Novel Rhamnogalacturonan I and Arabinoxylan Polysaccharides of Flax Seed Mucilage

The viscous seed mucilage of flax (Linum usitatissimum) is a mixture of rhamnogalacturonan I and arabinoxylan with novel side group substitutions. Therhamnogalacturonan I has numerous single non reducing terminal residues of the rare sugar L-galactose attached at theO-3 position of the rhamnosyl residues instead of the typicalO-4 position. The arabinoxylan is highly branched, primarily with double branches of non reducing termina lL-arabinosylunits at the O-2 and O-3 positions along the xylan backbone. While a portion of each polysaccharide can be purified by anion-exchange chromatography, the side group structures of both polysaccharides are modified further in about one-third of the mucilage to form composites with enhanced viscosity. Our finding of the unusual side group structures for two well-known cell wall polysaccharides supports a hypothesis that plants make a selected few ubiquitous backbone polymers onto which a broad spectrum of sidegroup substitutions are added to engender many possible functions. To this end, modification of one polymer may be accompanied by complementary modifications of others to impart functions to heterocomposites not present in either polymer alone. (Authors abstract)

Seed mucilages provide a system to examine the physical properties of pectin gels in the absence of cellulose and its cross-linking glycans. The epidermal cells of the seed coats of flaxseed release large quantities of mucilage that form a gel-like capsule. Seed mucilages vary in their chemical composition and the objective of the present study was to determine their characteristics in flax.  They report the finding of high degree of t-Araf residues attached to both the O-2 and O-3 of the (1/4)-b-D-xylan chain to form doubly branched residues the arabinoxylan backbone of the mucilage. The arabinoxylan is weakly acidic, owing to thet-GlcA residues also attached to the xylan chain. The side group structure of the flax RG I is also atypical, with single nonreducing terminal L-Gal and L-Fuc residues attached to the O-3 position instead of the O-4 position. While some of the arabinoxylan and RGI can be separated chromatographically, a composite of the two polymers not separated by anion-exchange chromatography was found to have enhanced viscosity unable to be reproduced in mixtures of the two isolated polymers. Is modified in diverse ways to fine-tune their physical, biological, and physiological functions. Flax mucilage serves as an example that alterations in the fundamental structure of one polymer are complemented by alterations in others to form composites to yield a property not present in either polymer alone. For the flax mucilage, the fact that both the arabinoxylan and RG I polymers have been altered from structures normally found in the primary wall suggests that each contains complementary alterations from typical to accommodate new functions unrelated to wall physics. That flax has at least two forms of mucilage suggests that multiple physical properties provide multiple functions.  (Editors comments)