Gene expression of NAD-dependent sorbitol dehydrogenase during fruit development of apple (Malus pumila Mill. var. domestica Schneid.)

Abstract

The purpose of this study is to establish the mechanism of expression of NAD-dependent sorbitol dehydrogenase (NAD-SDH) activity during the development of apple (Malus pumila Mill. var. domestica Schneid, cv. Fuji) by investigating the NAD-SDH activity and the levels of NAD-SDH protein and transcript. The gene expression was investigated by RNA blot analysis using NAD-SDH cDNA and the protein level was determined by Western blot using the antibody against NAD-SDH protein expressed in E. coli. The activity of NAD-SDH was very weak up to 80 days after flowering (DAF), then increased temporarily at 104DAF. Thereafter, it declined with fruit enlargement; it rose rapidly again from 160 DAF with fruit maturation and remained high up to 207 DAF. That the levels of the immunoreactive protein of NAD-SDH per g fresh weight were high at 104, 181, and 207 DAF indicates a close correlation between the activity and the protein of NAD-SDH. Furthermore, RNA blot analysis indicated that the NAD-SDH transcript was proportionate to the level of the activity and protein through the developmental stages. These results indicate that the rise in the activity may depend upon the expression of the NAD-SDH gene.