Down-regulation of a ripening-related beta-galactosidase gene (TBG1) in transgenic tomato fruits

Exo-galactanase/beta -galactosidase (EC 3.2.1.23) activity is thought to be responsible for the loss of galactosyl residues from the cell walls of rip ening tomatoes. Transgenic tomato plants (Lycopersicon esculentum Mill cv. Ailsa Craig) with reduced exo-galactanase/beta -galactosidase mRNA were gen erated to test this hypothesis and to investigate the role of the enzyme in fruit softening. A previously identified tomato beta -galactosidase cDNA c lone, TBG1, was used in the experiments. Heterologous expression of the clo ne in yeast demonstrated that TBG1 could release galactosyl residues from t omato cell wall galactans. Transgenic plants showed a reduction in TBG1 mRN A to 10% of normal levels in the ripening fruits. However, despite the redu ction in message, total beta -galactosidase and exo-galactanase activities were unaffected. Furthermore, there was no apparent effect on levels of cel l wall galactosyl residues when compared with the control. It was concluded that during the ripening of tomato fruits a family of beta -galactosidases capable of degrading cell wall galactans are active and down-regulation of TBG1 message to 10% was insufficient to alter the degree of galactan degra dation.