Search

Main Conclusion: Chemical analyses and glycome profiling demonstrate differences in the structures of the xyloglucan, galactomannan, glucuronoxylan, and rhamnogalacturonan I isolated from soybean ( Glycine max ) roots and root hair cell walls. The root hair is a plant cell that extends only at its tip. All other root cells have the ability to grow in different directions (diffuse growth). Although both growth modes require controlled expansion of the cell wall, the types and structures of polysaccharides in the walls of diffuse and tip-growing cells from the same plant have not been determined. Soybean (Glycine max) is one of the few plants whose root hairs can be isolated in amounts sufficient for cell wall chemical characterization. Here, we describe the structural features of rhamnogalacturonan I, rhamnogalacturonan II, xyloglucan, glucomannan, and 4-O-methyl glucuronoxylan present in the cell walls of soybean root hairs and roots stripped of root hairs. Irrespective of cell type, rhamnogalacturonan II exists as a dimer that is cross-linked by a borate ester. Root hair rhamnogalacturonan I contains more neutral oligosaccharide side chains than its root counterpart. At least 90% of the glucuronic acid is 4-O-methylated in root glucuronoxylan. Only 50% of this glycose is 4-O-methylated in the root hair counterpart. Mono O-acetylated fucose-containing subunits account for at least 60% of the neutral xyloglucan from root and root hair walls. By contrast, a galacturonic acid-containing xyloglucan was detected only in root hair cell walls. Soybean homologs of the Arabidopsis xyloglucan-specific galacturonosyltransferase are highly expressed only in root hairs. A mannose-rich polysaccharide was also detected only in root hair cell walls. Our data demonstrate that the walls of tip-growing root hairs cells have structural features that distinguish them from the walls of other roots cells.

Xyloglucan is a polysaccharide that has important roles in the formation and function of the walls that surround growing land plant cells. Many of these plants synthesize xyloglucan that contains galactose in two different side chains (L and F), which exist in distinct molecular environments. However, little is known about the contribution of these side chains to xyloglucan function. Here, we show that Arabidopsis (Arabidopsis thaliana) mutants devoid of the F side chain galactosyltransferase MURUS3 (MUR3) form xyloglucan that lacks F side chains and contains much less galactosylated xylose than its wild-type counterpart. The galactose-depleted xyloglucan is dysfunctional, as it leads to mutants that are dwarfed with curled rosette leaves, short petioles, and short inflorescence stems. Moreover, cell wall matrix polysaccharides, including xyloglucan and pectin, are not properly secreted and instead accumulate within intracellular aggregates. Near-normal growth is restored by generating mur3 mutants that produce no detectable amounts of xyloglucan. Thus, cellular processes are affected more by the presence of the dysfunctional xyloglucan than by eliminating xyloglucan altogether. To identify structural features responsible for xyloglucan dysfunction, xyloglucan structure was modified in situ by generating mur3 mutants that lack specific xyloglucan xylosyltransferases (XXTs) or that overexpress the XYLOGLUCAN L-SIDE CHAIN GALACTOSYLTRANSFERASE2 (XLT2) gene. Normal growth was restored in the mur3-3 mutant overexpressing XLT2 and in mur3-3 xxt double mutants when the dysfunctional xyloglucan was modified by doubling the amounts of galactosylated side chains. Our study assigns a role for galactosylation in normal xyloglucan function and demonstrates that altering xyloglucan side chain structure disturbs diverse cellular and physiological processes., (© 2015 American Society of Plant Biologists. All Rights Reserved.)

Cotton fiber is an important natural textile fiber due to its exceptional length and thickness. These properties arise largely through primary and secondary cell wall synthesis. The cotton fiber of commerce is a cellulosic secondary wall surrounded by a thin cuticulated primary wall, but there were only sparse details available about the polysaccharides in the fiber cell wall of any cotton species. In addition, Gossypium hirsutum (Gh) fiber was known to have an adhesive cotton fiber middle lamella (CFML) that joins adjacent fibers into tissue-like bundles, but it was unknown whether a CFML existed in other commercially important cotton fibers. We compared the cell wall chemistry over the time course of fiber development in Gh and Gossypium barbadense (Gb), the two most important commercial cotton species, when plants were grown in parallel in a highly controlled greenhouse. Under these growing conditions, the rate of early fiber elongation and the time of onset of secondary wall deposition were similar in fibers of the two species, but as expected the Gb fiber had a prolonged elongation period and developed higher quality compared to Gh fiber. The Gb fibers had a CFML, but it was not directly required for fiber elongation because Gb fiber continued to elongate rapidly after CFML hydrolysis. For both species, fiber at seven ages was extracted with four increasingly strong solvents, followed by analysis of cell wall matrix polysaccharide epitopes using antibody-based Glycome Profiling. Together with immunohistochemistry of fiber cross-sections, the data show that the CFML of Gb fiber contained lower levels of xyloglucan compared to Gh fiber. Xyloglucan endo-hydrolase activity was also higher in Gb fiber. In general, the data provide a rich picture of the similarities and differences in the cell wall structure of the two most important commercial cotton species.

Xyloglucan is an important hemicellulosic polysaccharide in dicot primary cell walls. Most of the enzymes involved in xyloglucan synthesis have been identified. However, many important details of its synthesis in vivo remain unknown. The roles of three genes encoding xylosyltransferases participating in xyloglucan biosynthesis in Arabidopsis (Arabidopsis thaliana) were further investigated using reverse genetic, biochemical, and immunological approaches. New double mutants (xxt1 xxt5 and xxt2 xxt5) and a triple mutant (xxt1 xxt2 xxt5) were generated, characterized, and compared with three single mutants and the xxt1 xxt2 double mutant that had been isolated previously. Antibody-based glycome profiling was applied in combination with chemical and immunohistochemical analyses for these characterizations. From the combined data, we conclude that XXT1 and XXT2 are responsible for the bulk of the xylosylation of the glucan backbone, and at least one of these proteins must be present and active for xyloglucan to be made. XXT5 plays a significant but as yet uncharacterized role in this process. The glycome profiling data demonstrate that the lack of detectable xyloglucan does not cause significant compensatory changes in other polysaccharides, although changes in nonxyloglucan polysaccharide amounts cannot be ruled out. Structural rearrangements of the polysaccharide network appear responsible for maintaining wall integrity in the absence of xyloglucan, thereby allowing nearly normal plant growth in plants lacking xyloglucan. Finally, results from immunohistochemical studies, combined with known information about expression patterns of the three genes, suggest that different combinations of xylosyltransferases contribute differently to xyloglucan biosynthesis in the various cell types found in stems, roots, and hypocotyls.