Pectin Modification in Cell Walls of Ripening Tomatoes Occurs in Distinct Domains

The class of cell wall polysaccharides that undergoes the most extensive modification during tomato (Lycopersicon esculentum) fruit ripening is pectin. De-esterification of the polygalacturonic acid backbone by pectin methylesterase facilitates the depolymerization of pectins by polygalacturonase II (PGII). To investigate the spatial aspects of the de-esterification of cell wall pectins and the subsequent deposition of PGII, we have used antibodies to relatively methylesterified and nonesterified pectic epitopes and to the PGII protein on thin sections of pericarp tissue at different developmental stages. De-esterification of pectins and deposition of PGII protein occur in block-like domains within the cell wall. The boundaries of these domains are distinct and persistent, implying strict, spatial regulation of enzymic activities. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of proteins strongly associated with cell walls of pericarp tissue at each stage of fruit development show ripening-related changes in this protein population. Western blots of these gels with anti-PGII antiserum demonstrate that PGII expression is ripening-related. The PGII co-extracts with specific pectic fractions extracted with imidazole or with Na2CO3 at 0[deg]C from the walls of red-ripe pericarp tissue, indicating that the strong association between PGII and the cell wall involves binding to particular pectic polysaccharides.