Influence of the β-subunit Protein on Pericarp Softening, Cell Wall Hydrolysis, and Ultrastructural Changes in Ripening Tomato Fruit

Abstract

The function of the β-subunit protein in the cell wall metabolism of ripening tomato fruit is unknown. We examined pericarp softening, and polygalacturonase isozyme 2-mediated cell wall and pericarp tissue hydrolysis using tomato fruit (`Ailsa Craig') expessing an antisense gene for the cell wall-subunit protein. Softening of pericarp tissue was accelerated in the β-subunit-deficient compared with wild-type fruits, decreasing from an average of 22 N at the mature-green stage (both lines) to 9 N (12.7, wild) at the turning-pink stage to 2.5 N (5.1, wild) at the ripe stage. Pectin solubilization was enhanced significantly in the antisense line compared with the wild type, but differences in depolymerization patterns were minimal. Scanning electron microscopy of pericarp tissue revealed no differences in the tomato lines at the mature-green stage. During ripening, the pericarp tissue of the antisense fruit exhibited a more extensive loss in cell wall rigidity evident from the collapsed appearance of cell walls. Application of purified PG2 to the cut surface of mature-green pericarp tissue induced ultrastructural changes paralleling those observed during ripening of the two fruit lines. Refracture of PG2-treated, mature-green pericarp tissue showed that enzyme activity was confined to the tissue surface in wild type fruit, but penetrated at least several cell layers in the antisense line. Although some workers have suggested that the role of the β-subunit is not related to PG1(PG2+subunit) formation, we conclude that the protein influences pericarp tissue softening and cell wall hydrolysis by limiting the mobility and catalytic capacity of PG2.