Xyloglucan in the cell walls of cotton fiber

Abstract

Xyloglucans occur widely in the primary walls of higher plant cells where they are bound in close association with cellulose microfibrils. Evidence was recently presented that the polysaccharide functions in the organization of cellulose microfibril assembly by preventing the fasciation of the fibrils and could aid in spreading the fiber network over the cell surface’. In pea stems, auxin treatment induces the development of endo-(1+4)-/3-D-glucanase activities and these are responsible for xyloglucan turnover 2J. The integrity of xyloglucan could then control the ability of microfibrils to loosen, and of cells to elongate and expand during growth. Cotton fibers are highly elongated cells similar in shape to pollen tubes and root hairs. However, unlike these latter cell types, cotton fibers elongate not by tip growth, but by deposition of wall components and elongation throughout the length of the fiber”‘. During elongation, the fibers deposit a primary wall similar in composition to that of many other dicot species 5 that includes xyloglucan as one component6,7. We recently showed that xyloglucan deposition is restricted to the elongation phase of fiber development ‘. The mechanism by which cell shape is determined in cotton fibers is not understood, but the interaction of xyloglucan with cellulose microfibrils and the orientation of such tibrils might be expected to play some role. In this report, we have examined the fine structure of cotton fiber xyloglucan and compared its structure to that found in cell walls of other dicotyledonous plants. Cotton fiber cell wall polysaccharides were sequentially fractionated into hot water-soluble and 24% potassium hydroxide, and an insoluble fraction. Each fraction was analyzed by methylation analysis8 and the iodine-sodium sulfate method9 in order to determine the xyloglucan content. The results indicated that 95% of total xyloglucan was concentrated in the 24% potassium hydroxide extract.