Modification of expansin protein abundance in tomato fruit alters softening and cell wall polymer metabolism during ripening

The role of the ripening-specific expansin Exp1 protein in fruit softening and cell wall metabolism was investigated by suppression and overexpression of Exp1 in transgenic tomato plants. Fruit in which Exp1 protein accumulat ion was suppressed to 3% that of wild-type levels were firmer than controls throughout ripening. Suppression of Exp1 protein also substantially inhibi ted polyuronide depolymerization late in ripening but did not prevent the b reakdown of structurally important hemicelluloses, a major contributor to s oftening. In contrast, fruit overexpressing high levels of recombinant Exp1 protein were much softer than controls, even in mature green fruit before ripening commenced. This softening was correlated with the precocious and e xtensive depolymerization of structural hemicelluloses, whereas polyuronide depolymerization was not altered. These data are consistent with there bei ng at least three components to fruit softening and textural changes. One c omponent is a relaxation of the wall directly mediated by Exp1, which indir ectly limits part of a second component due to polyuronide depolymerization late in ripening, perhaps by controlling access of a pectinase to its subs trate. The third component is caused by depolymerization of hemicelluloses, which occurs independently of or requires only very small amounts of Exp1 protein.