was isolated as nascent polysaccharide before its incorporation into the tertiary cell wall of bast fibers was observed as long unbranched objects. RG-I entrapped by cellulose microfibrils in tertiary cell wall was visualized as compact micellar complexes. All types of flax RGs-I tended to aggregate. Relationships between RG-I structure and morphology are discussed.

Gelation of rhamnogalacturonan I is based on galactan side chain interaction and does not involve chemical modifications.

The article presents the structural principles of microwave-induced formation of new gel type from pectic rhamnogalacturonan I (RG-I). The backbone of gel-forming RG-I does not contain consecutive galacturonic residues and modifying groups that can be the cause of junction zone formation as it occurs in course of classical ways of pectin gelation. Microwave irradiation does not cause destruction and chemical modifications of RG-I. Removal of half of galactan chains from RG-I leads to loss of gelling capability pointing out on their leading role in this process. Rising of intensity of the bands attributed to galactose and glycosidic linkages in RG-I gel comparing to solution where this polymer exists as molecule associate indicates that the spatial organization of galactans in gel is changed. A model of the RG-I gelation is proposed: being destabilized at volumetric microwave heating RG-I associates are repacked forming network where RG-I molecules are entangled by galactan chains.

Physicochemical properties of complex rhamnogalacturonan I from gelatinous cell walls of flax fibers.

The physicochemical properties of flax fiber cell wall rhamnogalacturonan I (RG-I) and its fragments, obtained after galactanase treatment (fraction G1), were characterized. RG-I retains its hydrodynamic volume after its molecular weight decreases by approximately half, as revealed by SEC. Two techniques, DLS and NMR, with different principles of diffusion experiment were used to establish the reasons for this property of RG-I. Three possible types of particles were revealed by DLS depending on the concentration of the RG-I and G1 solutions (2-2.5, 15-20, and 150-200 nm). It was determined by BPP-LED experiments that the backbone of the RG-I was 1.3-1.9-fold more mobile than the side chains. The obtained data suggest a novel type of pectin spatial organization-the formation of RG-I associates with the backbone at the periphery and the interaction between the side chains to form a core zone.