beta-galactosidase II activity in relation to changes in cell wall galactosyl composition during tomato ripening

Activity of beta-galactosidase II (EC 3.2.1.23), which can hydrolyze beta-galactan from tomato cell walls, increased markedly during ripening of 'Roma' and 'Rutgers' tomatoes (Lycopersicon esculentum Mill.). Activity of two other beta-galactosidase isozymes, incapable of galactan hydrolysis, was present in green fruit and remained unchanged throughout ripening. beta-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 beta-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 beta-galactosidase II activity. Nonetheless, the observation that softening commenced before beta-galactosidase II activity or galactose Loss was detectable suggests some other basis for the earliest stages of ripening-related fruit softening in tomato.