CHANGES IN CELL-WALL ARCHITECTURE DURING CELL ELONGATION

A model of the plant cell wall of dicotyledons and non-graminaceous monocotyledons contains two principal structural polysaccharide networks: a stretch-resistant load-bearing cellulose/xyloglucan network, and a compression-resistant pectic polysaccharide network. Structural modifications to either polysaccharide network are likely to have profound consequences for cell wall properties, including tensile strength and extensibility. We have described dynamic changes in the pattern of esterification of pectic polysaccharides that correlate with cell growth in both carrot and tobacco suspension cells. During elongation, cellulose microfibrils become oriented transversely with respect to the long axis of the cell. Other matrix polymers such as pectin and protein also acquire a net orientation, but in a tissue-specific manner. Preliminary results from Zinnia mesophyll cells suggest that some wall modifications such as the removal of fucose from xyloglucan may be developmentally regulated, but are not required for elongation per se. Our observations lead us to propose a tentative model for the molecular rearrangements that occur within these walls during cell elongation. Our use of methodologies which are suitable for analysis at the single cell wall level has demonstrated that the apoplastic continuum is a mosaic of different architectures, with major architectural differences between species, between tissues, and between domains within a single wall, and we discuss the implications of this heterogeneity for our model.