Reduced levels of β-subunit protein influence tomato fruit firmness, cell-wall ultrastructure, and PG2-mediated pectin hydrolysis in excised pericarp tissue

Tomato fruit expressing an antisense gene for the tomato fruit β-subunit protein were examined to determine the effect of this protein on fruit and pericarp firmness, the susceptibility of excised pericarp tissue to hydrolysis by purified PG2, and pericarp ultrastructure. Firmness of β-subunit-antisense and wild-type fruit and pericarp tissue was similar at the mature green stage. When ripe, β-subunit-antisense fruit firmness averaged 22% lower than firmness of wild type fruit, whereas pericarp firmness was nearly 50 % lower in the antisense line. Pericarp discs from wild-type and β-subunit-deficient mature-green fruit showed firmness decreases of 9 and 16 %, respectively, in response to treatment with purified PG2. PG2 treatment increased levels of soluble pectins recovered from mature-green discs but molecular mass differences between the β-subunit-antisense and wild type tissues were minimal. Ultrastructural differences between β-subunit-antisense and wild-type fruits, both during normal ripening and in mature-green pericarp discs treated with PG2, confirmed that the influence of the β-subunit protein on wall metabolism was dependent on the presence of PG2. Evidence indicated that the enzyme was capable of greater mobility within the tissue of the subunit-deficient compared with the wild-type line.