Polysaccharides in germination. Xyloglucans (‘amyloids’) from the cotyledons of white mustard

Abstract

Two xyloglucan fractions have been isolated from the cotyledons of resting white-mustard seeds, the first by extraction with hot EDTA, and the second by subsequent extraction with alkali or lithium thiocyanate. Although both appear to have the ‘amyloid’ type of structure in which chains of (1→4)-linked β-d-glucopyranose residues carry d-xylose-rich side chains through position 6, these side chains are rather different in structure in the two polysaccharide fractions, and the second or ‘insoluble’ xyloglucan has fewer of them. The side chains in both polysaccharides are also different from those in other seed amyloids, especially in having xylose linked through positions 3 and 4 (instead of through position 2 as usual) and in containing fucose residues. Both polysaccharides show the characteristic blue ‘amyloid’ colour with iodine in the presence of sodium sulphate, and it is suggested that this arises by the interaction of iodine molecules and possibly iodide ions within the interstices between aggregated xyloglucan chains. ‘Soluble’ xyloglucan is metabolized during germination and is presumed to have a reserve function. ‘Insoluble’ xyloglucan is metabolized less completely over the period studied but its lack of turnover during cell-wall differentiation indicates that it also is a reserve. These and other β-(1→4)-linked reserve polysaccharides of seeds might also have a structural function which is of particular value for the survival of the dormant seed.