The structure, composition and development of collenchyma cell walls in celery (Apium graveolens L.) petioles

Abstract

Collenchyma cells are one of the main mechanical supporting cell types in growing organs of herbaceous and woody plants. When mature, their cell walls are heavily thickened and have a polylamellate structure. However, little is known about the chemical composition of collenchyma cell walls and how they change during development. Furthermore, how the cellulose microfibrils are deposited and arranged to form ordered polylamellate structures in collenchyma cell walls is uncertain. In the current study, subepidermal collenchyma strands from celery (Apium graveolens L.) petioles were isolated and used as starting material. The monosaccharide and glycosyl-linkage compositions of isolated collenchyma cell walls from fully elongated celery petioles were determined. The compositions of fractions obtained by sequentially extracting walls with CDTA, Na2CO3, 1 M KOH and 4 M KOH were also determined. This showed that the collenchyma cell walls have similar polysaccharide compositions to those of parenchyma cell walls of many eudicotyledons. Changes in the thickness and polysaccharide compositions of collenchyma walls at four developmental stages were also studied using transmission electron microscopy, chemical analysis, immunofluorescence and immunogold microscopy with monoclonal antibodies, and by solid-state 13C NMR. During development, no wall thickening occurred after the cells had stopped elongating, demonstrating that collenchyma cell walls are primary walls. During development, the degree of methyl esterification of homogalacturonan, and the proportions of the rhamnogalacturonan-I side chains decreased, whereas the proportions of cellulose, xyloglucans and heteroxylans increased. These changes were associated with an increased rigidity of the cellulose and homogalacturonan. Remarkable changes in the fine structure of cell walls also occurred during development. Oblique sections stained with a carbohydrate-specific stain and examined by transmission electron microscopy showed a “V-shaped” herringbone pattern that became increasingly evident in the collenchyma walls during development. Examination of the orientation of cellulose microfibrils immediately under the plasma membrane by atomic force microscopy and field emission scanning electron microscopy showed they were oriented transversely. However, in the layer beneath, they were oriented at a high angle. Small angle X-ray scattering (SAXS) was also conducted on whole collenchyma strands to obtain the angular distribution of cellulose microfibrils relative to the long axis of the cells. This showed that the cellulose microfibrils were predominately oriented longitudinally and the proportion oriented in this direction increased gradually during development.