DISTRIBUTION OF PECTIC POLYSACCHARIDES THROUGHOUT WALLS OF SUSPENSION-CULTURED CARROT CELLS - AN IMMUNOCYTOCHEMICAL STUDY

A monoclonal antibody (2 F 4) recognizing a conformational epitope of polygalacturonic acid was used for immunogold localization of pectins in walls of suspension-cultured carrot (D. carota L.) cells at the electron microscopic level. In microcolonies of "young" or "mature" cells, polygalacturonic acid was essentially located on the middle lamella material expanded at three-way junctions between cells or lining intercellular spaces but was not found in primary walls. Middle lamellae far from junction zones and intercellular spaces were not recognized. Largely esterified pectic polymers, only detected by the 2 F 4 antibodies after on-grid de-esterification treatment by pectin methyl esterases, were present within all primary cell walls. Golgi bodies and associated vesicles were also labeled by the 2 F 4 antibodies only after de-esterification treatment, which indicates that pectic polymers are synthesized and secreted in a highly esterified form. A decrease of pectin esterification, which results probably from an in situ enzymatic de-esterification of the pectic polymers of the primary walls, was observed in "senescent" cells. These results are discussed in relation to biochemical analyses showing changes of the methyl ester content of pectins during the cell-wall growth.