Partial cloning and expression analysis of genes encoding NAD+-dependent sorbitol dehydrogenase in pear bud during flower bud formation

Partial fragments of five NAD+-dependent sorbitol dehydrogenase (NAD-SDH) genes were isolated from the developing buds of Japanese pear (Pyrus pyrifolia cv. Kosui) by RT-PCR (tentatively named PpySDH1-5). The derived amino acid sequences of PpySDH1-5 have 94 to 98% homology with NAD-SDH genes from apple. PpySDH1-4 were clearly distinguished from PpySDH5 based on primary structure, which showed 88-95% identity with each other, but only 69-71% identity with PpySDH5. Shading of limbs for 20 days increased NAD-SDH enzymatic activity in the buds and correspondingly increased the expression detected by PpySDH1. On the other hand, shoot bending increased NAD-SDH activity in lateral buds, and also increased the expression detected by PpySDH1. The expression of PpySDH5 was barely detected in any of these tissues. These results suggest that PpySDH1-5 encodes the isoforms of NAD-SDH protein which can be categorized into at least two groups and that PpySDH5 may not be requisite to bud growth, though some of the other NAD-SDH gene(s) may be. The increases of NAD-SDH activity, both by shading and shoot bending, are suspected to be regulated at the transcriptional level.