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.

Differential expression of α-L-arabinofuranosidases during maize (Zea mays L.) root elongation.

MAIN CONCLUSION: Specific α- l -arabinofuranosidases are involved in the realisation of elongation growth process in cells with type II cell walls. Elongation growth in a plant cell is largely based on modification of the cell wall. In type II cell walls, the Ara/Xyl ratio is known to decrease during elongation due to the partial removal of Ara residues from glucuronoarabinoxylan. We searched within the maize genome for the genes of all predicted α-L-arabinofuranosidases that may be responsible for such a process and related their expression to the activity of the enzyme and the amount of free arabinose measured in six zones of a growing maize root. Eight genes of the GH51 family (ZmaABFs) and one gene of the GH3 family (ZmaARA-I) were identified. The abundance of ZmaABF1 and 3-6 transcripts was highly correlated with the measured enzymatic activity and free arabinose content that significantly increased during elongation. The transcript abundances also coincided with the pattern of changes in the Ara/Xyl ratio of the xylanase-extractable glucuronoarabinoxylan described in previous studies. The expression of ZmaABF3, 5 and 6 was especially up-regulated during elongation although corresponding proteins are devoid of the catalytic glutamate at the proper position. ZmaABF2 transcripts were specifically enriched in the root cap and meristem. A single ZmaARA-I gene was not expressed as a whole gene but instead as splice variants that encode the C-terminal end of the protein. Changes in the ZmaARA-I transcript level were rather moderate and had no significant correlation with free arabinose content. Thus, elongation growth of cells with type II cell walls is accompanied by the up-regulation of specific and predicted α-L-arabinofuranosidase genes, and the corresponding activity is indeed pronounced and is important for the modification of glucuronoarabinoxylan, which plays a key role in the modification of the cell wall supramolecular organisation.