ß-Galactosidase II Activity in Relation to Changes in Cell Wall Galactosyl Composition during Tomato Ripening

Abstract

Activity of ß-galactosidase II (EC 3.2.1.23), which can hydrolyze ß-galactan from tomato cell walls, increased markedly during ripening of `Roma' and `Rutgers' tomatoes (Lycopersicon esculentum Mill.). Activity of two other ß-galactosidase isozymes, incapable of galactan hydrolysis, was present in green fruit and remained unchanged throughout ripening. ß-Galactosidase II activity was not detectable in green fruit of either cultivar, appearing first at the breaker stage of `Roma' fruit and not until the pink stage of `Rutgers' fruit. Consistent with this, galactose loss from Na2CO3-soluble pectin (NSP) was detectable at an earlier stage in `Roma' vs. `Rutgers' fruit. A greater decline in NSP galactose was evident in `Roma' fruit compared to `Rutgers' fruit, in keeping with the higher levels and longer period of ß-galactosidase II expression in the former. Significant galactose loss from trans -1,2-diaminocyclohexane-N,N,N',N' -tetraacetic acid-soluble pectin, in contrast, was not seen until the last stage of ripening. These results indicate that the long-reported, net galactosyl loss from the cell walls of ripening tomatoes correlates with ß-galactosidase II activity. Nonetheless, the observation that softening commenced before ß-galactosidase II activity or galactose loss was detectable suggests some other basis for the earliest stages of ripening-related fruit softening in tomato.