Further observations on cell wall morphogenesis and polysaccharide arrangement during plant growth

Abstract

The three-dimensional arrangement of the polysaccharide chains in cell walls was investigated, using ultracryotomy and cytochemistry, in order to test the validity of the previously postulated “ordered fibril hypothesis” and to analyze the characteristics of the primary wall morphogenesis. Both in mung bean hypocotyl (Phaseolus aureus) and pea root (Pisum sativum) cultured in defined conditions, cell to cell endogenous specificity is marked by differences in the numbers of layers, thickness, rhythm and direction of deposition. The occurrence of bow-shaped arrangements and of strata of orientation intermediate between the main crisscrossed multifibrillar layers suggests that the sequential changes of the morphogenetic activity of the cells is progressive. The twisted polysaccharide disposition evokes certain mesomorphic states; a part of the mechanism responsible for the wall arrangement may result from a self-assembly process as in the orientation of the molecules in a liquid cristal. This possibility finds experimental support in the fact that a three-dimensional association of the hemicellulose chains spontaneously appears when precipitated in acellular conditions. Polysaccharide removal associated with shadowing indicates that the ordered disposition within the wall is extensively altered by even a slight extraction. These data may invalidate diverse results which are generally brought forward to explain the wall organization during growth.