Fruit softening occurs by several mechanisms, including modifications of cell wall structure by wall degrading enzymes. The most prominent change in tomato fruit pericarp wall composition is the loss of galactosyl residues throughout development and especially during ripening. In order to understand the role of galactosyl turnover in fruit softening, we successfully produced three recombinant tomato β-galactosidase/exo-galactanase (TBG) fusion proteins in yeast. TBG1, 4 and 5 enzyme properties and substrate specificities were assessed. Optimum pH of TBG1, 4 and 5 was 5.0, 4.0, and 4.5 and optimum temperature was 40∼50, 40, and 40 °C, respectively. The K ms for TBG1, 4 and 5 were 7.99, 0.09, and 2.42 mm, respectively, using p-nitrophenyl-β-D-galactopyranoside as substrate. Using synthetic and plant-derived substrates, TBG1 and 5 released galactosyl residues from 1 → 4 linkages. TBG4 released galactosyl residues from a wide range of plant-derived oligosaccharides and polysaccharides. Using tomato fruit cell wall material, TBG1, TBG4 and TBG5 released galactosyl residues from a variety of fruit stages and cell wall fractions. TBG4 released the most galactosyl residues from the ASP fraction and especially the ASP fraction from fruit at the turning stage. Interestingly, even though walls from Turning fruit stage contain less total galactosyl residues than at the Mature Green stage, TBG4 released 3–4 fold more galactose from the CSP and ASP fractions from Turning fruit. These results suggest that changes in structure of wall pectic polysaccharides leading up to the Turning stage may cause the wall to become more susceptible to hydrolysis by the TBG4 product.
An Intracellular Laccase is Responsible for the Epicatechin Mediated Anthocyanin Degradation in Litchi Fruit Pericarp
