Structural Features of Different Functional Types of Flax Rhamnogalacturonan I

Rhamnogalacturonans I (RGs-I) are complex, multifunctional pectic polysaccharides found in plant cell walls and seed mucilages. Their structure varies based on the source, tissue type, and growth stage. This study investigates the structural peculiarities of different functional types of RG-I from flax, focusing on NMR analysis of RGs-I from seed mucilage, primary cell walls of hypocotyls, and tertiary cell walls of fibers. RG-I from flaxseed mucilage features single-residue side chains of Galp and Fucp, with 55% Rhap substitution at the rare O-3 position, no homogalacturonan, and the presence of acetyl groups. Primary cell wall RG-I includes homogalacturonan (up to 50% of the total backbone), 54% Rhap substitution at O-4, and short β-(1 → 4)-galactan and α-(1 → 5)-arabinan side chains (averaging 2-3 residues), with 17% represented by single Ara and Gal units, along with acetyl, feruloyl, and benzoyl groups. The tertiary cell wall RG-I has a pure backbone without homogalacturonan and the highest Rhap substitution at O-4 (72%) by long galactan side chains (up to 92 residues). Modifications of this RG-I in the cell wall suggest the removal of acetyl groups and trimming of galactan chains (47% reduced to a single residue; longer side chains of up to 16 residues remaining). This research provides insights into the structural diversity of RG-I within a single plant, contributing to the understanding of their functional roles in plant growth and development.