Abstract
To clarify the roles of the five sorbitol- and sucrose-related enzymes important to the development of ‘La France’ pear fruit, we traced the fluctuations in the activities and mRNA levels of these enzymes. Such an approach of simultaneously assaying five sugar-metabolizing enzymes is a first in pear fruit and the Rosaceae family. Both the activity and the mRNA level of NAD+-dependent sorbitol dehydrogenase (NAD-SDH) were high in young fruit, but decreased during fruit enlargement and rose again with fruit maturation. Sucrose synthase (SS) showed nearly the same fluctuation pattern as did NAD-SDH. The changes in both NAD-SDH and SS activities, based on fresh weight, were proportional to that of the relative growth rate (RGR) of the fruit. This suggests that they function closely in supplying hexose from the unloaded sugars in the phloem for active growth. Two isogenes of soluble acid invertase (S-AIV), S-AIV1 and S-AIV2, were present in pear fruit; the former was expressed highly in young fruit, whereas the latter was expressed both in young and mature fruits. The fluctuation pattern of the activity for sorbitol-6-phosphate dehydrogenase (S6PDH) was similar to that of NAD-SDH. However, the role of S6PDH in fruit is unclear. Although sucrose phosphate synthase (SPS) activity was higher in young fruit, it decreased and remained low with fruit maturation. This may be attributed to the finding that ‘La France’ pear fruit accumulates hexose but not sucrose. These results showed that NAD-SDH, SS, and S-AIV are of prime importance in supplying sugars for the development and accumulation of hexose necessary for pear fruit enlargement.
