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

Abstract

The aim of this study is to clarify the roles of NAD+-dependent sorbitol dehydrogenase (NAD-SDH) and NADP+-dependent sorbitol-6-phosphate dehydrogenase (S6PDH) in fruit development, and the regulatory mechanism(s) underlying 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 contained 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 that of an apple fruit. The activity of NAD-SDH is a function of fruit development, i.e. the increase in protein of this enzyme synchronized consistently with that of activity showing no posttranslational modification throughout the developmental stages. Furthermore, NAD-SDH activity correlated with the mRNA levels, indicating that the key regulatory step of the activity is at the transcriptional level. The increase in NAD-SDH with fruit development plays a dominantly important 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 the regulation of the activity is mainly at the transcriptional level like NAD-SDH. However, the role of S6PDH in fruit is not yet clear.