Gene expression of NAD(+)-dependent sorbitol dehydrogenase and NADP(+)-dependent sorbitol-6-phosphate dehydrogenase during development of loquat (Eriobotrya japonica Lindl.) fruit

The aim of this study is to clarify the roles of NAD(+)- dependent sorbitol dehydrogenase (NAD-SDH) and NADP(+)-dependent sorbitol-6-phosphate dehydro genase (S6PDH) in fruit development, and the regulatory mechanism(s) underl ying their expression during development using loquat as the study material . The cDNA of NAD- SDH, cloned from loquat fruit, consisted of 1,572 bp and c ontained an open reading frame of 1,023 bp capable of encoding a protein of 371 amino acids. The deduced amino acid sequence has 97.8% identity to tha t of an apple fruit. The activity of NAD-SDH is a function of fruit development, i.e. the increa se in protein of this enzyme synchronized consistently with that of activit y showing no posttranslational modification throughout the developmental st ages. Furthermore, NAD-SDH activity correlated with the mRNA levels, indica ting that the key regulatory step of the activity is at the transcriptional level. The increase in NAD-SDH with fruit development plays a dominantly i mportant role during fruit maturation and sugar accumulation. The trend of S6PDH activity during fruit development paralleled that of NAD - SDH activity. It correlated with protein and mRNA levels revealing that t he regulation of the activity is mainly at the transcriptional level like N AD- SDH. However, the role of S6PDH in fruit is not yet clear.